DESIGN AND TEST OF VEGETABLE OIL IMPREGNATEDgits.kmutnb.ac.th/ethesis/data/4710185069.pdf ·...

DESIGN AND TEST OF VEGETABLE OIL IMPREGNATED

POLYPROPYLENE FILM CAPACITORS

MR. BOONCHOO SOMBOONPEN

A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS

FOR THE DEGREE OF MASTER OF SCIENCE

IN ELECTRICAL POWER ENGINEERING

SIRINDHORN INTERNATIONAL THAI-GERMAN GRADUATE SCHOOL OF ENGINEERING

(TGGS)

GRADUATE COLLEGE

KING MONGKUT'S UNIVERSITY OF TECHNOLOGY NORTH BANGKOK

ACADEMIC YEAR 2007

COPYRIGHT OF KING MONGKUT'S UNIVERSITY OF TECHNOLOGY NORTH BANGKOK

ii

Name : Mr.Boonchoo Somboonpen

Thesis Title : Design and Test of Vegetable Oil Impregnated Polypropylene

Film Capacitors

Major Field : Electrical Power Engineering

King Mongkut’s University of Technology North Bangkok

Thesis Advisor : Assistant Professor Dr.Teratam Bunyagul

Co-Advisor : Dr.Thanapong Suwanasri

Academic Year : 2007

Abstract

The principal objective of the thesis is to do about a feasibility study about the

manufacturing of the polypropylene film capacitors impregnated with three different

types of fluid. The types of fluid are sunflower oil, soybean oil and Envirotemp FR3

fluid. Sunflower oil and soybean oil were filtrated by a small experimental purifier.

The electrical attributes of both oils is qualified according to the fluid insulation

standards (ASTM D6871). The design of model capacitors is single-element with 1.9

microfarad capacitance and AC voltage at 1500 volt. The dielectric used in capacitor

element is double-layered, double-side rough type polypropylene film wounding

together and then flattened. Two capacitor elements were pressed, in order to have

distinct space factor, and packed together in a model tank capacitor. Each model

capacitor of each liquid dielectric was impregnated at four different treated levels of

temperature. From the comparison capacitor characteristic of test results in

accordance with IEC 60871-1 standard, it was found that the space factor, the

temperature level during impregnation, and the vegetable oil type affected to the

capacitance and dissipation factors in capacitor.

(Total 101 pages)

Keywords : Vegetable oil, Impregnating temperature, Space factor,

Polypropylene film capacitor

______________________________________________________________ Advisor

iii

ช่ือ : นายบุญชู สมบุญเพ็ญ ช่ือวิทยานพินธ : ออกแบบและทดสอบตัวเกบ็ประจุโพลีโพรไพลีนแบบใช น้ํามันพืชแชอ่ิม สาขาวิชา : วิศวกรรมไฟฟากําลัง มหาวิทยาลัยเทคโนโลยีพระจอมเกลาพระนครเหนือ อาจารยที่ปรึกษาวิทยานิพนธหลัก : ผูชวยศาสตราจารย ดร.ธีรธรรม บุณยะกุล อาจารยที่ปรึกษาวิทยานิพนธรวม : อาจารย ดร.ธนพงษ สุวรรณศรี ปการศึกษา : 2550

บทคัดยอ

งานวิจยันี้เปนการศึกษากระบวนการผลิต ตัวเก็บประจุโพลีโพรไพลีนแบบแชอ่ิมในฉนวนน้ํามันพืช 3 ชนิด คือน้ํามันทานตะวัน น้าํมันถ่ัวเหลือง และฉนวนไฟฟาเหลวที่ผลิตจากน้ํามนัพืช

(Envirotemp FR3 Fluid) โดยทําการกรองน้ํามันทานตะวนั และน้ํามันถ่ัวเหลืองให ดวยเครื่องกรองน้ํามันฉนวนไฟฟาขนาดเล็กที่สรางขึ้น ผลการทดสอบคุณสมบัติทางฉนวนไฟฟาของน้ํามนัพืชทั้งสองชนิดไดคาตามมาตรฐานของฉนวนเหลว (ASTM D6871) การออกแบบตัวเก็บประจุเปนแบบตวัเดยีว มีคาความจุ 1.9 ไมโครฟาราด คาพิกดัแรงดันไฟฟากระแสสลับ 1,500 โวลทใชแผนฟลมโพลีโพรไพลีนแบบมีความขรุขระสองดาน สองช้ันเปนไดอิเล็กตริก สรางแบบพันมวนและบีบแบน มีขั้วตอแบบยืน่ออกสลับดานกัน บรรจุในถัง ถังละ 2 ตัว แตละตัวถูกบีบใหมีคาตัวประกอบชองวางตางกนั กระบวนการแชอ่ิมใชวิธีการดดูอากาศออกแลวเติมน้ํามันทีม่ีความบริสุทธิ์ ดวยความแตกตางของอุณหภูมิ 4 ระดับ

ผลการทดสอบและเปรียบเทียบคุณลักษณะของตวัเก็บประจุ ตามมาตรฐาน IEC 60871-1

ระหวางตัวเกบ็ประจุที่ใชฉนวนน้ํามันพืชแชอ่ิมทั้งสามชนิด พบวาตัวประกอบชองวาง อุณหภมูิของน้ํามันขณะแชอ่ิม และชนิดของน้ํามัน มีผลตอความเปลี่ยนแปลงคาความจุไฟฟา และคาตัวประกอบกําลังไฟฟาสูญเสียในไดอิเล็กตริกของตัวเก็บประจ ุ

(วิทยานพินธมีจํานวนทั้งสิ้น 101 หนา) คําสําคัญ : น้ํามันพืช,อุณหภูมิขณะแชอ่ิม,ตัวประกอบชองวาง,ตัวเกบ็ประจุแบบโพลีโพรไพลีน

_____________________________________________อาจารยที่ปรึกษาวิทยานิพนธหลัก

iv

ACKNOWLEDGEMENTS

I would like to express the profound and sincere gratitude to my advisors,

Assistant Professor Dr.Teratam Bunyagul and Dr.Thanapong Suwannasri, for the

invaluable information about the step-by-step design and in-depth testing process of

vegetable oil impregnated polypropylene film capacitors which is the crucial subject

of this dissertation. This work would not have been possible without the support,

which also brought about my improvement and advancement of scientific knowledge,

particularly in the area of an empirical investigation and experimentation.

I owe my most honest thankfulness to Assistant Professor Sarawut Kleesuwan,

for his on structive comments and useful solutions to this study, including vegetable

oil purification system, the methodology in dielectric test and the impregnation

process of capacitors, the investigation of their characteristics, and above of all that

matter, his cordial encouragement when in time of difficulty.

In addition, I would like to state my appreciation officially to the Electrical

Engineering Department, Pathumwan Institute of Technology, for all materials and

equipments provided. I am deeply grateful to Nissin Electric (Thailand) Co., Ltd., for

the production of model capacitors which consists of the support on raw materials,

winding process the capacitor’s element, especially Mr. Pravich Prachumthes, for

advice capacitor impregnation process. I am also considerably indebted to Tusco

Trafo Co., Ltd., Tira Thai Co., Ltd., and High Voltage Laboratory, Transmission Line

Maintenance Division, Electricity Generating Authority of Thailand (EGAT) for their

supports on specific instruments for dielectric test and the measurement of the

dissipation factor in vegetable oil and model capacitors applied in this study.

Boonchoo Somboonpen

v

TABLE OF CONTENTS

Page

Abstract (in English) ii

Abstract (in Thai) iii

Acknowledgements iv

List of Tables vii

List of Figures ix

Chapter 1 Introduction 1

1.1 High voltage capacitor 1

1.2 Oil impregnated polypropylene film capacitor 2

1.3 Vegetable oil in power capacitor 3

1.4 Vegetable oils 4

1.5 Purpose of the study 8

1.6 Scope of the study 8

1.7 Methods 9

1.8 Tools 9

1.9 Utilization of the study 9

1.10 The structure of thesis 10

Chapter 2 Material property in high voltage capacitor 11

2.1 Previous work on power capacitor 11

2.2 Dielectric materials 18

2.3 Liquid dielectric 22

2.4 Insulating oil purification system 26

2.5 Polypropylene dielectric 27

2.6 Composite dielectric 30

2.7 Wound capacitor flat winding 34

2.8 Capacitor impregnation process 41

2.9 Standard test method of capacitor (IEC 60871-1) 43

Chapter 3 Design and test of impregnated capacitor 45

3.1 Vegetable oil purification system 45

3.2 Vegetable oil properties 49

3.3 Design of capacitor element 54

vi

TABLE OF CONTENTS (CONTINUED)

Page

3.4 Correction factor of capacitance for the reference temperature 60

3.5 Assembly of model capacitor 62

3.6 Impregnation process 66

3.7 Experimental test 68

Chapter 4 Testing of the capacitors 75

4.1 Capacitance of capacitors 75

4.2 Voltage test between terminals, and between terminals and

container 86

4.3 Dissipation factor of capacitor 87

4.4 Short circuit discharge test 92

Chapter 5 Conclusion and recommendations 95

5.1 Conclusion 95

5.2 Recommendations for further work 97

References 99

Biography 101

vii

LIST OF TABLES

Table page

1-1 Envirotemp FR 3 fluid Values, and specification limits for

natural ester fluid and mineral oil 7

1-2 Typical initial Envirotemp FR3 fluid properties 8

2-1 Dielectric properties of some liquid dielectrics 25

2-2 Physical, chemical and thermal properties of polypropylene film 28

2-3 Electrical properties of polypropylene film 28

2-4 Dimension properties of polypropylene film 29

2-5 Space factor of polypropylene film 29

2-6 Electric withstanding capability of BOPP film 36

3-1 Vegetable oil properties from test results compared with natural

ester fluid standard and synthesis aster fluid for capacitor application 49

3-2 Relative permittivities of vegetable oils subjected to temperature

level as per IEC 60247 standard 51

3-3 Dissipation factors of vegetable oils subjected to

temperature level as per IEC 60247 standard 53

3-4 Correction factor of capacitance for 20 °C reference temperature 61

4-1 Capacitances of model units at the element thickness of 9.0 mm,

with rated capacitance of a capacitor designed at 1.9 µF at 30 °C 75

4-2 Capacitances of model units at the element thickness of 9.4 mm,

with rated capacitance of a capacitor designed at 1.9 µF at 30 °C 76

4-3 Capacitance of Sunflower oil impregnated polypropylene film capacitor

on surrounding temperature 77

4-4 Capacitance of soybean oil impregnated polypropylene film capacitor

on surrounding temperature 78

4-5 Capacitance of Envirotemp FR3 Fluid impregnated polypropylene film

capacitor on surrounding temperature 79

4-6 Capacitances of sunflower oil impregnated model capacitors at four

different impregnating temperatures 82

viii

LIST OF TABLES (CONTINUED)

Table page

4-7 Capacitance of soybean oil impregnated model capacitors at four

different impregnating temperatures 83

4-8 Capacitances of Envirotemp FR3 fluid impregnated model capacitors

at four different impregnating temperatures 84

4-9 Dissipation factors of sunflower oil impregnated model capacitors


4-10 Dissipation factors of soybean oil impregnated model capacitors


4-11 Dissipation factors of Envirotemp FR3 fluid impregnated model

capacitors at four different impregnating temperatures 89

4-12 Summary of changes in capacitance of model capacitors impregnated

with three fluids at four different impregnating temperatures 92

ix

LIST OF FIGURES

Figure page

1-1 Comparison of dietary fats 4

2-1 Insulating oil purification system 26

2-2 Capacitor comprising two layers of different dielectric materials 32

2-3 Capacitor Element of flattened wound capacitor 34

2-4 Two layer PP film wound capacitor 38

2-5 PP film and Al. foil of element capacitor wound type 38

2-6 Dimension of flattened type element wound capacitor 40

3-1 Small vegetable oil purification diagram 47

3-2 Small vegetable oil purification system 48

3-3 Dielectric breakdown voltage test of vegetable oils 50

3-4 Measurement of relative permittivity and dielectric dissipation factor 50

3-5 Dielectric breakdown voltages of vegetable oils 51

3-6 Relative permittivities of vegetable oils in temperature 52

3-7 Dissipation factors of vegetable oils 53

3-8 Rolling of capacitor element 55

3-9 Capacitor element 59

3-10 Model capacitor 62

3-11 Element pressing diagram 63

3-12 Capacitor element pressing process 64

3-13 Insert element to model capacitor 65

3-14 Impregnation process diagram 66

3-15 Impregnation process system 67

3-16 Model capacitors 68

3-17 RLC bridge meter and measurement 69

3-18 Capacitance and dissipation factor measurement at 1.5 kV. 70

3-19 Capacitance of capacitor at differential temperature test 70

3-20 Capacitance and dissipation factor test at 500 V for the first of test 71

3-21 AC withstand voltage test between terminals 71

3-22 DC withstand voltage test between terminals 72

x

LIST OF FIGURES (CONTINUED)

Figure page

3-23 Short circuit discharge test 73

4-1 Plotted capacitances of sunflower oil impregnated capacitors


4-2 Plotted capacitances of soybean oil impregnated capacitors


4-3 Plotted capacitances of Envirotemp FR3 impregnated capacitors


4-4 Plotted capacitances of sunflower oil impregnated model capacitors


4-5 Plotted capacitances of soybean oil impregnated model capacitors


4-6 Plotted capacitances of Envirotemp FR3 impregnated model capacitors


4-7 Plotted capacitances of vegetable oils impregnated model capacitors

for element thickness 9.0 mm 85

4-8 Plotted capacitances of Vegetable oils impregnated model capacitors

for element thickness 9.4 mm 85

4-9 Plotted dissipation factors of sunflower oil impregnated model


4-10 Plotted dissipation factors of soybean oil impregnated model


4-11 Plotted dissipation factors of Envirotemp FR3 fluid impregnated

model capacitors at four different impregnating temperatures 90

4-12 Plotted dissipation factors of model capacitors impregnated with

vegetable oils for element thickness 9.0 mm 91

4-13 Plotted dissipation factors of model capacitors impregnated with

vegetable oils for element thickness 9.4 mm 91

4-14 Plotted changes in capacitance of model capacitors impregnated

with three fluids at four different impregnating temperatures 93

CHAPTER 1

INTRODUCTION

In this chapter, basic terms and structures related to oil impregnated capacitors

are explained. These are high voltage capacitor, oil impregnated polypropylene film

capacitors, vegetable oil in power capacitor, and vegetable oils. An overview

including the purpose and the background has also been done to illustrate the

orientation of this research.

1.1 High voltage capacitor

The high voltage capacitor is a required element in the electrical power system.

It is applied to power factor correction in medium voltage and high voltage, DC high

voltage system and impulse generator. Most of power factor correction capacitors use

polypropylene film as the dielectric. Because polypropylene film is the least of

dielectric loss. For a better capability in their applications, It is impregnated with the

synthesis ester fluid.

Polychlorinated biphenyl are the synthesis ester fluids that used in the high

voltage capacitor. But they are toxic to human and the environment such as being a

carcinogenic substance, it was considered to be banned [1]. As a result it is necessary

to search for a substitution which should be an environmental and human friendly

impregnant. This also never causes any pollution problem in the future.

Castor oil is the first replaceable material used as an impregnant inside Kraft

paper capacitors. Although this fluid is superior in the application for the DC high

voltage and discharge capacitor, its viscosity is significantly high. Developers found

that there are five interesting vegetable oils to consider whether they are suitable to

use as an impregnant in high voltage capacitors. These are sunflower oil, soybean oil,

canola oil, rapeseed oil, corn oil and Envirotemp FR3. All of these fluids are

biodegradable and have good electrical attributes. In addition, their viscosities are

lower than the castor oil. At 100 ํC, their viscosities are nearly the same as synthesis

ester fluid.

2

Typically, high voltage capacitors are applied in the field of induction heating,

pulse, commutation, broadcast transmission, drives, bypass equipment, igniters,

frequency converters, filters, high voltage power supplies, snubbers, couplers, voltage

dividers, spark generators and harmonic filters. Voltages applied to these capacitors

are in the range of 1 to 300 KV, and capacitances from 100 pF to 5000 µF.

1.2 Oil impregnated polypropylene film capacitor

The oil impregnated polypropylene film capacitor is constructed of oil

impregnated polypropylene films wound together inside the element. The

determination of polypropylene film used in an oil impregnated capacitor relates to a

stretched, rough electrical insulating film of polypropylene, comprising zones having

different degrees of roughness which lie side by side and form fine channels between

each other. The polypropylene film is particularly suitable for the fabrication of

impregnated capacitors and for the sheathing of cables. The determination relates also

to a process for the manufacture of such film. Materials presently used inside these

impregnated capacitors are often combinations of paper-aluminum, paper-

polypropylene film-aluminum, or paper-metalized polypropylene film [2].

Generally, there are various types of polypropylene film capacitors made by

such material combinations. When considering the trend in constantly decreasing

dimensions of electrical components, development tends toward capacitors which are

constructed of polypropylene films and aluminum or of metalized polypropylene

films only and which are called "all-film capacitors".

