1 A Project Report on HIGH TEMEPERATURE SUPERCONDUCTIVITY IN ELECTRICAL POWER APPLICATIONS; A DETAILED STUDY OF HTSC SYNCHRONOUS MACHINE & EXPERIMENTATION ON 1 ST GENERATION AND 2 ND GENERATION HTSC WIRES Submitted by Srinagadatta Srikrishna R. (Reg. No. 011205146) In partial fulfillment for the award of the degree of Bachelor of Technology in Electrical and Electronics Engineering Under the esteemed guidance of Mr. B. V. A. S. Muralidhar Mr. R. Rajesh Sr. Engineer (EMC) Assistant Professor III BHEL Corporate R&D SEEE, SASTRA University School of Electrical & Electronics Engineering SASTRA University Thanjavur, India – 613 401 April, 2012
Transcript
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A Project Report on
HIGH TEMEPERATURE SUPERCONDUCTIVITY IN ELECTRICAL
POWER APPLICATIONS; A DETAILED STUDY OF HTSC SYNCHRONOUS MACHINE &
EXPERIMENTATION ON 1ST GENERATION AND 2ND GENERATION HTSC WIRES
Submitted by Srinagadatta Srikrishna R. (Reg. No. 011205146)
In partial fulfillment for the award of the degree of
Bachelor of Technology
in
Electrical and Electronics Engineering
Under the esteemed guidance of
Mr. B. V. A. S. Muralidhar Mr. R. Rajesh Sr. Engineer (EMC) Assistant Professor III BHEL Corporate R&D SEEE, SASTRA University
School of Electrical & Electronics Engineering
SASTRA University Thanjavur, India – 613 401
April, 2012
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BONAFIDE CERTIFICATE
Certified that the project work entitled “HIGH TEMPERATURE
SUPERCONDUCTIVITY IN ELECTRICAL POWER APPLICATIONS;
A DETAILED STUDY OF HTSC SYNCHRONOUS MACHINE &
EXPERIMENTATION ON 1ST GENERATION AND 2ND GENERATION
HTSC WIRES” submitted to SASTRA University, Thanjavur by
SRINAGADATTA SRIKRISHNA R (Reg. No:011205146), in partial
fulfillment for the award of the degree of Bachelor of Technology in Electrical
and Electronics Engineering is the work carried out independently under my
guidance during the period Jan 2012 – April 2012.
Project Guide
[Project Guide Name] [Designation & Affiliation]
External Examiner Internal Examiner
Submitted for the University Exam held on ___________
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DECLARATION
I submit this project work entitled “HIGH TEMPERATURE SUPERCONDUCTIVITY
IN ELECTRICAL POWER APPLICATIONS; A DETAILED STUDY OF HTSC
SYNCHRONOUS MACHINE & EXPERIMENTATION ON 1ST GENERATION
AND 2ND GEENRATION HTSC WIRES” to SASTRA University, Thanjavur in partial
fulfillment of the requirements for the award of the degree of “Bachelor of Technology” in
“Electrical and Electronics Engineering”. I declare that it was carried out independently
by me under the guidance of Mr. B. V. A. S. Muralidhar, Senior Engineer, BHEL Corporate
R&D, Vikasnagar, Hyderabad - 500093
SRINAGADATTA SRIKRISHNA R (Reg. No. :011205146 )
Date: Signature: Place:
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ACKNOWLEDGEMENTS
I wish to thank SASTRA University for providing me this opportunity to carry out my project work at BHEL Corporate R&D and for supporting my work.
I also thank BHEL Corporate R&D, Hyderabad for granting me permission to carry out my project work under their aegis.
I wish to express my heartfelt gratitude to Mr. M Seetaram, Additional General Manager (AGM), Electrical Machines Laboratory (EMC), BHEL Corporate R&D for being with me through the entire tenure of the project providing both technical and non – technical guidance required to initiate, to progress through and to complete this project. Without his guidance, this work would not have taken place.
I thank my guide Mr. B. V. A. S. Muralidhar, Senior Engineer, Electrical Machines Laboratory (EMC), BHEL Corporate R&D, Hyderabad for giving me this project and for guiding me throughout the project tenure. I am very grateful to him for identifying my capabilities and entrusting me with this project. It has been a very great learning experience under his guidance
I also wish to thank Mr. S. Ramacharyulu, Additional Engineer, Mr. Ramesh, Draftsman and Mr. Prakash, Junior Engineer, Electrical Machines Laboratory for patiently entertaining my technical queries and for the invaluable technical discussions that I had with them.
I wish to thank Dr. J. L. Bhattacharya, Ex – AGM, EMC, BHEL Corporate R&D for being a source of inspiration to me. The few interactions that I had the opportunity of having with him gave me a deep insight not only into technical aspects, but also were enlightening on the aspects of work process, research methodologies, professionalism and also about life.
I also wish to thank my internal guide Prof. R. Rajesh, Assistant Professor III, SEEE, SASTRA University for being a great help and encouragement.
I also thank Dr. Umakant Choudhary, GM(PEC & HR), BHEL Corporate R&D for his kind help and concern.
Finally I wish to thank my parents, friends and all those who associated with me over the project tenure, inspired me, advised me and helped me at BHEL Corporate R&D and at SASTRA University.
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ABSTRACT
This report is a study on High Temperature Superconductivity (HTSC) applications in
electrical power industry with special focus on the HTSC Synchronous Motor. A basic
theoretical treatise on superconductivity is first presented followed by a detailed
discussion on the 1st generation and 2nd generation HTSC wires. The manufacturing
procedures and technologies involved have been discussed in detail. Basic
experimentation was conducted on HTSC wires at Electrical Machines Laboratory,
BHEL Corporate R&D, Hyderabad. The data has been presented. Following this, I have
conducted a brief study on various power applications of HTSC i.e. HTSC Power cables,
HTSC Magnetic Energy Storage (MES), HTSC Fault Current Limiter (FCL), HTSC
Transformer and HTSC Machines. Next, an effort was made to understand the concept,
working, conceptual design, design process, the difficulties and complexities involved in
the manufacture and assembly of a HTSC Synchronous Motor currently under research
at the lab. The observations from the above field study at the research lab have been duly
reported in this work.
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LIST OF FIGURES
Figure No. Description
1.1 Discovery of superconducting materials
3.1 Defining parameters for Superconductivity
3.2 Resistivity graph of Superconductors vs. normal conductors
3.3 Behavior of type 1 superconductors to external magnetic field
3.4 Resistivity, Internal magnetic field and Magnetization of type 1 SC
3.5 Behavior of type 2 SC in an external magnetic field
3.6 Illustration of type 2 SC structure
3.7 Different states in the transition of type 2 SC materials
3.8 Resistivity, Internal magnetic field and Magnetization of type 2 SC
4.1 IBAD
4.2 RABiTS
4.3 Application specific requirements of the Superconducting wire for commercial applications
4.4 Manufacture of BSCCO wire
4.5 Cross section of BSCCO wire
4.6 Molecular structure of YBCO
4.7 Properties of YBCO superconductors
4.8 Manufacture of YBCO wires
4.9 Structure of YBCO wire
5.1 S.C.C. tests on the prototype transformer
5.2 O.C.C. tests on the prototype transformer
6.1 Cold type HTCS Power cable
6.2 Warm type HTSC Power cable
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6.3 Difference between AC & DC HTSC cables & conventional cables
7.1 Block diagram of a SMES based system
8.1 Conceptual design of resistive type SFCL
9.1 Reduction in size in transformers with HTSC
9.2 Difference in the conceptual designs of conventional vs. HTSC transformers
9.3 State – of – the Art of HTSC transformer projects
10.1 Loss comparisons of air core (HTSC) and iron – core (conventional) 7.5MW, 3600rpm, 60Hz for full load rated speed operations
11.1. Model of HTSC race track with coil copper encasing
11.2 Race track coils fabricated by Siemens
11.3 Neon based cryo cooling technique
11.4 Illustration of cooling process in Neon based system
11.5 Conceptual design adopted by Electrical Power Research Institute (EPRI)
11.6 Conceptual Design adopted by Siemens AG
11.7 Conventional Synchronous Rotor
11.8 HTSC Synchronous Rotor
11.9 Projected Efficiencies of a HTSC Synchronous Motor compared to a conventional Synchronous Motor
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LIST OF TABLES
Table no. Description
4.1 Application specific requirements of the Superconducting wire for commercial applications
5.1 S.C.C. tests on prototype transformer
5.2 O.C.C. tests on the prototype transformer
6.1 State – of – the Art HTSC Power cable projects
6.2 Present status of Power cables vs required specifications
7.1 Comparison of various energy storage technologies
7.2 State – of – the Art SMES projects
8.1 State – of – the Art HTSC FCL projects
9.1 State – of – the Art HTSC transformers projects
10.1 State – of – the Art HTSC Machine projects
11.1 Current projects on HTSC Synchronous Machines
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TABLE OF CONTENTS
Acknowledgements 4
Abstract 5
List of Figures 6
List of Tables 8
1. Introduction……………………………………………………………………………………………………….. 12 1.1. Organization of this report……………………………………………………………………….…….15
2. Review of Related Literature………………………………………………………………………………..16 SECTION 1
3. Introduction to Superconductivity…………………………………………………….………………….19 3.1. Superconductivity…………………………………………………………………….……….…………..19 3.2. Superconductors……………………………………………………………………..…………………….19 3.3. Defining parameters of Superconductivity……………………………………………….……19 3.4. Hallmarks of Superconductivity………………………………………………………………..……20 3.5. Theory governing superconductivity (BCS Theory)………………………………………...21 3.6. Classification of Superconductors…………………………………………………………….…….21 3.7. Type 1 Superconductors…………………………………………………………………………………22 3.8. Type 2 Superconductors…………………………………………………………………….…………..23 3.9. Applications of Superconductivity………………………………………………………..…………24
4. High Temperature Superconducting (HTSC) wires………………………………………………..25 4.1. Introduction………………………………………………………………………………………………..….25 4.2. Demands on conductors for coil applications………………………………………………….28 4.3. BSCCO wire…………………………………………………………………………………………………..…29 4.4. Manufacture of BSCCO wire……………………………………………………………………………30 4.5. Disadvantages of BSCCO wire……………………………………………………………………..….31 4.6. YBCO wire………………………………………………………………………………………………..……..31 4.7. Manufacture of YBCO wires…………………………………………………………………………...31 4.8. Properties of HTSC wires………………………………………………………………………..……….33
5. Experimentation on 1st generation & 2nd generation HTSC wires………………………..…36 5.1. Testing of 1G HTSC (BSCCO) wire……………………………………………………………….……36 5.2. Testing of 2G HTSC (YBCO) wire……………………………………………………………….………36 5.3. S.C.C. tests on the prototype transformer……………………………………………………...37 5.4. O.C.C. tests on the prototype transformer……………………………………………………..38
SECTION 2
6. High Temperature Superconducting (HTSC) power cables…………………………………….41 6.1. Introduction……………………………………………….…………………………………………………..41 6.2. Types of HTSC power cables……………………………………………………………………….…..41 6.3. Benefits of HTSC power cables…………………………………………………………………………42
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6.4. State – of – the – Art HTSC power cables………………………………………………………….43 7. High Temperature Superconducting (HTSC) Magnetic Energy Storage (MES)………..45
7.1. Introduction……………………………………………………………………………………………………..45 7.2. Concept and working of a SMES…………………………………………………………………….…46 7.3. Advantages of SMES…………………………………………………………………………………………46 7.4. State – of – the – Art SMES projects………………………………………………………………...47
8. High Temperature Superconducting (HTSC) Fault Current Limiter (FCL)………………...48 8.1. Introduction……………………………………………………………………………………………………..48 8.2. Ideal fault current limiter characteristics………………………………………………..………..49 8.3. Types of fault current limiters…………………………………………………………………………..49 8.4. Passive limiters…………………………………………………………………………………………………49 8.5. Solid – state limiters………………………………………………………………………………………...50 8.6. Hybrid limiters………………………………………………………………………………………………….51 8.7. Introduction to Superconducting Fault Current Limiters………………………………….51 8.8. Conceptual design of Resistive type SFCL…………………………………………………………53 8.9. Basic design aspects…………………………………………………………………………………………53 8.10. State – of – the – Art SFCL projects……………………………………………………………54 8.11. Current status of SFCL technology…………………………………………………….………54
9. High Temperature Superconducting (HTSC) transformer………………………………………56 9.1. Introduction…………………………………………………………………………………………………….56 9.2. Benefits of HTSC transformer………………………………………………………………………..…56 9.3. Design tradeoffs and cost drivers of HTSC Transformers………………………………….57 9.4. Achieving cryogenic temperatures and maintaining it………………………………………59 9.5. Practical issues in the design of HTSC transformers………………………………………….60 9.6. State – of – the – Art HTSC transformer projects……………………………………………..60
10. High Temperature Superconducting (HTSC) Machines…………………………………………..63 10.1. Introduction……………………………………………………………………………………………….63 10.2. Concept behind HTSC machines…………………………………………………………………63 10.3. Efficiency……………………………………………………………………………………………………65 10.4. Introduction to HTSC machine design…………………………………………………………66 10.5. Machine design challenges…………………………………………………………………………67 10.6. State – of – the – Art HTSC machine projects……………………………………………..68
SECTION 3
11. A detailed study of the High Temperature Superconducting (HTSC) synchronous motor……………………………………………………………………………………………………………………..…71 11.1. Background of this study………………………………………………………………………………71 11.2. Prior experience of the Electrical Machines Laboratory in Superconducting
Applications…………………………………………………………………………………………………..71 11.3. Conceptual design of the HTSC motor…………………………………………………………..71 11.4. Variations in the design followed by R&D organizations worldwide……………..73 11.5. Conceptual design adopted by BHEL R&D………………………………………………………75 11.6. Steps involved in performing the detailed design of a HTSC motor……………….82 11.7. Advance design variations developed by R&D institutions worldwide…………..83
12. Summary and conclusion…………………………………………………………………………………………….90
Appendix A: List of all available superconductors..........................................................92
Appendix B: Glossary associated with superconductivity..............................................98
Appendix C: A case study on the R&D work on HTSC transformers with fault
current limiting capability at Waukesha Electric Systems, USA ..............107
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CHAPTER 1
INTRODUCTION
Superconductivity is a phenomenon occurring in certain materials at low temperatures, characterized by the complete absence of electrical resistance and the damping of the interior magnetic field (Meissner effect). Superconducting materials have the unique property of being able to carry current with negligible resistive losses. . The critical temperature for SCs is the temperature at which the electrical resistivity of a SC drops to zero. Some critical temperatures of metals are: aluminium (Al) Tc = 1:2 K, tin (Sn) Tc = 3:7 K, mercury (Hg) Tc = 4:2 K, vanadium (V) Tc = 5:3 K, lead (Pb) Tc = 7:2 K, niobium (Nb) Tc = 9:2 K. Compounds can have higher critical temperatures, e.g., Tc = 92 K for YBa2Cu3O7 and Tc = 133 K for HgBa2Ca2Cu3O8. Superconductivity was discovered by Dutch scientist H. Kamerlingh Onnes in 1911 (Nobel Prize in 1913). Onnes was the first person to liquefy helium (4.2 K) in 1908. The near no-loss property occurs when the superconducting material is operated below a critical temperature, magnetic field, and current density level. The SC state is defined by three factors:
critical temperature Tc; critical magnetic field Hc; critical current density Jc.
Maintaining the superconducting state requires that the magnetic field and the current density, as well as the temperature, remain below the critical values, all of which depend on the material. For most practical applications, SCs must be able to carry high currents and withstand high magnetic field without averting to their normal state. Meissner effect (sometimes called Meissner-Ochsenfeld effect) is the expulsion of a magnetic field from a SC. When a thin layer of insulator is sandwiched between two SCs, until the current becomes critical, electrons pass through the insulator as if it does not exists. This effect is called Josephson effect and can be applied to the switching devices that conduct on-off operation at high speed. In type I SCs the superconductivity is ‘quenched’ when the material is exposed to a sufficiently high magnetic field. This magnetic field, Hc , is called the critical field. In contrast, type II SCs has two critical fields. The first is a low-intensity field Hc1, which partially suppresses the superconductivity. The second is a much higher critical field, Hc2, which totally quenches the superconductivity. The upper critical field of type II SCs tends to be two orders of magnitude or more above the critical fields of a type I SC. Some consequences of zero resistance are as follows: _ When a current is induced in a ring-shaped SC, the current will continue to circulate in the ring until an external influence causes it to stop. In the 1950s, ‘persistent currents’ in SC rings immersed in liquid helium were maintained for more than five years without the addition of any further electrical input.
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_ A SC cannot be shorted out, e.g., a copper conductor across a SC will have no effect at all. In fact, by comparison to the SC, copper is a perfect insulator. _ The diamagnetic effect that causes a magnet to levitate above a SC is a consequence of zero resistance and of the fact that a SC cannot be shorted out. The act of moving a magnet toward a SC induces circulating persistent currents in domains in the material. These circulating currents cannot be sustained in a material of finite electrical resistance. For this reason, the levitating magnet test is one of the most accurate methods of confirming superconductivity. _ Circulating persistent currents form an array of electromagnets that are always aligned in such as way as to oppose the external magnetic field. In effect, a mirror image of the magnet is formed in the SC with a North pole below a North pole and a South pole below a South pole. The main factor limiting the field strength of the conventional (Cu or Al wire) electromagnet is the I2R power losses in the winding when sufficiently high current is applied. In a SC, in which R = 0, the I2R power losses practically do not exist. The only way to describe SCs is to use quantum mechanics. The model used is the BSC theory (named after Bardeen, Cooper and Schrieffer) was first suggested in 1957 (Nobel Prize in 1973) [5]. It states that: _ lattice2 vibrations play an important role in SCs; _ electron-phonon interactions are responsible. Photons are the quanta of electromagnetic radiation. Phonons are the quanta of acoustic radiation. They are emitted and absorbed by the vibrating atoms at the lattice points in the solid. Phonons possess discrete energy (E = hv) where h = 6:626 068 96(33) Js is Planck constant and propagate through a crystal lattice. Low temperatures minimize the vibrational energy of individual atoms in the crystal lattice. An electron moving through the material at low temperature encounters less of the impedance due to vibrational distortions of the lattice. The Coulomb attraction between the passing electron and the positive ion distorts the crystal structure. The region of increased positive charge density propagates through the crystal as a quantized sound wave called a phonon. The phonon exchange neutralizes the strong electric repulsion between the two electrons due to Coulomb forces. Because the energy of the paired electrons is lower than that of unpaired electrons, they bind together. This is called Cooper pairing. Cooper pairs carry the supercurrent relatively unresisted by thermal vibration of the lattice. Below Tc, pairing energy is sufficiently strong (Cooper pair is more resistant to vibrations), the electrons retain their paired motion and upon encountering a lattice atom do not scatter. Thus, the electric resistivity of the solid is zero. As the temperature rises, the binding energy is reduced and goes to zero when T = Tc. Above Tc a Cooper pair is not bound. An electron alone scatters (collision interactions) which leads to ordinary resistivity. Conventional conduction is resisted by thermal vibration within the lattice. In 1986 J. Georg Bednorz and K. Alex Mueller of IBM Ruschlikon, Switzerland, published results of research [7] showing indications of superconductivity at about 30 K (Nobel Prize in 1987). In 1987 researchers at the University of Alabama at Huntsville (M. K. Wu) and at the University of Houston (C. W. Chu) produced ceramic SCs with a critical temperature (Tc = 52:5 K) above the temperature of liquid nitrogen. As of March 2007, the current world record of superconductivity is held by a ceramic SC consisting of thallium, mercury, copper, barium, calcium, strontium and oxygen with Tc = 138 K. There is no widely-accepted temperature that separates high temperature superconductors (HTS) from low temperature superconductors (LTS). Most LTS superconduct at the boiling point of liquid helium (4.2 K = �2690C at 1 atm). However, all the SCs known before the 1986 discovery of the superconducting oxocuprates would be classified LTS. The barium-lanthanum-cuprate Ba-La-Cu-O fabricated by Mueller and Bednorz, with a Tc = 30 K = �2430C, is generally
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considered to be the first HTS material. Any compound that will superconduct at and above this temperature is called HTS. Most modern HTS superconduct at the boiling point of liquid nitrogen (77 K = �1960C at 1 atm). All HTS are cuprates (copper oxides). Their structure relates to the perovskite structure (calcium titanium oxide CaTiO3) with the general formula ABX3. Perovskite CaTiO3 is a relatively rare mineral occurring in orthorhombic (pseudocubic) crystals. Economic Considerations Before 1986, the critical temperature of superconducting materials was in the range where liquid helium (at 4.2 Kelvin) was the required cooling fluid for superconducting coils. The cost to cool superconducting coils using these low temperature superconducting (LTS) materials was prohibitive when considering their use in industrial electric motors. In 1986 the discovery of high temperature superconducting (HTS) materials raised the interest of rotating machinery manufacturers as the critical temperature of these materials exceeds the boiling point of liquid nitrogen (77 Kelvin or 77 K). Figure 1.1. Progress in the discovery of Superconducting materials.
Source: [Sheahen] Fig. 1.1 shows the critical temperature of superconducting materials versus their date of discovery. The discovery of the Yttrium and Bismuth based materials, YBaCuO and BiSrCaCuO, respectively in Fig. 1, resulted in active development of HTS wire and coils for industrial electric motor and utility generator applications. Although these materials do superconduct in liquid nitrogen (77 K), they can carry higher currents at higher magnetic fields when their operating temperature is dropped. For superconducting motors, these materials are typically cooled to the 30 to 40 K temperature range. For a 6000 hp (4480 kW)
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industrial motor, the input power to the cooling system for the superconducting coils operating at 30 K will be about seven kW or 0.16% of the rated output power of the motor. In power engineering, superconductivity can be practically applied to synchronous machines, homopolar machines, transformers, energy storages, transmission cables, fault-current limiters, linear synchronous motors and magnetic levitation vehicles. The use of superconductivity in electrical machines reduces the excitation losses, increases the magnetic flux density, eliminates ferromagnetic cores, and reduces the synchronous reactance (in synchronous machines). Research directed at the development of economically viable HTS based industrial motors has been going on for over 18 years and has included the demonstration of a number of motor prototypes in ratings up to 5000 hp (3730 kW). Superconducting technology promises substantial loss reduction for motors in the rating range of 5000 hp and above accompanied with motor volume reduction. At 60 to 77 K (liquid nitrogen) thermal properties become friendlier and cryogenics can be 40 times more efficient than at 4.2 K (liquid helium). Loss reduction of a factor of two compared to energy efficient induction motors of the same rating appears feasible. Along with loss reduction, significant volume and weight reductions are also possible, thereby making HTS based motors a technology that will impact large motor users. 1.1. Organization of this report This report consists of three sections
1. Section 1 This section consists of three chapters which introduces superconductivity and a discussion on HTSC wires and presents the results of the experiments conducted on them.
Chapter 3: Introduction to Superconductivity Chapter 4: High Temperature Superconducting (HTSC) wires Chapter 5: Experimentation on 1st generation and 2nd generation HTSC wires
2. Section 2 This section consists of five chapters focusing on the applications of HTSC in electrical power industry
Chapter 6: High Temperature Superconducting (HTSC) Power Cables Chapter 7 High Temperature Superconducting (HTSC) Magnetic Energy Storage
(MES) Chapter 8: High Temperature Superconducting (HTSC) Fault Current Limiter
(FCL) Chapter 9: High Temperature Superconducting (HTSC) Transformer Chapter 10: High Temperature Superconducting (HTSC) Machines (Generators &
Motors)
3. Section 3 This section consists of chapter 11: A Detailed study of High Temperature Superconducting (HTSC) Synchronous machine.
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CHAPTER 2
REVIEW OF RELATED LITERATURE
Since the discovery of superconductors, the application of superconductivity has found its way into many areas. Besides the most popular applications in medical devices and instruments, such as magnetic resonance imaging (MRI) and superconducting quantum interference device (SQUID), superconductivity has led to demonstrations of several electrical power devices including motors, generators, transformers, fault current limiters, power transmission cables, and superconducting magnetic energy storage systems (SMES). Past three decades has witnessed a large volume of literature in this field in the form of research paper published in journals, papers presented at conferences, reports of the pilot projects undertaken at organizations worldwide, presentations by companies in this field and the specifications sheet of the superconducting products being manufactured by industries. All the above mentioned sources have become great resources in this study.
Each chapter in this report has a different set of literature that was referenced for this study. In general, some sources have been a great resource for the reading, namely
1. SuperPower Inc
www.superpower-inc.com
It is a world leading developer and producer of second-generation high-temperature superconducting (2G HTS) wire, providing enormous advantages over conventional conductors of electric power - high efficiency, smart grid compatible, green, clean, safe and secure.
The numerous presentations provided by SuperPower Inc.on HTSC wires and on each of the power applications of HTSC have been very influential in making this report.
2. American Superconductor
www.amsc.com
It is a provider of ideas, technologies and solutions that meet the world’s demand for smarter, cleaner and better energy.
The information shared by them on the HTSC wires and the various projects; namely the 5MW HTSC Synchronous Motor Project and the 36.5MW HTSC Synchronous Motor project have provided deep insight into the subject
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3. Siemens AG
www.siemens.com
Siemens has been one of the pioneer organizations to experiment the possibility of HTSC in electrical machines. The 4MVA 3600rpm HTSC generator project and 36.5 MW 120 rpm ship propulsion motor have been a great resource to understand the HTSC machine design details
4. Nexans
www.nexans.com
It is a worldwide leader in the cable industry, it offers an extensive range of cables and cabling systems to raise industrial productivity, improve business performance, enhance security, enrich the quality of life, and assure long-term network reliability.
The HTSC cable projects and HTSC FCL projects undertaken by Nexans have been one of the first in the industry providing great information on the art.
5. Electric Power Research Institute
www.epri.com
The Electric Power Research Institute, Inc. (EPRI) conducts research and development relating to the generation, delivery and use of electricity for the benefit of the public. The EPRI, through its establishment, Reliance Electric Ltd. has been the first in the industry to experiment with HTSC motors. The 2MW and the 5MW HTSC motor projects reports published in the 1997 report titled ‘Electric Motors using High Temperature Superconducting Materials Applied to Power Generating Station Equipment’ prepared by Reliance Electric for EPRI and published in January, 1997 has been one of the first project reports on the pioneer HTSC projects which describes the machine design in detail.
6. A book titled “Introduction to High Temperature Superconductivity” by Thomas P. Sheahen published by Kluwer Academic Publishers has been a great resource for all the chapters in this report.
Other projects carried out at General Electric, Zenergy, Wetinghouse, Waukesha Electric Systems, Converteam and many other companies have made great contribution to this report through their respective project reports and associated numerous publications.
Apart from these, many IEEE papers, independent papers, reviews and publications in various other journals and conferences have assisted in the making of this report. In general, www.wikipedia.org has been a great source of information on few topics.
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SECTION 1
INTRODUCTION TO SUPERCONDUCTIVITY
DETAILED DISCUSSIONS ON 1ST GENEARTION AND 2ND GENERATION HTSC WIRES AND EXPERIMENTS
CONDUCTED ON THEM
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CHAPTER 3
INTRODUCTION TO SUPERCONDUCTIVITY
3.1. Superconductivity
The field of superconductivity, once a mere laboratory curiosity has moved into the realm of applied science since the last 40 years. Many more applications have become possible because of the discovery of ceramic superconductors which operate at comparatively at high temperatures and even more will be possible.
3.2. Superconductors
For most materials which are normal conductors whenever an electrical current flows there is some resistance to the flow of electrons through the materials. It is necessary to apply a voltage to keep the current going; to replace the energy dissipated by the resistance in the material. A superconductor; in contrast is a material with no resistance at all.
A lot of metals show modest resistance at room temperatures but turn into superconductors when cooled to almost near to absolute zero temperatures. The first superconductor to be discovered was mercury, soon after the invention of a cryo refrigerator that could attain temperatures below the boiling point of Helium i.e. 4.2K. In the subsequent years many more materials were found to be superconducting at these very low temperatures. By the 1960’s certain allows of Niobium were made which became superconductors in the temperature range of 10 – 25K. It was then generally believed according to theoretical grounds that there would be no superconductors above 30K.
Since a superconductor has no resistance, it carries current indefinitely without requiring voltage or expenditure for electricity. Once the current is started it continues, provided the superconductor is kept below the critical temperature. The cost of running a superconducting persistent loop is simply the cost of refrigeration; which for low temperature superconductors being Helium proved to be very expensive.
3.3. Defining parameters of Superconductivity
1. Critical Current The ideal physical definition of critical current is the current where a material has a phase transition from a superconducting phase to a non-superconducting phase. For practical superconducting wire, the transition is not infinitely sharp but gradual. In this case, the critical current is defined as the current where the voltage drop across the wire becomes greater than a specific electric field, usually 1 microvolt/cm. Sometimes, for low-loss magnet applications, a lower electric field criterion is used. The critical current is represented by the variable Ic.
2. Critical Temperature It is the temperature below which the transition takes place from normal conducting state to the superconducting state.
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3. Critical Field A superconducting material can tolerate only a certain maximum field over which it loses its superconducting nature. So, the domain of superconductivity is restricted within the limits of critical current, critical temperature and critical field. Figure 3.1. Defining Parameters for Superconductivity
Source: SuperPower Inc.
3.4. Hallmarks of Superconductivity
A material is said to have transitioned into the superconducting phase when it exhibits certain characteristic properties. In essence, experimentally when these properties have been identified in the material, then it can be stated that the material has transitioned into the superconducting state.
1. Zero Resistivity 2. Perfect Diamagnetism
3.4.1. Zero Resistivity
Figure 3.2. Resistivity graph of Superconductors vs. normal conductors
Source: www.wikipedia.org
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A normal conductor when cooled near to absolute zero temperatures the resistivity decreases gradually and at absolute zero temperature reaches a finite value which can be found for that material by analyzing the dependence of resistivity of that material with temperature. Unlike, for a superconducting material as the temperature is cooled; at a finite temperature called the critical temperature the resistance abruptly falls to zero.
3.4.2. Perfect Diamagnetism It is more popularly known by a name i.e. the Meissner effect. This plays a central role in the magnetic phenomenon associated with superconductivity. Meissner effect is the expulsion of magnetic field from within the superconductor. This expulsion is very different from not letting any magnetic field inside the material; any metal with infinite conductivity would do the later. If a magnetic field is already present and a material is cooled below the critical temperature to become a superconductor; all the magnetic field within that material is expelled out of it. Hence, it behaves like a perfect dia – magnet; i.e. all the magnetic field applied over the material simply passes over the material without entering into it giving the effect of the superconductor behaving like a magnet.
3.5. Theory governing superconductivity (BCS theory)
BCS theory proposed by Bardeen, Cooper and Schrieffer (BCS) was the first microscopic theory of superconductivity. This theory describes superconductivity as a microscopic effect caused by a condensation of electrons into a boson – like state.
Roughly speaking the superconducting phenomenon has been given the following explanation by BCS. An electron moving through a conductor will attract nearby positive charges in the lattice. The deformation of the lattice causes another electron of opposite spin to move into the region of higher positive charge density. The two electrons then become correlated. There are many such pairs in a superconductor so that they overlap very strongly forming a highly collective condensate. Breaking of one pair results in a change in the energies of the remaining macroscopic number of pairs. If the required energy is greater than the energy gained by the collisions with the oscillating atoms, then the electrons will stay paired, thus nit experiencing any resistance. Thus the collective behavior of the condensate is the crucial ingredient of superconductivity.
In many superconductors, the attractive interaction between electrons (necessary for pairing) is brought about indirectly by the interaction between the electrons and the vibrating crystal lattice (the phonons)
Any more discussion in the field of superconductivity will lead into Quantum Mechanics, hence we stop here.
3.6. Classification of Superconductors
3.6.1. Based on their response to magnetic field
1. Type 1 They have a single critical magnetic field HC above which superconductivity is lost
22
2. Type 2 They have two critical fields HC1 and HC2 between which there is partial superconductivity. Above HC2 there is no superconductivity and below HC1 it is a perfect superconductor
3.6.2. Based on their governing theory
1. Conventional They can be explained by the Bardeen Cooper Schreiffer (BCS) theory
2. Non – conventional BCS theory fails in explaining the superconductivity in them.
3.6.3. Based on their critical temperature
1. Low temperature They have a critical temperature below 77K
2. High temperature They have a critical temperature above 77K
3.7. Type 1 Superconductors
The behavior of the material is described in the figure on the left In the presence of an external magnetic field, the superconducting material generates surface currents
which oppose the existing magnetic field making it perfectly diamagnetic. Figure 3.3. Behavior of type 1 superconductors to external magnetic field (left) Figure 3.4. Resistivity, Internal magnetic field and Magnetization of type 1 SC (right)
Source: Sheahen (left) Source: SuperPower Inc. (right)
23
3.8. Type 2 Superconductors
The behaviour of type 2 superconductors is very complex and cannot be explained by BCS theory. They have small gaps within the volume of the material which allows the magnetic field to pass
through it. These gaps are called vortices. Magnetic field when in between the two critical magnetic fields HC1 and HC2 passes through the
vortices thereby defeating the Meissner effect Figure 3.5. Behavior of type 2 SC in an external magnetic field (left) Figure 3.6. Illustration of type 2 SC structure (right)
Source: Introduction to superconductivity, Sheahen
Figure 3.7. Different states in the transition of type 2 SC materials (left) Figure 3.8. Resistivity, Internal magnetic field and Magnetization of type 2 SC (right)
Source: SuperPower Inc.
24
With this, the discussion on the superconductivity and its theories ends since the main focus of this report are the applications of superconductivity.
3.9. Applications of Superconductivity
3.9.1. Industrial a. High Gradient Magnetic Separator b. In chemical industries, in the synthesis of certain reactions
3.9.2. High Energy Physics
a. High Power magnets b. SQUID c. In R&D laboratories
3.9.3. Medical a. MRI ( Magnetic Resonance Imagery) b. NMR (Nuclear Magnetic Resonance)
3.9.4. Electronics
a. Antennas b. Filters c. Microcontrollers etc.
3.9.5. Automobile
a. MHD (Magneto Hydrodynamic )motors b. Magnetic Levitation
3.9.6. Electrical
a. Superconducting Power Cables b. Superconducting Magnetic Energy Storage (SMES) c. Superconducting Fault Current Limiter (SFCL) d. Superconducting Transformer e. Superconducting Synchronous Machine
25
CHAPTER 4
High Temperature Superconducting (HTSC) Wires
4 .1. Introduction
In 1986 two IBM scientists, Georg Bednorz and Alex Müller, announced the discovery of a material that was superconducting at 34 K, 11 degrees warmer than had ever before been observed. Within a year, scientists in the U.S. and Japan created new compounds with yet higher superconducting transition temperatures. In fact, by March 1987 eight new materials were produced that are superconducting above 77 K, the boiling point of liquid nitrogen at standard atmospheric pressure. (Liquid nitrogen is an efficient cryogen, inexpensive, easy to insulate, inexhaustible, readily available, and non-polluting.) One of these, YBa2Cu3Ox (YBCO), has all the desired characteristics for use in the electronics industry but lacks one feature essential for use in power applications: ability to be formed into wires by thermo- mechanical means.
In the late 1980’s, scientists turned their attention to the Bi(Pb)SrCaCuO superconductor, a family of HTS that has plate-like grains that align easily when wire-forming processes are used. This family of wires is produced by what is commonly referred to as the oxide powder-in-tube (OPIT or PIT) process. For this, a silver or silver alloy tube is loaded with precursor powder. The tube is then sealed and drawn into a fine wire. These round wires are cut and re-stacked into another hollow tube and, after a series of additional drawing, rolling, and heat treatment steps, multi-filamentary ribbons (or tapes) are produced with the desired superconducting phase assemblage and texture. Lengths of BSCCO wire as long as 1 km are now routinely produced by companies in the U.S. and Japan. At liquid nitrogen temperatures, these wires can have overall engineering current densities in excess of 100 A/mm2 with no applied magnetic field. This performance degrades by an order of magnitude at 77 K upon application of just a few tenths of a tesla magnetic field. Thus, in order to use these wires in electric machinery, such as motors, generators, transformers, and energy storage magnets, the wires must be cooled to temperatures in the neighborhood of 20-30 K using helium gas or a closed-cycle cryo cooler. Since superconducting, rotating electric machines may need fields as high as 5 tesla, and since today’s magnetic resonance imaging machines typically generate fields of 1 to 4 tesla, new wires are needed that can take advantage of the simpler, less-costly cryogenics requirements associated with operation at liquid nitrogen temperatures (65-77 K).
The YBCO compound has the unfortunate problem that its grains are difficult to align. In HTS, electric current doesn’t flow well from grain to grain through high-angle grain boundaries. Coatings on silver and silver alloys have also proven to make poor superconductors, due to low superconductor densities and poor grain alignment. So, while YBCO is useful for making thin films on single-crystal substrates for electronics applications or for small discs for bearings, something else is needed for wires.
In 1988 Lawrence Berkeley National Laboratory initiated work to form YBCO tape conductors by depositing films on metal substrates. This was a modest effort, and was regarded as risky since it seemed likely at the time that a way would be found to make more conventional wires of the YBCO compound. However the weak link problem, caused by incomplete alignment of film crystallites, proved highly intractable. It thwarted the conventional approaches, and nearly prevented success with deposited film conductors as well. Fortunately, YBCO film growth itself was not a problem; there were literally hundreds of papers reporting the successful growth of high-current films by epitaxial film growth on single-crystal substrates. Single-crystal
26
substrates are useful for electronic applications. However, for electrical applications (that is, long wires) strong temperature-resistant nickel-alloy substrates coated with yttria stabilized zirconia (YSZ) buffer layers took the place of the single-crystal substrates. The films of YBCO and YSZ were deposited with the pulsed laser deposition (PLD) technique. The YBCO crystallites readily formed with the correct c-axis orientation normal to the substrate, but the in-plane orientation was random. As a result the critical current density of YBCO films on metal and polycrystalline YSZ substrates investigated by Oak Ridge appeared to be limited to about 100 A/mm2 (77 K, 0 T). Thus, in-plane orientation appeared to be necessary. Ion beam assisted deposition (IBAD), as applied by Lawrence Berkeley National Laboratory, and an independent group at Fujikura in Japan, proved to be a solution to the texturing problem. This increasingly popular technique utilizes the bombardment of a growing film with energetic ions, resulting in improved texture. While a normally incident beam is usually used but, the Berkeley group found that an oblique ion beam can introduce the needed in-plane orientation in the YSZ buffer layer. Epitaxial growth of the superconducting YBCO film then resulted in critical current densities up to 6,000 A/mm2, an enormous improvement. Two new processes have been under development since 1991 that promise a new way to manufacture flexible, high current density wires made from YBCO, something that has eluded researchers since the discovery of YBCO in 1987. These wires offer impressive performance opportunities at liquid nitrogen temperatures. In both cases, the key is to prepare a textured substrate, or template on which the YBCO may be deposited as a thick film. Done correctly, the YBCO grains are well-aligned, mimicking the alignment of the underlying substrate, resulting in the prospect of long-length wires that are strongly-linked. Biaxial-textured substrates, where the atomic planes of the grains in each layer of the substrate are well-aligned in the surface of the tape, represent one potential solution to the shortcomings to fabrication of long-length YBCO wires. The national laboratories attacked the YBCO weak-link problem in two different ways.
The Los Alamos group worked to improve the IBAD process, refining the quality of the angular alignment of the YSZ crystallites, and introducing an additional cerium oxide buffer layer which eliminates the tendency of a few YBCO grains to crystallize with a 45 degree misalignment angle. The Los Alamos process is illustrated in figure. With this process current densities reached 8,000 A/mm2 in 1994 and 13,000 A/mm2 in 1995.
27
Figure 4.1. IBAD
Source: [Sheahen]
Oak Ridge National Laboratory researchers turned their attention to developing sharp biaxial textures in metals, such as nickel and copper, and then depositing on them additional, chemically benign metal layers with epitaxial orientation similar to that of the underlying metal strip. In the most recent architecture, Oak Ridge deposits the oxide buffer layers directly on the nickel tape, with no intervening metal coating on the nickel. Like Los Alamos, the thin oxide buffer layers are placed on top in order to transfer the alignment to the superconducting layer while avoiding chemical degradation, but Oak Ridge relies on the alignment of the first metal strip instead of the IBAD process to provide the template for the superconductor (see figure 4.2). Oak Ridge calls its substrate technology .RABiTS.,. or rolling-assisted, biaxial-textured substrates. The Oak Ridge group produced the simplest version of their substrate using dual metal oxide buffer layer architecture and a common industrial film growth technique, called electron beam evaporation. For this, extremely thin layers of two ceramic materials are rapidly deposited sequentially using a laboratory-scale electron beam system. A cerium oxide layer as thin as 100 angstroms is placed almost instantaneously on the rolled nickel, followed by a 140 nm layer of yttria-stabilized zirconia. In the lab environment, this layer takes about 20 minutes to grow. The ceramic layers in the RABiTS sandwich are, therefore, remarkably thin.
28
Figure 4.2. RABiTS
Source: [Sheahen]
4.2. Demands on conductors for coil applications
A superconducting material that is being investigated for coil applications must possess the following properties in general
1. High current densities 2. High mechanical strengths 3. Must be able to fabricate in long lengths 4. Must tolerate high operating currents 5. Low ac losses
Table 4.1. Application specific requirements of the Superconducting wire for commercial applications
The structure of the BSCCO molecule is shown in figure 4.1.