In the past, there was a development in improving impregnate ability of oil

polypropylene film used in this type of capacitor, in which the film was roughened by

systematically influencing the morphology. Although it has been possible to improve

the impregnation of capacitors produced from films manufactured according to these

processes, non-impregnated areas cannot be completely eliminated and, as a

consequence, the above-described disadvantages experienced with smooth films will

still occur.

The following development in polypropylene film used in this oil impregnated

capacitor was by adopting super high-purity capacitor-grade homo-polypropylene

resin; the rough film is produced by biaxial-orientation tented process. This kind of

3

film features excellent physical and electrical properties, such as good profile

evenness, high electric strength, low dielectric dissipation and good wind ability etc.

The film can be compatible well with several types of oil impregnated capacitors.

According to the investigation by National Power Capacitor Test Center [3], this new

developed polypropylene film conforms to all the requirements of capacitor insulation

materials. Thus, it is widely used as dielectric in the oil impregnated polypropylene

film capacitor as well.

1.3 Vegetable oil in power capacitor

In general, energy storage power capacitors are designed to meet the needs of

each specific application. These include magnetizing equipment, laser, fusion

research, metal farming equipment, strobes, and defibrillators. Power capacitors used

in the light duty and low repetition rate applications that required high energy density

are made with either metalized polypropylene or metalized Kraft paper. Besides, an

aluminum foil is functioned as the extended electrodes with soldered connection. The

dielectric for these can be selected from either polypropylene or Kraft paper with a

specially refined castor oil impregnant; for example, impulse capacitor for impulse

generator.

Power capacitors are important elements that can help in proper design of

transmission and distribution networks, reduce system losses, control the voltage, and

reactive current. Due to the facts, the capacitor fluid is the core material in a capacitor

which eliminates voids by permeating through the solid dielectric, as internal voids

results in electrical discharges leading to premature failure of dielectric system. In

addition, it serves as a heat transfer medium by dissipating heat generated inside the

windings. Since two decades, the search for a reliable, environmental friendly

capacitor fluid was on throughout the world.

This research aimed to determine a metalized film power capacitor used in an

alternating current, more particularly to such the capacitor constructed with dry film

capacitor bodies employing a vegetable oil as an impregnant. Appropriate vegetable

oils which may be applied in the oil mixture of the present invention are selected from

sunflower seed oil, rapeseed oil, soybean oil, castor oil and maize or corn oil.

Comparison of Dietary FatsDIETARY FATCastor oil** 2%→Canola oilSafflower oilSunflower oilCorn oilOlive oilSoybean oilPeanut oilCottonseed oilLard*Beef tallow*Palm oilButterfat*Coconut oil

MONOUNSATURATED FAT

Source : POS Pilot plant Corporation Saskatchewan, CANADA June 1994

commercial approach these oils are extracted primarily from seeds of oilseed plant

** Refer to wikipedia free encyclopedia: http://en.wikipedia.org and not food ingredient

(Oleic acid almost)

POLY UNSATURATED FAT SATURATED FAT

1.4 Vegetable oils

Vegetable oils are triglyceride-based fluid-staged materials extracted from

plants. Although there are several parts of plants that can yield oils, but in a

4

Linoleic Acid

Alpha-Linolenic Acid (An Omege-3 Fatty Acid)

FIGURE 1-1 Comparison of dietary fats

* Cholesterol content (mg/tbsp):Lard 12 ;Beef tallow 14;Butter fat 33

Fatty acid content normalized to 100 %

cosmetics, pharmaceuticals, discharge capacitor and other industrial purposes [4].

Although there are glycerin esters and various mixtures of fatty acids, these oils

contained free fatty acids and diglycerides as well. Vegetable oils are increasingly

in actual. Vegetable oils, of food grade, are distinct by dietary fats which are

expressed in Figure 1-1. Triglyceride fats are contained not only in edible vegetable

oils, but also in some other inedible vegetable oils such as processed linseed and

castor oil. These inedible oils are widely used as the elements in lubricants, paints,

←1% 47%

←Trace 48%

←1% 39%←Trace% 39%

←1% 28%

19%

23%←1% 75%

29%16%14%

10%

68% 3%→91% 2%→

51% 48% 2%→43% 27% 19% 15% 15% 13% 12% 10% 7%

21%

76% Trace→ 71% 1%→ 57% 1%→

9% 54% 8%

11%

33% 54% Trace→

9%

7%

←3% 94% ( 90% Ricinoleic acid 4% Oleic acid ) 1%→ 61%

8% 33%

5

used in the electrical industry as an insulator since they are non-toxic to the

environment, biodegradable if spilled and have high flash and fire points. Vegetable

oils, however, have to be traded-off their benefits with biodegradable characteristics.

Thus, they are generally used in systems with no exposure to the atmospheric oxygen

and are much more expensive than crude oil distillated ones.

As mentioned, vegetable oils have high stability to an oxidation reaction so they

have found to be used as engine lubricants. Vegetable oils are mostly applied to

produce bio-degradable hydraulic fluid and lubricant. Normally, vegetable oils have

also been used experimentally as cooling agents in PCs. The only limit factor in

industrial purposes for vegetable oils is that all such oils can eventually be

decomposed and turned into rancid by their chemical reaction. Vegetable oils that are

more stable, such as mineral oil and synthesis ester fluid, are preferred for some

industrial uses. Some vegetable oils are suitable for being the liquid dielectric since

they have a high dielectric breakdown voltage and high flash point characteristics.

These oils are, for example, castor oil, rapeseed oil, sunflower oil and soybean oil.

1.4.1 Sunflower oil: Sunflower oil is the non-volatile oil extracted from

sunflower seeds. It is commonly used for frying foods and being an element of some

cosmetics such as a skin softener. Predominantly, it contains linoleic acid in

triglyceride formatted. The British Pharmacopoeia had listed that these sunflower oils

produced can be categorized into two types regarding their linoleic acid containments.

These are high linoleic and mid oleic sunflower oil, which typically contained at least

82% and 69% of linoleic acid respectively. A variation in fatty acid profile is strongly

influenced by both genetics and climate.

In addition, sunflower oil contains high essential vitamin E and low saturated

fat. The two most common types of sunflower oil are linoleic and high oleic. Linoleic

sunflower oil is common cooking oil that has high levels of the essential fatty acids

called polyunsaturated fat. It is also known for having a clean taste and low levels of

trans fat. High oleic sunflower oils are classified as having monounsaturated levels of

80% and above. Recently, sunflower oil has been developed as a hybrid containing

linoleic acid. They have monounsaturated levels lower than other oleic sunflower oils.

The hybrid oil also has lower saturated fat levels than linoleic sunflower oil.

Sunflower oil of any kind has been shown to have cardiovascular benefits as well.

6

1.4.2 Soybean oil: To the world, soybean is an important cereal crop, which

also provided oil and protein. Solvent-extracted soybeans turned them into the

vegetable oil and defatted soybeans can also be used as animal feed. A small

proportion of the crop is consumed directly by humans. Soybean products do appear

in a large variety of processed foods.

The major unsaturated fatty acids in soybean oil triglycerides are 7% linolenic

acid (C18:3); 51% linoleic acid (C-18:2); and 23% oleic acid (C-18:1). It also

contains the saturated 4% of fatty acids and 10% of stearic and palmitic acid. The

soybean oil has a relatively high proportion, 7–10%, of oxidation prone linolenic acid,

which is an undesirable property for continuous service, such as in a restaurant. This

kind of oil contains 1% of linolenic acid. There were three companies, Monsanto,

DuPont/Bunge, and Asoyia who introduced low linolenic, (C18:3; cis-9, cis-12, cis-15

octadecatrienoic acid) Roundup Ready soybeans to the world in 2004. In the old days,

hydrogenation was used to reduce the instauration in linolenic acid, but this produced

the unnatural trans-fatty acid trans fat configuration, whereas in nature the

configuration is in cis formatted.

1.4.3 Envirotemp FR3 fluid: This fluid is a Fire Resistant Natural Ester based

dielectric coolant specifically formulated for use in distribution transformers [5]. It is

unique environmental, fire safety, chemical, and electrical properties are

advantageous. Envirotemp FR3 fluid is produced from edible seed oils and food grade

performance enhancing additives. Hence, it does not contain any petroleum,

halogens, silicones or any other hazardous material. It can be quickly and thoroughly

biodegraded in both soil and aquatic environments. This fluid showed non-toxic

characteristics in aquatic toxicity tests. Artificially, it is tinted green to reflect its

favorable environmental profile.

Also, Envirotemp FR3 unique characteristics are its high fire point of 360°C

and flash point of 330°C. It has the highest ignition resistance of less-flammable

fluids currently available. Being referred as a High Fire Point or “less-Flammable”

fluid and listed as a Less-Flammable Dielectric liquid by Factory Mutual and

Underwriters Laboratories made it suitable applications used in complying with the

National Electric Code (NEC) and insurance requirements.

7

In addition, Envirotemp FR3 fluid is also compatible with standard transformer

insulation materials, components and with liquid processing equipment and

procedures. It has particular and most preferable thermal characteristics with a

viscosity closer to conventional transformer oil, superior dielectric strength in new

and continued service applications, and excellent chemical stability overtime.

TABLE 1-1 Envirotemp FR 3 fluid Values, and specification limits for natural

ester fluid and mineral oil

Envirotemp FR3 is the trademark of COOPER Power systems

Since that it has excellent environmental, fire safety and capable characteristics,

applications for Envirotemp FR3 fluid have been expanded into a variety of other

equipments, including sectionalizing switches, transformer rectifiers, electromagnets,

and voltage supply circuits for luminaries. Other potential applications under previous

studies include voltage regulators, high voltage cables, and power substations.

New AS-Received Fluid Tested property ASTM

Method

Typical Envirotemp FR3 fluid ASTM

D6871 ASTM D3487

Dielectric Breakdown(kV)

1 mm gap 2 mm gap

D877 D1816

50-55

28-33 60-70

≥ 30

≥ 20 ≥ 35

≥ 30

≥ 20 ≥ 35

Kinematic Viscosity (cst)

40°C 100°C

D445 32-33 7-8

≤ 50 ≤ 15

≤ 12.0 ≤ 3.0

Water Content (mg/kg) D1533 20-30 ≤ 200 ≤ 35

Dissipation Factor (%)

25°C 100°C

D924 0.02-0.06

1-3

≤ 0.20 ≤ 4.0

≤ 0.05 ≤ 0.30

Volume Resistivity (Ω-cm)

D1169 20-40 x 1012

-

-

Pour Point(°C) D97 -18 - -21 ≤ -10 ≤ -40 Flash Point (°C) D92 325-330 ≥ 275 ≥ 145

Fire Point (°C) D92 355-360 ≥ 300 -

8

TABLE 1-2 Typical initial Envirotemp FR3 fluid properties

Electrical Property Value Test Method

Dielectric Strength 56 kV@25°C (0.080 gap) 47 kV @25°C

ASTM D1816 ASTM D877

Kinematic Viscosity 33 cSt @ 40°C 8 cSt @ 100°C ASTM D445

Relative Permittivity [Dielectric Constant] 3.2@25°C ASTM D924

Moisture Content 20 mg/kg ASTM 1533B Dissipation Factor [Power Factor] 0.05%@25°C ASTM D924

Volume Resistivity 30 x 1012 Ω-cm @ 25°C ASTM D1169

Pour Point -21°C ASTM D97 Flash Point (Open Cup) 330°C ASTM D92

Fire Point 360°C ASTM D92 Envirotemp FR3 is the trademark of COOPER Power systems

1.5 Purpose of the study

1.5.1 To study the electrical properties of vegetable oil and oil purification

system

1.5.2 To study the design of vegetable oil impregnated polypropylene film

capacitor

1.5.3 To study the capacitor impregnation process and find the most suitable

impregnating temperature for producing vegetable oil impregnated polypropylene

film capacitor

1.5.4 To determine characteristics of vegetable oil impregnated polypropylene

film capacitor

1.6 Scope of the study

This research investigates the process of producing the polypropylene film

capacitors impregnated with three types of vegetable oils. Sunflower oil, Soybean oil

and Envirotemp FR3 fluid are taken into consideration. The process, prior to this

research, contains the purification of Sunflower oil and Soybean oil to have

9

electrical properties with respect to the standard specifications of natural ester fluids;

design and production of the polypropylene film capacitors impregnated at four

different levels of temperature, determination of space factor effect, suitable

impregnating temperature and capacitor characteristics.

1.7 Methods

1.7.1 Purify vegetable oil and determine typical values of electrical properties

by standard test method

1.7.2 Design and produce capacitor elements according to manufacturing

1.7.3 Impregnate the elements of polypropylene film capacitors with Sunflower

oil, Soybean oil and Envirotemp FR3 fluid at four different levels of temperatures

1.7.4 Capacitor test in accordance with IEC 60871-1 standard

1.7.5 Analyze results of the test by calculation and graphic plot

1.8 Tools

1.8.1 Small vegetable oil purification system

1.8.2 Sunflower oil, Soybean oil and Envirotemp FR3 Fluid

1.8.3 Elements of impregnated polypropylene film capacitor

1.8.4 Model capacitors impregnated tank

1.8.5 Heating tank with temperature control

1.8.6 Boiler tank with temperature control and clear vacuum chamber

1.8.7 Two stage vacuum pump

1.8.8 High volt laboratory with AC and DC medium voltage testing.

1.8.9 Liquid insulation tester

1.8.10 Electric strength of insulating materials tester

1.8.11 Capacitance bridge meter

1.8.12 Capacitance and dissipation factor tester

1.9 Utilization of the study

1.9.1 The knowledge of vegetable oil purification system

1.9.2 The designing of the polypropylene film capacitor impregnated with

vegetable food oil

10

1.9.3 The suitable impregnating temperature of sunflower oil, soybean oil and

Envirotemp FR3 fluid for impregnated polypropylene film capacitors

1.9.4 The vegetable oil impregnated polypropylene capacitor characteristics.

1.10 The structure of thesis

This research presents the study of a feasible occasion in the manufacturing of

polypropylene film capacitors impregnated with three of vegetable oil types.

Capacitor elements are designed and impregnated at four different levels of

temperature, space factor, and capacitor characteristics.

This paper contains five chapters. In the following chapters are:

Chapter 2: Previous works on power capacitors, material properties in high

voltage capacitors and flattened wound capacitors

Chapter 3: Vegetable oil purifier, electrical property test, design and assembly

of model capacitors, impregnating process and testing method

Chapter 4: Measured results of capacitance, dissipation factor, withstand

voltage and short circuit discharge test

Chapter 5: Conclusion of the research and recommendation for further work

CHAPTER 2

MATERIAL PROPERTY IN HIGH VOLTAGE CAPACITOR

2.1 Previous works on power capacitor

This session focuses on support information from previous researches involved

with oil impregnated polypropylene film power capacitors. The summary of related

papers will be included in this chapter as well.

One interesting literature focused on oil-impregnated film HV capacitors [6].

The purpose of this journal was to determine the breakdown behavior of

Polypropylene film with/without rapeseed oil impregnation as function of

temperature. The aging test was measured with their life-time acceleration under

higher electric fields and temperatures compared with operating field and

temperature. Polypropylene film was chosen as dielectric because of its superiority in

dielectric strength and permittivity. Dehydrated rapeseed oil was used as an

impregnant processed in closed glass containers within a circulation air oven.

The breakdown strength of this experiment can be measured in accordance with

DIN 0303 standard. The authors found that when the temperature was increased, the

breakdown strength of dry and impregnated PP films was decreased accordingly.

Beside, the impregnant, rapeseed oil, help enhanced the breakdown strength of the

films by 25% or more. They also found that increasing oil impregnation temperature

brought to the increase in breakdown strength. In addition, the impurity of the PP film

itself from additives brought to the abrupt change of slope in breakdown strength.

They concluded that additives in PP film can cause the unfavorable breakdown

behavior at higher temperatures. This study recommended further researches that oil

solubility should be high enough so that the impregnation is completely distributed

throughout the polymer. Also, oil and PP film should be well and carefully chosen to

provide no influence from any reactive group or additive to the experiment.

The other interesting paper focused on the rapeseed oil derivative as a new

capacitor impregnant [7]. This research was subjected to evaluate other types of fluids

as a replacement of the well-known toxic and non-biodegradable agent, Poly

12

Chlorinated Biphenyls (PCB) which has been used for decades. Methyl ester from

rapeseed oil (MRSO in short term) was chosen as an impregnant. The evaluation has

been done in comparison to other commercial capacitor fluids (i.e. Midel, Baylectrol,

and PCB). This MRSO was made by transesterificating process using excess

methanol. Its flash point, pour point, and lower viscosity are imperative to capacitor

applications. In addition, the dielectric constant of MRSO is at moderate level which

is suitable to this application. With its low value of dissipation factor, the power loss

in a capacitor can be optimized.