Figure 4.3. Molecular structure of BSCCO
Bi2Sr2Ca2Cu3O8
Source: www.wikipedia.org
30
Covalent Cu-O bonding and Cu3+ valence states leads to partly filled energy bands Oxidation to Cu3+ leaves a hole in the conduction band for p-type conductivity Weak localization of valence electrons (low ionic character) Alkaline and rare earth metals act as charge reservoirs
4.4. Manufacture of BSCCO wire
Figure 4.4. Manufacture of BSCCO wire
Source: SuperPower Inc.
Figure 4.5. Cross section of BSCCO wire
Source: [DOE 1997]
31
4.5. Disadvantages of BSCCO wire
The first generation HTSC wires are made from BSCCO. They are very delicate and expensive.
BSCCO is by no means an ideal material since it carries very little current at 77K in high magnetic fields.
To get appreciable current densities in BSCCO, it must be cooled to 20 -30K. This necessitated the need for developing new SC materials
4.6. YBCO wire
Figure 4.6. Molecular structure of YBCO (left)
Source: www.wikipedia.org
Figure 4.7. Properties of YBCO superconductors (right)
4.7. Manufacture of YBCO wires
Figure 4.8. Manufacture of YBCO wires
32
Figure 4.9. Structure of YBCO wire
Source: SuperPower Inc.
33
4.8. Properties of HTSC wires
4.8.1. Performance The word performance applied to superconducting wire refers to the critical current
density Jc of the wire. The early elemental (Type I) superconductors were never of practical interest, mainly because they carried very little current; and a magnetic field of a few hundred gauss (0.03 tesla) would quench superconductivity completely.
Type II superconductors have the very important property of having high Jc even in magnetic fields of several tesla. All the HTS materials fall within the Type II category. Samples of YBCO, BSCCO, etc. from 1988-1990 were plagued with crystal imperfections and mechanical irregularities, and showed Jc values below 10 A/mm2. The advantage of contemporary BSCCO wire is that it has Jc > 1,000 A/mm2 (at sufficiently low temperatures and modest magnetic fields). YBCO coated conductors made via IBAD techniques have Jc > 10,000 A/mm2 in short samples. RABiTS technology is equivalent in Jc in short samples, and may offer other advantages, which are further discussed below. The high performance of these samples is due to very good grain-alignment, which in turn is due to the substrate conditioning achieved by IBAD and RABiTS. If the alignment of consecutive grains deteriorates (i.e., disorientation of adjacent grains by more than 5 to 10 deg.), the value of Jc drops sharply, and the material is no longer useful for high-current applications. One objective of the second-generation wire program is to extend these coated conductors to very long lengths ( > 1 km) while still preserving high Jc
values. YBCO coated conductors require a completely new type of production equipment and "thin film" processing techniques (common in the metalized can label, snack food bag, and recording tape industries but quite different from much of the equipment used to make BSCCO- 2223 .OPIT. wires). This capitalization represents a barrier to the YBCO coated conductor development business that few companies can afford to overcome without strategic partnerships with other companies, the national laboratories, and universities. For example, estimates of the capital cost to install a pilot line for coated conductors range from as low as $5 million to as high as $50 million.
4.8.2. Real wire Considerations
Increasing the thickness of the conductor film is an important issue. Total current, as contrasted to current density, is what is needed in practical applications, and so a high Jc must be accompanied by a large film cross sectional area in order to deliver the total current. Typically, thin films are perhaps 0.4m in thickness, so even a 1m film borders on the category of .thick. These conductors may require thicknesses total) of 5 or 10 µm to achieve the total current needed (unless values of Jc can be increased substantially), and this introduces a new worry. As the YBCO layer thickens, is there a possibility that mis-oriented grain growth will occur, defeating the purpose of the original textured substrate? Also, will film mechanical properties (cracks in YBCO film over 3-5 µm thick) similarly limit the overall thickness? It may turn out that 1 or 2 :m is the maximum practical thickness. Any real wire includes some .overhead. for insulation, etc., and therefore we distinguish between the critical current in the superconducting material itself Jc , and the .engineering. critical current Je. For practical applications, the figure of merit is Je , not Jc, because Je relates to how much actual current flows through a real conductor with a certain cross- ectional area. In the case of BSCCO made by the Powder-in-Tube (PIT) method, the amount of silver surrounding the BSCCO reduces Je compared to Jc. In the case of YBCO coated conductors, the thickness of the substrate and buffer may be ten times the
34
thickness of the YBCO itself, in which case the reduction from Jc to Je will exceed a factor of 10 -- the penalty for .overhead. is very severe. 4.8.3. Magnetic Properties
In the first generation of wire, the sheathing material (silver) is non-magnetic. Using RABiTs, the first thing to note is that the substrates are often magnetic materials (e.g., nickel). Therefore, it is important to investigate the interactions between substrate and HTS in a magnetic field. Alternative choices of substrate (Hastelloy, stainless steel) may be used to minimize adverse effects of external magnetic fields, although these alloys may be difficult to align by rolling. 4.8.4. AC Losses
The subject of AC losses is complex and depends on the application. Experimental measurements are usually needed to verify theoretical expectations. Often differences between theory and experiment are interpreted in terms of conductor non-uniformity. Generally, AC losses are associated with changing magnetic fields. Self-field losses are those which occur due to the magnetic fields produced by the conductor acting on itself. Other losses are caused by the interactions of the different components of a system. For a tape conductor, the orientation of the magnetic field is important; the losses are usually larger when the field has a significant component normal to the plane of the conductor.
Eddy current losses are due to currents induced in normal metal as the result of time-varying magnetic fields. In conventional motors, generators, and transformers, for example, these losses are reduced by the use of laminated steel for the magnetic circuit and the use of thin conductor strands for the copper conductors. The steel is formulated with high electrical resistivity and minimum magnetic hysteresis in mind. Transposition of the copper conductors also can be used to reduce eddy currents. Second generation coated conductor technology offers reduced eddy current losses relative to BSCCO powder-in-silver tube technology due to the higher resistivity of the nickel or nickel alloys (relative to silver) used to support the superconductor. However, the ferromagnetism of pure nickel may lead to hysteretic losses if it is used as a substrate. The physical picture is that changes in the externally imposed magnetic field cause flux penetration into the superconductor. The losses are proportional to the frequency, since the loss per cycle is fixed, and are also proportional to the inverse of the critical current density. Since second generation conductors are expected to have high critical current densities, the inverse dependence of loss on Jc is a beneficial aspect. Initial measurement of self field losses on YBCO coated conductors seem to be encouragingly low. 4.8.5. Geometry
The geometrical considerations have mostly to do with the matter of flexibility. Truly useful wire will be bent in most applications. The parameters of layer thickness, bend radius, bending strain, and tensile/compressive strain all come together under the umbrella of .geometry.. At first, it seems desirable to coat the substrate with a very thick film of YBCO. Doing so increases Je, hopefully without sacrificing Jc. But this is not assured. The total current flowing in the full conductor is the key figure of merit; if very thick films accumulate defects and then succumb to poor grain alignment, for example, the anticipated Je will not be realized. Maximizing the useful film thickness is a key goal.
35
Bending: When a layer is 5µm thick, and bent on a radius of 5cm, the strain is of the order of 1 part in 104. However, these coated conductors including substrate may have total thicknesses as large as 100 µm, and it is the total conductor that will be bent around the specified radius in each application.
Following this discussion on HTSC wires, basic experimentation on 1st generation and 2nd generation HTSC wires were carried out, the results of which are presented in the next chapter.
36
CHAPTER 5
EXPERIMENTATION ON 1ST GENEARTION & 2ND GENERATION HTSC WIRES
In this chapter, the results of the tests performed on the HTSC winding are presented.
5.1. Testing of 1G HTSC (BSCCO) wire
A BSCCO (1G wire) of 10 cm length was taken and 150 current was passed in it with a
potential difference of 1µV/cm. A separate test apparatus was prepared for this application.
5.2. Testing of 2G HTSC (YBCO) wire
A prototype transformer has been manufactured and tested. This test was done to study the
behavior of high temperature superconducting winding in superconducting stage
particularly for ac operation like a coil in transformer.
A rectangular frame was fabricated with the help of 0.5mm CRGO steel. One limb of the
transformer was wound with normal air cooled copper winding acting as a primary
winding. Second limb is having a 2G YBCO superconducting winding.
In this case core is air cooled and the winding is liquid nitrogen cooled. A double wall
vacuum insulated FRP container has been used for accommodating two coils of HTSC
winding immersed in liquid nitrogen. After the winding attained a temperature of 77K,
primary winding is excited with a 50 Hz supply. OCC tests were done and SCC test has
been carried for a rated current of 135 Amps.
Special type of connectors and soldering of copper connector and HTSC tape is required
for taking out the connection from HSC coil at 77K to outside at room temperature.
37
5.3. S.C.C. tests of the prototype transformer
Table 5.1. S.C.C. tests on prototype transformer
S.C.C. Test
Primary Secondary
Voltage Current Power PF Current
0 0 0 0 0
28.53 0.3826 7.11 0.651 11.82
43 0.637 14.52 0.52 20.37
64.3 0.928 28.6 0.48 30.37
84.2 1.202 45.6 0.45 39.97
107.3 1.488 75.1 0.471 49.96
130.1 1.792 105.8 0.454 60.6
150.3 2.067 135.4 0.436 70.4
172 2.349 170 0.421 80.4
192.8 2.639 203 0.399 90.7
215.3 2.941 249 0.392 101.7
233.9 3.213 291 0.387 110.7
253.1 3.481 336 0.382 119.9
275.3 3.802 391 0.373 131
294.4 4.054 447 0.375 139.6
316.4 4.37 521 0.377 150.3 (Coil opend)
38
0
20
40
60
80
100
120
140
160
0 1 2 3 4 5
SEC
ON
DA
RY
CU
RR
EN
T I
N A
MPS
PRIMARY CURRENT IN AMPS
SCC TEST
Secondary Current(A)
PRIMARY TURNS : 272SECONDARY TURNS: 09TURNS RATIO : 30
I MAX =150A
Figure 5.1. S.C.C. tests on prototype transformer
5.4. O.C.C. tests on the prototype transformer
Table 5.2. O.C.C. tests on the prototype transformer
Primary Secondary
Voltage Current Power PF Coil- V Coil- V
(V) (mA) 9-T 9-T
0 0 0 0 0 0
19.67 21.6 0.195 0.46 0.635 19.19
39.95 36.8 0.78 0.53 1.3 39.29
61.13 49.8 1.76 0.58 1.997 60.35
80.9 59.1 3.01 0.63 2.649 80.05 Turns ratio 30.22222
99.5 67.1 4.42 0.663 3.264 98.64
39
121.4 76.1 6.37 0.69 3.99 120.58
141.5 84 8.4 0.707 4.65 140.52
160.8 91.9 10.61 0.718 5.29 159.86
181.1 101 13.71 0.72 5.98 180.72
201 111.6 15.08 0.71 6.64 200.66
211.5 117.7 17.49 0.702 6.96 210.33
222.1 124.2 19.08 0.692 7.31 220.91
230.1 129.2 20.37 0.685 7.58 229.07
241.2 136.9 22.2 0.672 7.96 240.55
251 144.3 23.9 0.66 8.27 249.92
Coil R= 1.5 Ohms Primary turns 150 (cu)
Leads R= 1.48 Ohms Secondary turns
9 (HTSC)
Figure 5.2. O.C.C. tests on the prototype transformer
0
1
2
3
4
5
6
7
8
9
0 25 50 75 100 125 150 175 200 225 250 275 300
SEC
ON
DA
RY
VO
LT
AG
E I
N V
OL
TS
PRIMARY VOLTAGE IN VOLTS
OCC TEST
Secondary Voltage(V)
PRIMARY TURNS : 272SECONDARY TURNS : 09TURNS RATIO : 30
40
SECTION 2
HIGH TEMPERATURE SUPERCONDUCTIVITY IN ELECTRICAL POWER APPLICATIONS
HTSC POWER CABLES
HTSC MES
HTSC FCL
HTSC TRANSFORMER
HTSC MACHINES
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CHAPTER 6 High Temperature Superconducting (HTSC) Power Cables
6.1. Introduction
Projects to demonstrate superconducting power cable in utility power grids have increased internationally as the technology improves and the need to ease issues related to power congestion in densely populated urban centers is realized by power system operators. Superconducting power cables are a possible solution to these congestion issues because they ca provide three to five times more capacity than conventional underground power cables in the same physical space. One application touted for High Temperature Superconducting (HTSC) power cables is underground cable retrofits, where the cost of expanding existing tunnels or digging new ones outweighs the initial cost of superconducting system. Additionally HTSC Power Cables may be an excellent option where rights of ways (ROW) are difficult or impossible to obtain.
6.2. Types of HTSC Power Cables
6.2.1. Cold A cold dielectric superconducting power cable employs concentric layers of HTS wire separated by the high voltage insulation material, commonly referred to as the dielectric. Superconducting tapes (cooled by liquid nitrogen) are both inside and outside the dielectric, and consequently the dielectric itself is also immersed in liquid nitrogen. This ‘cold dielectric’ gives the cable design its name. The inner, high voltage layer(s) of superconductor tapes are transmitting power while the outer layer(s) are grounded. In the outer layers, currents equal in magnitude but opposite in phase to the inner layers are being induced. These induced currents completely cancel the electromagnetic fields of the inner layers, so that a cold dielectric HTS power cable has no stray electromagnetic fields outside the cable, no matter how high its current (and thus transmission power) rating. This is one of the key benefits of the cold dielectric design. The fact that the electromagnetic field is contained inside the superconducting screen also significantly reduces the cable inductance, another important benefit of HTS power cables.
6.2.2. Warm This simpler design of HTS power cables is the ideal choice when electromagnetic stray fields can be tolerated and a slightly lower transmission capacity than that of a cold dielectric cable is acceptable. Its high voltage phase layer(s), consisting of superconducting tapes, are stranded around a core that also serves as the channel for the liquid nitrogen coolant. Unlike in the cold dielectric design, there are no superconducting screen layers requiring cooling, and consequently the dielectric is kept at ambient temperature, or warm. As this cable designs has higher electrical losses and a higher inductance when compared to a cold dielectric design, it has its place in applications where conventional cables have reached their limits but not all the features of a cold dielectric design are necessary. In such situations, it can be the choice that makes the best economical sense, owing to its simpler overall design, cheaper manufacturing cost, and reduced superconductor length.
42
6.3. Benefits of HTSC Power Cables Lower voltages Because of the higher capacity of VLI (Very Low Impedance) cable – approximately three to five times higher ampere carrying capacity than conventional cables – utilities may employ lower voltage equipment, avoiding both the electrical (I²R) losses typical of high current operation and the capital costs of step up and step down transformers. High current VLI cables at 115 kV or even 69 kV may solve problems that would ordinarily require a 230 kV or 345 kV conventional solution. Easier installation HTS cables are actively cooled and thermally independent of the surrounding environment. Life extension and improved asset utilization Over time, thermal overload ages and degrades cable insulation. By drawing flow away from overtaxed cables and lines, strategic insertions of VLI cable can „take the heat off“ urban power delivery networks. Reduced electrical losses In optimized designs, lower net energy losses occur in VLI cables, than in either conventional lines and cables or unshielded HTS cables with a single conductor per phase, offering a transmission path with high electrical efficiency. Because VLI circuits tend to attract power flow, they will naturally operate at a high capacity factor, reducing the losses on other circuits and further magnifying their efficiency advantage. Indirect and non monetary savings In addition to these “hard cost“ savings, VLI cables may result in other “soft cost“ savings. For example, time to install may be shortened because of reduced siting obstacles associated with compact underground installations and less burdensome siting requirements for lower voltage facilities. VLI cables might be routed through existing, retired underground gas, oil or water pipes, through existing (active or inactive) electrical conduit, along highway or railway rights-of-way, or through other existing corridors. Reduced regional congestion costs Finally, and perhaps most significantly, the ability to complete grid upgrade projects more quickly will translate into the earlier elimination or relaxation of grid bottlenecks. Solving physical bottleneck problems will sharply reduce the grid congestion costs that, in today‘s unsettled, imperfectly competitive marketplace, can impose huge penalties on consumers and the economy at large. Underground installation The underground installation of VLI cable eliminates the visual impact of overhead lines. Environment friendly dielectric Liquid nitrogen, the coolant/dielectric of choice for VLI cables, is inexpensive, abundant and environmentally benign. Elimination of EMF The coaxial design of VLI cold dielectric cables completely suppresses electromagnetic fields (EMF). Refer figure 6.3. for a detailed comparison between the HTSC cables and conventional cables
43
6.4. State – Art – of - Art HTSC Power Cables Table 6.1. State – of – the Art HTSC Power cable projects Manufacturer Place/Country/Year Type Data HTSC Innopower Yunnan, CN, 2004 WD 35kV, 2kA,
33m, 3Φ Bi – 2223
Sumitomo Albany, US, 2006 CD 34.5kV, 800A, 350m, 3Φ
Sumitomo Gochang, KR, 2006 CD 22.9kV, 1.25kA, 100m, 3Φ
Bi – 2223
LS Cable Gochang, KR,2007 CD 22.9kV, 1.26kA, 100m, 3Φ
Bi – 2223
Sumitomo Albany, US, 2007 CD 34.5kV, 800A, 30m, 3Φ
YBCO
Nexans Hannover, D, 2007 CD 138kV, 1.8kA, 30m, 1Φ
YBCO
Nexans Long Island, US, 2008
CD 138kV, 1.8kA, 600m, 3Φ
Bi – 2223
Nexans Spain, 2008 CD 10kV, 1kA, 30m, 1Φ
YBCO
Ultera New York, US, 2010 Triax 13.8kV, 4kA, 240m, 3Φ
YBCO
Ultera Amsterdam, NL Triax 50kV, 2.9kA, 6000m, 3Φ
YBCO
Nexans Long Island, US, 2011
CD 138kV, 2.4kA, 600m, 1Φ
YBCO
LS Cable Gochang, KR, 2011 CD 154kV, 1GVA, 100m, 3Φ
YBCO
LS Cable Seoul, KR, 2011 CD 22.9kV, 50MVA, 500m, 3Φ
YBCO
Sumitomo Yokohama, JP, 2012 CD 66kV, 200MVA, 200m, 3Φ
Bi – 2223
Sumitomo TEPCO, JP CD 66kV, 5kA TBD Furukawa TEPCO, JP CD 275kV, 3kA Bi – 2223 Sumitomo Chubu U., JP, 2010 CD 10kV, 3kA DC,
20m, 200m Bi – 2223
VNIIKP Moscow, RU, 2010 CD 20kV, 200m Bi – 2223 Nexans Spain CD 10kV, 3.2kA,
30m, 1Φ Bi – 2223
Table 6.2. Present status of Power cables vs required specifications Present Value Required Value Cryostat losses 1.5 – 2 W/m 0.5 W/m AC losses ( at 2.9kA) 1.4 W/m/phase 0.2 W/m/phase
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Figure 6.1.Cold type HTCS Power cable
Figure 6.2.Warm type HTSC Power cable
Figure 6.3. Difference between AC & DC HTSC & conventional cables
Source: Nexans Superconductors
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CHAPTER 7 High Temperature Superconducting (HTSC) Magnetic Energy Storage (MES)
7.1. Introduction
The desirability of electric energy storage is by now a given, and a number of recent studies have examined the economics associated with various methods of storage. Some are conventional, such as charging and discharging lead-acid batteries; other methods are more innovative. In the storage method known as pumped hydro, electricity is generated at night and used to pump water uphill to a basin above a hydroelectric dam; later on, during peak demand hours, the water flows downward through turbines and generates electricity at the time it is needed. In all cases, the figure of merit by which competing methods of storage are evaluated is the round-trip efficiency, which means simply the ratio of power delivered upon exit to the power input at the start. The round-trip efficiently is weighed along with both initial capital cost and annual operating costs to perform a cost/benefit analysis of any particular energy storage pathway. In the case of pumped hydro, for example, Virginia Electric Power has obtained3 a round-trip efficiency over 80%, but they incurred capital costs in acquiring land and building dams and hydroelectric generators; and, of course, there are finite operating costs of their system. A lifecycle cost analysis incorporates some expected-use profile, and amortizes capital costs over the lifetime of the equipment, so as to arrive at a net cost per kilowatt figure. That can then be compared with cost estimates for other forms of storage, and with the option of having no storage at all. Such factors as the estimated future price of coal and natural gas enter into the calculation. The options available to a utility are many. Although a blackout is to be avoided through astute advanced planning, gentle reductions in line voltage are not entirely out of the question. Clearly, however, it is better to actually meet the full demand. Doing so may or may not require electricity to be stored. One variation of the no-storage option is to buy power from other utilities to meet peak demand. Not everyone can do that. In any case, storage of electricity has a place in the utility sector. SMES is attractive because it has a round-trip efficiency of over 90% under the right circumstances.
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Table 7.1. Comparison of various energy storage technologies
Source: KIT 7.2. Concept and Working of a SMES SMES is a device for efficiently storing energy in the magnetic field associated with a circulating current. An inverter/convertor is used to transform AC power to direct current, which is used to charge a large solenoid or toroidal magnet. Upon discharge, energy is withdrawn from the magnet and converted to AC power. Figure is a schematic diagram of a SMES system. The components include a DC coil, a power conditioning system (PCS) required to convert between DC and AC, and a refrigeration system to hold the superconductor at low temperature. The inverter/converter accounts for about 2–3% energy loss in each direction. Figure 7.1. Block diagram of a SMES based system
Source: Sheahen 7.3. Advantages of SMES
Rapid response for either charge or discharge Power is available when needed, not only when generated Minimal resistive energy losses in the superconducting coil and solid state power
conditioning Ability to go to high fields i.e. allow high power density High hoop strength of 2G HTSC
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Continued price improvements in HTSC materials Development in enabling technologies like cryocoolers, insulation etc Safe – no chemical reactions, no toxins produced.
There is a further economic advantage associated with larger SMES units. Denoting the magnetic induction by B, the energy stored in a magnetic field is proportional to the dimensions of the SMES unit go up only linearly with B, and the refrigeration requirement is proportional to size. Therefore, larger SMES units have the economic advantage of less refrigeration need per stored megawatt. 7.4. State – of – the – Art SMES projects Table 7.2. State – of – the Art SMES projects Institution Country Year Data SC Application KIT Germany 1997 320kVA, 203kJ NbTi Flicker
compensation AMSC US 2MW, 2.6MJ NbTi Grid Stability KIT Germany 2004 25MW, 237kJ NbTi Power Modulator Chubu Japan 2004 5MVA, 5MJ NbTi Voltage stability Chubu Japan 2004 1MVA, 1MJ Bi 2212 Voltage Stability KERI Korea 2005 750kVA, 3MJ NbTi Power stability Ansaldo Itlay 2005 1MVA, 1MJ NbTi Voltage stability Chubu Japan 2007 10MVA, 19MJ NbTi Load compensation CAS China 2007 0.5MVA, 1MJ Bi - 2223 - KERI Korea 2007 600kJ Bi - 2223 Power, Voltage
quality CNRS France 2008 800kJ Bi - 2212 Military
Application KERI Korea 2011 2.5MJ YBCO Power quality ABB/SP US 2013 2.5MJ, 20kW YBCO -
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CHAPTER 8
High Temperature Superconducting (HTSC) Fault Current Limiter (FCL)
8.1. Introduction
Damage from short circuit currents is a constant threat to any electric power system, since it threatens the integrity of its generators, bus-bars, transformers, switchgears, and transmission and distribution lines . Building on this statement, the FCL is described below. The role of the FCL is to limit prospective fault current levels to a more manageable level without a significant impact on the distribution system. Consider a simple power system model, as shown in figure, consisting of a source with voltage VS, internal impedance ZS, load Zload, and fault impedance Zfault.
In steady state. Iline = VS / ( Zs + Zload ) Eq 1 When a fault occurs in a system, Iline = VS / ( ZS + Zfault ) Eq 2 Where Zfault << Zload
Since the supply impedance ZS is much smaller than the load impedance, Equation (2) shows that the short circuiting of the load will substantially increase the current flow. However, if a FCL is placed in series, as shown in the modified circuit, Equation (3) will hold true; Iline = VS / ( ZS + ZFCL + Zfault ) Eq 3 Equation (3) tells that, with an insertion of a FCL, the fault current will now be a function of not only the source ZS and fault impedance Zfault, but also the impedance of the FCL ZFCL. Hence, for a given source voltage VS and increasing will decrease the fault current Iline.
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8.2. Ideal fault current limiter characteristics Before discussing any further, it is important that some of the ideal characteristics be laid out for an FCL. An ideal FCL should meet the following operational requirements:-
1. Virtually inexistent during steady state. This implies almost zero voltage drop across the FCL itself
2. Detection of the fault current within the first cycle (less than 16.667ms for 60Hz and 20ms for 50Hz) and reduction to a desirable percentage in the next few cycles.
3. Capable of repeated operations for multiple faults in a short period of time 4. Automatic recovery of the FCL to pre-fault state without human intervention 5. No impact on voltage and angle stability 6. Ability to work up to the distribution voltage level class 7. No impact on the normal operation of relays and circuit breakers 8. Finally, small-size device that is relatively portable, lightweight and maintenance free
In reality, one would like to have an FCL that would satisfy all of the foregoing characteristics. However, certain trade-offs and compromises have been made in nearly all categories and types.
8.3. Types of fault current limiters This section presents a brief review of the various kinds of FCL that has been implemented or proposed. FCL(s) can generally be categorized into three broad types:
1. Passive limiters 2. Solid state type limiters, and 3. Hybrid limiters
In the past, many approaches to the FCL design have been conducted ranging from the very simple to complex designs. A brief description of each category of limiter is given below.
8.4. Passive limiters Fault limiters that do not require an external trigger for activation are called passive limiters. The current limiting task is achieved by the physics involved in the FCL itself. The simplest of all kinds of fault current limiter is the inductor. The current limiting strategy is achieved by inserting impedance Z = jωL. Since current cannot change instantaneously in an inductor, current is therefore limited at the moment of a fault. Figure shows an inductor in series with the load and source.
There are a few pros and cons in using an inductor for FCL application:
1. Technique has been well known, installed, field tested and commissioned for many years
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2. Relatively low cost and maintenance, but 3. Bulky to handle and replace 4. Produces a voltage drop in steady state and causes lagging power factors
Another kind of passive limiter that is gaining attention is the superconducting fault current limiter (SFCL). SFCL(s) work on the principle that under steady state, it allows for the load current to flow through it without appreciable voltage drop across it. During a fault, an increase in the current leads to a temperature rise and a sharp increase in the impedance of the superconducting material. Below are a few advantages and disadvantages of using an SFCL:
1. Virtually no voltage drop in steady state 2. Quick response times and effective current limiting, but 3. Cooling technologies still at infancy, leading to frequent break downs 4. Commercial deployment is still to be witnessed 5. Superconducting coils can saturate and lead to harmonics
8.5. Solid-state limiters Recent developments in power switching technology have made solid state limiters suitable for voltage and power levels necessary for distribution system applications. Solid state limiters use a combination of inductors, capacitors and thyristors or gate turn off thyristors (GTO) to achieve fault limiting functionality. An example of a solid state limiter is shown in Figure. In this type of limiter, a capacitor is placed in parallel with an inductor and a pair of thyristors.
In steady state, the thyristors are turned off and all current flows through the capacitor. The placement of the capacitor is also useful by nature because it provides series compensation for the inductive transmission line. Hence, equation (2.4) holds true: ZFCL (NORMAL) = -j / ωC However, when a fault occurs the thyristors are switched on, which forces most of the current to flow through the inductor branch. The net FCL impedance seen by the circuit is as follows. Z FCL (FLT) = jωL / ( 1 - ω2 LC ) Below are a few advantages and limitations of solid state limiters in general:-
1. Provide significant fault current limiting impedance 2. Low steady state impedance as capacitors and inductors can be tuned for a particular frequency to
show virtually no impedance and voltage drops. 3. Harmonics introduced due to switching devices 4. Voltage drop introduced during faults
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8.6. Hybrid limiters As the name implies, hybrid limiters use a combination of mechanical switches, solid state FCL(s), superconducting and other technologies to create current mitigation. It is a well know fact that circuit breakers and mechanical based switches suffer from delays in the few cycles range. Power electronic switches are fast in response and can open during a zero voltage crossing hence commutating the voltage across its contacts in a cycle. In 2001, Shi et al proposed a novel Triggered Vacuum Switch (TVS) based FCL. Figure shows the circuit arrangement of one such device.
In their work, they state that the reactance of the capacitor C1 and reactor L is about zero at nominal power frequencies. In steady state, the TVS and SW2 are in the off state. SW2 is a quick permanent magnetism vacuum contactor with a 3-10ms closure delay, which prevents TVS from long-time arc erosion. When a fault occurs, a trigger signal is sent to both TVS and the contactor turning on the bypass capacitor C1. This creates a situation where the reactor L will limit the fault current immediately. The ZnO arrestor is used for over voltage protection and capacitor C2 and switch SW1 are set-up as a conventional series compensation.
8.7. Introduction to Superconducting Fault Current Limiters (SFCL)
8.7.1. Types of SFCLs 8.7.1.1.Resistive The operation of this type of SCFCL is based on the quench of the superconducting material, which describes its transition from the superconducting state to the normal conducting state. The quench occurs rapidly when the short circuit current flowing through the SCFCL exceeds the superconductor’s critical current. This variation of the SCFCL utilizes a resistor in parallel with the superconducting material that protects the superconductor from hotspots that may develop during the quench, as well as avoiding overvoltage over the SCFCL that may damage it. These SCFCLs are considered fail safe and can be built to exhibit negligible impedance during normal system operation. A recovery time is however required following a quench, which can range
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from one second to under one minute, depending on the material employed. One present disadvantage is that there is energy loss caused by the current leads passing from room temperature to cryogenic temperature that will result in a loss of approximately 40-50 W/kA heat loss per current lead at cold temperature (Noe and Steurer, 2007, p. 17). This would equate to a maximum operating loss of approximately 80kW for a three phase SCFCL operating in series with a 10MW generator connected at 11kV. 8.7.1.2. Resistive Magnetic This variation of the SCFCL utilizes a parallel inductance with the superconducting material. Their paper describes how the increasing magnetic field, caused by the growing current flowing in the inductor under fault conditions, accelerates the quench and mitigates the hot spot phenomenon in the superconducting material. 8.7.1.3. Bridge Type SCFCL This SCFCL employs solid state technology to control the flow of current through a superconducting inductance. The disadvantages of this Bridge Type SCFCL are that it is not considered to be fail-safe device, and it exhibits relatively high total energy losses. 8.7.1.4. DC biased Iron core SCFCL These devices incorporate two iron-core coils that are driven into saturation by introducing a DC bias current under normal operating conditions. These two cores are placed in the series path of the potential fault current. While these two cores are in operating in saturation mode, their (and hence the SCFCL) inductances are low. When fault current flows, these coils will be driven out of saturation resulting in an increase in the apparent coil inductance. This concept has the advantage of requiring relatively less superconductor material, and a smaller cryogenic system is required to cool the device. The requirement for the iron cores does however make the device bulky when compared to other SCFCL devices 8.7.1.5. Power Electronics Power electronic components may be used to interrupt the fault current and direct it through limiting superconducting impedance, thereby controlling the magnitude of the fault current along the particular path. Once again, these devices will not be considered fail-safe as the failure of one power electronic device can lead to mal – operation of the fault current limiting device.
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8.8 Conceptual Design of Resistive type SFCL
Figure 8.1. Conceptual design of resistive type SFCL
Source: Converteam
8.9. Basic Design Aspects SCFCL passively limits a fault current by intrinsically developing resistance under over-current. The rated power of a SCFCL is defined by P = IN UN; where IN is the nominal current (current in normal operation) and UN is the voltage of the system protected by SCFCL, which approximates the total voltage developed across the conductor during a fault. IN is given by Ajc / √2, with A being the cross-section of HTS and jc its critical current density. UN is given by Lemax / √2, where L is the length of HTS and Emax is the designed maximum electric field. Power application is most practically realized both by a high Emax and a long length L. As a practical approach, long length can be achieved by structuring a plate into a long meander. For YBCO thin film, an Emax value of around 25 V/cm has been reported. However, in reality a much more compromised value is taken because designs with high Emax are more prone to hot spot. SCFCL with distinctively different limitation behaviors can be tailored by simply varying the Emax. For economical HTS conductors, a current carrying capability, expressed as Ampere per width, higher than 100 A/cm would be required. This can be achieved either by high jc and/or large cross section. The exploitation of cross-section, A, has its limitation firstly, because SCFCL components usually take the form of plates where a compact design calls for a minimized width and secondly, the thickness is limited because of AC-losses. For Bi-2212 with a typical jc in the range of 1000–10,000 A/cm2 at 77 K, sufficient current capability can be achieved with bulk conductor (thickness in millimeter range, which can still be tolerated from the AC-losses point of view).
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8.10. State – of – the – Art SFCL projects Table 8.1. State – of – the Art HTSC FCL projects
8.11. Current status of SFCL Technology
In 2001 ABB reported the successful test of an 8kV, 6.4 MVA resistive SCFCL. No new information regarding this development was available, with ABB concluding that the widespread application of such devices would only be achieved with the realization of low cost superconductors and cost effective and reliable cooling. Nexans Superconductors have developed a 3Φ, 10MVA, 10kV resistive SCFCL that was field tested in Germany for one year from 2003. It was named CURL 10 and the test was deemed successful for MV applications. The device is currently undergoing further testing in Germany. Following on from the Nexans CURL 10 resistive SCFCL development described in the paragraph above, the company have moved to develop a resistive type SCFCL with magnetic field assisted quench (i.e. resistive magnetic ).The aim of this project is to develop a 110kV, 1.8kA demonstrator. Following earlier successful research relating to the Matrix Fault Current Limiter Project, an American based project is developing a 138kV SCFCL using the pure resistive SCFCL concept and the latest second generation (2G) superconducting components (Superpower, 2006). This project forms part of the US Department of Energy’s Superconductivity Partnership Initiative program and the use of the 2G components promise to make this development more cost effective and commercially viable. A national project is currently underway in Japan to develop and demonstrate a 6.6kV, 600A resistive SCFCL application. In Korea, the ten-year “Dream of Advanced Power Systems by Applied
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Superconductivity (DAPAS) Technology Program” is aiming to commercialize superconducting power equipment. During the first phase of the program they have successfully built and tested a 6.6kV SCFCL. Innopower in China are developing a 35kV prototype DC biased iron core SCFCL. Many challenges lay ahead for developers and manufacturers of SCFCLs. As utility (substation) based solutions will be required to have a life in excess of 30 years, the ageing and long term behavior of the superconducting material needs to be understood. As this is relatively new and unexploited technology, such information is not available at this stage. As a result of the relatively high cost of these superconducting devices, research and development is currently focused on the MV and HV applications where large technical and economic benefits are to be achieved.
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CHAPTER 9
High Temperature Superconducting (HTSC) Transformer
9.1. Introduction
Transformers utilizing High Temperature Superconductors are perceived as a “breakthrough” technology coming at an “opportune time”.
High Temperature Superconductor (HTSC) properties, improved refrigeration reliability and lower refrigeration costs make it possible to overcome the limitations experienced in the Low Temperature Superconducting (LTSC) designs of the 70’s and the 80’s. But commercial success will depend on demonstrated reliability of operation and the scale up of HTSC manufacturing.
9.2. Benefits of a HTSC Transformer
9.2.1. Greater Effective Capacity
One major advantage of HTSC transformer is reduced size and weight. Another is a distinct environmental plus – in the conventional transformer, oil is a fire hazard and a potential contaminant, whereas in the HTSC Transformer, the only substance present in large volume is the non – inflammable and environmentally benign liquid nitrogen. But perhaps the key advantage is the capability for over – capacity operation, due in part to the low temperatures at which the HTSC windings operate.
Heat is the principle enemy of the paper oil electrical insulation system of conventional power transformers. In order to meet the desired life of 30 or more years, transformer capacity ratings are based on holding the temperature of the hottest part of the insulation under 1100 C. Thermal damage is cumulative, so that operation at only 200 C over the limit for a total of 100 days – less than 1% of 30 years – will reduce the transformers life by 25%.
In view of this sensitivity, the thermal management of conventional transformers has received much attention in the recent years. This is also because utility customers are making much heavier use of air – conditioning systems, even in colder climates, giving rise to peak loading conditions that can last 10 hours or more on the hottest days of the year. Loss of insulation life can be significant under these conditions. So transformers are increasingly being purchased with excess capacity, just to meet maximum temperature limits that may occur only on a few days. The upshot is that they operate well below an optimal level most of the time.
In contrast, HTSC Transformers, their windings and insulations necessarily operate in the ultra – cold range of 20K to 77K, where insulations will not degrade. HTSC units can run at rated power continuously and efficiently. In fact, at up to twice rated power, they can run for
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indefinite periods of time without any loss of operating life, albeit at greatly reduced efficiency because of a disproportionate increase in the use of liquid nitrogen or an increased refrigeration load. Thus one HTSC transformer can in emergencies carry the loads normally handled by two, and HTSC Transformer lifetime can be greatly extended.
Refer figure 9.1. for a better visualization
9.2.2. Low impedance with immunity
HTS Transformers will normally be designed to operate as one – for – one replacements for conventional transformers, complete with an ability – limited only by their own internal impedance – to operate through a fault current of 10 – 12 times the rated current. But they also can be configured to provide additional power system advantages.
Preliminary analyses done by labs worldwide indicate that they can be built to have very low internal impedance and still, through an alternative fault current limiting transformer design, be self protecting against the higher fault currents that could result. It may be possible, if needed, to limit the low – voltage side current to the rating of existing breakers.
Low impedance makes the transformer better at maintaining output voltage levels over a wide range of operating power levels and better able to transmit power downstream through the power system. Utilization of this feature will involve consideration of transformer interfaces with the grid and the load in each situation, and may especially apply to a new power construction where a complete system of compatible components can be installed in an economical way.
Conventional transformers are efficient (typically 99.3 – 99.7 % for the 30MVA class, depending upon loading), but there is considerable room for improvement. About 25% of the 7 – 10% losses in transmission and distribution systems occur in power transformers. The transformer loss costs more than $2 billion annually in United States alone.
Most of the conventional transformer losses are due to resistive heating in its windings – and HTSC transformers have zero winding resistance. Admittedly, the HTSC versions still have ac losses in the iron core and low levels of other kinds of ac losses in the windings that require refrigeration power. Nonetheless, they can be substantially higher in efficiency than conventional transformers, to the extent that the reduced loss in each HTSC unit can more than pay for its initial capital cost over its lifetime.
9.3. Design tradeoffs and cost drivers of HTSC Transformers
Zero resistance and 10 – 100 times greater current density promise striking advantages in transformer size and performance. Classical resistive losses are eliminated, and the quantity of conductor in the HTSC Transformer windings can be reduced to tens as against thousands of kilograms for the conventional transformer. Since the windings in principle require little space and generate little resistive heat, it should be possible to make superconducting transformers inexpensively, with greatly reduced power capacity, much increased efficiency,
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and very much smaller size. While these advantages can be realized in large part, they cannot all be achieved to the same degree in the same transformer. As always, there are practical limitations and tradeoffs
Ultimately, reductions in size will be limited by dielectric design considerations. The transformer must meet the international standard dielectric tests for system voltages and the basic impulse insulation test levels that are specified. For example, a 138kV winding may need to withstand impulse voltages of 650kV. The design of the transformer winding must include sufficient space for insulation if it is to accommodate these high voltages with commercially available dielectric materials and proven design approaches.
Iron core size, which is related to winding size, mainly determines overall transformer size and weight. Eddy current and magnetic hysteresis losses are produced in the core in direct proportion to the core volume. These losses tend to be on the order of tens of kilowatts, much too large to be economically removed by low – temperature refrigerators. HTSC transformers are consequently designed to operate with cores near ambient temperature and isolated thermally from the windings. If the core is too large, its losses occur regardless of whether current (power) is drawn from the transformer, they contribute strongly to the total owning costs. So there are strong incentives to reduce core and winding size.
But reducing core diameter adds to the number of turns and so to the total length and cost of the HTSC conductor. Though the superconductor winding has no classical resistive losses, there are several forms of eddy current and hysteresis losses, which depend on the magnitude of the ac magnetic flux density in the transformer windings, typically a maximum of 0.1 – 0.3T. Compared to conventional resistive and eddy current losses, ac losses in the HTSC transformer winding are small; but because they occur at low temperatures, it takes many times their value in refrigeration power to extract the heat produced. The multiplier is 20 at 77K, increasing to over 100 at 20K.
Great care is therefore given to the design of low-loss conductor and winding configurations. At a fixed transformer power rating, the ac flux density in the windings is increased as the size of the transformer core (and windings) is reduced. Dielectric ac losses in insulating materials also tend to increase as the volume of the winding is reduced. HTSC transformer are therefore made with the core large enough so that the conductor quantity and the cost are reasonably low and the fields on the windings are low enough to keep ac losses within reasonable limits.
Another trade off involves the current density of superconductors, which increases as their operating temperatures are increased. Clearly, the lower the operating temperature, the less HTSC material is needed to provide the ampere-turns of the transformer windings, and the lower its cost becomes. But, as noted earlier, the lower the operating temperature, the higher are both refrigeration capital costs and the refrigeration power needed to remove the ac losses that are generated.