In their empirical process, there were two coupling capacitors impregnated with

MRSO fabricated in a close collaboration from the manufacturer. The investigation

was all referred to IEC 358-1971 standard. It was resulted that the capacitance and

voltage acquired was obtained well within specified limits. The discharge magnitudes

showed no change during the last ten minutes of voltage application. In addition, the

measurement of temperature coefficient ranged from -10 to 65 °C exhibited a very

small value which were not significant nor exceed the manufacturer’s restriction. The

investigation recommended further studies to pursue on the in-depth research of this

MRSO impregnant, which was proved that it can be replaced to existing fluids. They

concluded that MRSO’s electrical properties were mostly compatible with those

commercial fluids used in capacitors. Pursuing previous investigation on MRSO [8], the same authors made a

consequential study of this fluid impregnated in power capacitors. The reason was

that during these two decades the demand of power generation has outgrown the

supply and there was also a requirement in energy conservation by improving the

system design. Due to the facts, Central Power Research Institute or CPRI

(Bangalore) had developed 10 kVAR MRSO impregnated power capacitors which are

already qualified as per IS2834-1986. Some had been installed in related industries

and worked satisfactory. This paper aimed to study about the production and

evaluation of MRSO impregnated power capacitors in pilot scale, which consisted of

two experimental tests.

The first test of this paper was done related to MRSO properties itself. They

found that all of its properties were satisfied when compared to other fluids and can

be used as an alternate impregnant in power capacitors. The second test was arranged

13

according to IS2834-1986 standard, to investigate two types of MRSO impregnated

capacitors; the LT and HT power capacitor. For LT capacitor test, it showed that all

figure was passed and qualified. The HT capacitor test result, however, was failed.

Explanations were that its close proximity to the edge of aluminum foil electrode and

the close proximity to an electrode or from the top metallic plate used for stacking

might cause this failure. It was concluded that the review of design and manufacturing

process for HT capacitors is required. Besides, the test of MRSO properties itself

showed satisfactory result when compared to other existing fluids. In addition, it was

encouraged to any future research to determine the opportunity of using rapeseed oil

extracted agent in pilot scale, especially for the capacitor industry.

Back in 1999, the application of polypropylene film with capacitors was

determined [9]. This literature aimed to summarize research results of Biaxially

Oriented Polypropylene Film (BOPP) and provide an experimental test in the

application performance of this material. This paper stated that the performance in

reserving energy of capacitors mainly depends on dielectric constant (ε) and the

square of the electric withstanding capability (E2). There are several drawbacks

caused by its physical properties which lead to the lower performance and the

breakdown of capacitors. These disadvantageous attributes are voids and impurities.

Because it is important for the capacitor design to increase the electric withstanding

capability, those newer designs of BOPP filmed capacitors are required to be

evaluated.

Focusing on their experimental tests, the first investigation was on the capacity

performance. There were two methods for this necessary evaluation; the electrode and

element method. The first method was inexpensive and easy to carry out, while the

second method was much more expensive and difficult in carrying out than the first

one but intensively significant in its accuracy and certainty. With both methods

applied, authors found that, in terms of breakdown and field intensity, some

capacitors were qualified using the electrode method but proved tin pot under the test

using the second method. The second investigation was to check out if capacitor’s

surface condition itself could influence the impregnate performance and the capability

of electric withstanding in BOPP film or not. The setup of this test was to impregnate

three types of BOPP filmed capacitors; the non-roughened film, one side roughened

14

film and double sides roughened film, into an impregnant and measure their

capacitance values in every hour. The figures showed that the double sides roughened

filmed capacitor was best performed in impregnate performance, followed by the one

side roughened filmed and non-roughened filmed one. The explanation was that the

double sides roughened film help increased voids between the film and aluminum

foil, which brought to an improvement of impregnate performance of the capacitor.

With the electric withstanding capability comparison test of the one side and double

sides roughened filmed capacitor, their average electric withstanding values (MV/μm)

showed no significant difference. The next investigation was on the compatibility of

BOPP film with impregnant. In this case, the authors used PXE and M/DBT as

impregnants. It was found that swelled and resolved degree of BOPP film in M/DBT

impregnated was smaller than in PXE due to the lower in molecule weight and

viscosity. The final investigation was to optimize the combination usage of BOPP

film with impregnants. For the test, 4 model capacitors was produced from BOPP film

with the same physical attributes; the width of the film and aluminum foil, number of

layers, impregnant applied, and the capacitance. Three constraints were taken into

consideration; the relation between dielectric loss (tanδ) and capacitance, the relation

between partial discharge and temperature, and the endurance of capacitors. From this

experiment, it was found that tanδ values of all tested capacitors within all range of

temperature were smaller than 5x10-4 and failed within the designed restriction.

Additionally, the partial discharge performance of all capacitors was up to the demand

within the temperature limitation. For the endurance investigation as per GB 11024-

89 Chinese National Standard, there was no breakdown in any model capacitor. It also

showed that there was a small influence from the overload and over voltage to

capacitors, but the purity effect brought the tanδ values down to about 50%. This

paper concluded that no matter the capacitor was one side or two sides roughened, it

took no effect to the performance. Besides, the element method was rather

recommended than the electrode method due to its accuracy and certainty. For any

further study, the authors’ guideline was to focus mainly on electric withstanding

capability and the compatibility with impregnant of BOPP film because these were

important indexes when considering about the development of power capacitors.

15

In 2006, there was a research on the loss tangent (tan δ) on cleaning effect for

oil in oil impregnated all-film capacitors [10]. This value was said to be the life or

aging acceleration factor in any oil-impregnated all-film shunt capacitor. To lengthen

the life expectancy value of a shunt capacitor, this parameter should be considered

when designing a new capacitor. The tanδ value of capacitor film and oil affects

directly to the tanδ value of a capacitor, which means that the lower value in tanδ of

the oil and film applied leads to the lower value in tanδ of a capacitor, also. The

partial discharge process (PD) must be carried out in order to clean the oil

impregnated in shunt capacitors so that their tanδ value were lower. In this paper,

there was a particular design and production of model capacitors. Three model

capacitors impregnated with oils of different tanδ were investigated. In each model

capacitor, eight element capacitors were contained. Connected through one common

terminal sharing, the average values of these element capacitors were measured. For

this study, Polypropylene (PP) was used as the capacitor films and Phenyl Ethyl

Phenyl Ethane (PEPE) was applied as the impregnant inside. The aging of capacitors

were accelerated by placed them in an oven at the temperature of 75 °C for 500 hours.

Finally, aged capacitors were taken out and the oil inside model capacitors but outside

element capacitors was measured for tanδ values by the method of Chromatography-

Mass Spectrometry (GC-MS).

Upon the test, result was that after applying specified voltage, measured tanδ

value of first model capacitor was the same, while second capacitor and third

capacitor was lower to nearly the same as the first unit though their calculated tanδ

value should be at 10 and 20 times of its pre-measured value, respectively. The tanδ

value of aged model capacitors showed that there was a small change in each

capacitor when compared between the figures just before and after the process.

Furthermore, the PD value of all capacitors was close to each other after aging

process with a little higher amount than before the aging had been done. The PDIV

measured values of model units also showed the similar pattern; rising at the

beginning and falling at the end of this aging process. It was concluded that though

elements capacitors inside the second and third capacitor, impregnated with inferior

oil than the first one, were not deteriorated much and their qualities were in good

shape as element capacitors inside the first model capacitor after the aging process.

16

This phenomenon was caused by the cleaning effect for oil during impregnation and

the aging process. The reason was that most of impurities (larger than 1 μm) were

mainly blocked by the elements. However, some small impurities could invade into

these elements which brought to the effect mentioned. The oil inside the elements was

then trapped and could not diffuse with the outside oil. This caused to the reduction of

tanδ values in other model capacitors as well.

There was an investigation of vegetable oil characteristics in HV AC capacitors

in 1995 [11]. In this study, commercial-grade rapeseed oil and polypropylene film

was chosen as the impregnant and the dielectric of the test. The fluid was purified and

filtered, and impregnated into the film. The capacitor models were accelerately aged

and measured for their electrical losses, average breakdown voltages, and discharge

characteristics. Along with this setup, there was a parallel investigation of an existing

impregnant such as benzyltoluence (BT) which was mainly composed of an ester of

pentaerythritol (EPE) and the mixture between rapeseed oil and BT. Finally, figures

of these two impregnants obtained from the measurements were compared and

analyzed. In term of the absorption, tested rapeseed oils nearly proportioned to the

concentration. In this case, there was no interaction between the oils and PP films. In

the breakdown strength, PP films impregnated with rapeseed oil with 10% BT were at

normal agreed range, with larger breakdown strength when films used were thinner.

Their discharge characteristics of rapeseed oil impregnated films itself can be

improved by addition of an aromatic liquid, which could be achieved and qualified

near the characteristics of M/DBT. With additional concentration in BT of more than

25%, impregnated films made the metallization cracked by the swelling. Finally, the

capacitor models were tested. The models, made of two rough PP films pressed, were

placed with the temperature at 80ºC in Q2 atmosphere to accelerate the electrical

degradation of PP films inside.

From the measured figures, the breakdown voltage of rapeseed oil impregnated

PP films was somewhat inferior when compared to those BT impregnated. Also, it

was found that the breakdown voltage of PP films with rapeseed oil/BT mixture

impregnated was about 10% larger than PP films with BT impregnated. The

conclusion of this paper stated that although rapeseed oil impregnant’s electrical

characteristics were less favorable than BT, these characteristics could be improved.

17

Some attributes of rapeseed oil fluid such as resistivity and the inception voltage of

discharges could be treated to the optimization of usage by additional aromatic liquid.

The most important attribute and was advantageous to rapeseed oil impregnant was

that, there was no crack of the metallization when increasing its concentration or

swelling. This study also recommended further research to investigate impregnated

liquids other than this rapeseed oil, or the mixture between the rapeseed oil and other

liquids, to study about the influence of various additives.

Also in 1996, there was an investigation on increasing the breakdown,

especially in DC voltage, in oil-impregnated filter capacitors [12]. The reason was

that DC filter capacitors are widely used in electronic apparatus, which are subjected

to hard service conditions with their capacitance of about 600 µF. In this paper, model

units consisted of the PP film, the Kraft paper/PP mixed dielectric, and the M/DBT

impregnant with additional cleaning to help increasing their electric stresses. The

authors claimed that there were two advantages in using M/DBT as an impregnant.

Firstly, its distribution is well uniformed for AC electric stress in the film and layer.

Secondly, its electric loss is low. The model dielectric was made of a basic resin. The

rough film was 7 to 7.8 μ in compromised thickness with controlled surface roughness

to facilitate the completeness of its oil impregnation. For the measurement of

dispersion in breakdown stress, it was found that values of DC breakdown stress were

higher than AC ones. The more of DC voltage applied to DC filter capacitors, the

higher to their failure rate. In the pollution effect investigation of DC dielectric

strength, results were that the all-film unit with polluted liquid impregnated was

increase in their dielectric strength when compared with a reference capacitor which

had no impurity in its impregnant. Moreover, the DC dielectric stress was varied

directly to the liquid resistivity at certain linear rate. When reducing its resistivity, it

was also meant to an increase in the total dielectric breakdown of the capacitor,

especially at the low temperature. It was also an important note that, when increasing

the additive cleaning in their impregnants, model units tended to be decreased in the

number of element failures because of an improvement in its dielectric stress

property.

Due to the satisfactory results of all investigation on power electronic capacitors

obtained from this study, which was according to the IEC 1071-1 standard, the

18

authors concluded that the model capacitors were qualified and achieved in their

successful records.

2.2 Dielectric materials

A dielectric material is a substance that is inferior in electrical conduction, but

efficiently support in electrostatic field property. An electrostatic field can help store

energy if the flow of current between opposite electric charge poles is minimally kept

while the electrostatic lines of flux are not impeded or interrupted. This property is

useful in capacitor applications and the construction of radio-frequency transmission

lines.

Most of dielectric materials are actually in solid state. These materials include

ceramic, mica, glass, plastics, and the oxides of various metals. Besides, some liquids

and gases can serve as good dielectric materials. In practice, an excellent dielectric is

dry air, and is used in variable capacitors and some types of transmission lines. Also,

distilled water can be used as a fair dielectric. A vacuum is an exceptionally efficient

dielectric.

As mentioned, one of the most important properties in a dielectric is its ability

to support an electrostatic field while dissipating minimal energy in the form of heat.

The lower the dielectric loss (the proportion of energy lost as heat), the more effective

is a dielectric material. Another important electrical property to be considered is the

dielectric constant, the extent to which a substance concentrates the electrostatic lines

of flux. Low dielectric constant materials include a perfect vacuum, dry air, and most

pure, dry gases such as helium and nitrogen. Moderate dielectric constant materials

include ceramics, distilled water, paper, mica, polyethylene, and glass. Among high

dielectric constant materials, metal oxides can be included.

Practically, there are three types of electrical properties for dielectric materials

which should take into consideration; the dielectric constant (or relative permittivity),

dissipation factor and dielectric strength.

2.2.1 Dielectric constant. This factor is the ratio of capacitance of a capacitor

with test material as the dielectric to the capacitance of a capacitor with a vacuum as

the dielectric. When determine the performance of a capacitor, its dielectric materials

should have dielectric constant should be high so that the capacitor dimensions can be

19

dA

C roεε=

minimized. This factor can be calculated using: εr = Cs/Cv where Cs is the capacitance

with the specimen as the dielectric, and Cv is the capacitance with a vacuum as the

dielectric.

2.2.2 Dissipation Factor. This value is the ratio of the power dissipated in the

test material to the power applied, which is equal to the tangent of the loss angle, or

the cotangent of the phase angle. The dissipation factor can be calculated using;

)CfRπ2(1θcotδtanDF

pp

=== Eq.2-1

where δ = the loss angle,

θ = Phase angle,

f = Frequency,

Rp = Equivalent parallel resistance,

and Cp = Equivalent parallel capacitance.

The determination of chosen dielectric materials for capacitors depends on the

capacitance value, frequency of application, maximum tolerable loss, and maximum

working voltage. The size and cost of required capacitors is also additional external

constraints. In practice, selection criteria of high voltage power capacitors are

distinctly different than those used in small integrated circuits. Large capacitance

values can be acquired at low frequencies due to low-frequency polarization

mechanisms such as interfacial and dipolar polarization. On the other hand, it

becomes more difficult to achieve large capacitances at high frequencies, and at the

same time maintain acceptable low dielectric loss, in as much as the dielectric loss per

unit volume is εoεrωE2 tan δ.

The principles of capacitor design can be determined from capacitance of a

parallel plate capacitor as following,

Eq.2-2

There are various applications for these dielectric loss capacitors, which can

be also made of many different materials in many different styles. Basically,

20

capacitors can be classified into three types; AC capacitors, DC capacitors, and

capacitors for pulse applications. AC capacitor is the most widely used because it can

also work with DC and pulse applications. For AC capacitors, it is important to

consider losses in their applications. Losses of a dielectric (except vacuum) can be

divided into two types; conduction loss, and dielectric loss. A conduction loss

represents the flow of actual charge through the dielectric. A dielectric loss is

occurred due to movement or rotation of the atoms or molecules in an alternating

electric field. It is the reason why food and drink gets hot in a microwave oven. One

way of describing dielectric losses is to consider the permittivity as a complex

number, defined as;

δ−ε=ε ′′−ε′=ε jej Eq.2-3

where

ε′ = AC capacitivity

ε″ = Dielectric loss factor

δ = Dielectric loss angle

Capacitance is a complex number C* in this definition, becoming the expected

real number C as the losses approach zero. That is, it can be defined as;

CjCC* ′′−′= Eq.2-4

One reason for defining a complex capacitance is that we can use the complex

value in any equation derived for a real capacitance in a sinusoidal application, and

get the correct phase shifts and power losses by applying the usual rules of circuit

theory. Equation 2-3 expresses the complex permittivity in two ways, as real and

imaginary or as magnitude and phase. The magnitude and phase notation is rarely

used. Instead, people usually express the complex permittivity by ε′ and tan δ,

ε′ε ′′

=δtan Eq.2-5

21

Where tan δ is called either the loss tangent or the dissipation factor DF. The

real part of the permittivity is defined as ε′ = εrεo where εr is the dielectric constant

and εo is the permittivity of free space.

They also need to have a lower dissipation factor than capacitors used as DC

filter capacitors. The AC circuit term power factor PF may also be defined for AC

capacitors. It is given by the expression PF = cos θ where θ is the angle between the

current flowing through the capacitor and the voltage across it.

( ) ( )22

cosε′+ε ′′

ε ′′=θ Eq.2-6

For good dielectric, ε′ >> ε″

δ=ε′ε ′′

≈θ tancos Eq.2-7

Therefore, the term power factor is often used interchangeably with the terms loss

tangent or dissipation factor, even though they are only approximately equal to each

other. We can define the apparent power flow into a parallel plate capacitor as

*22

CjVjXc

VVIS ω=−

==

( )ε ′′−ε′ω

= jdAjVS 2

( )ε ′′−ε′ω

= jdAjVS 2 Eq.2-8

By analogy, the apparent power flow into any arbitrary capacitor is

( )DFjCVjQPS 2 +ω=+= Eq.2-9 The power dissipated in the capacitor is

( )DFCVCVP 22 ω=′′ω= Eq.2-10

22

Where εr infers εr′. Large capacitances can be achieved by using high εr

dielectrics, thin dielectrics, and large areas. 2.2.3 Dielectric strength. This term can be defined as the maximum electric

field strength of an insulating material that it can withstand intrinsically without

breakdown, or without failure of its insulating properties. In a given configuration of

dielectric material and electrodes, this factor can be assumed as the minimum electric

field that produces breakdown.