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Figure 9.3.illustrates the difference in conceptual design between the HTSC and conventional transformers
9.4. Achieving Cryogenic temperatures and maintaining it
One of the preferred approaches to cooling and electrically insulating the windings is to surround them with liquid nitrogen (LN2), which is the only safe and low – cost cryogen available in liquid form in the 20 – 77K temperature range of interest. It is an excellent electrical insulator and low – loss dielectric, provided that gas bubbles are not allowed to form. The tendency for gas to bubble is sub – cooling the nitrogen to below its boiling point (77K at 1atm) to as low as 63K, whereupon it solidifies, this can be done by passing though a bath of the same stuff boiling at reduced pressure and temperature.
The nitrogen can be either refrigerated and re – condensed within the transformer, in a closed cycle, or allowed to boil away, in open cycle. In the second case, it serves as a portable, storable refrigerant that absorbs 44Wh of heat per liquid litre. In this mode, it needs replenishment by periodic transfer of liquid element through insulated pipes from remote storage vessels. The vessels can be refilled from on – site liquefiers, but a least – cost approach is to have liquid nitrogen delivered as needed several times per year from large efficient liquefaction plants. Such deliveries to hospitals and industrial concerns are highly reliable and are common in place. Since transformer losses cooled by liquid nitrogen may be hundreds of watts, cooling costs by this approach can be on the order of $1/h.
Cryo coolers can also serve to directly cool the HTSC windings. Their use allows the operating temperature of the windings to be selected at optimal levels without restriction of the liquid nitrogen range. On the other hand, electrically insulating the windings and providing a refrigeration reserve becomes more challenging with this approach.
A preferred thermal insulation between ambient and cryogenic environments is vacuum. Evacuated metallic double walled vessels (dewars) are routinely used for the storage and transportation of cryogenic liquids. No active pumping of the vacuum spaces is required over years of operation. To prevent heating due to induced current, vacuum – tight non – metallic vessels will be required to contain the liquid nitrogen surrounding the transformer windings.
Regardless of the general thermal insulation approach, heat from the warm outside world and heat generated in the electrical leads will be conducted down the leads to the winding area. This adds both fixed and load – dependant components that may account for half of more of the refrigeration load of the transformer. The economics of refrigeration on a cost – per – unit power basis thus becomes more favorable for designs of higher voltage (lower current) and higher power rating.
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9.5. Practical Issues involved in the design of HTSC transformers
The challenge is to obtain the substantial advantages of HTSC in a transformer that operates simply, reliably, and uninterruptedly. Several – day on – board refrigeration reserve of liquid nitrogen will ensure continuous operation without a break in service to the customer.
The least – cost mode of refrigeration – open – cycle cooling with liquid nitrogen – will require about 100 refills of a standard storage tank near the substation over a 30 – year period. (Note that the utilities are already accustomed to regularly checking and, as needed, replacing cylinders of nitrogen gas used to protect the oil and insulation systems in conventional transformers)
Alternatively, closed cycle operation with cryo coolers should make the cryogenics virtually unseen by the customers. Operation of this expediency has been achieved in the medical field with superconducting magnetic resonance imaging (MRI); where thousands of superconducting magnets, about the size of transformer windings, now operate continuously with minimum of attention and maintenance, using Gifford McMahon cycle cryo coolers. The first of these systems have operated for more than 10 – 15 years of service. Existing refrigeration technology must be extended in the future to produce rugged, reliable, low – maintenance, cost – effective transformer packages for indoor and outdoor use, where the standard is a conventional transformer with a typical lifetime of 30 years.
HTSC costs are also of very much importance. The collaborative efforts of industry, government, and the utilities are successfully bringing HTSC transformer closer to commercialization. Each application and installation will provide valuable feedback and operating experience to the designers, manufacturers, and users alike.
9.6. State of the Art HTSC Transformer project
Table 9.1. State – of – the Art HTSC transformers projects
Country Institution Application Data Phase Year HTSC
USA Waukesha Demonstrator 1MVA/13.8kV/6.9kV 1 Bi -2223
USA Waukesha Demonstrator 5MVA/24.9kV/4.2kV 3 ΔY Bi -2223
Japan Fuji Electric & U Kyushu
Demonstrator 1MVA/22kV/6.9kV 1 <2001 Bi –2223
Germany Siemens Railway 1MVA/25kV/1.4kV 1 2001 Bi – 2223
EU CNRS Demonstrator 41kVA/2050V/410V 1 2003 P – YBCO
S – Bi2223
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Korea U - Seoul Demonstrator 1MVA/22.9kV/6.6kV 1 2004 Bi – 2223
Japan RTRI Railway 4MVA/25kV/1.2kV 1 2004 Bi – 2223
China CAS Demonstrator 630kVA/10.5kV/0.4kV 3 2005 Bi – 2223
Japan U Kyushu Demonstrator 400kVA/6.9kV/2.3kV 1 - YBCO
Japan U Nagoya Demonstrator 2MVA/ 22kV/6.6kV 1 2009 P – Bi 2223
S - YBCO
Germany KIT Demonstrator 60kVA 1 2010 P – Cu /
S – YBCO
New Zealand
IRL Prototype 1MVA/11kV/415V 3 2012 YBCO
USA Waukesha Prototype 28MVA/69kV 3 2012 YBCO
Figure 9.1. Reduction in size in transformers with HTSC
Courtesy: Waukesha Electric Systems, USA
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Figure 9.2. Difference in the conceptual designs of conventional vs. HTSC transformers
Source: Waukesha Electric Systems, USA
Figure 9.3. State – of – the Art of HTSC transformer projects
Source: KIT, Germany
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CHAPTER 10 High Temperature Superconducting (HTSC) Machines
10.1. Introduction
Because the greatest single use of electricity is in electric machines, it is of great interest to utilize superconductors so as to capture the highest possible efficiency in an electric machine design. Electric machine-driven applications account for over 50% of the electricity used in the United States. To place this in perspective, U.S. consumers spend $90 billion annually on electricity converted to shaft power by machines, and over $7 billion on new electric machines.
Efficient electric machine systems have the potential to reduce industrial electricity consumption by over 240 billion kWh annually by the year 2010. Electric machines are rather efficient to start with, so a superconducting machine has to do even better in order to offset the cost of refrigeration. There is competition between steadily improving conventional machine designs and new technologies (such as superconductivity). 10.2. Concept behind HTSC machines
Superconducting machines are best understood by comparing them to conventional machine design. In conventional machines, magnetic steel is used to increase the magnetic field produced by the machine coils. This is termed an iron core machine. However, because iron saturates magnetically at 2.2 T, the maximum field strength in a conventional device is about 2 T. In general, the power output of any rotating machine can be expressed
Po = Co * V * Bgmax * Jbmax
Where:
V = machine active volume
Bgmax = peak air gap radial flux density
Jbmax = peak linear current density at the air gap
If the air gap magnetic field Bgmax can be increased, the machine size will decrease for the same machine power rating. In conventional machines Bgmaxis limited by iron core saturation, core loss, and the ability to create a magnetic field with windings that have a high tendency of producing losses. Jbmax is limited by the ability to cool the machine coils and by the space available for the current carrying conductors. The main advantage of using superconductors in electric machines is that they can create an air gap magnetic field without any losses. This advantage must be weighed against the added cost and complexity of having to cool the superconducting windings to cryogenic
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temperatures. Assuming the superconducting windings are held at 77 K, somewhat below TC it is important to remember that the critical current JC falls off steeply with magnetic field B for any of the HTSC conductors in this temperature range In machines, superconductors are used only in DC windings so as to minimize the necessary cooling costs. Under these conditions only the heat leak from outside the winding cryostat must be compensated by the refrigeration system, because the winding itself (once cooled below its transition temperature) is lossless. Taking all of these attributes of the superconductor into account, an HTSC machine will have the following features DC superconducting windings to produce a large Bgmax
Air core construction to eliminate the problems of iron core saturation and core loss at the high Bgmax levels.
Normal (copper) AC windings to provide similar to that of a conventional machine The performance advantages of an HTSC machine over that of a conventional machine include the following:
Higher power density than a conventional machine due to the large Bgmax produced by the lossless HTSC winding.
Higher efficiency than an conventional machine due to the lossless superconducting winding and smaller machine size. Superconductors can be utilized in any machine type that results in steady-state operation with at least one coil carrying only DC current. A synchronous machine with a superconducting field winding (carrying DC current), and a normal conducting armature winding (carrying AC currents), both fit this description. There are other machine architectures that were discarded in the past which can be revisited. Six different architectures that might be used: Homopolar DC Synchronous AC Induction Induction/synchronous hybrid Reluctance Homopolar inductor
The first two machine types, homopolar DC and synchronous AC, have been shown (using low-temperature superconducting materials) to be viable design concepts for the application of superconductivity. Each of the other types was considered at least qualitatively. The conclusion was reached that the homopolar DC and synchronous AC are the best choices. With the design somewhat constrained in this way, it is appropriate to further delimit the design by seeking to optimize the efficiency of the machine. 10.3. Efficiency
In order to discuss what kind of efficiency one expects from an electric machine, it is first necessary to decide what size machine is to be used. The emphasis here is on applying superconducting materials to large machines used in central power generating plants and
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large-horsepower industrial applications. The selection of large machines was based on two principles: One is that a large machine should be better able to absorb the overhead costs associated with a liquid nitrogen cooling system. The second is that the anticipated space savings and efficiency improvements (which result from machine designs utilizing superconducting material) will have a significant impact on operating economics. The large machines currently being used in many of these applications take up valuable floor space and are operating at a high percentage of the time under load, so machine efficiency improvements are quickly converted to monetary savings. Typical applications of these machines are for pump and fan drives. Large pump and fan drives are increasingly being served by adjustable speed machine drives, due to the increased system efficiency (compared to throttling) when used for flow control. For pump and fan applications the superconducting machine should be designed for adjustable speed use, because this technology is expected to be commonplace when these machines become commercially available. Consequently, the HTSC machine will be started and controlled by an adjustable frequency drive. This means that across-the-line starting is not required. A conventional large-horsepower machine typically has an efficiency of 97%. For one specific superconducting machine design, the efficiency increase vs. a conventional machine is shown in Figure. The losses in the superconducting machine are only 52% of those in the conventional machine for this 10,000 hp machine design. Figure 10.1. Loss comparisons of air core (HTSC) and iron – core (conventional) 7.5MW, 3600rpm, 60Hz for full load rated speed operations
Source: Sheahen
The machine designs compared here are rated in thousands of horsepower and are large enough that the cryogenic support system is a fraction of the total machine cost. Another major advantage of these machines is that typically they operate continuously (or nearly so).
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Loss reductions of greater than 50% can be realized by an HTSC machine compared with a large conventional induction machine. Over the operating life of the machine, the resulting cost savings can equal one to two times the initial HTSC machine cost. The superconducting winding is only one part of this machine. A large part of the loss reduction occurs due to the small machine size. This is an extremely important point. The estimated size of the 10,000 hp superconducting machine is 54% of the volume of the equivalent induction machine. Friction and windage loss, as well as armature I2R loss, are reduced simply because the machine is smaller. The only increase in loss is the additional armature-winding eddy current loss in the superconducting machine due to the increased air gap magnetic field. This is the solid black layer on the right of Figure. All the other loss contributions decrease in going to the superconducting design. This comparison is made at full-load conditions. For continuous running full-load applications, the reduced loss of the 10,000 hp superconducting machine represents approximately 1 million kWhr per year saved compared to the conventional iron core machine. 10.4. Introduction to HTSC Machine Design
The idea of a superconducting electric machine is not new; as soon as superconductors were discovered, it became an obvious goal. However, not until NbTi multi – filamentary wire became available could an actual implementation be considered. (Previous superconductors didn't carry enough current to be interesting.) The first superconducting generator designs in the 1970s (using LTSCs, of course) provided guidance for contemporary HTSC machine designs. At the outset, it is essential to recognize that designing a superconducting machine demands much more than a trivial substitution for copper wire. In conventional machines, a lot of iron and copper are used, and the iron saturates. In a superconducting machine, iron would be eliminated in favour of an air core design, with fields well above the iron saturation limit. This calls for rethinking the entire design.
10.4.1. Initial Design Concepts
In Section above, it has been stated that the homopolar DC and synchronous AC machines are the best candidates. This statement deserves some explaining. Both the homopolar DC and synchronous AC machine types have been constructed using LTSC materials, and the design concepts have been verified by tests by various institutions across the globe. Reference to the same from the numerous papers published on these laboratory attempts The homopolar DC machine is attractive for using HTSC materials because the coil wound with HTSC materials is stationary, and liquid nitrogen cooling of a stationary coil is a comparatively easy task. This advantage is offset by the low-voltage, high-current power supply requirements imposed by this machine type. High currents must be supplied to the rotating member of the machine, which poses significant sliding current collector problems. Further, the low-voltage, high-current characteristics require an expensive power converter and increase the on-site power cable installation costs. For example, a homopolar DC machine rated at 430 V and 5800 A for 3000 hp. The comparable rating for a 3000 hp synchronous machine is three-phase, 4000 V and 350 A, which is much more manageable from existing power distribution systems.
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The synchronous AC machine also is well suited to the use of HTSC materials. Advantages of this construction include: (1) Easily adapted to air core design (2) Armature is a copper winding designed for a voltage rating appropriate to the horsepower (3) Increased space available for the armature-winding (due to absence of stator iron teeth) reduces the primary I2R loss and increases the power density (for a given armature-winding current density). The disadvantages are the difficulties associated with (1) Achieving a liquid nitrogen cryostat construction on the rotating member (2) Designing to withstand the mechanical forces that the HTSC coil experiences. On balance, the synchronous AC machine concept seemed to have the edge, and therefore further conceptual design followed this path. This does not represent an outright rejection of the homopolar DC machine, but rather exhibits the need to commit to a single design in order to make progress The machine design features a superconducting exciting field winding and a copper armature winding, separated by appropriate flux and thermal shields. There is no magnetic material in the main flux path except for an outer magnetic shield to contain stray flux fields. This construction is referred to as an air core design. 10.5. Machine Design Challenges 10.5.1. Magnetic Fields Large synchronous machines with superconducting field windings have very little magnetic material in them. Conventional machine design techniques that are based on calculating magnetic fields in a small air gap will not work for HTSC machines. New design techniques based on two-dimensional and three-dimensional magnetic field calculations throughout the entire machine are necessary to model steady-state and transient machine performance. Beyond the unique challenges that the air core geometry presents to machine design, superconducting synchronous machine analysis requires detailed knowledge of the magnetic field distribution in the HTSC winding area. This is because HTSC wire has JC values that vary with magnetic field. Both steady-state DC as well as transient AC magnetic fields must be determined for all machine operating conditions. This is quite unlike the conventional case where copper wire performance does not depend on magnetic field. 10.5.2. Liquid/Gas Coolant Flow The combined actions of pressure change, centrifugal force, heat generation, and so on, establish the flow conditions and liquid/gas phase boundaries internal to the rotor cryostat. The fluid mechanics and heat transfer properties of liquid nitrogen in a non – uniform centrifugal force field are especially challenging. Present day design uses multichannel two-phase flow. In that case, channel distribution, size, and flow resistance must be adjusted to compensate for radial and axial pressure gradients, so that the liquid/vapour discharge has
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uniform quality and density. Otherwise, instabilities result that lead to dynamic unbalance problems. 10.5.3. Thermal Analysis The temperature rise of the field winding must be determined under both steady-state and transient conditions. The analysis must quantify the following:
Heat influx sources: armature I2R, field leads, outside ambient, etc.; Heat conduction resistances; Coolant surfaces and thermal resistances based on the results of the liquid/gas coolant flow
study; Heat storage, propagation and dissipation during a quench
10.5.4. Mechanical Analysis Conventional electric machines operate with fields up to 2 T. The HTSC machine will contain fields up to 5 T. This will increase the stress levels both on the conventional copper winding and the HTSC winding to values well above those experienced due to Lorentz forces in ordinary machines. Stress levels in the superconducting winding are especially critical because these windings are also exposed to other mechanical and thermal forces in addition to the Lorentz forces. Specifically, the wire sheath material must have mechanical strength to withstand both centrifugal and Lorentz forces, without suffering deformation. Furthermore, the field coil, field support structure and inner flux shield must comprise a pre - stressed assembly for dimensional stability at rated current, field and speed. The collection of these challenges dictates the direction of the design concept. 10.6. State – of – the – Art HTSC Machine Projects
Table 10.1 State – of – the Art HTSC Machine projects
Year Institution Country Type Power (MVA)
Speed (rpm)
HTSC Application
2011 Converteam UK Generator 1.79 214 - Hydro generator
2010 Siemens Germany Motor 4 120 - Ship propulsion motor
2010 Converteam UK Generator 8 12 ReBCO Wind generator design study
2008 AMSC USA Motor 36.5 120 BSCCO Ship propulsion motor
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2008 General Atomics
USA Motor 5 Homopolar induction motor
2008 Reliance Electric
USA Generator 10.5 120 BSCCO Generator design study
2007 IHI Marine, SEI Japan Motor 0.365 220 BSCCO Ship propulsion motor
2007 Doosan, KERI Korea Motor 1 3600 BSCCO Demo motor
2007 Doosan, KERI Korea Generator 1 1800 Demo generator
2007 Kawasaki Japan Motor 1 190 BSCCO Ship propulsion motor
2006 Long electromagnetics
USA Generator 5 10000 ReBCO High speed generator
2005 AMSC USA Generator 8 1800 Synchronous condenser
2004 AMSC USA Motor 5 230 BSCCO motor
.
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SECTION 3
A DETAILED STUDY ON HIGH TEMPERATURE SUPERCONDUCTING (HTSC) SYNCHRONOUS MOTOR
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CHAPTER 11 A Detailed study of the High temperature Superconducting (HTSC) Synchronous Motor
11.1. Background of this study
Bharat Heavy Electricals Ltd. (BHEL) is a Navaratna Public Sector Undertaking in India which is an integrated power plant equipment manufacturer and one of the largest engineering and manufacturing companies in India. It is engaged in the design, engineering, manufacture, construction, testing, commissioning and servicing of a wide range of products and services for the core sectors of the economy, viz. Power, Transmission, Industry, Transportation, Renewable Energy, Oil & Gas and Defense.
BHEL Corporate R&D at Hyderabad leads BHEL's research efforts in a number of areas of importance to BHEL's product range. The Electrical Machines Laboratory is a state – of – the Art established laboratory carrying out product oriented research in the field of Electrical Machines.
A project order was received by Electrical Machines Laboratory, BHEL R&D from the Ministry of Defense, India to develop a 200kW HTSC Synchronous Motor for defense applications.
The following work presented in this reported is the result of my association with the EMC laboratory during the execution of the above mentioned project for the fulfillment of my Bachelors project work.
11.2. Prior Experience of the Electrical Machines Laboratory in Superconducting Applications
The EMC Lab has been associated with superconducting applications apart from other areas of research, for the last decade with various projects to their credit.
1. High Gradient Magnetic Separator 2. 132 kW Low Temperature Superconducting (LTSC) Synchronous Motor 3. 5 MW Low Temperature Superconducting (LTSC) Synchronous Generator 4. 1 MVA High Temperature Superconducting (HTSC) transformer 5. 200 kW High Temperature Superconducting (HTSC) Synchronous Motor
11.3. Conceptual design of the HTSC Motor
The motor has an air core (i.e. nonmagnetic) construction so that the air gap field can be increased without the core loss and saturation problems imposed by a laminated stator and rotor iron core. Only the outer layer, the laminated frame and flux shield, is made of magnetic material which acts as a flux shunt to confine the high magnetic fields within the
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motor. A nonmagnetic and non – conducting support structure for the copper armature winding is located inside the frame. The normal conducting (copper) armature winding lies just outside of the air gap. It must be constructed from transposed filaments to reduce eddy current losses. The armature conductors will experience field levels that are about an order of magnitude above those experienced in a conventional motor. Conventional motor conductors lie in high permeability teeth which redirect the flux away from the conductors so that only the slot leakage flux actually penetrates the copper. In the superconducting motor, since the armature conductors see the entire air gap flux density, the armature winding must be carefully designed to minimize eddy current losses. Under steady state operation the motor rotor rotates in synchronism with the rotating field created by the three phase armature currents and the superconducting field winding experiences only DC magnetic fields. Under load or source transients, however, the rotor will move with respect to the armature created rotating field and it will experience AC fields. In a conventional synchronous motor these AC fields induce currents in damper windings or bars that create restoring torques to bring the rotor back into synchronism after a disturbance. These damper windings also serve as the rotor cage winding for across-the-line starting. The superconducting motor will be started by ramping the output frequency of the armature inverter, therefore a starting cage will not be necessary. Damping will be provided by concentric conducting shells located outside of and rotating with the field winding. The shells must also act to shield the field winding from all AC fields created during transients to prevent AC losses from occurring in the superconductors. It is expected that a two layer shielding structure will be utilized to accomplish the damping and shielding effectively. The outer layer will be a high strength material at room temperature (a warm shield) which will act as the damper winding and provide some AC flux shielding. Inside of the outer warm shield will be a thermal insulation space (vacuum) that surrounds the rotor cryostat. The inner layer of the rotor damper/shield structure will be a high conductivity shell that is near the operating temperature of the superconducting coils. This inner shield (the high conductivity AC flux shield) will provide some damping and, most importantly, acts to shield the superconducting field winding from any AC fields that pass through the outer warm shield. Inside the inner shield is the superconducting field winding on a nonmagnetic support structure. The superconducting field coils will be immersed in some cryogenic coolant or cooled through some mechanism. The coolant will be transferred into and out of the rotor through a rotating transfer coupling and be refrigerated by some means outside the motor.
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11.4. Variations in the design followed by R&D organizations worldwide 11.4.1. Rotor Design topologies 11.4.1.1. Magnetic core rotor design This design uses iron as the rotor core. This is opposite to the air core rotor design used in present day designs. This was one of the first designs tested by R&D organisations since the construction of this s very simple. It involves simply substituting the copper field winding with the HTSC winding.
A careful study of the history of superconducting machine design; the design constants in use in present day HTSC machines was developed from this design
Advantages Amount of the HTS material used for the field winding is greatly reduced Possibility to use the rotor steel of a conventional machine Use this design, if the aim is reducing the amount of superconducting material (cost savings)
Disadvantages
Limits the amount of flux achievable due to the saturation of the core Types of magnetic core rotor design
I. Warm rotor Warm rotor implies that keeping the core of the machine at room temperatures thereby cooling only the HTSC winding.
Advantages
Cold mass is minimized (amount of material to be cooled) Cool – down time (amount of time to reduce the temp of rotor from ambient to operating
temperature) is reduced Suitable for large machines like turbo generators
Disadvantages
Need for a complex support structure that connects the ambient temp rotor core and the operating temp field winding
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II. Cold Rotor This is another variation where the core is also maintained at cryogenic temperatures along with the HTSC winding. In this design, since it is a magnetic core design the entire core mass and the HTSC winding is cooled by the cryogenic coolant.
Advantages
A simple support structure to connect the field winding to the rotor core Disadvantages Cold mass is increased highly Load on the cryo cooler increases highly Cool down time also increases 11.4.1.2. Non – magnetic core rotor design Non – magnetic core rotor / air core rotor / ironless rotor core does not use the rotor core as part of the magnetic circuit of the machine The support for the HTS field coils is made from non – magnetic materials like alloys of aluminium which has excellent thermal conduction Currently this is in much use in designs world wide. Advantages High flux densities are possible as there is no saturation effect, hence full utilization of HTS technology can be obtained
Disadvantages
More HTS material is required which increases the cost
Removing the magnetic core introduces complexities and challenges in the design of the support structure. Since there is no rotor iron core, the support structure must now be designed to transmit all the torque produced 11.4.2. Stator design topologies 11.4.2.1. Stator with magnetic teeth
Conventional machine stator design with iron teeth Flux density is limited by the saturation of the iron teeth and core loss Used only for low flux machines
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Advantages Possibility to Retrofit existing machines Provision for easy up gradation of conventional machines Existing conventional design expertise can be put to use
Disadvantages
Due to iron teeth, the flux that can be obtained is severely limited Reduction in size of the motor is hardly possible Presence of harmonics in the magnetic field ( slot harmonics ) due to the alternating slots
and teeth which leads to losses Use this design only to improve overall efficiency
11.4.2.2. Stator without magnetic teeth
Best scheme for getting the full advantage of HTSC Technology Non – magnetic teeth / air core rotor Air core winding is situated in the air gap between the stator (yoke) and the rotor without
any magnetic teeth The stator slot is composed of non – magnetic FRP having the same permeability with air
Advantages
Compactness can be achieved Air gap flux density is not limited. This implies that high operating flux can be achieved Negligible harmonics is generated in the magnetic field due to the absence of slots ( highly
reduced slot harmonic ) Very small additional losses are generated Noise due to vibration of teeth is eliminated
Disadvantages
Heat removal is difficult as teeth which helps in heat conduction is absent This is a new design, so it needs to be established
11.5. Conceptual Design adopted by BHEL R&D
The design adopted by BHEL R&D relied heavily on its previous experience with Low Temperature Superconducting Motor. Few modifications were made in the conceptual design of the LTSC motor to accommodate the HTSC feature.
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The design consists of the following features
1. Race track type field coils 2. Stainless Steel coil support structure with cryo pipe and field lead holes, bolted to the inner
periphery of a Stainless Steel cylinder which forms the inner rotor 3. Coil support structure with bolt for lid and bottom is welded to the lid 4. Air core rotor with shaft inside the inner rotor. 5. Torque tubes on either sides of the inner rotor 6. The coolant pipes are within the shaft and are brought out from within it inside the inner
rotor. 7. Three piece shaft i.e. non drive end, inner rotor shaft, drive end. 8. Air core stator with EHV winding above the outer rotor with non - magnetic support
structure 9. Brushless exciter at the non drive end. 10. Cold rotor type construction 11. Flow type cryo cooling system with helium transfer coupling for transferring cryo coolant
from stationary medium to rotating medium and radiation screen between the inner and outer rotor to neutralise the incoming heat and outgoing heat at the torque tube
12. Series type cryo coolant transfer between the individual support structure 13. Rotating vacuum system with bulk vacuuming and with provision for connecting seal off
valve for re – vacuuming
11.5.1. Race track type field coils
Second Generation (2G) (YBCO) wire is used for making the field coils. The YBCO wire is sandwiched between two SS tapes to enhance the mechanical strength of
the coil. A Teflon tape is then wound over this SS/2G wire/SS structure to provide insulation. This makes the starting material for winding the coil. This wire is then wound in turns on a former to make a layer. Many such layers stacked one above the other form a coil. Since this coil is in the shape of a race track; it is called a race track coil This particular shape for the coil is chosen because it resembles the shape of the field
winding in a conventional synchronous machine After making a coil, a copper encasing is provided around it keeping a provision for taking
out the field leads. This encasing serves many purposes.
i. It enhances the mechanical strength of the machine ii. It acts as an interface which absorbs the heat from the wires and brings it to the surface. In
essence, it is the wall between the wires and cryogenic coolant; hence plays a major role in the cooling process.
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A few such race track coils are stacked one above the other and the field leads are connected in series to form a single pole.
Spacers are placed between the coils to help the cryo coolant to get in contact with a larger area of the coil which leads to better coolant distribution and enhanced cooling.
The end result of this process is a completed field pole made of a stack of race track coils comprising the 2G HTSC wires and encased by copper sheet, with only two field leads visible outside the field pole.
Figure 11.1. shows a model of a HTSC race track coil with copper encasing
Figure 11.2 shows race track coils in reality
11.5.2. Stainless Steel coil support structure with cryo pipe and field lead holes, bolted to the inner periphery of a Stainless Steel cylinder which forms the inner rotor
11.5.3. Coil support structure with bolt for lid and bottom is welded to the lid The field pole is then placed inside an SS block having two parts i.e. the lid and the bottom. The bottom is having a pole shoe and pole body made of SS. The field pole is lifted and placed on the pole body, so that the pole body snugly fits into the inner
space of the race track structure. This pole body rests on the pole shoe. Hence essentially the Copper pole body is in contact with the inner periphery of the SS support
structure This bottom of the block has two holes; one each for entry and exit from the support structure. A hole has two concentric pipes through each one of it. The inner pipe carries the field leads which is connected to the two leads of the field pole The outer pipe carries the cryogenic coolant which cools the field coils. It also cools the field leads. The field leads are to be maintained always at cryogenic temperature and hence the field leads are
always inside the cryogenic coolant pipe throughout the machine till the end. This keeps the leads also in superconducting state.
The field leads and cryo coolant pipe can be seen on the support structure outer periphery. The lid consists of a SS plate which is flat on one side where it joins with the face of the bottom part
and is angular on the other side where it needs to be bolted to the inner periphery of a SS cylinder. Hence the outer radius of the lid is equal to the inner radius of the SS cylinder. After the pole is place through the pole body and rests on the pole shoe, the bottom part is covered
by the lid. The lid and the bottom part are welded together at the contact surface of the two bodies. A bolt which extends into the lid from the upper angular side is then fixed to the inner periphery of
the SS cylinder (i.e. the inner rotor) After all the field poles along with their SS support structures are attached to the inner periphery of
an SS cylinder, the connections to the concentric pipes intended to carry the field leads in the inside and the cryo coolant on the outside are given.
This is a very complicated process since there is already a shaft inside the inner rotor and hence the inlet and outlet field leads and cryo coolant pipes must be inside the shaft; concentric to it.
Hence the workspace that is available for carrying out the connections is very less.
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Note: A discussion on the coil support structure with bolt for lid and bottom is welded to the lid
One major disadvantage of this arrangement is that, the entire weight of the coils either in stationary state or rotating state falls solely on the welding between the lid and the bottom.
Moreover the behavior of the weld at cryogenic temperatures is unknown. Hence there is a possibility of the weld giving away partially or completely during operation If the weld gives away partially, pores will be formed in the weld. One side of the weld is the enclosure having gaseous He and the other side is vacuum, hence any
pores in the weld will create a channel between the two which is dangerous in perspective of the cooling system. Since the coils are not designed to withstand vacuum, there is every possibility of the copper enclosure to puncture exposing the gaseous He to the vacuum. (moreover the copper enclosure is designed to be as thin as possible to minimize the mass that needs to be cooled to cryogenic temperatures along with the Superconducting coils and also to minimize the diameter of the motor. Hence they are not thick enough to sustain exposure to vacuum. This will lead to a puncture in the copper enclosure). This will cause all the He gas to be sucked into the vacuum and hence the temperature of the coils increases. This will initiate a quench. And the superconductor will undergo a transition into the normal conductor state destroying the superconductivity. Simultaneously, as the vacuum of the machine gets diminished the heat load on the cryo refrigerator increases tremendously. After reaching its limit, the cryo refrigerator also fails.
This will ultimately shut down the machine If the weld gives away completely, the entire structure consisting of the pole shoe, pole body, field
coils will eventually collapse into the center of the rotor and the cryo pipe and field lead connections will break and if there is a series mode of cooling then the entire coolant will start flowing into the vacuum which might cause serious damage; not only will the machine get destroyed but there is a chance of explosion since a large volume of gas is openly in contact with a very high vacuum all of a sudden.
All these problems must be given a thought and corresponding design changes or replacement must be carried out subsequently
11.5.4. Air core rotor with shaft inside the inner rotor. The core of the inner rotor experiences magnetic fields as high as 5T, hence any magnetic
material used will get saturated. To avoid this problem, the core is left hollow (i.e. air core). This hollow space that is referred above is the inner volume of the SS cylinder to which the
field poles are bolted. This volume is later filled with vacuum and is cooled to the operating temperatures (in case
of a cold rotor) or left as it is at room temperature ( in case of a warm rotor) The axis of the cylinder also contains the shaft that exists throughout the length of the
machine.
11.5.5. Thermal Neutralizer above the inner rotor Above the inner rotor, a copper cylinder is weld to the torque tubes which serves a specific
purpose of neutralizing the temperature across the torque tube The torque tube is at the room temperature and the inner rotor is at operating temperature,
hence there is a high temperature gradient across the torque tube.
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The heat from the outer atmosphere has a high tendency to enter into the inner rotor thereby increasing its temperature.
To solve this problem, two points having the same temperature gradient on each torque tube are connected together by a copper cylinder by means of welding.
It creates a by – pass route for the heat to flow across the inner rotor without entering into it and damaging the operating temperature
This is done to maintain the cryogenic load constant and to keep the electrical and magnetic characteristics of the HTSC wire constant, since it depends on the operating temperature
11.5.6. Outer rotor (AC Damper) above the thermal neutraliser Above the thermal shield, lies another copper shell at room temperature which serves as the
AC Damper for the synchronous motor Under steady state operation the motor rotor rotates in synchronism with the rotating field
created by the three phase armature currents and the superconducting field winding experiences only DC magnetic fields.
Under load or source transients, however, the rotor will move with respect to the armature created rotating field and it will experience AC fields.
In a conventional synchronous motor these AC fields induce currents in damper windings or bars that create restoring torques to bring the rotor back into synchronism after a disturbance.
These damper windings also serve as the rotor cage winding for across-the-line starting. Whereas the superconducting motor will be started by ramping the output frequency of the
armature inverter, therefore a starting cage will not be necessary. Damping will be provided by concentric conducting shells located outside of and rotating
with the field winding. The shell must also act to shield the field winding from all AC fields created during
transients to prevent AC losses from occurring in the superconductors.
11.5.7. Torque tubes on either sides of the rotor On both side of the rotor, Solid SS cylinders having the diameter the same as the inner rotor
(SS cylinder). These are called torque tubes. It serves the following purposes
It is the intermediate structure which provides connectivity between the outer shaft and the inner rotor.
The torque tubes have the shaft on the outer side and the inner rotor (SS cylinder) welded to the inner side
It transmits the torque across the machine. Provides mass to the rotor structure to attain the required moment of inertia It is also the interface between the room temperature mass (outer shaft) and the
operating temperature mass (inner rotor).
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The drive end torque tube, field poles, inner rotor, shaft inside the inner rotor, thermal neutralizer, outer rotor (AC damper) and non – drive end torque tube all put together constitute the rotor of the machine
11.5.8. The coolant and field lead pipes are taken into the inner from within the shaft. The shaft within the inner rotor also has another function apart from providing the
mechanical support for rotation. This shaft is hollow and contains two concentric pipes, the inner one carrying the field leads
and the outer one carrying the coolant. Inside the inner rotor, the pipes are bent perpendicular to the axis and then connected to the
field pole support structures. The field leads are also made of HTSC material, hence must be cooled and maintained at
cryogenic temperatures. Hence, the field leads are placed n the inner pipe and the coolant in the outer. The coolant pipes are made of copper to enhance thermal conduction
11.5.9. Three piece shaft i.e. non drive end, rotor shaft, drive end. The most important point to be noted is that the shaft of the machine is actually made up of
three pieces i.e. the shaft on the non –drive side, the rotor shaft and the drive end shaft. This means that during the construction, the three pieces are manufactures individually and
then coupled together during the assembly. This aspect further increases the complexity of the machine since the shaft rotates at high
speeds. The balancing of the rotor becomes complex because of the three piece feature. During the occurrence of faults in a machine, vibrations occur in the rotor of the machine. In
a conventional machine, the shaft is a single entity. Whereas in a HTSC motor, the shaft is made up of three pieces, hence there are high chances of any vibrations occurring in the shaft to get amplified and throw the machine out of balance if the shaft is not balanced properly.
Most important of all is that, the shaft in this case has other arrangement along with it (i.e. the coolant and field lead pipes) hence; it is not just a simple solid rod as in a conventional machine.
Coupling and balancing this complicated structure requires more elaborate calculations and experience.
11.5.10. Larger Air gap The HTSC machine has a larger air gap compared to a conventional machine. Typically, the conventional machine has an air gap of around 0.5mm while the HTSC
machine has an air gap of 20mm This magnetic field generated is this machine is very large. Typically around 5T for a fully
optimized machine.
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If the stator winding is placed in a high magnetic field of around 5T, the copper conductor will get saturated.
Hence, a large air gap is maintained. As a result, the magnetic field weakens over the air gap and reaches sustainable values.
11.5.11. Air core stator with EHV winding above the outer rotor with non - magnetic
support structure The armature winding lies just outside of the air gap. It is constructed from transposed filaments to reduce eddy current losses. The armature conductors will experience field levels that are about an order of magnitude
above those experienced in a conventional motor. Conventional motor conductors lie in high permeability teeth which redirect the flux away
from the conductors so that only the slot leakage flux actually penetrates the copper. In the superconducting motor, since the armature conductors see the entire air gap flux
density, the armature winding must be carefully designed to minimize eddy current losses. Hence, extra high voltage windings are used in this machine. The most important feature is that this machine has no teeth in its stator. Hence, a separate arrangement is needed to support the winding. A support structure made of Hardened Glass Laminate (HGL) is used. HGL is used because it is a non – magnetic material.
Note: Discussion on HGL
o Glass is first made into fibers. o The fibers are then woven into a cloth o This cloth is then mixed with a special resin to become hard. o This hardened resin cloth is stacked in many layers and pressed at around 200-3000C and up to 300
atm. pressure. o It becomes a hard material which is used as insulation, spacers, etc.
11.5.12. Brushless exciter at the drive end. To excite the machine, that is to provide the DC supply to the field winding, a brushless
exciter is placed on the drive end shaft side.
11.5.13. Cold rotor type construction With reference to the section (insert the number), the cold rotor type construction is used. The implications of this design are
The volume inside the inner rotor and the space between the inner rotor and the thermal neutralizer are filled with vacuum.
The coolant sees the field poles (HTSC Winding), field support structure and the inner rotor as the cold mass ( material to be cooled )
It implies that these parts will be at operating temperatures (25K – 30K)
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11.5.14. Flow type cryo cooling system with helium transfer coupling for transferring cryo coolant from stationary medium to rotating medium In this type, the coolant flows along the entire cooling path. The coolant pipe inside the shaft, the perpendicular pipes from the shaft to the field poles,
the field poles and then back again to the coolant pipe in the shaft forms the path of the coolant in the machine.
The difficulties in this system are obvious. The design must be very efficient to accommodate the contraction in the material when
cooled to cryogenic temperatures There must be very minimum bends and welds in this arrangement as it needs to carry a
cryogenic liquid through a vacuum chamber. The pipes and bends and welds must have a finish of very high smoothness to sustain the
vacuum
11.5.15. Series type cryo coolant transfer between the individual support structure The coolant flows from one pole to the other in series i.e. after passing through all the poles
it again enters into the pipe inside the shaft. This arrangement has the advantage of minimizing the complexity involved if it were to be
designed in parallel type, wherein each pole structure is connected independently to the shaft.
In the parallel type, the number of connections are more, hence the welds, bends are also more increasing in the complexity.
Series type is very simple compared to the parallel type
11.5.16. Stationary Vacuuming with provision for connecting seal off valve for re – vacuuming A seal of valve is connected to the non – drive end of the rotor. Its function is to connect the vacuum pump to the rotor to re – establish vacuum in the rotor. The seal of valve has O – rings which tightly bind to the vacuum pump when the vacuuming
process is initiated. A drawback of this system is that, there is no provision to do re – vacuuming while the rotor
is in rotation. To do re – vacuuming, the machine has to be stopped. From previous experience with superconducting machine projects, it has been roughly
estimated that the machine will have to be stopped once every 4 days to do the re – vacuuming.
11.6. Steps involved in performing the detailed design of a HTSC Motor
11.6.1. Rotor Design 1. Field Winding electrical design 2. Field winding thermal design 3. Field winding mechanical design 4. Field winding support structure design
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5. Heat bus design 6. EM shield design 7. Vacuum establishment 8. Torque tube design 9. Three piece rotor design 10. Commutator Design 11. Non - drive end - bore design for entry of heat pipe 12. Magnetic Analysis of rotor
11.6.2. Stator Design
1. Design of EHV windings 2. Design of winding support structure 3. Outer shield design
11.6.3. Cryogenics Design
1. Hot Head 2. Cold head 3. Heat pipe 4. Cryo - refrigerator
11.6.4. Vacuum Technology
1. Establishing very high vacuum in a rotating drum
11.7. Advanced design variations developed by R&D institutions worldwide
11.7.1. Neon based cryo cooling technique
Salient features: Works on the principle of heat exchange as shown in figure 11.3. A cryo refrigerator (Gifford McMahon or Pulsed Tube) forms the cold head (Condenser) of
this heat exchanger. A cylinder inside the machine (inside the rotor) forms the hot head (Evaporator) A heat pipe carrying liquid Neon forms the heat carrying medium between the cold head. The heat from the poles and pole structures is transported to the heating cylinder by means
of conduction through copper rods. Figure 11.4. shows an illustrations of cooling process in this system Advantages of this design
11.7.1.1. No coolant flow throughout the machine 11.7.1.2. No need to collect the coolant since it is a closed system. 11.7.1.3. Neon having boiling point at 20K is the best suitable fluid to maintain 25K – 30K
temperature range. 11.7.1.4. No need for advanced precision welding since there are no complex bends and welds
involved. 11.7.1.5. Simple to construct and assemble
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11.7.1.6. A study conducted by Siemens says that the Helium cryogenic system fails in maintaining constant temperature in the rotor; while the Neon based system is successful
Discussion on enabling technologies
1. Cryo cooling Current status of the technology for obtaining vacuum for HTSC motor is 1. One way to obtain vacuum is to manufacture the rotor as a sealed vacuum chamber. This approach does not require that the rotor be connected to an external vacuum pump during operation. However, it does require that the welds and joints be of very high quality. In addition, the composite materials commonly used in HTSC have inherently high out – gassing rates that rapidly compromise the vacuum level. This requires that the motor be stopped and the rotor vacuum chamber be pumped out periodically to maintain a sufficient level of vacuum. 2. Another way to obtain a vacuum surrounding the HTSC material is to enclose the entire rotor (sometimes even the stator) in a stationary vacuum chamber. This allows that vacuum space to be constantly pumped out to maintain the requisite vacuum level. The major disadvantage of this approach is that it requires rotating vacuum seals for the rotor shaft. The cost and complexity of the rotating vacuum seals increases as the size of the shaft increases. Hence, for very large motors (for which HTSC is deemed to be viable) the use of rotating vacuum seals becomes prohibitively expensive.