2.3 Liquid dielectric

2.3.1 Liquid dielectric: In high voltage applications from molecular

arrangement point of view, liquids can be described as ‘highly compressed gases’ in

which the molecules are closely arranged. It is known as kinetic model of the liquid

structure. For the movement of charged particles, however, their microscopic streams

and interface conditioned with other materials cause a distortion in the otherwise

undisturbed molecular structure of the liquids. The well known terminology

describing the breakdown mechanisms in gaseous dielectrics, such as, impact

ionization, mean free path, electron drift etc. is, therefore, also applicable for liquid

dielectrics.

Liquid dielectrics can be classified in between the two states of matter, which is

a gaseous and solid insulating material. This intermediate position of liquid dielectrics

also characterized by its wide range of application in power and instrument

transformers, power cables, circuit breakers, power capacitors etc. They function as

elements in parts of various systems as following:

2.3.1.1 Insulation between the past carrying voltage and the grounded

container, as in transformers.

2.3.1.2 Impregnation of insulation provided in thin layers of paper or

other materials, as in transformers, cables and capacitors, where oils or impregnating

compounds are used.

2.3.1.3 Cooling action by convection in transformers and oil filled

cables though circulation.

2.3.1.4 Filling up of the voids to form an electrically stronger integral

past of composite dielectrics.

23

2.3.1.5 Arc extinction in oil circuit breakers.

2.3.1.6 High capacitance provides by liquid dielectrics whit high

permittivity to power capacitors.

Many natural and synthetic fluids can be used as dielectrics. Physical and

electrical attributes such as electric strength, viscosity and permittivity can be varied

in the wide range. The appropriate application of a liquid dielectric in an apparatus is

determined by its physical, chemical and electrical properties. In addition,

applications also depend upon the requirements of the functions to be performed.

Apart from mineral oils, there are some vegetable oils found to be fitted their

applications in electrical equipments. These available oils are castor, linseed,

rapeseed, soya, groundnut, corn, olive, sunflower, mustard, clove, almond,

mangoseed, cottonseed oils, etc. Basically, there are fatty acids accumulated in

vegetable seeds. Chemically, these are ester compounds produced form sebacic acids

and glycerine. Some volatile vegetable oils, however, have a strong odor and are

extracted from leaves, wood and roots of special plants. Higher the molecular weight

of these oils, more is the specific resistance and lower is the loss tangent (tanδ ).

Most of an important component for the production of ‘oil modified alkaline

resins’ is made from rapeseed, soybean and castor oils. Such resins incorporate the

advantages of oils to improve their elasticity as against the hard dried resins. Soybean

oil whit epoxy resin is known as ‘softener’ for some synthetic materials. Castor oil,

which has hydroxide content of about 5%, is an important polyisocyanide reagent.

The chemical composition of this unsaturated oil which has a high relative

permittivity between 4.2 and 4.5. Castor oil has therefore found wide application as

impregnating agent in power capacitors.

2.3.2 Breakdown in liquids: Liquids are used in high voltage equipment to serve the

dual purpose of insulation and heat conduction. They have the advantage that a puncture path

is self-healing. Temporary failures due to over voltages are reinsulated quickly by liquid

flow to the attacked area. However, the products of the discharges may deposit on solid

insulation supports and may lead to surface breakdown over these solid supports. The

causes of breakdown voltage in liquids are classified into three types; electronic

breakdown, cavitation breakdown and suspended particle mechanism [13]

24

Highly purified liquids have dielectric strengths as high as 1 MV/cm. Under actual

service conditions, the breakdown strength reduces considerably due to the presence of

impurities. The breakdown mechanism in the case of very pure liquids is the same as the

gas breakdown, but in commercial liquids, the breakdown mechanisms are significantly

altered by the presence of the solid impurities and dissolved gases.

The most common insulating liquids are made from petroleum refinery oils. Askarels,

fluorocarbons, silicones, and organic esters including castor oil, however, are used in

significant quantities. The selection of dielectric liquid in any application can be considered

by its inherit properties. The important electrical properties of the liquid include the

dielectric strength, conductivity, flash point, gas content, viscosity, dielectric constant,

dissipation factor, stability, etc. Polybutanes are widely used in the electrical industry

Because of their low dissipation factor and other excellent characteristics. Askarels and

silicones are particularly useful in transformers and capacitors and can be used at

temperatures of 200 °C and higher. The suitable application for castor oil is high voltage

energy storage capacitors because of its high corona resistance, high dielectric constant,

non-toxicity, and high flash point.

In practical, most of these liquids are used at voltage stresses of about 50-60 kV/cm

when the equipment is continuously operated. On the other hand, in applications like high

voltage bushings, where the liquid only fills up the voids in the solid dielectric, it can be

used at stresses as high as 100-200 kV/cm. [14]

2.3.3 Standard test method of insulating liquid

There are three electrical properties of liquid taken into consideration; dielectric

constant, dissipation factor and dielectric breakdown. These properties are scoped

within the two standards shown below;

2.3.3.1 Standard testing methods of insulating liquids for relative

permittivity, dielectric dissipation factor (tan δ) and DC resistivity (IEC 60247): This

International standard describes methods for the determination of the dielectric

dissipation factor (tan δ), relative permittivity and DC resistivity of any insulating

liquid material at the test temperature. It is primarily intended for making reference

tests on unused liquids, which can also be applied to liquids in service in transformers

cables and other electrical apparatus. The standard, however, is applicable to a single

25

phase liquid only. With insulating liquids other than hydrocarbons, alternative

cleaning procedures may be required.

2.3.3.2 Standard test method for dielectric breakdown voltage of

insulating liquids (ASTM D877-02, 2007): The dielectric breakdown voltage is a

measurement in the ability of an insulating liquid to withstand electrical stress. This

factor can be reduced by contaminants such as water, conducting particles, and dirt.

Lower value of this factor in this test method indicates significant concentrations of

contaminants in the liquid investigated.

2.3.4 Electrical property of liquid dielectric: Core electrical properties of the

liquid considered in this paper, when used as a dielectric, are withstand breakdown

capability under electrical stress, electrical capacitance per unit volume determined by

its relative permittivity, power factor or loss tangent which is an indication of the

energy loss under AC conditions, and its resistivity. Some electrical properties of

sampled liquids as a dielectric are shown in Table 2-1[15].

TABLE 2-1 Dielectric properties of some liquid dielectrics

Property Transformer oil

Cable oil

Capacitor oil

PETEP oil

Silicone oil

Breakdown strength at 20ºC on 2.5 mm standard sphere gap

15 kV/mm

30 kV/mm

20 kV/mm

>15 kV/mm

30-40 kV/mm

Relative permittivity (50Hz)

2.2-2.3

2.3-2.6

2.1

2.7

2-73

Tan δ (50Hz) (1kHz)

0.001 0.0005

0.002 0.0001

0.25×10-3

0.10×10-3 0.1×10-3

0.5×10-3 10-3

10-3

Resistivity (ohm-cm) 1012-1013 1012-1013 1013-1014 >1014 3×1014

Specific gravity at 20ºC 0.89 0.93 0.88-0.89 0.96-

0.97 1.0-1.1

Viscosity at 20ºC (cst) 30 30 30 80 10-1000

26

2.4 Insulating oil purification system

Impurities in liquid dielectrics mostly mean dust, moisture, dissolved gases and

ionic impurities. Purification and filtration process such as centrifuging, degassing

and distillation, and chemical treatment are required to provide uniformity and purity

of these liquids. Dust particles, which reduce the breakdown strength of the liquid

dielectrics, can be removed by careful filtration.

In normal condition, liquids contain a small amount of moisture and dissolve

gases, which can significantly affect the breakdown strength of the liquids, also. This

impurity can be treated by distillation and degassing. Water vapour, an ionic impurity

in liquid, which leads to high conductivity and heating of the can be removed using

drying agents or vacuum drying. A commonly used closed-cycle liquid purification

system to prepare liquids as per the above requirements is shown in Figure 2-1.

INLET

OUT LET

HEATER

INLETPUMP

SAMPULINGVALVE

AFTERFILTEROFFTION( 0.5 MICRON )

DISCHARGEPUMP

CHECKVALVE

VACUUMPUMP

BOOSTERPUMP

TRAP

PRE FILTEROFFTION( 5 MICRON )

TEMPERATUREGAUGETEMP

GAUGE

PRESSUREVACUUM GAUGE

VACUUM BREAKSOLENOID VALVE

VACUUMCHAMBER

PUMPRELIEFVALVE

TEMPGAUGE

PRESSUREVACUUM GAUGE

LEVELCONTRONVALVE

CHAMBERBY-PASS

FIGURE 2-1 Insulating oil purification system

This system can provide purification process of liquid in cycling loops. From

the reservoir, the liquid flows though the distillation column to remove ionic

27

impurities. Getting rid of water, applied drying agents or frozen out in the low

temperature bath can be employed. To remove dissolve gases, the liquid is passed

through the cooling tower and/or pumped out by the vacuum pumps. There are two

system filters; pre-filter and after filter, which are taken care of particles screening of

5 and 0.5 micron, accordingly. In this stage, the liquid is purified ready to be used in

the test cell. The liquid used then flows back into the reservoir. The system will begin

purification process again and so on.

Description of process in brief; oil, at ambient or elevated temperature, is

introduced into the vacuum chamber, where by vacuum distillation, water, dissolved

air and gases, and other low-boiling-range volatile contaminants are removed.

There are four functions for chemically-inert accelerator cartridges inside the

vacuum chamber; coalescence filtration before the evaporation stage, expand liquid’s

surface area using glass fibers, releasing gases and vapors from the liquid quickly by

sharp points of the glass fibers, and fine filtration removing solid contaminants. This

method is more efficient than previously used spray nozzles and baffles.

2.5 Polypropylene dielectric

For the experiment, we applied with bi-axially oriented polypropylene film,

two-side rough designed for impregnated film-foil capacitors.

2.5.1 Physical, chemical and thermal properties of polypropylene film: Some

other distinct characteristics of PP film present a crucial influence to test result, too.

There are five characteristics in our determination; specific gravity, water absorption,

maximum continuous working temperature, maximum peak working temperature, and

melting point. Table 2-2 shows these characteristics in details.

2.5.2 Electrical properties of polypropylene film: Five electrical characteristics

are taken into consideration; permittivity, loss factor, volume resistivity, dielectric

strength, and electrical defects. The values of each characteristic of PP film are shown

in Table 2-3.

2.5.3 Dimension properties and space factor of polypropylene film: The

dimension of PP film, especially its thickness, can affect test result during

impregnation process. The impact of different space factor (≥5μm) can also influence

our investigation. These two characteristics; are shown in Table 2-4 and Table 2-5.

28

TABLE 2-2 Physical, chemical and thermal properties of polypropylene film

Source: Bollore′ Inc.

TABLE 2-3 Electrical properties of polypropylene film

Characteristics 20°C 100°C Tolerance/ Typical value

2.20 2.15 50Hz Permittivity

1MHz 2.20 2.15 Typical value

1.5 1.6 Loss factor tg δ (10-4)

50Hz

1MHz 1.6 2.5 Typical value

Volume resistivity (Ω-cm) 1018 1017 Typical value

Dielectric strength (Vdc / μm) 560 Typical value 1.3 Number

at 6 μm 1 Number at 8 μm

0.6 Number at 1.2 μm

Electrical defects at 300Vdc /μm

0.5 Number at 15-18 μm

Maximum value

Source: Bollore′ Inc.

Characteristics Value Tolerance/ Typical value

Measurement method or

reference norm

Specific gravity 0.905 Typical value ASTM D 92

Water absorption (%) 0.005 Typical value ASTM D570

Max. working temperature

(continuous) (°C) 100 Typical value

Max. working temperature (peak, °C)

120 Typical value

Melting point (°C) 167 to 169 Typical value DSC

29

TABLE 2-4 Dimension properties of polypropylene film

Characteristics Value Tolerance / Typical value

Thickness by weight (μm) 5 go 17.8 +/- 6% on 1m2 of film on roll

weight

Thickness variation in % of nominal

thickness

GR 0 GR + GR -

+2 to -2 % on roll weight +2 to +6% on roll weight -2 to -6% on roll weight

Film width (mm) 8 to 239.5 240 to 398 ≥ 400

+/- 0.5 +/- 0.7 +/- 1

Source: Bollore′ Inc. TABLE 2-5 Space factor of polypropylene film

Source: Bollore′ Inc. The value of space factor can be calculated from Eq.2-3 as following;

% Eq.2-11

Tpm = Micrometer thickness at pressure = 1 Bar (µm)

Tpg = Thickness by weight, by weighing 1 m2 of film, for density = 0.905 (µm)

For Example; two-side roughened polypropylene film is SF = 10 % ,thickness

by weight Tpg = 17.8 µm for density = 0.905 therefore Tpm = 19.58 µm.

2.5.4 Standard test method of solid insulating material

2.5.4.1 Electrical strength of insulating materials - Test methods - part 1

(IEC 60243-1): Tests at power frequencies: It is set to determine the short-time

electric strength of solid insulating materials at power frequencies between 48 Hz and

Characteristics Value Tolerance / Typical value

Measurement method or reference norm

Space factor for thickness 5μm (%) 7.5 +/- -2.5

Space factor for thickness >5μm (%) 10 +/-3

Space factor Measurement

100T

TTSF

pg

pgpm ×−

=

30

62 Hz. There is no consideration of liquids and gases testing methods, although these

are specified and used as impregnants or surrounding media for the solid insulating

materials. This standard includes methods for the determination of breakdown

voltages along with the surfaces of solid insulating materials.

2.5.4.2 IEC 60243-2: electric strength of insulating materials - testing

methods - part 2: Additional requirements for tests using direct voltage: This standard

gives methods of test for the determination of the short-time electric strength of solid

insulating materials at direct current voltage. There is no consideration of liquids and

gases testing methods, although these are specified and used as impregnants or

surrounding media for the solid insulating materials.

2.6 Composite dielectric

2.6.1 What is composite dielectric

In general, insulation system can not achieve its task perfectly without

composing of more than two insulating materials. These different materials can be in

parallel with each other, such as air or insulating oil in parallel with solid insulation or

in series with one another. These insulation structures are called composite dielectrics.

Being a part of mechanical requirements, the composite pattern of an insulation

system concerned with separating electrical conductors at different potentials. In

actual, single materials will commonly have at least small volumes of another

elemental material such as gas or voids in a solid. Meanwhile, there may be dust

particles or gas bubbles in a liquid or gas. One common composite dielectric is the

solid/liquid combination or liquid impregnated flexible solid like thin sheets of paper

or plastic. It is widely used in low and high voltage equipments such as cables,

capacitors, transformers, oil-filled switchgear, bushings etc.

2.6.2 Properties of composite dielectric

General speaking, a common composite dielectric can comprise plenty of layers

superimposed one over the other. It is called “layered construction” which can be

found in many applications such as cables, capacitors and transformers. There are

three important properties of composite dielectrics which affect the performance as

following;

31

2.6.2.1 Effect of multiple layers: This is advantageous property of

composite dielectric based on the fact that two thin sheets have a higher dielectric

strength than a single sheet of the same total thickness. The advantage is significant

particularly when materials have a wide variation in dielectric strength values

measured at different points on its surface.

2.6.2.2 Effect of layer thickness: For this property, a composite

dielectric’s breakdown voltage will be increased if there is an increase of its layer

thickness. This is because in this layered construction a breakdown can be occurred at

the interfaces only, not directly through another layer. In addition, a discharge

penetrated through one layer can never enter the next one until the interface also

reaches the discharge channel. This is certainly important to the insulating paper since

its thickness can be varied and consequently the dielectric strength across its surface

to the paper which can produce an electric field stress comparable to that of the

discharge channel. Furthermore, impregnation process can be enhanced by the rough

surface of the paper when tightly wound. Whereas the lower thickness areas within

the paper can also cause breakdown even at considerably lower voltages. Previous

studies on composite dielectrics indicated that the discharge inception voltage

depends on the thickness of the solid dielectric, together with the dielectric constant of

both the liquid and solid dielectric. The difference between the liquid and solid

dielectrics in the dielectric constants does not significantly affect the change of

electric field at the electrode edge with the change in the dielectric thickness.