2. FRP molding for the required shape
FRP implies Fiberglass Reinforced Concrete It is a composite of a resilient durable resin with an immensely strong fibrous glass. The resin is the main component and is normally a polyester resin. It is supplied in the form of viscous syrup, which when suitably activated sets to a hard
solid. Just as concrete may be reinforced with steel rods, so polyester resin may be reinforced with
glass fibers to form FRP.
Fabrication of FRP A single surface mould or form is used on which layers of glass mat with liquid resin is
impregnated until the required thickness is built up, so forming a laminate or moulding. After removal of this product from the mould many more in the same way can be made.
In the HTSC Motor FRP is used as a support and insulation for the armature winding. But fabricating FRP for this application requires considerable skill since the shape is complex and the operating conditions to which it is subjected are adverse. Hence, further work must be carried out to improve this design.
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Figure 11.1. Model of HTSC race track with coil copper encasing
Developed with Autodesk® Inventor® Fusion
Figure 11.2. Race track coils fabricated by Siemens
Source: Siemens
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Figure 11.3. Neon based cryo cooling technique
Source: Siemens Figure 11.4. Illustration of cooling process in Neon based system
Source: Siemens
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Figure 11.5. Conceptual design adopted by Electrical Power Research Institute (EPRI)
Figure 11.6. Conceptual Design adopted by Siemens AG
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Comparison between the rotors of a conventional vs HTSC synchronous motor Figure 11.7. Conventional Synchronous Rotor
Figure 11.8. HTSC Synchronous Rotor
Source: Siemens
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Table 11.1. Current projects on HTSC Synchronous Machines
Figure 11.9. Projected Efficiencies of a HTSC Synchronous Motor compared to a conventional Synchronous Motor
Source: Siemens
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CHAPTER 12
SUMMARY AND CONCLUSION
12.1. Summary
In the above work, superconductivity was first introduced as an abstract theory. The BCS theory which is the governing theory for superconductivity was briefly explained. The classifications of superconductors based on their critical temperature, response to magnetic field and governing theory have been described. The behavior of type 1 and type 2 superconductors in the presence of an external magnetic field has been explained in detail.
The superconducting wires have been discussed in detail. The manufacturing processes, the technologies involved in the fabrication of the wires have been described. A detailed discussion has been made on the electrical, mechanical, physical, geometry, thermal and magnetic properties of the wires; both 1G (BSCCO) and 2G (YBCO). Basic experiments have been conducted on the same, the results of which have been presented.
Following the above work, superconductors were investigated for their use in electrical power applications in the section 2. The applications like HTSC power cables, HTSC magnetic energy storage, HTSC fault current limiter, HTSC transformer, HTSC machines have been discussed briefly. Each application being unique has been discussed in its own style. A detailed case study on the R&D work at Waukesha Electric Systems, USA on HTSC transformers i.e. the 5/10 MVA project and the present 28 MVA project have been presented in detail in Appendix C. This and similar case studies help in the design of new projects on such applications.
The last section consists of the detailed discussion on the concepts, design process, conceptual design, constructional details and challenges involved in the design, construction and assembly of HTSC Synchronous Motor project. This section is the result of the active interactions of the author with the engineers and technicians at Electrical Machines Laboratory, BHEL Corporate R&D, Hyderabad on the 200kW HTSC Synchronous Motor project currently under progress at the lab.
This report attempts to portray superconducting materials as being one of the most eligible candidates for use in high power rating electrical utility applications with the ultimate goal of minimizing losses and achieving better efficiency thereby resulting in large economical benefits for the industry. The superconducting applications are also the best suited solution in the aspect of environmental impact which is being considered as of very high importance lately. Further developments in this field are possible with the collective efforts of R&D organizations, universities, industry and the government.
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REFERENCES
[Sheahen] Thomas P. Sheahen “Introduction to High Temperature Superconductivity”, Kluwer
Academic, United States.
[SuperPower Inc] www.superpower-inc.com (Referencing to all the documents shared by SuperPower)
[AMSC] www.amsc.com (Referencing to all documents shared by AMSC)
[Siemens] www.siemens.com (Referencing to all the documents shared by Siemens)
[Nexans] www.nexans.com (Referencing to all the documents shared by Nexans)
[Converteam] www.converteam.com (Referencing to all the documents shared by Converteam)
[DOE, 1997] The university of Tennessee space institute “Research and Development Roadmap to
Achieve Electrical Wire Advancements from Superconducting Coatings”, Department of
Energy, 1997
[Virginia, 2009] Manish Verma, “A comprehensive overview, behaviour model and simulation of Fault Current Limiters, Virginia Polytechnic and state university, 2009
[Florida, 2006] Mohit Mathur, “Cooling concept for the Armature winding of a High Temperature Superconducting Motor”, Florida State University, 2006
[EPRI,1997] Reliance Electric Company, “Electric Motors using High Temperature Superconducting Materials Applied to Power Generation Station Equipment
[Waukesha] www.waukesha.com (Referencing to all the documents shared by Waukesh)
[General Electric] General Electric, “Design and development of a 100MVA HTS Generator for commercial entry”, US department of Energy, 2006
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Appendix A
List of all available Superconducting materials
Type 1 Superconductors
Material Critical Temperature (K)
Lead (Pb) 7.196
Lanthanum (La) 4.88
Tantalum (Ta) 4.47
Mercury (Hg) 4.15
Tin (Sn) 3.72
Indium (In) 3.41
Palladium (Pd)+ 3.3
Chromium (Cr)+ 3
Thallium (Tl) 2.38
Rhenium (Re) 1.697
Protactinium (Pa) 1.40
Thorium (Th) 1.38
Aluminium (Al) 1.175
Gallium (Ga) 1.083
Molybdenum (Mo) 0.915
Zinc (Zn) 0.85
Osmium (Os) 0.66
Zirconium (Zr) 0.61
Americium (Am) 0.60
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Cadmium (Cd) 0.517
Ruthenium (Ru) 0.49
Titanium (Ti) 0.40
Uranium (U) 0.20
Hafnium (Hf) 0.128
Iridium (Ir) 0.1125
Beryllium (Be) 0.023
Tungsten (W) 0.0154
Platinum (Pt)+ 0.0019
Lithium (Li) 0.0004
Rhodium (Rh) 0.000325
Type 2 Superconductors
Material Critical Temperature
(Tl5Pb2)Ba2Mg2Cu9O17+ +28 C
(Tl5Pb2)Ba2MgCu10O17+ +18C
(Tl4Pb)Ba2MgCu8O13+ +3C
(Tl4Ba)Ba2MgCu8O13+ ~265 K
(Tl4Ba)Ba2Mg2Cu7O13+ ~258 K
(Tl4Ba)Ba2Ca2Cu7O13+ ~254K
(Tl4Ba)Ba2Ca2Cu7O13+ ~254 K
(Tl4Ba)Ba4Ca2Cu10Oy ~242 K
Tl5Ba4Ca2Cu10Oy ~233 K
(Sn5In)Ba4Ca2Cu11Oy ~218K
(Sn5In)Ba4Ca2Cu10Oy ~212K
Sn6Ba4Ca2Cu10Oy ~200K
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(Sn1.0Pb0.5In0.5)Ba4Tm6Cu8O22+ ~195K
(Sn1.0Pb0.5In0.5)Ba4Tm5Cu7O20+ ~185K
(Sn1.0Pb0.5In0.5)Ba4Tm4Cu6O18+ ~163K
Sn3Ba4Ca2Cu7Oy ~160K
(Hg0.8Tl0.2)Ba2Ca2Cu3O8.33 138 K*
HgBa2Ca2Cu3O8 133-135 K
HgBa2Ca3Cu4O10+ 125-126 K
HgBa2(Ca1-xSrx)Cu2O6+ 123-125 K
HgBa2CuO4+ 94-98 K
Tl2Ba2Ca2Cu3O10 127-128 K
(Tl1.6Hg0.4)Ba2Ca2Cu3O10+ 126K
TlBa2Ca2Cu3O9+ 123K
(TlSn)Ba4TmCaCu4O14+ ~121 K
(Tl0.5Pb0.5)Sr2Ca2Cu3O9 118-120 K
Tl2Ba2CaCu2O6 118 K
TlBa2Ca3Cu4O11 112 K
(SnTl0.5Pb0.5)Ba4Tm3Cu5O16+ 105 K
TlBa2CaCu2O7+ 103 K
Tl2Ba2CuO6 95 K
TlSnBa4Y2Cu4Ox 86 K
Sn4Ba4(Tm2Ca)Cu7Ox ~127 K
Sn2Ba2(Tm0.5Ca0.5)Cu3O8+ ~115 K
SnInBa4Tm3Cu5Ox ~113 K
Sn3Ba4Tm3Cu6Ox 109 K
Sn3Ba8Ca4Cu11Ox 109 K
SnBa4Y2Cu5Ox 107 K
Sn4Ba4Tm2YCu7Ox ~104 K
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Sn4Ba4TmCaCu4Ox ~100 K
Sn4Ba4Tm3Cu7Ox ~98 K
Sn2Ba2(Y0.5Tm0.5)Cu3O8+ ~96 K
Sn3Ba4Y2Cu5Ox ~91 K
SnInBa4Tm4Cu6Ox 87 K
Sn2Ba2(Sr0.5Y0.5)Cu3O8 86 K
Sn4Ba4Y3Cu7Ox ~80 K
Bi1.6Pb0.6Sr2Ca2Sb0.1Cu3Oy 115 K
Bi2Sr2Ca2Cu3O10 110 K
Bi2Sr2(Ca0.8Y0.2)Cu2O8 95-96 K
Bi2Sr2CaCu2O8 91-92 K
CaSrCu2O4 110 K
YSrCa2Cu4O8+ 101 K
(Ba,Sr)CuO2 90 K
Pb3Sr4Ca3Cu6Ox 106 K
Pb3Sr4Ca2Cu5O15+ 101 K
(Pb1.5Sn1.5)Sr4Ca2Cu5O15+ ~95 K
Pb2Sr2(Ca, Y)Cu3O8 70 K
AuBa2Ca3Cu4O11 99 K
AuBa2(Y, Ca)Cu2O7 82 K
AuBa2Ca2Cu3O9 30 K
YBa3Cu4Ox 177 K
YCaBa3Cu5O11+ 107 K
(Y0.5Lu0.5)Ba2Cu3O7 106 K
(Y0.5Tm0.5)Ba2Cu3O7 104 K
Y3Ba5Cu8Ox 105 K
Y3CaBa4Cu8O18+ 99 K
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(Y0.5Gd0.5)Ba2Cu3O7 97 K
Y2CaBa4Cu7O16 96 K
Y3Ba4Cu7O16 96 K
Y2Ba5Cu7Ox 96 K
NdBa2Cu3O7 96 K
Y2Ba4Cu7O15 95 K
GdBa2Cu3O7 94 K
YBa2Cu3O7 92 K
TmBa2Cu3O7 90 K
YbBa2Cu3O7 89 K
YSr2Cu3O 62 K
GaSr2(Ca0.5Tm0.5)Cu2O7 99 K
Ga2Sr4Y2CaCu5Ox 85 K
Ga2Sr4Tm2CaCu5Ox 81 K
La2Ba2CaCu5O9+ 79 K
(Sr,Ca)5Cu4O10 70 K
GaSr2(Ca, Y)Cu2O7 70 K
(In0.3Pb0.7)Sr2(Ca0.8Y0.2)Cu2Ox 60 K
(La,Sr,Ca)3Cu2O6 58 K
La2CaCu2O6+ 45 K
(Eu,Ce)2(Ba,Eu)2Cu3O10+ 43 K
(La1.85Sr0.15)CuO4 40 K
SrNdCuO 40 K
(La,Ba)2CuO4 35-38 K
(Nd,Sr,Ce)2CuO4 35 K
Pb2(Sr,La)2Cu2O6 32 K
(La1.85Ba.15)CuO4 30 K
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GdFeAsO1-x 53.5 K
(Ca,Sr,Ba)Fe2As2 38 K
LiFeAs 18 K
MgB2 39 K
Ba0.6K0.4BiO3 30 K
Nb3Ge 23.2 K
Nb3Si 19 K
Nb3Sn 18.1 K
Nb3Al 18 K
V3Si 17.1 K
Ta3Pb 17 K
V3Ga 16.8 K
Nb3Ga 14.5 K
V3In 13.9 K
PuCoGa5 18.5 K
NbN 16.1K
Nb0.6Ti0.4 9.8K
MgCNi3 7 - 8 K
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Appendix B
Glossary of terminology associated with Superconductivity
1G (1st Generation) HTS Wire
BSCCO 2223 is a commonly used name to represent the HTS material Bi(2-
x)PbxSr2Ca2Cu3O10. This material is used in AMSC's multi-filamentary composite HTS wire and has a typical superconducting transition temperature around 110 Kelvin.
2G (2nd Generation) HTS Wire
Second generation wire has a significantly different architecture compared to 1G wire. The architecture of 2G HTS wire, which comprises multiple coatings on a base material, or substrate, is designed to achieve the highest degree of alignment possible of the atoms in the superconductor material. The objective is to achieve the highest possible electrical current carrying capacity. While the high degree of alignment of the atoms in an HTS material can be achieved by many different laboratory techniques, the business challenge is to achieve this alignment with a high volume, low cost manufacturing methodology. The AMSC wire’s “three-ply” architecture — consisting of slit 2G tape sandwiched between thin copper or stainless steel strips — is similar to that of AMSC’s commercial 1G HTS wire. AMSC’s 2G HTS wire is called 344 superconductors, as it relates to the 3-ply, 4.4 cm width of the finished wire.
AC
Alternating current. An electric current that flows back and forth, typically changing direction 50 or 60 times per second.
Amp
Ampere. The standard unit for measuring the strength of an electric current.
Back Tension
Back tension is the amount of force that is placed on the wire along its longitudinal axis during processing (such as wire transfer, reeling, winding, ec.). So an "x" amount of back tension is "x" amount of force applied along the longitudinal direction of the wire.
Ballooning
Ballooning is damage to the wire caused by the rapid expansion of cryogenic gases within the wire when it is warmed to room temperature from a cryogenic temperature. When the wire is exposed to a pressurized liquid cryogenic environment, the liquid cryogen can diffuse into the wire unless the wire is hermetically sealed. Upon warming to room temperature, this cryogenic liquid turns to gas within the wire causing it to balloon. This
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balloon process will disrupt the material and thus the superconducting properties of the wire. If you are considering using the wire in a pressurized liquid cryogenic environment such as liquid Nitrogen, please contact us about the advantages of using AMSC's Hermetic Wire (See Hermetic Seal). Hermetic Wire has been designed specifically to withstand pressurized liquid cryogenic environments.
Bend Diameter
The bend diameter is the inner diameter of the wire when bent around a circular mandrel, spool, or reel. The critical bend diameter is the smallest diameter to which the wire can be wound without damaging the wire. For non-circular winding, there cannot be any section of the winding where the arc of the bend is tighter than the arc of the corresponding circle having the critical bend diameter.
BSCCO 2223
BSCCO 2223 is a commonly used name to represent the HTS material Bi(2-
x)PbxSr2Ca2Cu3O10. This material is used in our multi-filamentary composite HTS wire and has a typical superconducting transition temperature around 110 Kelvin.
Coated Conductors
Ribbon-shaped wires that show promise as a next generation wire technology. These wires are made by depositing a thin layer of HTS material, along with a protective coating, onto a flexible metal ribbon.
Coil
A wound spiral of wire that forms an electromagnet when energized. Specific coils are often named for their particular geometric design. For example, a "racetrack" coil is oval shaped.
Compressive Stress
Compressive stress arises from forces that attempt to condense or shrink the material or wire (squeezing as opposed to pulling). A critical compressive stress for HTS wire is the amount of compressive stress that can be applied to the wire before the critical current is reduced below a specified level (such as 95% Ic retention).
Compressive Strain
Compressive strain is the percentage change in a material dimension that is under compression. A critical compressive strain for HTS wire is the amount of compressive strain that occurs before the critical current is reduced below a specified critical level (such as 95% Ic retention).
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Critical Current
The ideal physical definition of critical current is the current where a material has a phase transition from a superconducting phase to a non-superconducting phase. For practical superconducting wire, the transition is not infinitely sharp but gradual. In this case, the critical current is defined as the current where the voltage drop across the wire becomes greater than a specific electric field, usually 1 microvolt/cm. Sometimes, for low-loss magnet applications, a lower electric field criterion is used. The critical current is represented by the variable Ic.
Critical Current Density
The critical current density, Jc is the critical current of a superconductor divided by the cross sectional area of the superconductor material. The critical current density is useful when characterizing the quality of a superconductor material. The critical current density, Jc should not be confused with the engineering critical current density, Je (see Engineering Critical Current Density)
Critical Current Retention
The critical current retention is the relative ratio of the critical currents before and after a specific process or test. For example, assume the initial critical current, Ico, of a wire is 100 amps. After the wire is subjected to a certain test, the post-test critical current, Ic, is measured at 95 amps. The critical current retention, Ic/Ico, is 95/100 = 0.95 or 95% for this wire under the test conditions.
Cryogenics
The branch of engineering that pertains to materials and equipment that are used at very low temperatures.
Current Leads
Conductors that carry large amounts of power but minimal heat into ultra-low temperature cryogenic environments. Current leads address the critical problem of heat leak.
Current Limiter
A device used to instantaneously limit the flow of excessive electrical current (fault current) in a circuit, thereby protecting expensive electrical equipment. Fault currents are typically caused by short circuits, lightning or common power fluctuations.
Dielectric
An insulating substance such as oil, liquid nitrogen or paper.
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Dirty Power
Fluctuations in the voltage or current on a power system that can affect the performance and operation of electrical equipment.
Electric Motor
Equipment that converts electrical energy into useful mechanical power.
EMF
Electric and magnetic fields surrounding any wire that conducts electricity.
Energy Storage
Reserving electric energy for later use to avoid blackouts or fluctuations in power. Storage methods include batteries, flywheels, pumped hydro-power and Superconducting Magnetic Energy Storage (SMES).
Engineering Critical Current Density
The engineering critical current density, Je, is the critical current of the wire divided by the cross sectional area of the entire wire, including both superconductor and normal metal materials. This is different from the standard critical current density, Jc, which is the critical current divided by the cross sectional area of just the superconductor material of the wire. The engineering critical current density is an important parameter used in the design of applications based on HTS wires.
EPRI
Electric Power Research Institute. Founded in 1972, EPRI identifies and pursues advanced technology for the U.S. electric utility industry to improve power production, distribution and use. EPRI serves approximately 700 member utilities.
Fault Current
The momentary flow of excessive electrical current in a circuit. It is usually caused by short circuits, lightning or common power fluctuations.
Four Probe Resistance
A four probe resistance measurement is a technique for measuring the resistance of low resistance material. Unlike two probe resistance measurements where the voltage drop is measured by the same probe that does the current excitation, the four probe technique separates the voltage drop measurement probes from the current excitation probes. This separates the probe resistance from the actual sample resistance measurement. For low resistance measurements, such as for highly conductive metals and especially for
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superconductors, it is necessary to perform the resistance measurement using a four probe technique.
Generator
Equipment that converts rotational mechanical input power, such as that from a steam turbine, into electricity by using electromagnetic force.
Grid
The electric power industry infrastructure of interconnected electrical systems and services that provide power to all users.
Hermetic Seal
HTS wire can be hermetically sealed to prevent ballooning issues (see Ballooning). To establish a hermetic seal, American Superconductor has created a special lamination technique. This HTS Hermetic Wire prevents liquid nitrogen from entering the system even under a pressurized environment.
High Temperature Superconductor (HTS)
Resistance-free conductors made of ceramic materials that exhibit superconducting properties at temperatures between 20 to 130 Kelvin (-423? to -225?F), therefore requiring less expensive cooling systems than those needed for low temperature superconductors (<10 Kelvin, -441? F). The first high temperature superconductor was discovered in 1986.
hp
Horsepower. A measurement of power used to rate motors. HTS technology will be most effective at first in motors rated 1,000 hp and higher.
Inductive Winding
When wire is wound into a coil configuration, a current flowing in the coil produces a field around the coil, and the coil can be characterized by an inductance determined by the stored energy in that magnetic field. Such a coil is termed an inductive winding, in contrast to special windings which cancel out the field and are thus “non-inductive.” A flat inductive pancake coil is a convenient configuration for measuring n-values of long wire lengths at extremely low voltage levels. Inductive winding is also the standard technique used to make superconductor magnets using HTS wires.