2.6.2.3 Effect of interfaces: Determining pre-breakdown and breakdown

strengths of dielectrics, the interface between two dielectric surfaces in a composite

dielectric system is one of the key factors. Discharges usually occur at the interfaces

and the magnitude of the discharge depends on the associated surface resistance and

capacitance. When the surface conductivity increases, the discharge magnitude also

increases, resulting in damage to the dielectric. 2.6.3 Permittivity of composite dielectric

As the result of reducing electrical breakdown at the interface of two different

insulation materials[18], the interfaces in highly stressed field regions should be

normal to the field lines. The ‘parallel-plate capacitor’ which contains two layers of

32

different materials represented by the permittivity 1ε and 2ε is therefore typical for

many applications. Figure 2-2 (a) shows the arrangement and dimensions assume. For

usual dielectric materials with power frequency AC voltage application, the

conductivity of the materials can be ignored and hence no free change is built up at

the interface between the two layers. The displacement vectors D1 and D2 are then

equal, starting from and ending ay the equal free changes on the plate only. As

,ED ε= which is the same in both materials, the ratio of the field strength becomes;

1

2

2

1

EE

εε

= Eq.2-12

(a) (b)

FIGURE 2-2 Capacitor comprising two layers of different dielectric materials:

(a) Smooth parallel plates (b) Parallel plates with oil filled

As the field remains uniform in each layer, the voltage V or potential difference

between the two plates is

2211 dEdEV += Eq.2-13

where d1, d2 are the individual values of the thickness of the two dielectrics.

Introducing Eq. 2-12 into Eq. 2-13, we obtain the following absolute values of E1 and

E2 with reference to the voltage applied:

33

⎟⎟⎠

⎞⎜⎜⎝

⎛εε

+=

+⎟⎟⎠

⎞⎜⎜⎝

⎛−

εε

=

⎟⎟⎠

⎞⎜⎜⎝

⎛ε

+ε

ε=

2

121

1

21

2

2

1

11

1

dd

V

11dd

1dV

ddVE Eq.2-14

11dd

1dV

ddVE

1

21

2

2

1

11

2

+⎟⎟⎠

⎞⎜⎜⎝

⎛−

εε

=

⎟⎟⎠

⎞⎜⎜⎝

⎛ε

+ε

ε= Eq.2-15

In theoretical approach, either Eq.2-14 or Eq.2-15 shown above may be applied

in our calculation for the mean value of dielectric materials’ permittivity in

homogeneous mixture such as resin- or oil-impregnated Kraft papers shown in Figure

2-2 (b). These layers mentioned are usually oriented in parallel to the electrodes,

any multiple-dielectric system can be separated into an infinite number of layers

with materials designated by their intrinsic properties ε1 and ε2 and the resultant

permittivity (εres) of composite dielectric can be defined as;

ED resε= Eq.2-16

where D and E are macroscopic mean values. As the microscopic values E1 or

E2 will remain unchanged by multiple layers, we can write

2211res EEED ε=ε=ε= Eq.2-17

or after replacement of E1 or E2 in Eq.2-14 or Eq.2-15 and rearranging the numbers

( ) ( )2

2

1

1res d/dd/d

1dVE

ε+

ε

⎟⎠⎞

⎜⎝⎛=ε Eq.2-18

As before, V/d represents the mean value of the field strength within the

mixture, and the distances can be replaced by relative volumes v1 and v2 as the

relationships d1/d and d2/d represent also the volumes of the two materials. Therefore;

34

( ) ( )2211res /v/v

1ε+ε

=ε Eq.2-19

for a mixture of n materials

( ) ( ) ( )nn2211res /v.../v/v

1ε++ε+ε

=ε Eq.2-20

with ∑=

=n

1ii 1v or 100 percent.

Example: An oil impregnated capacitor using a dielectric made from two rough

side polypropylene film that space factor (SF) = 10 % , Tpg = 17.8 µm and Tpm =

19.58 µm. We can calculate the volume of polypropylene film by v1 = 17.8WL , and

space for oil impregnation v2 = (19.58-17.8) WL .

The percent volume of each value is, v1 = (17.8WL)/(19.58 WL) × 100 =

90.91 % or 0.9091 p.u. and v2 = (19.58-17.8)WL/(19.58 WL)×100 = 9.09 % or

0.0909 p.u. , where relative permittivity of polypropylene εr1 = 2.2 and Vegetable

oil εr1 = 3.1. The resulted permittivity εres can be defined according to Eq.2-19 ;

εres = 2.26.

2.7 Wound capacitor flat winding

2.7.1 Basic construction of impregnated polypropylene capacitor

Extended part of foil

Folded part of foil

Extended part of foil

Folded part of foil

Aluminum Foil

Polypropylene Films

Aluminum Foil

FIGURE 2-3 Capacitor Element of flattened wound capacitor

35

The element has been pressed flat in the height direction and is called flattened

pressed element. Element or (active) foil length is obtained on unwinding the element

in the length direction. Height is always determined from the side on which the

bushings are fitted, to the opposite side. The process is shown in Figure 2-3.

Normally, the length direction of the flattened element corresponds to the depth

direction of a container. Depending on the design, the element width direction may

correspond either to container height direction or to container width direction.

After being processed, the dielectric and electrodes are ready in thin foil form,

then they can be wound to achieve a technically suitable shape. The principle of a

wound capacitor is shown in Figure 2-3. Generally, the capacitor mentioned will

consist of electrodes made of about 6 μm thick aluminum foil. Sometimes dielectrics

can be made from impregnated paper or plastic films with several layers. The

effective area of an individual winding will become smaller caused by the winding

with each electrode used on both sides. With projected electrodes or so called

extended foils, external connections can be achieved as shown in the figure.

2.7.2 Design of wound capacitors

2.7.2.1 Rated voltage of capacitor

Rated voltage can be defined as the operating field strengths of AC and DC

voltage capacitors. For high-voltage capacitors, the most important dimensional factor

is the operating field strength E as it is applied frequently in the calculations for

power and energy density.

Relating to polypropylene film properties, in 1992 the research made by Lu

Youmeng and Li Zhaolin had studied about testing methods of plastic films for

electrical applications [19]. The result of this research is shown in Table 2-6. It is

mentioned that when compared between two testing methods of electric withstanding

capability of BOPP film (Biaxially Oriented Polypropylene film), the electrode and

the element method, it was found that the maximum breakdown voltage of the model

capacitor when using the electrode method was less than a half value resulted in the

element method. They also found that the breakdown voltage from the element

method was less than a half value of the unit done by the electrode method. It means

that the maximum voltage from their voltage test between terminals of a capacitor

36

without safety factor was not more than a half of calculated maximum breakdown

voltage for the dielectric strength of a film.

TABLE 2-6 Electric withstanding capability of BOPP film

Note: 1# electrode method; 2# element method

Referred to the research of testing methods of plastic film for electric usage in 1992

The rated voltage of capacitor (UN) is the R.M.S. value of the alternating

voltage which is very important for a capacitor design. It is a half value of maximum

voltage in an AC voltage test between terminals (as refer to IEC 60871-1, voltage test

between terminals = 2UN for AC test and = 4UN for DC test). Ubmin = k×2UN at k is

the safety factor used in a capacitor design. According to IEC 60243-1 and IEC

60243-1 testing method, the reported value of dielectric strength in insulating

materials is the median of the dielectric strength (Eb) in kV/mm. Sometimes Eb value

of a dielectric material may be less than 15 % of reported value or electrical property

data of materials. In that case, Eb value for a capacitor design becomes less than 15 %

of Eb value on electrical property data of material.

Example: In a case of polypropylene film with thickness by weight (Tpg) = 17.8

µm and Dielectric strength (Eb) = 560 Vdc/ µm (IEC 60243-2), then the designed E =

0.85Eb = 0.85×560 = 476 Vdc/µm.

From our research, Eb of the chosen element = 0.5 Eb of PP film = 238

Vdc /µm. According to IEC 60871-1; dielectric strength at UN ;E = 238/4 Vdc/µm =

59.5 V/µm. The safety factor applied in the study is between 130 %, with E = 45.77

19.5μm 18.0μm 15.0μm 12.0μm 10.0μm 9.0μm Sample

1# 2# 1# 2# 1# 2# 1# 2# 1# 2# 1# 2#

average 12.8 7.8 11.8 7.2 9.2 6.2 7.2 4.6 5.4 3.8 5.2 3.3 Breakdown voltage (kv)

min 11.8 5.0 11.0 5.0 8.2 4.1 6.9 3.2 4.9 2.8 4.8 2.1

average 666 - 657 - 618 - 610 - 592 - 569 -

medium - 400 - 400 - 413 - 383 - 380 - 367

Field

intensity (MV/m)

min 606 256 614 278 537 273 585 267 534 280 530 233

37

kV/ µm to the safety factor = 150 % , with E = 39.67 kV/ µm. Therefore, the

dielectric strength of our designed capacitor can be defined as; Eb = 40 - 45 Vrms/µm

for UN and, Eb = 80 - 90 Vrms/µm for UNdc .

For AC voltage application, the following effective field strengths are chosen;

power capacitors: Eb = 15-20V/μm for Askarel-impregnated paper and Eb = 35-40

V/μm for Askarel-impregnated paper/foils, and measuring and coupling capacitors:Eb

= 10-15V/μm for Mineral oil impregnated paper.

For DC voltage application, the instance of usual operating field strength is;

smoothing capacitors: Eb = 80…100V/μm for Mineral oil impregnated paper

Example: The vegetable oil impregnated polypropylene film capacitor designed

to use two-side rough PP film with thickness by weigh (Tpg) = 17.8 µm and dielectric

strength ( Eb ) = 560 Vdc/µm (IEC 60243-2). When applied with Eb = 42 V/µm,

then UN = 747.6 V for the dielectric of a single layer PP film and UN = 1495.2 V

for the dielectric double layer PP film.

2.7.2.2 Capacitance of a winding: The winding process, in

manufacturing basis of a wound capacitor, is show in Figure 2-3 with a cross section

in Figure 2-4. In this case, a four-layer film (two-layer PP film between aluminum

foil; Sp = 2) are wound one above the other on the mandrel. Assuming the dielectric

has total thickness of d and the overlapping width of both the metal foils is W. For the

capacitance of the winding besides d and εr, it is only the value of W as well as the

length of the upper metal foil L (the measuring foil) that are certainly significant.

Doubling the effective capacitance can be achieved by the winding process. From the

winding capacitance we have;

dL×W

2=C roεε Eq.2-21

The edge spacing We is put to prevent an external flashover and is usually

chosen to be about 5 to10 mm.

From the designed flattened type element wound capacitor shown in Figure 2-5,

all dimension is taken into consideration for its capacitance (C) in μF as following;

38

Al. Foil

Al. Foil

PP Film PP Film

(a) (b)

FIGURE 2-4 Two-layer PP film wound capacitor; (a) Cross section of

two-side roughened PP film between Al plates; (b) Cross

section of four-layer PP film with two folded Al. foils

FIGURE 2-5 PP film and Al. foil of element capacitor wound type

pgTpmT

d

pgTpmT

W

L1V 2V

1VFoil PP

Foil PP

39

pmp

res6

TSWLε10854.82C

×××××

=−

μF Eq.2-22

Wε10854.82

TSCL

res12-

pmp

××××××

= m Eq.2-23

2DπLL 1

T += m Eq.2-24

L Average length of aluminum foil used in capacitor (m)

LT Total length of aluminum foil

Tpg Thickness by weight of polypropylene film (m)

Tpm Micrometer thickness of polypropylene film (m)

Ta Thickness of aluminum foil (m)

Wp Width of polypropylene film (m)

Wa Width of aluminum foil (m)

Wpe Width of polypropylene film edge between aluminum foil (m)

W Width of capacitor plate (m)

We Width of folded edge foil (m)

Wct Width of capacitor terminal (m)

LT Length of aluminum foil (m)

[ ( ) ]T1-noDπnLT += m Eq.2-25

A = LW Area of capacitor plate (m2)

DR Roller diameter (m)

Do Roller diameter with start only polypropylene film (m)

D1 Average diameter of first aluminum winding (m)

Nps Number of turns in first winding of polypropylene film (4)

Npe Number of turns in last winding of polypropylene film (4) Sp Number of layers of polypropylene film placed between foil

n Number of turns wound in aluminum foil

pmppsR0 TSN4DD += m Eq.2-26

40

apmp T3TS2T += m Eq.2-27

T Total thickness of PP film and aluminum foil before winding

2T Increase in diameter per turn

ln Length of ‘n’ turns of aluminum Foil

apmp01 T3TS2DD ++= m Eq.2-28

(3Ta for Folded foil and 2Ta for non Folded foil)

( )[ ]T21nDl 1n −+π= m Eq.2-29

( )[ ]T1nDπnL 1 −+= m Eq.2-30

Furthermore, number of turns wound in aluminum foil can be defined as;

1TD5.0

TπL1

TD5.0n 1

21

21 −⎟

⎠⎞

⎜⎝⎛−

⎥⎥⎦

⎤

⎢⎢⎣

⎡+

⎭⎬⎫

⎩⎨⎧

−⎟⎠⎞

⎜⎝⎛= Eq.2-31

2.7.5 Calculation of element thickness and width

FIGURE 2-6 Dimension of flattened type element wound capacitor

TE = total PP thickness + total aluminum foil thickness

+ total kraft paper thickness

then, TE = ∑Tpms+ ∑Tpmd +∑Tpme+∑Ta + ∑Tkp Eq. 2-32

TE = Element thickness

TE

TCE

WE

41

While ∑Tpms = Total start PP insulation thickness ∑Tpms = 2Sp× 2Nsp×Tpm Eq. 2-33

And ∑Tpme = Total end PP insulation thickness ∑Tpme = 2Sp× 2Nep×Tpm Eq. 2-34

in most case, Nsp = Nep therefore ∑Tpme = ∑Tpms ∑Tpmd = Total polypropylene film dielectric thickness

∑Tpmd = 2Sp× 2n×Tpm Eq. 2-35

∑Ta = Total aluminum foil thickness with folded edge

∑Ta = 2n×3Ta Eq. 2-36

∑Tkp = total kraft paper thickness

∑Tkp = 2Nkp×Skp×Tkp Eq. 2-37

Tkp = Kraft paper thickness

Nkp = Number of turns wound in insulating kraft paper

Skp = Number of layers in insulating kraft paper

If Nsp = Nep = 4 and Sp = 2 (For double layer of PP film)

then, TE = 32Tpm+ 2n(4Tpm+3TA) + 2 Nkp ×Skp×Tkp Eq. 2-38

Complete capacitor element thickness (TCE) can be determined as;

TCE = TE + ∑Tpb Eq. 2-39

∑Tkp = Total insulating Kraft paper thickness

Tpb = Pressed board thickness

Complete capacitor element width (WE)

ER

E T2DπW += Eq. 2-40

42

Example: where PP Film and Al. Foil; Tpm=19.58 µm, TA = 6 and n = 35

turns, and Tkp = 50 µm, Skp = 3 , and Nkp = 6; so capacitor element thickness

(TE) = 9.168 mm and two pressboards with 0.5Tpb = mm are used; then the value

of complete capacitor element thickness (TCE) = 19.168 mm. and capacitor

element width (WE) = 103.41 mm

2.8 Capacitor impregnation process

An impregnated electrical capacitor is a capacitor having a dielectric at least

partly consists of polypropylene film and aluminum foil, which is expanded by an

impregnation filled with liquid dielectric. To produce such capacitor, the temperature

during and after impregnation and the degree of winding compression or tightness

may be varied to provide desired control of the expansion and assure complete

impregnation and control of the final volume comparable with the original capacitor

structure. In addition, it comprises at least a pair of electrodes. Moreover, at least one

thin insulator sheet should solely consists of a polypropylene plastic film wound

between electrodes mentioned and is impregnated with an insulating oil. The

insulating oil features an oil having physical properties which allow the impregnated

film to expand at 80° C by an amount no greater than 0.5% over the non-impregnated

film at room temperature, and physical properties which allow insulating oil to be

presented in the film at 80° C in an amount no greater than 10% by weight of the film.

There are developments in which to enhance and optimize the drying time of the

capacitor stack after it has been placed inside the insulator housing without

influencing the dryness of the active part before impregnation. In the past, capacitors

were dried together in a large drying chamber. Presently, there is an adjustment to this

process so that fewer capacitors are dried in smaller drying chambers, while the

number of chambers has been increased to maintain the same overall capacity. This

results to shorter drying and impregnation cycle which might take only a couple of

days instead of 2 to 3 weeks as it was made before. Furthermore, using independent

modular units enhances different technologies in impregnation process at the same

time. This allows optimal fabrication lots and number of fabrication units to be

defined. Therefore, cycles of drying and impregnating capacitors can be maximized

which also leads to superior flexibility. The assembly of the complete capacitors can

43

be done with “dry” parts. And full drying and impregnation can be performed on the

assembled unit. [16]

During impregnation process, a liquid dielectric capacitor impregnant; including

the halogenated aromatic hydrocarbon, castor oil, and mineral oil is filled into

polypropylene resin and diffuse throughout the film. After a capacitor has been made

with the pre-incorporated impregnant, it is impregnated to increase the amount of

liquid dielectric therein.[17]

2.9 Standard test method of capacitor (IEC 60871-1)

The international standard, IEC 60871-1: Shunt capacitor for AC power

systems, is applied to an investigation at a rated voltage above voltage 1000 V part 1.

The scope of this standard in details is shown below;

2.9.1 Test conditions: Unless otherwise specified for a particular test or

measurement, the temperature of the capacitor dielectric shall be in the range + 5°C to

+35 °C.

2.9.2 Capacitance measurement: The capacitance shall be measured at 0.9 to

1.1 times the rated voltage, using a method that excludes errors due to harmonics. The

final capacitance measurement shall be carried out after the voltage test. For

capacitance tolerances, The capacitance shall not differ from the rated capacitance by

more than -5 % to +10 % for capacitor units.

2.9.3 Measurement of the tangent of the loss angle ( tan δ ) of the capacitor: The

capacitor losses ( tan δ ) shall be measured at 0,9 to 1,1 times rated voltage, using a

method that excludes errors due to harmonics. The accuracy of the measuring method

and the correlation with the values measured at rated and frequency shall be given.

2.9.4 Voltage test between terminals: The AC test shall be carried out with a

substantially sinusoidal: Utac = 2 UN ;and the DC test voltage shall be as follows:

Utdc = 4 UN

2.9.5 Short- circuit discharge test: The unit shall be charged by means of DC

and then discharged through a gap situated as close as possible to the capacitor. It

shall be subjected to five such discharges within 10 min. The test voltage shall be 2.5

UN, and to find differences of capacitance within 5 minutes after this test.

CHAPTER 3

DESIGN AND TEST OF IMPREGNATED CAPACITOR

The discussion in this chapter includes design and test of vegetable oil

impregnated polypropylene film capacitors. There are three type of vegetable oils in

our study; sunflower oil, soybean oil, and Envirotemp FR3 fluid. For the two

commercial oils, sunflower oil and soybean oil, purification process is required to

provide appropriate electrical properties according to specifications of natural ester

fluids. During the test, model capacitors were impregnated with mentioned fluids at

four impregnating temperatures, and then test results, their electrical properties, were

measured.

3.1 Vegetable oil purification system

3.1.1 Function of insulating oil purification system

In this research, insulating oil purification system can be defined as the system

in which to filter and purify selected vegetable oils impregnated with model

capacitors for their better liquid dielectric characteristics. It was designed with similar

to principles of liquid insulating purification system utilized in dielectric

manufacturing industry for a commercial purpose. Elements and procedures of this

system are shown previously in chapter 2. It was designed to be a small-scale system

with clarified view for an observation to reduce expensive automatically-controlled

mechanisms. Also, it is made particularly for an impregnant preparation of this study.

This system performs a purifying treatment to remove impurities contained in

vegetable oils, which help them in improving the insulating property. In addition, it

can also help remove water content, bubble gases or moisture in these fluids. These

vegetable oils are for human consumption, which are generally required fillings of

nitrogen gas inside.

The system consists of large particle removal filter which can screen most of 5

micron or larger particles from a fluid. This filter is made from polypropylene, which

46

is also called ‘PP sediment removal cartridge water purifier’. The impurities contained

in vegetable oils can cause to lower dielectric property of these fluids, too.

3.1.2 Elements of the system

Constructs and elements consisted in this system are shown in Figure 3-1 with

its actual process shown in Figure 3-2.

3.1.2.1 Filtering processes: There are two steps of filtration; pre-filter

and after-filter. The first pre-filter will operate when the machine is rotated. Its filter

element inside the cartridge is made from polypropylene. In addition, it works well

within the pressure range of 1.4 to 6.9 bars, at the maximum temperature 52°C, with

screen size of 5 micron. Another filter consisted is called the after-filter or fine filter.

Besides, it is made from polyethylene with screen size of 0.3 micron. This filter

operates particle removal from a fluid before it flows back to the anti-moisture

reservoir.

3.1.2.2 Heater: It is an equipment used for heating vegetable oils with

the temperature limit at 60°C. There are copper tubes inside this equipment, which

help in the heat-exchanging process.

3.1.2.3 Vacuum chamber: This part is made from clear acrylic with its

diameter of 15 cm and 140 cm long. It functions as a vacuum chamber for degassing

or moisture removal and a reservoir or oil tank in this system. There are glass balls

inside the chamber to increase surface area of the fluid, along with nozzles to quickly

remove moisture contained.

3.1.2.4 Oil pump: The power steering pump of a car is used to compress

the oil to have suitable pressure and flow rate. It is driven by a DC motor to adjust the

oil pressure controlled by speed adjust. The suitable pressure of oil is important for oil

filter, oil spray in vacuum chamber and oil flow rate in closed-cycle oil purification

system.

3.1.2.5 Two-stage vacuum pump: It is used for vacuuming a fluid so

that moisture contained can be removed by the extremely low temperature at 55°C. It

can also help degas bubbles by 0.2 Torr pressure conditioned.

3.1.2.6 Pressure vacuum gauge: This equipment displays the minimum

pressure in the system for an investigation of leakages or broken seams which lead to

lower performance or failure of operation.

47

3.1.3 Operations

In purification process within the system, two vegetable oils; sunflower oil and

soybean oil, will be brought into the consideration to find out their dielectric strengths

M

TWO STAGEVACUUMPUMP

WATER TRAP

PRESSURE VACUUM GAUGE

OIL INLET

FINE FILTER0.3 MICRON

FILTER5 MICRON

HEATER

TEMPERATURE

MOTOR

OIL PUMP

VACUUMCHAMBER

OIL OUTLET

DRAIN

OIL SPRAY

GAUGE

FIGURE 3-1 Small vegetable oil purification diagram

(breakdown voltages) as per standard testing methods for dielectric breakdown

voltage of insulation liquids using disk electrodes (ASTM D877). From test results,

breakdown voltage of food grade sunflower oil and soybean oil is less than 30 kV.

Then chosen oil is filled into the system at a half volume of the vacuum chamber. A

fluid in the system will be conditioned certainly at 55 ±3 °C and 0.2 Torr of pressure

for 24 hours. During purification process, pre-filter will screen out particles sizing at

least 5 micron from the system. Then the fluid will flow through a fine filter to screen

out 0.3 micron particles by suction back to the vacuum chamber. The measured

outcomes of dielectric breakdown voltages of these oils are shown in Table 3-1. When

48

oil in the system is changed, it is also necessary to change filter elements inside the

cartridge and flood all the system with the replaced fluid. In every test, dielectric

breakdown voltages will be recorded before applying into model capacitors.

3.1.4 Vegetable oil preparations

After purification process, each fluid is then impregnated to model capacitors.

Electrical property test measured are shown in Table 3-1.

FIGURE 3-2 Small vegetable oil purification system

The process shown in Figure 3-2 leads to measured results in Table 3-2 and

Table 3-3. It is referred to IEC 60247 standard; Insulating liquids – measurement of

relative permittivity, dissipation factor (tan δ) and DC resistivity. Dissipation factors

and dielectric constants are demonstrated in Figure 3-5 and Figure 3-6.

49

3.2 Vegetable oil properties

Vegetable oil properties, in this case, are according to standard testing methods

for dielectric breakdown voltage of insulating liquids using disk electrodes. These

vegetable oils were previously prepared by filtering and tested with dielectric oil

tester as per ASTM: D 877, until their dielectric breakdown values measured are close

to 47 kV as shown in Table 3-1.

TABLE 3-1 Vegetable oil properties from test results compared with natural

ester fluid standard and synthesis aster fluid for capacitor application

Typical Tested

property Method Sun- flower

oil

Soybean oil Envirotemp

FR3 Fluid

Natural ester fluid

(vegetable oil) standard

(ASTM D6871)

Synthesis ester (PXE)

(Nissin Electric)

Dielectric Breakdown (kV)

ASTM D877

53.52, 57.34

50.00, 46.96, 46.76, 55.48

40.27, 37.12

(47 from Specification)

≥ 30

80

Dielectric constant (20°C)

IEC 60250 3.105 3.091 3.062 - 2.5

at 80 °C

Kinematic Viscosity (cst)

40°C100°C

ASTM D445

- -

- -

33.98 8.22

≤ 50 ≤ 15

5.2 -

Water Content (mg/kg) D1533

-

-

20-30

≤ 200

-

Dissipation Factor (%)

25°C80°C

100°C

D924

0.508 6.452

-

0.496 5.768

-

0.033 -

0.689

≤ 0.20 -

≤ 4.0

- 0.02

-

Volume Resistivity (Ω-cm)

D1169

-

-

3.9×1013

no (≥1×1013

from specification)

9.4×1014

Pour Point (°C) D97 - - -24 ≤ -10 ≤ -40

Flash Point (°C) D92 - - 316 ≥ 275 145

Fire Point (°C) D92 - - 364 ≥ 300 167

50

FIGURE 3-3 Dielectric breakdown voltage test of vegetable oils

Figure 3-3 and Figure 3-4 shows actual dielectric breakdown voltage test of

these fluids and measurement (with a courtesy of Tira Thai Co., Ltd.) of their relative

permittivities (dielectric constants) and dissipation factors, accordingly.

FIGURE 3-4 Measurement of relative permittivity and dielectric dissipation factor

51

57.3

4

50

37.1

2

53.5

2

46.7

6

40.2

7

46.7

6 55.4

8

0

10

20

30

40

50

60

70

Sunflower Soybean oil Envirotemp FR3fluid

Die

lect

ric b

reak

dow

n vo

ltage

(kV

)

1 2 3 4 1 2 1 2

FIGURE 3-5 Dielectric breakdown voltages of vegetable oils

TABLE 3-2 Relative permittivities of vegetable oils subjected to temperature

level as per IEC 60247 standard

Relative permittivity Temperature (°C)

Sunflower oil Soybean oil Envirotemp FR3 Fluid

5 3.211 3.245 3.228 10 3.175 3.193 3.178 15 3.140 3.143 3.119 20 3.105 3.091 3.062 25 3.071 3.044 3.012 30 3.039 2.999 2.971 35 3.006 2.965 2.940 40 2.978 2.941 2.917 50 2.923 2.896 2.880 60 2.882 2.865 2.853 70 2.845 2.841 2.825 80 2.807 2.816 2.801 90 2.773 2.793 2.780 100 2.751 2.768 2.758

52

2.6

2.7

2.8

2.9

3

3.1

3.2

3.3

0 20 40 60 80 100 120

Temperature (°C)

Rel

ativ

e pe

rmitt

ivity

Sunflower oil

Soybean oil

Envirotemp FR3 fluid

FIGURE 3-6 Relative permittivities of vegetable oils in temperature

According to test results shown in Figure 3-3 and Figure 3-4, relative

permittivities and dielectric dissipation factors of these fluids as per IEC 60247

standards are summarized in Table 3-2 and Table 3-3, respectively. Their dielectric

breakdown voltages are plotted graph demonstrated in Figure 3-5. Please note that in

each measurement, dielectric breakdown voltages of these fluids are slightly different.

In addition, outcomes of these fluids shown as plotted curves in Figure 3-6 indicate

that their relative permittivities subjected to temperature levels ranged from 5 °C to

100 °C are similar to each other without any significant difference.

In Table 3-3 and Figure 3-7, outcomes from the measurement of dissipation

factors of vegetable oils subjected to temperature level and their plotted graph are

shown. As the adjusting temperature rose from 20 to 80 °C, dielectric losses of these

fluids are increased continuously. The overall maximum measured dissipation factor

of 6.452% is from sunflower oil at 80 °C, and the minimum value of 0.0536% comes

from Envirotemp FR3 at 20 °C. From Figure 3-7, it is noticeable that a plotted curve

of sunflower oil is highest among these fluids, while the lowest plotted curve is of

Envirotemp FR3 fluid.

53

TABLE 3-3 Dissipation factors of vegetable oils subjected to

temperature level as per IEC 60247 standard

* Corrected by k = 0.77 of Envirotemp FR3 Fluid’s correction factor

0

1

2

3

4

5

6

7

8

0 10 20 30 40 50 60 70 80 90

Oil Temperature (°C)

Diss

ipat

ion

fact

or(%

)

Sunflower oil Soybean oilEnvirotemp FR3 Fluid

FIGURE 3-7 Dissipation factors of vegetable oils

Dissipation factor (%) Temperature (°C) Sunflower oil Soybean oil Envirotemp

FR3 fluid 20 0.442* 0.432* 0.0536 30 0.574 0.561 0.069 40 1.189 1.124 0.227 50 2.126 2.000 0.595 60 3.275 3.099 1.162 70 4.741 4.421 1.880 80 6.452 5.786 2.600

54

3.3 Design of capacitor element

In studying about the production of polypropylene film capacitor impregnated

with three vegetable oils; sunflower oil, soybean oil, and Envirotemp FR3 fluid, the

selected rated voltage of a capacitor element is at 1.5 kV with its capacitance of 1.9

µF. This production process can be described as following;

3.3.1 Capacitor dielectric material: Because capacitors of this production are to

be impregnated with vegetable oils which have high viscosity, an appropriate

dielectric material used in this research is chosen to be two-side roughened

polypropylene film. The breakdown voltage property of this material, from

manufacturer’s specifications, is shown in Table 2-3. In addition, studies of this

property in polypropylene film are collected in Table 2-6. The breakdown voltage of

capacitor element effective field strength made from this material is between 40-45

Vrms/µm.

According to this research, the chosen value of this property is E = 42 Vrms/µm

with polypropylene film thickness of 1500 V/(42 Vrms/µm) which is equal to 35.71

µm. Thus, appropriate thickness of a dielectric in the capacitor can be obtained by an

engagement of two PP film (two layer film) with maximum thickness of 17.8 µm

each, which is equal to 17.8×2 = 35.6 µm. For the selection of electrode materials,

aluminum foils with thickness of 6 µm are considered based on the fact that this is the

only available standard size in the market.

3.3.2 Wound capacitor element design: In this section, properties of selected

materials are described for better understanding of capacitor element design in details.

3.3.2.1 Aluminum foil: This material is used as dielectrodes of

capacitors. The dimension of aluminum foil in our test is 6 μm thick, with its width of

316 mm.

3.3.2.2 Polypropylene film: PP film applied in the element design is

two-side roughened film with the thickness of 17.8 µm by weight and 300 mm wide.

Its specific space factor is 10%, which brings to its total thickness of 19.58 µm when

this factor is taken into account, with its relative permittivity (εr) of 2.2.

3.3.2.3 Vegetable oils: Relative permittivity(εr), measured at 20 °C, of

sunflower oil, soybean oil, and Envirotemp FR3 fluid is 3.105, 3.091, and 3.062,

55

respectively. The designed relative permittivity(εr) of all fluids is calculated to be

used at 3.086.

267±1 mm

351±1 mm

300 mm

25±1

25±1

17±1

7±1 7±1

17±1 60

FIGURE 3-8 Rolling of capacitor element

Figure 3-8 presents a design of the capacitor element. These elements are made

from aluminum foils with thickness of 6 µm and 316 mm wide. The selected

dielectric is a polypropylene film, double-sides rough designed. It is 17.8 µm thick by

weight, 300 mm wide, with total thickness (with space factor) of 19.58 µm. The

film’s permittivity is 2.2 ( )2.2r =ε . By the way, a wound element has a diameter of 60

mm of rolled film. For the first winding, polypropylene film was rolled in 4 turns, and

placed at the center. 25 mm wide aluminum foils used as extended terminals were

placed at both sides of the rolled film. Each aluminum foil was folded to have a

diameter of 7 mm.

For element winding process, a diameter-adjustable roller was used in order that

an element could be pulled out from the roller. There are 35 turns per one element in a

winding. After the process, an element was pulled out and then pressed as shown in

Figure 3-9. Terminals were then placed at each side by using copper wires weaved

together with aluminum foils, which were overlapped. By finishing the process, we

56

obtained one capacitor element. In our element design, some figures can be obtained

by the calculation. From Figure 3-8 , The rolling of capacitor element and below that are

specifications of the designed element taken into our consideration, Tpg Thickness by weight of polypropylene film (17.8 µm)

Tpm Micrometer thickness of polypropylene film (19.58 µm at SF = 10 %)

Wp Width of polypropylene film (300 mm)

Wa Width of aluminum foil (316 mm)

Wpe Width of polypropylene film edge between aluminum foil (17 mm)

We Width of folded edge foil (7 mm)

Wct Width of capacitor terminal (25 mm)

Nps Number of turns in first winding of polypropylene film (4)

Npe Number of turns in last winding of polypropylene film (4)

DR Roller diameter with polypropylene film (60 mm)

A= LW Area of capacitor plate (m2)

where; 20 °C Relative permittivity of polypropylene (εr1) = 2.2,

Relative permittivity of sunflower oil (ε21) = 3.105,

Relative permittivity of soybean oil (ε22) = 3.091,

and Relative permittivity of Envirotemp FR3 fluid (ε23) = 3.062,

hence, The average of relative permittivity of three vegetable oils (ε2) = 3.086

According to section 2.6.3 in previous chapter, resultant permittivity (εres), when

taken into Eq. 2-23, can be obtained as εres = 2.259

From Eq.2-23, the length of aluminum foil can be defined as following;

W××10×854.8×2T×S×C

=Lres

12-pmp

ε

where L Average length of aluminum foil used with capacitor (m)

Tpm Micrometer thickness of polypropylene film = 19.58 ×10-6 m

W Width of capacitor plate = 0.267 m

57

Sp Layers number of polypropylene film dielectric = 2

(from designed)

therefore, 267.0259.210854.82

1058.192109.1L 12-

66

××××××××

=−−

L = 6.966 m

from Eq. 2-24, 2D

+L=L 1T

π

Where the term LT is the total length of aluminum foil

from Eq.2-26 pmppsRo TSN4DD += m

where, Do Roller diameter with start only polypropylene film (m)

D1 Average diameter of first aluminum winding (m)

DR Roller diameter = 6 × 10-2 m

(Source: Nissin Electric (Thailand) Co., Ltd.)