Inductor
A device that stores electrical energy in a magnetic field.
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Inverter
A type of power converter that converts dc power into ac power.
Ion-Beam Steering Magnets
Devices used for guiding particle beams in systems such as particle accelerators for physics research or ion-implantation equipment for semiconductor manufacturing.
I-V Curve
The I-V curve or the “Applied Current vs the Voltage Drop” curve is the standard curve measured to observe a material’s superconductivity and critical current. The voltage drop across the superconductor material is measured as a function of the applied current. The I-V curve is used to determine physical parameters such as the superconducting critical current, the critical current density and the n-value of the material.
Joule (kJ+MJ)
A unit of measurement of energy equal to a watt-second. kJ represents a kilojoule (1,000 joules); MJ is a megajoule (one million joules).
Kelvin
Kelvin is a unit of temperature starting at absolute zero and having the same scale as Celsius. Thus zero degrees Celsius is 273.15 Kelvin. Similarly -489.67 degrees Fahrenheit is absolute zero on the Kelvin scale. Under ambient pressure, Helium liquefies at about 4.2 Kelvin, Nitrogen liquefies at about 77 Kelvin and water freezes at about 273.15 Kelvin. The scientific symbol for the Kelvin unit is K.
kVA
Kilovolt-amperes. A unit of measure of apparent power.
kW
Kilowatt. A unit of power equal to 1,000 watts or about 1.34 horsepower.
Liquid Nitrogen
An inexpensive, inert and non-toxic liquid cryogen formed by chilling gaseous nitrogen to 77 Kelvin (-320? F). Liquid nitrogen is used to cool HTS wires and components to achieve superconducting performance in applications such as power cables and transformers.
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Longitudinal
For our HTS wire the longitudinal direction is the direction along the length of the wire. For example the longitudinal tensile stress is produced by a force stretching the wire along its length. The longitudinal axis is also referred to as the axial, or rolling direction for HTS wire.
LTS
Low Temperature Superconductivity
Metallic Precursor
HTS materials are compounds of metals with oxygen. A metallic precursor is an alloy of the metallic components that is formed into a desired wire shape and then reacted with oxygen to create the HTS compound.
MVA
Megavolt amperes. A unit of measure of apparent power.
N-Value
The n-value describes the relationship of the voltage drop across the wire to the applied current. For the transition from zero resistance (zero voltage drop) to a finite resistance (finite voltage drop), the I-V curve of HTS wires can almost always be fit with the power law
E(j) = Ec (j/jc)n
Here E(j) is the longitudinal voltage drop across the superconductor, Ec is the electric field criterion (see Critical Current), j is the applied current density, jc is the critical current density and n is the exponent. For a sharper transition, the I-V curve has a higher n-value. The n-value is often used to determine the quality of a bulk superconducting material (see inductive winding).
Power Converter
Devices that are used for converting power from dc to ac or vice versa. They are used in motor controllers, chemical processing, energy storage and large industrial processes.
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Power Quality
A smooth, even flow of electricity – free of variations in voltage, current or frequency that provides a competitive advantage for utilities and their industrial customers.
Power Semiconductors
Solid-state devices, such as transistors, used for the conversion and control of electric power.
Resistance
An obstruction to the free flow of electrons in a material. Resistance causes electric current to lose energy in the form of heat.
SMES
Superconducting magnetic energy storage. Devices based on the principle that an electric current introduced into a resistance-free superconducting coil lasts indefinitely and never dissipates energy – which provides a backup power supply that's always charged and ready to respond instantly to power fluctuations.
Tesla
A unit of measure of magnetic field strength. For example, an ordinary kitchen magnet has a magnetic field strength of less than 0.05 Tesla. American Superconductor has produced magnet coils surpassing 2 Tesla, the threshold rating for commercial HTS motors and generators.
Transformer
A device that converts power from one voltage and current level to another. Transmitting energy at higher voltages is more efficient, but consumers need low voltage power. Electricity experiences several voltage changes en route to an end-user.
Self Field
The self field is the magnetic field that is induced when there is finite current flowing in a wire. In the case of critical current measurements, the self field is the magnetic field that is induced in a straight piece of wire that is being measured.
Splice
A splice is a joint between two wire segments. AMSC's High Strength HTS wires can be spliced together using a special soldering technique. Typical splices have an electrical resistance less than 200 nOhm at 77 Kelvin.
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Superconducting Transition Temperature
The superconducting transition temperature Tc is the temperature at which a material undergoes a thermodynamic phase transition from a superconducting state to a non-superconducting or normal state. High temperature superconductors are usually defined as those with transition temperature above 20 K - 40 K, when measured with small currents and no applied magnetic field. It should be noted that Tc is suppressed by both applied current and magnetic field.
Tensile Stress
The tensile stress results from a force that would result in elongation of a material. The critical tensile stress for HTS wire is the amount of tensile stress that can applied to the wire before the critical current is reduced below a specified level (such as 95% Ic retention).
Tensile Strain
The tensile strain is the percentage change in dimension that results from tensile stress on a material. A critical tensile strain for HTS wire is the amount of tensile strain that occurs before the critical current is reduced below a specific critical value (such as 95% Ic retention).
Transverse
For HTS wire, the transverse direction is usually designated as being orthogonal to the length of the wire. The two major transverse axes are along the width of the tape (also known as the ab-trans, lateral, or long transverse directions), and perpendicular to the face of the tape (also known as the c-axis, normal, or short transverse directions).
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Appendix C
A case study of the R&D work on HTSC Transformers with fault current limiting capability at Waukesha Electric Systems, USA
Waukesha Electric Systems is one of the largest U.S. manufacturers of medium and large power transformers and a valued supplier of transformer accessories including a line of Transformer Health Products®, LTC and breaker components and complete transformer service solutions for the transmission and distribution of electric power
http://www.waukeshaelectric.com/
Chronological overview of the projects at WES
1994 – 2000
1MVA , 1 φ prototype tested in 1998 13.8kV HV/6.8kVLV; Bi-2212 at 25K HV, Vacuum, AC loss testing and cold mass assembly at ORNL HV breakdown caused by MLI
2000 – 2005
5/10MVA prototype tested in 2003 – 04 24.9kV HV/4.6kV LV ; Bi – 2223 at 25K HV, AC loss testing, cooling system design/fabrication at ORNL Transformer failed HV dielectric tests
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The epoxy insulation was cracked
2005 – 2010
Conceptual design rework with 70K, YBCO tapes
HV, cryogenic, dielectric & ac loss testing, composite dewar development at ORNL
2010 – Present
Construction of 28MVA FCL Smart grid demo transformer for South California Edison
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Specifications of the HTSC Transformers at WES
1MVA HTSC Transformer with FCL capability at WES
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Conceptual Design of the 5/10 MVA HTSC Transformer with FCL capability
Collaborations made for the design of 5/10 MVA HTSC Transformer
ORNL: Oak Ridge National Laboratory WES: Waukesha Electric Systems
IGC – SP: SuperPower Inc
RG&E: Rochester Gas & Electric
Description of the cool down process for the windings
The Phase B was cooled in WES custom made vacuum test tank To cool the phase test L-N2 /cold N2 gas was used During the cool down, radial temperature gradients < 5K and the axial temperature gradients
< 10K were maintained during the initial stages of cooling and later temperature gradients < 5K typically were maintained
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Tests were conducted with certain pre – defined requirements which have been explained in the succeeding paragraph
Winding resistances were monitored during cool down. The superconducting portion of the coils achieved zero resistance around 90K
Once the final temp was achieved, the coil was held for 38min at 15kV – operating voltage without breakdown
Predefined requirements for the tests conducted on the cooled windings
Test Description
Normal Operation
The transformer needs to remain at a constant temperature indefinitely during normal operation (26.3kV, 63Arms for the primary) with only the cryocoolers operating. The temperature needs to be sufficiently cold for superconducting operation in the magnetic fields of the transformer.
Overload Operation
The transformer needs to run for an unspecified time, greater than 24 hours at twice operating current, same voltage at a constant temperature with only the cryocoolers running. If needed, a small amount of additional external cooling can be applied. The temperature needs to be sufficiently cold for superconducting operation in the magnetic fields of the transformer.
Short Circuit Fault
The transformer needs to sustain a short circuit fault current of up to 10 times the normal operating current for up to 0.3 sec. After 5 sec, with no power into the transformer (simulating a breaker trip and reclose), the windings and leads must be cold enough to resume superconducting operation at normal voltage and current.
Basic Impulse Level
The transformer needs to survive a BIL of 100kV, 1.2 X 50µs on each of the HV coils and 50kV, 1.2 X 50µs on each of the LV coils. The pulse is applied to the bushings outside the tank.
Over Voltage
The transformer needs to be able to operate at 1.5 times the rated voltage with no current.
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Preliminary tests conducted on the 5/10MVA HTSC Transformer and their results
a. Single phase tests
Short term 1-φ tests were conducted with the LV shorted, causing current to flow in both primary & secondary windings without applying large ac voltage
At 210V all phases carried about 67A on the primary; corresponding to a 0.84% impedance
Thermal response of phase set verses time during testing
b. Three phase tests Three phase test were carried out with all three LV bushings shorted together Initial test results show that heating occurs within the phase sets during operation of currents
of 63Arms This was due to a poorer than expected vacuum level leading to a higher than expected
background heat leak
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• Note that 116A into the bushings (line current) corresponds to 67A in the HV coils due to the delta connection on the primary side of the transformer
• All values are coil current/voltage
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Second Round of electrical tests on the 5/10MVA HTSC Transformer
Resistance tests for the individual phases
Resistance (mΩ)
Temp
Phase A Phase B Phase C
L – L L – N
L – L
L – N
L – L L – N
LV Cold 0.59 1.2 0.61 1.2 0.50 1.1
RT 56.9 29.8
54.9 27.7
54.9 27.6
HV Cold 2.02 2.03 1.85
RT 1791.6
1790
1787.7
All RT resistances were measured using 1A applied current.
All cold temp resistances were measured with 5A applied current to minimize the effect of contact voltages
Winding Capacitance, Dissipation Factor and Megger Test
Configuration Capacitance DF Resistance
pF % GΩ
HV to LV 28750 0.078
HV to Gnd 4734 0.241
LV to Gnd 40030 0.056
HV to LV & Gnd
33490 0.065 >510
LV to HV & Gnd
68780 0.050 >510
HV & LV to Gnd
>510
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Ratio & Exciting current Test
Phase Ratio HV exciting Current LV exciting Current
Bushing mA Bushing mA
A 10.40 H1 – H3
5.65 X0 – X1 81
B 10.40 H1 – H2
4.30 X0 – X2 64
C 10.40 H2 – H3
5.26 X0 – X3 81
The exciting current was measured with the transformer at operating temperatures
20Vac was applied line to line for the HV windings & line to neutral for the LV windings
HV – Δ LV – Y
Single Phase Impedance Test
Phase Voltage (VAC)
HV LV
Current (A)
% impedance
Current (A)
% impedance
A 16.1 5.0 0.87 52.6 0.86
32.25 10.0 0.87 106.8 0.85
64.8 20.0 0.87 209.9 0.86
B 15.65 5.0 0.84 53.7 0.82
31.22 10.0 0.84 107.4 0.81
62.4 20.0 0.84 211.4 0.83
C 15.60 5.0 0.84 54.1 0.81
30.65 10.0 0.82 106.7 0.80
62.00 20.0 0.83 212.2 0.82
Since the resistive component of the impedances is negligible, the impedances are all reactive
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Coil Refrigeration System for 5/10MVA HTSC Transformer
The coil refrigeration System is a modular unit that plugs into the top of a 5/10MVA HTS Transformer
Salient features of the coil refrigeration system
A Pro – Engineer model was prepared which plugs directly into the transformer model.
Cryo coolers are suspended on bellows & springs for vibration & thermal contraction
Electrically heated 350W dummy load was hung below the thermal shield to simulate the coils in proof tests
Designed for 3-g horizontal and 0.5-g vertical handling/shipping load
Cooling module could be installed easily in both the test tank and the 5/10MVA Transformer
L-He is used to cool the phase sets while L-N2 is used to cool the thermal shields
Testing process of the cooling system prior to the installation into the tank
Minimum standby mode temperature was determined
Dummy load temperature profile was determined for heater power loads corresponding to up to 2 times rated operation
Stability under non – uniform heating was investigated
Testing was performed with one cryocooler operating
Testing was performed with two cryocooler operating
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Transition from single cooler normal operation to dual cooler 2 times overload operation was simulated
Test results of the cooling module
Test showed acceptable operating temperatures and temperature differentials to the coolers
Transition from single cooler normal operation to dual cooler 2X overload operation was smooth
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Final points from the results of the cooling system
Cooled down automatically in 25hr – while being unattended overnight
Operated for a week with no manual attention overnight
Operated with static vacuum i.e. no pumping
For 100W background heat load - 30K was obtained with one cooler and 20K with two coolers
Maximum temperature were within acceptable limits for within 350W total load
No instability observed with 25% unbalance in phase A
Transition to 2 times overload operation was smooth
Installation of the cooling module in the test tank
The shipped package of the cooling system
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Installation into the tank
Desired Properties for the materials used in the HTSC Transformer
High Dielectric Strength Low partial discharge Low dissipation factor Thermal compatibility Mechanical strength for solid materials Thermal conduction
Test conducted on the dielectric materials
AC breakdown
Impulse breakdown
Partial discharge and aging
Dissipation factor
Thermal shock tests
Araldite 5808 was selected as the primary candidate for investigation as the dielectric material in the transformer
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1. HV testing of solid samples
2. Partial discharge studies with HVAC power supply
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3. Impulse testing
After these tests, the dielectric was simulated in the transformer model and the dielectric stress points were identified
Dielectric Stress Points in the 5/10 MVA HTSC Transformer
Calorimetrical AC loss tests and their results
Test coil were hanged in vacuum with good thermal isolation
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Calorimetric measurement avoids the influence of the metal Dewar
Loss sensitivity was observed to be lesser than 0.1W
Heating rates on thermometers were observed with 10s AC pulses with frequency ranges from 45 – 240Hz
Short pulses limit temperature drift and give losses on individual turns which is otherwise difficult to measure electrically
Calibration was done by comparing the ac loss data with heating rates for known dc heat inputs to heater
Components of the 5/10 MVA HTSC Transformer at WES
LV coil & HV coil
Structural frame of the Transformer developed by IGC – SP
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Assembly, installation and operation of the 5/10 MVA HTSC Transformer at WES
Assembling the core, bushings and vacuum tank
Before installation, all systems were vacuum checked for 10-9 atm-cc/sec
Installation of the transformer
Problems faced in the 5/10 MVA HTSC Transformer project
• Initial tests showed that phase A was operating at a higher heat load than the rest of the phases
• Since a single cooling system was used on all the phases, the additional heat generated in phase A had an impact on the other phases
• Without auxiliary cooling, the transformer was able to run at current levels corresponding to those necessary for the diurnal load cycle of the WES manufacturing plant.
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• Damage to some LV leads also occurred during over current testing.
• The leads were undersized which resulted in excess heating causing the insulation burning off the leads
• This damage occurred prior to any voltage testing
• 3-φ voltage testing was performed by energizing the LV bushings through a variable voltage source with the HV bushings open circuited
• During ramping up to rated voltage, a flashover occurred at 8.2kV (HV) damaging the phase B
• Diagnostic testing revealed that this was most likely a flashover from the low voltage coils to ground
• The lead suspect is the damaged leads insulation from the LV leads
• Two phase voltage testing was then performed on the remaining phases (A&C)
• A flashover occurred when ramping from 13kV to 15kV (HV) causing a short between the HV & LV windings
Conclusions drawn and lessons learnt from this project
The faults led to the termination of testing.
A failure mode analysis and root cause study was undertaken
Besides the thermal coupling of the leads which was already identified, prime suspects are the substantial decrease in the dielectric properties of the epoxy used ( Araldite 5808), the possibility of the inclusion of voids during the manufacturing process and the impacts of the very low impedance transformer (this) when internal shorts occur
With all this experience gained, they proceeded to the 28MVA project
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The 28 MVA HTSC Transformer project at WES
Milestones achieved in this project
Open top composite dewar for 28MVA transformer has successfully passed four month long term tests
AC loss tests on YBCO simulated HV coils were carried out
New insulation test coil was successfully tested for 550kV – BIL
Salient features of this project
Rating 28MVA, 70.5kV/12.47kV, 132A/1296A
13.1% impedance to match the existing conventional units
Increase to 26% impedance in HTS quench gives 50% reduction of fault current
Continuous operation at 40.6MVA (145% of rated load) with extra liquid nitrogen supply
Normal operation at 70K, 3.4bar
HTS is sized for significant margin on 40.6MVA current
Cryocoolers cycled to match heat loads at lower ratings
Current leads are sized for 125% of 28MVA current
Maximum current lead temperature < 1200 at 40.6MVA
Conceptual design of the Transformer
• 70K pressurized sub – cooled nitrogen is a good substitute for oil.
• HTS tape with parallel resistive conductor for stability & better fault handling are being used
• Coil dewar surrounds warm steel core
• Air cooled core with blower
• Proof of concept was demonstrated with 1Φ alpha – 1 ( with normal conductors) and alpha – 2 (HTSC conductors)
• WES design software and ORNL (Oak Ridge National Laboratory) design spreadsheet were used
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Windings of the transformer
The windings were similar to WES conventional design
HV – continuous disc winding, single conductor, 8-12turns/disc.
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LV – screw winding; 8 – 12 conductors in parallel, transposed to give uniform current sharing
Windings contained several individually tested modules to limit amount of conductor at risk in a test failure
Standard 6mm or 12mm 2G HTS tape with high resistance alloy strip was used
Provides robust conductor that can be insulated on high speed machine ( for winding)
Salient features of the open top composite dewar
• Dewar performed well in vacuum & thermal cycling tests.
• Small oil free molecular drag pump station (5l/s) could maintain 10-3 torr in warm condition and <10-4 torr in cold condition
• Dewar was refilled once a week for four months
• Boil off increased from 0.14 to 0.16l/hr in the first week then remained constant
• Could valve off pump for 8 hrs with no effect on boil off
• Could unplug pump to simulate power outage with only a small rise in pressure
• Now being used routinely as a lab facility for ac loss measurements
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Inset: Molecular drag pump
Test coils for predicting AC losses
AMSC 344S stainless clad HTS tape was used; co – wound with copper or stainless
HTS tape is bare, insulation is only around co – wound conductor to prevent buckling
4mm disc spacers machined from clear plastic were used
High voltage winding was simulated with the following specs – 26 , 6 – turn discs, continuously wound by WES, 4.4mm tape, 8.9cm ID, 21.4cm length, 50m of HTS tape, 75A of measured Ic (equivalent to 50A rms)\
Electronic loss measurement with lock – in amplifier – external torroidal air core bucking transformer to cancel inductive signal has been added
Validation was done by using third coil made of only copper which showed the expected temp rise at both RT and 77K
New cryostat in the composite dewar with pumped LN bath allowed testing at reduced temperature, with no effect on nearby metal wall
Coil with co – wound copper was tested at reduced temperature and several frequencies in this dewar
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Test Results of the coil prototypes
Non – linear current dependence of the losses in the HV prototype
• Losses α (Ipeak / Ic )n • Losses are similar for co-wound copper and stainless steel • For Ipeak / Ic < 0.4 , n ~ 1.5 • Consistent with ferromagnetic Ni – W substrate • For Ipeak / Ic > 0.4, n ~ 2. • Eddy current or coupling mechanism is the cause for this non - linearity
Variation of the losses in the HTSC/Cu coil as a square of the frequency
• Dividing AC loss by square of frequency collapses the loss into one line
• Loss proportional To square of frequency also suggests coupling or eddy mechanism
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• Temperature Independence of the losses in the HTSC/Cu coil
• Very little change in loss when temperature is lowered from 77K to 72K suggests that most of the losses is from the Cu or HTS substrate
• We get similar results from HTS/SS coil
• Losses are higher at the coil ends
• Each VT pair covers 2 discs • Radial fields at ends increase losses
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Insulation studies for the 550kV coil
• Similar 350kV BIL coil passed all tests
• Standard WES design pressboard structure
• Copper conductor with WES polymer insulation
• LV & HV disc windings
Commercial 650kV bushing successfully tested
Test results for the 550kV coil
HV coil was impulsed from top with bottom and LV coil grounded
77K LN bath was pressurized to 1.8bar absolute to stop boiling
Passed three impulse shots at -550kV and three shots at +501kV
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Warmed up and un – grounded the HV coil
Tests with ac voltage showed partial discharge of 2.2pC at 102kVac
Passed 1min withstand at 201kVac
650kV impulse tests are planned at WES as the 28MVA Transformer project is still in progress
This project is still in progress at Waukesha Electric Systems, USA.
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