Nps Number of turns in first winding polypropylene film = 4

06062.01058.1924406.0D 6o =××××+= − m

As per Eq. 2-28, apmpo1 T3TS2DD ++= m

(3Ta is referred to folded edge foil)

where, Ta = Thickness of aluminum foil = 6 × 10-6 m

661 10631058.192206062.0D −− ××+×××+=

D1 = 0.06071 m

061.720607.0π966.6LT =

×+= m

from Eq. 2-27, apmp T3TS2T += m

Therefore 566 10632.910631058.1922T −−− ×=××+×××= m

58

In addition, the number of turns wound in aluminum foil can be defined from

Eq. 2-31 as; n = Number of turns wound in aluminum foil.

1TD5.0

TL1

TD5.0n 1

21

T2

1 −⎟⎠⎞

⎜⎝⎛−

⎥⎥⎦

⎤

⎢⎢⎣

⎡

π+

⎭⎬⎫

⎩⎨⎧

−⎟⎠⎞

⎜⎝⎛= turn

110632.9

0607.05.010632.9

06.7110632.9

0607.05.0n 5

21

5

2

5 −⎟⎠⎞

⎜⎝⎛

×−

⎥⎥⎦

⎤

⎢⎢⎣

⎡

××π+

⎭⎬⎫

⎩⎨⎧

−⎟⎠⎞

⎜⎝⎛

×= −−− turn

( )[ ] 0955.31427.233310955.314n 21

2 −+= turn

[ ] 0955.31427.2333198.98655n 21−+= turn

( ) 0955.31425.121987n 21−= turn

0955.31426.349n −= = 35.17 turn

Thus, total number of turns wound in aluminum foil is chosen to be 35 turn.

3.3.3 Calculation of capacitor element thickness and width From Eq. 2-34 TE = Element thickness and Nsp = Nep = 4, Sp = 2

(double layers of PP film)

TE = 32Tpm+ 2n(4Tpm+3TA) + 2 Nkp ×Skp×Tkp where, Tkp = Kraft paper thickness

Nkp = Number of turns wound in insulating Kraft paper

Skp = Number of layers in insulating Kraft paper

Tkp = 50 µm, Skp = 3, and Nkp = 6;

Capacitor element thickness (TE) = 9.168 mm, and two pressboards with

0.5Tpb = mm are applied; complete capacitor element thickness (TCE) = 19.168

mm. If without kraft paper; a capacitor element thickness are shown in Figure 3-9

it has capacitor element thickness = 7.368 mm.

59

In this research, two element thickness (TE) = 9.0 mm and 9.4 m are

selected so that space factors of model elements can be divided into 2 terms. Distance

was measured inside between two pressboards.

Calculation of capacitor element width (WE) can be defined as;

from Eq. 2-36 ER

E T2DπW +=

therefore, Capacitor element width, 168.9260πWE +

×=

WE = 103.42 mm

Capacitor element width (WE) = 103.42 mm

Apart from the design mentioned, the dimension of our model capacitor element

was arranged. It is 103.4 mm wide, 3,500 mm long, with the thickness of 9.17 mm.

The entire dimension of capacitor element is shown in Figure 3-9.

Finally, after we obtained the dimension and specifications of the element, all

information were sent to a manufacturer, Nissin Electric (Thailand) Co., Ltd., for the

production of these model elements.

FIGURE 3-9 Capacitor element

60

3.4 Correction factor of capacitance for the reference temperature

Due to the surrounding temperature of model capacitors, relative permittivity of

polypropylene and vegetable oil dielectric was changed. εr value of polypropylene

was changed from 2.2 at 20 °C to 2.15 at 100 °C as shown in Table 2-3. In addition, εr

values of vegetable oils were changed according to figures shown in Table 3-2. A

calculation was made to achieve the rate of change in relative permittivity of

polypropylene which was constantly linear from 5°C to 100 °C as expressed in Table

3-4. Every step of temperature the Resultant permittivity (εres) are calculated by

Eq.2-19 that the volume of polypropylene v1 = 0.9091 and volume of vegetable oils

v2 = 0.0909 are used.

From Eq.2-22 C = K× εres μF Eq.3-1

As pmp

6

TSWL10854.82K

×××××

=−

Eq.3-2

where is L = 6.97 m , W = 0.267 m, Sp = 2 and Tpm =19.58 μm

8452.058.192

W267.097.610854.82K6

=×

××××=

−

All capacitance values at different temperature of these three vegetable oils

impregnated polypropylene film capacitor were calculated, while k = C20°C/ Ct as

shown in Table 3-4. Changes in capacitance of these values are plotted in Figure 4-1

to Figure 4-3. When a correction is applied, the reference temperature for the

consideration is + 20 °C , unless otherwise it agreed the manufacturer and purchaser

or standard.

T

AB

LE

3-4

C

orre

ctio

n fa

ctor

of c

apac

itanc

e fo

r 20

°C r

efer

ence

tem

pera

ture

Tem

pera

ture

(°C

) 5

10

15

20

25

30

35

40

50

60

70

80

90

100

Rel

ativ

e pe

rmitt

ivity

of

Pol

ypro

pyle

ne; ε

r1

2.20

94

2.20

63

2.20

31

2.2

2.19

68

2.19

37

2.19

06

2.18

75

2.18

13

2.17

5 2.

1687

2.

1625

2.

1562

2.

15

Rel

ativ

e pe

rmitt

ivity

; εr2

1

3.21

1 3.

175

3.14

3.

105

3.07

1 3.

039

3.00

9 2.

978

2.92

3 2.

882

2.84

5 2.

807

2.77

3 2.

751

Res

ulta

nt

perm

ittiv

ity; ε

res1

2.

2739

2.

2692

2.

2645

2.

2599

2.

2552

2.

2506

2.

2461

2.

2416

2.

2328

2.

2246

2.

2166

2.

2086

2.

2007

2.

1936

Cap

acita

nce

(μF)

C

= K

*εre

s1

1.92

18

1.91

80

1.91

39

1.90

99

1.90

59

1.90

21

1.89

83

1.89

45

1.88

70

1.88

01

1.87

34

1.86

66

1.85

99

1.85

39

Sunf

low

er

oil

Cor

rect

ion

fact

or

to 2

0°C

; k1

0.99

38

0.99

58

0.99

79

1.00

00

1.00

21

1.00

41

1.00

61

1.00

82

1.01

21

1.01

59

1.01

95

1.02

32

1.02

69

1.03

02

Rel

ativ

e pe

rmitt

ivity

;εr2

2 3.

245

3.19

3 3.

143

3.09

1 3.

044

2.99

9 2.

965

2.94

1 2.

896

2.86

5 2.

841

2.81

6 2.

793

2.76

8

Res

ulta

nt

perm

ittiv

ity; ε

res2

2.

2754

2.

2701

2.

2647

2.

2592

2.

2538

2.

2486

2.

2439

2.

2397

2.

2313

2.

2237

2.

2164

2.

2091

2.

2018

2.

1945

Cap

acita

nce

(μF)

C

= K

*εre

s2

1.92

31

1.91

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1.91

40

1.90

94

1.90

48

1.90

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1.89

64

1.89

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1.88

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1.87

93

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1.86

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; k2

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1.00

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1.00

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47

1.00

68

1.00

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25

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2.

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2.

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2.

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2.

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2.

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2.

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2.

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1.91

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1.90

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1.90

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1.89

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1.89

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1.89

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1.88

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1.87

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1.87

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rote

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61

62

3.5 Assembly of model capacitor

3.5.1 Model capacitor tank

The capacitor tank (or the container) for vegetable oil impregnation is 63 mm

wide, 150 mm long, with its height of 470 mm. Please note that all of precedent

dimension is inside the container. The design of this tank, made from 5 mm of

thickness stainless steel, is showed in Figure 3-10.

(a) (b)

FIGURE 3-10 Model capacitor

Figure 3-10 shows model capacitor diagram. In picture (a), the model capacitor

designed was during impregnation process, which consists of a cover sealed in

vacuum condition. Picture (b) illustrates a complete impregnated model capacitor

which contains the fluid inside.

For safety concern, a gap between each capacitor element must be at least 10

mm when these elements are put into the capacitor tank. From the particular design of

this research, one capacitor tank can hold capacitor elements up to two units.

63

3.5.2 Capacitor element pressing process: Before impregnating vegetable oils,

it is necessary to press these capacitor elements so that they will be exactly fitted in

the tank. Referring to previous related researches, there are two sizes of pressed

thickness in our consideration; 9.0 mm and 9.4 mm as showed in Figure 3-11. The

actual picture of this process is shown in Figure 3-12.

In our pressing process, there are two papers placed in between an element. The

capacitor element used in this step can be achieved from previous winding and

pressing process. Terminals at the end of both sides were curved out because their

aluminum foils ended were folded to 6 mm thick. At this stage, the dielectric with

total thickness of 50 micron was wound around a model capacitor element at 18 turns

FIGURE 3-11 Element pressing diagram

(a)

(b)

9.0 mm

9.4 mm

64

to compensate the gap inside. Then it was pressed to the designed thickness of 9.0

mm and 9.4 mm. In measuring pressed thickness, vernier meter was used as a

measuring tool. When achieve thicknesses mentioned, elements were wrapped to

prevent themselves from loosening.

Figure 3-11 (a) illustrates a side view of the wound element. After the winding

process, capacitor elements were pressed with two different thicknesses (9.0 and 9.4

mm). From figure 3-11 (b), it is remarkable that pressing elements without any

arrangement can cause a large gap between capacitor element and papers inside. To

solve this unanticipated gap, papers were wound firstly and put in between the

element to compensate the lack of this structure. This made the element being

proportionally pressed and left no space between.

Figure 3-12 illustrates a side view of the wound element. After the winding

process, capacitor elements were pressed with two different thicknesses (9.0 and 9.4

mm). From figure 3-13 it is remarkable that pressing elements without any

arrangement can cause a large gap between capacitor element and papers inside. To

solve this unanticipated gap, papers were wound firstly and put in between the

element to compensate the lack of this structure. This made the element being

proportionally pressed and left no space between.

FIGURE 3-12 Capacitor element pressing process

65

FIGURE 3-13 Insert element to model capacitor

3.5.3 Heating capacitor element

A complete model capacitor can be moisture contained at its surface area, which

leads to the decline in capacitance tolerance. Thus, when elements were placed into

the capacitor tank, a cover made from stainless steel should be sealed. Two valves

were connected to holes on the cover to begin moisture removal along with the

heating process.

At this stage, the element was placed in the container as shown in Figure 3-13.

When impregnating a model capacitor, moisture in each capacitor element should be

removed along with the process. Moisture removal process was operated by heating at

80 ± 5 °C and vacuuming with double-stage vacuum pump for 48 hours.

At this particular step, heat was transferred from 1500 watts spotlight used as a

heat source beside the element container. In controlling required temperature, a

thermometer was placed at the element tank. When it reached to the desired

temperature, heat was reduced by variable transformers shown in Figure 3-15.

66

3.6 Impregnation process

FIGURE 3-14 Impregnation process diagram

FIGURE 3-14 Impregnation process diagram

After model capacitors were well provided, these units would be ready for the

next process. Three impregnants; purified sunflower oil, soybean oil and Envirotemp

FR3 fluid were impregnated into each model unit under four different levels of

temperature. As previously mentioned, it is necessary to remove moisture in capacitor

elements by heating them at 80 ± 5°C and vacuuming with two-stage vacuum pump

throughout 48 hours.

Figure 3-14 shows an idea of moisture removal via heating process, which was

processed in the boiler. In a theoretical basis this impregnation process must be done

just through free ideal space, which cannot be accessible in reality. Thus, the process

was conditioned under a vacuumed pressure by two-state vacuum pump, instead.

67

Before impregnating them, specific vacuumed pressure should be down to 0.1 Torr.

In this process, oil storage reservoir was used to contain insulating oil. The insulating

oil passed through an electrical heater to heat the oil before flowing into the capacitor.

This electrical heater then heated up the oil to the setup temperature, after that the hot

oil flew into the model unit. Processing temperature can be checked by a thermometer

at the oil path and can be controlled manually. The temperature set point was setup

previously according to the scope of this study. Meanwhile, the capacitor was

conditioned at vacuumed pressure down to 0.1 Torr until the insulating oil was fully

filled into the tank.

FIGURE 3-15 Impregnation process system

It is remarkable that during this stage, the flow rate of this hot fluid would be

extremely slow and expected to be fully filled into the capacitor within 2 hours. When

this process was done, the overflowed oil would be shown in a clear chamber; the

whole processes was then automatically halted and wait until next 24 hours to restart

68

the whole process again. While in the process, pressure was always controlled as

vacuumed.

When the impregnation process was accomplished, the cover of each capacitor

was changed for additional connects at both poles as shown in Figure 3-16.

Afterwards, the unit would be ready for the next process as per IEC 60871-1 standard.

FIGURE 3-16 Model capacitors

In Figure 3-16 above, complete model capacitors are shown. After these model

units were achieved, they were then ready for the final investigation, our experimental

test.

3.7 Experimental test

The IEC 60871-1 standard aims to stipulate in electrical properties of high

voltage capacitors as discussed in chapter 2. For this research, there is an arrangement

in our experimental test for its capacitance, AC withstand voltage between terminals,

AC withstand voltage between terminals and the container, measurement of the

tangent of loss angle (tan δ) in each model capacitor, and its short circuit discharge

test.

69

3.7.1 Capacitance measurement.

All experimental tests in this study are according to the capacitance testing

methods as per IEC 60871-1 standard. This standard is applied to shunt capacitors for

AC power system having a rate voltage above 1000 V. Vegetable oil impregnated

polypropylene capacitors were preliminary measured with the voltage at 0.15 UN by

RLC bridge meter as shown in figure 3-17. Another voltage was measured at 0.9 to

1.1 times of the rated voltage, using the method that excludes errors due to harmonics

with Glynna bridge capacitance measurement as shown in Figure 3-18. Figure 3-19

demonstrates capacitance test with respect to four different impregnating temperatures

mentioned as a plotted graph previously shown in Figure 4-3. The preliminary

arrangement of capacitance and dissipation factor test is illustrated in Figure 3-20 (a)

and (b). The final capacitance measurement was carried out after the voltage test. The

accuracy of the measuring method is such that the tolerances can be met, and does not

differ from the rated capacitance by more than -5 % to +10 %.

FIGURE 3-17 RLC bridge meter and measurement

70

FIGURE 3-18 Capacitance and dissipation factor measurement at 1.5 kV.

FIGURE 3-19 Capacitance of capacitor at differential temperature test

71

FIGURE 3-20 Capacitance and dissipation factor test at 500 V for the first of test

3.7.2 Voltage test between terminals.

FIGURE 3-21 AC withstand voltage test between terminals

Every capacitor is subjected for 10 sec to both the test of AC test and DC test.

The AC test was carried out with a substantially sinusoidal voltage at 3.0 kV (Ut = 2.0

UN, UN= 1.5 kV), while the DC test was carried out with a substantially sinusoidal at

6.0 kV (Ut = 4.0 UN, UN= 1.5 kV), is shown in figure 3-21 and 3-22.

(a) (b)

72

FIGURE 3-22 DC withstand voltage test between terminals

3.7.3 AC voltage test between terminals and container.

Capacitor unit having all terminals insulated from the container is subjected for

10 sec to a test voltage applied between the terminals (joined together) and the

container. The test voltages is 13 kV applied. This process is shown in Figure 3-23.

3.7.4 Measurement of the tangent of the loss angle (tan δ) of the capacitor.

The capacitor losses (tan δ) was measured at 0.9 to 1.1 times rated voltage with

Glynna bridge capacitance measurement, using a method that excludes errors due

to harmonics. This testing method to find dissipation factor is shown in figure 3-20. In

capacitance and dissipation factor measurement, this equipment can be used to

measure and then calculate an actual value at the same time.

3.7.5 Short circuit discharge test.

The unit is charged by means of DC and then discharged through a gap situated

as close as possible to the capacitor. It is subjected to five such discharges within 10

minute. The test voltage is 3.75 kV (Ut = 2.5 UN). Within 5 minute after this test, the

unit is subjected to a voltage test between terminals. The capacitance was measured

before the discharge test and after the voltage test. The short circuit discharge test is

shown in figure 3-24.

73

FIGURE 3-23 Short circuit discharge test

CHAPTER 4

TESTING OF THE CAPACITORS

4.1 Capacitance of capacitors

For this research, The bridge meter is applied in order to measure capacitance

properties of model capacitors. Determining capacitance properties, there are two

attributes in our consideration, that is, capacitances and capacitance tolerances of

these units. Table 4-1 and Table 4-2 demonstrates capacitances of model units

measured at 30 °C, 0.9 to 1.1 times of rated voltage with Glynna bridge capacitance

measurement, with the element thickness of 9.0 and 9.4 mm, respectively.

TABLE 4-1 Capacitances of model units at the element thickness of 9.0 mm,

with rated capacitance of a capacitor designed at 1.9 µF at 30 °C

Capacitance (Cn) (µF) Vegetable oil

impregnating condition

Impregnating temperature

(°C) Measured at 30 °C

Corrected to 20 °C

Capacitance tolerance

(%)

60 1.930 1.938 + 2.00

70 1.974 1.982 + 4.32

80 2.000 2.008 + 5.69

Sunflower oil at 30°C

k =1.0041 90 2.010 2.018 + 6.22

70 1.739 1.747 - 8.04

80 1.822 1.831 - 3.65

90 1.904 1.913 + 0.68

Soybean oil at 30°C

k =1.0047 100 1.910 1.919 + 1.00

70 1.765 1.773 - 6.67

80 1.795 1.803 - 5.08

90 1.952 1.961 + 3.22

Envirotemp FR3 Fluid

at 30°C k =1.0047

100 1.983 1.992 + 4.86

76

TABLE 4-2 Capacitances of model units at the element thickness of 9.4 mm,

with rated capacitance of a capacitor designed at 1.9 µF at 30 °C

Capacitance (Cn) (µF) Vegetable oil

impregnating condition

Impregnating temperature

(°C) Measure at 30 °C

Correct to 20 °C

Capacitance tolerance

(%)

60 1.920 1.928 + 1.47

70 1.973 1.981 + 4.27

80 1.980 1.988 + 4.64

Sunflower oil at 30°C

k =1.0041 90 1.985 1.993 + 4.90

70 1.713 1.721 - 9.42

80 1.793 1.801 - 5.19

90 1.832 1.841 - 3.13

Soybean oil at 30°C

k =1.0047

100 1.761 1.769 - 6.88

70 1.744 1.752 - 7.78

80 1.783 1.791 - 5.72

90 1.935 1.944 + 2.32


at 30°C k =1.0047

100 1.971 1.980 + 4.22

As per testing methods, all measured temperature is required to be corrected

from 30 to 20 °C. In most cases, capacitances of model units tend to increase when

impregnating temperature is increased. From the overview, it was found that the most

satisfied model capacitor, with capacitance as the constraint, seems to be the unit with

element thickness of 9.0 mm rather than of 9.4 mm. Theoretically, this is right to the

statement that a capacitor with lower space factor should be superior to the other in

term of its capacitance property. Our test results also indicate that Envirotemp FR3

fluid is most favorable for this property, with sunflower oil as the worst one. This

might be because Envirotemp FR3 is the fluid developed to be used in electrical

applications but sunflower oil and soybean oil are necessary to be filtered and purified

to remove impurities contained. Remaining impurities i.e. small particles and water

content, if there is any, can then affect these measured figures, unanticipatedly.

77

T

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4-3

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1.

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2.

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1.

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2.

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1.

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30

1.

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1.

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1.

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1.

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1.

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1.

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1.

9004

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1.

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1.

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1.

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1.

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1.

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40

1.

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1.

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1.

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1.

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1.

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1.

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1.

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1.

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1.

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1.

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1.

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1.

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1.

9645

50

1.

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1.

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1.

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1.

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1.

9391

1.

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1.

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1.

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1.

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55

1.

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1.

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1.

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1.

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60

1.

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1.

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5 1.

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1.

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1.75

62

1.71

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1.83

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1.80

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1.91

54

1.83

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1.92

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1.75

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15

1.91

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1.74

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1.70

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1.82

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1.68

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1.80

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1.71

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1.76

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78

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5 1.

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1.

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1.

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1.

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1.

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1.

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1.

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2.

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10

1.

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1.

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1.

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1.

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1.

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1.

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2.

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1.

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15

1.

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1.

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1.

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1.

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1.

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1.

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1.

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1.

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1.

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1.

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25

1.

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1.

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1.

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1.

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1.

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1.

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1.

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1.

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1.

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1.

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1.

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1.

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1.

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1.

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1.

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1.

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1.

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1.

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1.

9453

55

1.

8819

1.

7133

1.

7372

1.

7690

1.

7532

1.

9150

1.

8978

1.

9663

1.

9386

60

1.

8788

1.

7126

1.

7363

1.

7668

1.

7515

1.

9137

1.

8965

1.

9629

1.

9339

79

80

1.85

1.90

1.95

2.00

2.05

2.10

0 10 20 30 40 50 60 70

Capacitor temperature ( °C)

Cap

acita

nce

(μF)

Calculate 60°C 60°C70°C 70°C 80°C80°C 90°C 90°C

FIGURE 4-1 Plotted capacitances of sunflower oil impregnated capacitors

at four different impregnating temperatures

1.65

1.70

1.75

1.80

1.85

1.90

1.95

2.00

0 10 20 30 40 50 60 70

Capacitor temperature (°C)

Cap

acita

nce

(μF)


FIGURE 4-2 Plotted capacitances of soybean oil impregnated capacitors


_______ Element thickness 9.0 mm _ _ _ _ _ Element thickness 9.4 mm


81

1.65

1.70

1.75

1.80

1.85

1.90

1.95

2.00

2.05

2.10

0 10 20 30 40 50 60 70

Capacitor temperature (°C)

Cap

acita

nce

(μF)


FIGURE 4-3 Plotted capacitances of Envirotemp FR3 impregnated capacitors


From Figure 4-1 to Figure 4-3, plotted capacitances of sunflower oil, soybean

oil and Envirotemp FR3 impregnated capacitors are illustrated. According to these

plotted graphs, it can be described that changes in capacitances of these three

vegetable oil are varied directly to their dielectric constants. Also, whenever these

capacitor temperature decrease the capacitances of all are increased the same

capacitance calculation, it means that fluids can be much more impregnated into

dielectrics. Plotted values of these charts can be found in Table 4-3 to Table 4-5 in

chapter 4. All type of vegetable oils that have lower space factor (9.0 mm element

thickness) are better than higher space factor (9.4 mm element thickness).

As subjected to the impregnation, capacitances of model units with sunflower

oil, soybean oil, and Envirotemp FR3 fluid impregnated, with four different

impregnating temperatures are expressed in Table 4-6 to Table 4-8, accordingly.

These tables and their plotted graphs are also shown in Figure 4-4 to Figure 4-6.

Capacitances of sunflower oil impregnated model capacitors and their plotted

graph measured at 1500 Vrms, 50 Hz, and 30 °C are shown in Table 4-6 and Figure 4-4.


82

According to test results, capacitances of 9.0 mm element thick model units

impregnated at 80 and 90°C are slightly higher than designed value (2.00 and 2.01 μF,

respectively). Also, capacitances of 9.0 mm element thick model units are higher than

ones with element thickness of 9.4 mm. The test results are shown in Table 4-3 to 4-5.

When compare capacitances from mathematical calculation to measured values,

it was found that there are significant differences between them but in the same

parallel direction. This might be due to physical properties of impregnants which

affect test results other than ideal calculations. A change in impregnating temperature

leads to higher relative permittivity because the spacing between two plates is

changed which is affected by an impregnating fluid inside.

TABLE 4-6 Capacitances of sunflower oil impregnated model capacitors


Capacitance (μF) Impregnating temperature

( °C )

Oil Dielectric breakdown (D877)

(kV) Element thickness

9.0 mm Element thickness

9.4 mm 60 57.34 1.930 1.920 70 57.34 1.974 1.973 80 53.52 2.000 1.980 90 57.34 2.010 1.985

1.60

1.70

1.80

1.90

2.00

2.10

2.20

50 60 70 80 90 100Impregnating temperature ( °C)

Cap

acita

nce

(µF).

Element thickness 9.0 mmElement thickness 9.4 mm

FIGURE 4-4 Plotted capacitances of sunflower oil impregnated model capacitors


83

Table 4-7 and Figure 4-8 shows capacitances and a plotted graph of these values

for soybean oil impregnated model units at four different impregnating temperatures

measured at the same condition. Please note that there is a decline of a measured

capacitance impregnated at 100 °C (1.761 μF) for the model unit with element

thickness of 9.4 mm. Based on theoretical assumption, the failure might be caused by

element pressing process so that it is not similar to the direction of model units with

element thickness of 9.0 mm.

TABLE 4-7 Capacitance of soybean oil impregnated model capacitors


1.50

1.60

1.70

1.80

1.90

2.00

2.10

60 70 80 90 100 110

Impregnating temperature ( °C)

Cap

acita

nce

(µF)

.


FIGURE 4-5 Plotted capacitances of soybean oil impregnated model capacitors



( °C )


(kV) Element thickness 9.0 mm

Element thickness 9.4 mm

70 50.00 1.739 1.713 80 46.96 1.822 1.793 90 46.76 1.904 1.832 100 55.48 1.910 1.761

84

Similarly, Table 4-8 and Figure 4-6 shows capacitances and a plotted graph of

units impregnated with Envirotemp FR3 at four different temperatures measured at

the same condition. There is an abrupt change of the capacitances in both element

thicknesses at the impregnating temperature ranged between 80 and 90 °C (from

1.795 to 1.952 μF in 9.0 mm element thick units and 1.783 to 1.935 μF in 9.4 mm

element thick units). This change indicates the most favorable range of temperature

for impregnating Envirotemp FR3 fluid into model units. The maximum capacitance

of 9.0 and 9.4 mm element thick model unit at 100 °C is 1.983 and 1.971 μF.

TABLE 4-8 Capacitances of Envirotemp FR3 fluid impregnated model capacitors



( °C )




9.4 mm 70 40.27 1.765 1.744 80 40.37 1.795 1.783 90 37.12 1.952 1.935 100 37.12 1.983 1.971

1.60

1.70

1.80

1.90

2.00

2.10

2.20

60 70 80 90 100 110


Cap

acita

nce

(µF)

.


FIGURE 4-6 Plotted capacitances of Envirotemp FR3 impregnated model

capacitors at four different impregnating temperatures

85

1.60

1.70

1.80

1.90

2.00

2.10

2.20

50 60 70 80 90 100 110


Cap

acita

nce

(µF)

Sunflower oil Soybean oil


FIGURE 4-7 Plotted capacitances of vegetable oils impregnated model

capacitors for element thickness 9.0 mm

1.60

1.65

1.70

1.75

1.80

1.85

1.90

1.95

2.00

2.05

2.10

50 60 70 80 90 100 110


Cap

acita

nce

(µF)

.



FIGURE 4-8 Plotted capacitances of Vegetable oils impregnated model

capacitors for element thickness 9.4 mm

86

Figure 4-7 and Figure 4-8 illustrates the overview of capacitance property in oil

impregnated model capacitors with the elements pressed to 9.0 and 9.4 mm thick,

respectively. A fluid with good measured capacitances is sunflower oil impregnated

start at 70 °C. As mentioned, these values are higher than the designed value (1.9 μF)

but are still in the acceptance level. Envirotemp FR3 fluid, however, is in a good

shape of its trend when considered to the change in impregnating temperatures.

Due to the scope of this study, we did not determine the effect of impregnating

temperature larger than 100 °C according to the fact that at that range of temperature,

other elements used in electrical applications can be damaged as well. Anyway, one

proven fact from results is that we can increase capacitance property by reducing the

element thickness (or so-called space factor) of a capacitor.

4.2 Voltage test between terminals, and between terminals and container

4.2.1 Voltage test between terminals

The test and measurement of AC withstand voltage test has been done between

terminals of model capacitors. In each model unit, our test was taken for 10 seconds

both in AC and DC applied voltage. The AC test was carried out with a substantially

sinusoidal voltage at 3.0 kV (Ut = 2.0 UN, UN= 1.5 kV), while the DC test was done

under a direct current voltage at 6.0 kV (Ut = 4.0 UN, UN 1.5 kV). The detailed testing

methods, designed process, and plan of voltage test between terminals are discussed

previously in chapter 3.

All vegetable impregnated polypropylene film capacitors, pressed in different

space factors and impregnated at four different temperatures, were passed with

favorable results both in AC and DC withstand voltage test. All soybean oil and

Envirotemp FR3 fluid impregnated polypropylene film model units were also

qualified in both tests as well.

4.2.2 Voltage test between terminals and container

In AC withstand voltage test between terminals and container (capacitor tank),

every model unit was subjected to a test interval of 1 minute. This test was carried out

with a substantially sinusoidal voltage at 8 kV (standard insulation levels for Um < 52

kV – series I). According to test results, it was found that all model units were

qualified without any failure, also.

87

4.3 Dissipation factor of capacitor

Dissipation factor is the specific value of tangent of the loss angle (tan δ) of

model capacitors. In our dissipation factor test, capacitor losses (tanδ) were measured

at 0.9 to 1.1 times rated voltage with Glynna bridge capacitance measurement, using a

method that excludes errors by harmonizing model units. All model units were

measured at 1500 Vrms, 50 Hz, 30 °C.

Envirotemp FR3 fluid, in this case, is the ready-to-use fluid which had been

improved in its dissipation factor (or dielectric loss) by the manufacturer as can be

seen in the manufacturer specifications. Thus, it was used particularly as the reference

to compare with two treated vegetable oils; sunflower oil and soybean oil.

Dissipation factors of sunflower oil impregnated model units are shown in Table

4-9. Additionally, these figures are plotted and demonstrated in Figure 4-9 below.

According to test results, dissipation factors of sunflower oil impregnated model units

with the element thickness of 9.4 mm are higher than those with the element thickness

of 9.0 mm. Please note that from 60 to 70 °C impregnating temperature, there is an

abrupt incline of dissipation factor (%) from 0.0264 to 0.0418 in the units with

element thickness of 9.0 mm and from 0.0306 to 0.0581 in the units with element

thickness of 9.4 mm. This indicates the poor impregnating temperature at 60 °C of

sunflower oil through the dielectric of model units due to higher viscosity of the fluid,

which leads to the unsuitable impregnation process of the model unit.

TABLE 4-9 Dissipation factors of sunflower oil impregnated model capacitors


Dissipation factor (%) Impregnating temperature

( °C )

Oil dielectric breakdown (D877)



9.4 mm 60 57.34 0.0264 0.0306

70 57.34 0.0418 0.0581

80 53.52 0.0437 0.0635

90 57.34 0.0468 0.0698

88

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

50 60 70 80 90 100


Diss

ipat

ion

fact

or (%

)Element thickness 9.0 mmElement thickness 9.4 mm

FIGURE 4-9 Plotted dissipation factors of sunflower oil impregnated model


Dissipation factors of soybean oil impregnated model units at four different

impregnating temperatures, measured at the same condition are showed in Table 4-10

and Figure 4-10. Though trends of these factors are mostly the same, the 9.0 mm

element thickness impregnated at 90 °C is very well because it less of dissipation

factor . Moreover, units with element thickness of 9.4 mm tend to be highly inclined,

which is caused by the space factor of the units in pressing process.

TABLE 4-10 Dissipation factors of soybean oil impregnated model capacitors



( °C )




9.4 mm 70 50.00 0.0252 0.0218

80 46.96 0.0281 0.0357

90 46.76 0.0367 0.0552

100 55.48 0.0367 0.0793

89

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

60 70 80 90 100 110Impregnating temperature ( °C)

Diss

ipat

ion

fact

or (%

)



FIGURE 4-10 Plotted dissipation factors of soybean oil impregnated model


Likewise, dissipation factors of Envirotemp FR3 fluid impregnated model units

are shown in Table 4-11. From a plotted graph in Figure 4-11, it was found that most

of dissipation factors of 9.4 mm element thick model units are greater than those of

9.0 mm. However, the greatest value of dissipation factor (0.0645 %) in the 9.4 mm

element thick unit impregnated at 100 °C is the lowest value, which indicates an

excellent performance of this fluid when impregnating temperature is increased.

TABLE 4-11 Dissipation factors of Envirotemp FR3 fluid impregnated model



( °C )

Oil dielectric breakdown (D877)



9.4 mm

70 40.27 0.0144 0.0145

80 40.37 0.0283 0.0372

90 37.12 0.0327 0.0623

100 37.12 0.0354 0.0645

90

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

60 70 80 90 100 110

Impregnate Temperature ( °C)

Diss

ipat

ion

fact

or (%

)Element thickness 9.0 mmElement thickness 9.4 mm

FIGURE 4-11 Plotted dissipation factors of Envirotemp FR3 fluid impregnated

model capacitors at four different impregnating temperatures

To summarize all measured results, these figures are plotted and illustrated in

Figure 4-12 and Figure 4-13. All capacitances of model capacitors impregnated with

each fluid at four impregnating temperatures, with element thickness of 9.0 and 9.4

mm are shown, respectively. According to test results, it should be noted that units

impregnated with soybean oil at 100°C has highest dissipation factor (0.0793).

The overview of these results is that, when thickness of the element is in our

consideration, the dissipation factor of the unit with thinner pressed element is lower

than the other. This indicates that the dissipation factor will be increased when the

thickness of the element inside is increased, also.

In addition, it should be noted from Figure 4-13 that dissipation factors of

soybean oil impregnated units with the element thickness 9.4 mm tend to increase

continuously when impregnating temperature is increased while units impregnated

with other two fluids at any element thickness, or impregnated with this fluid at the

element thickness of 9.0 mm, seems to be saturated with a small change at their last

impregnating temperature This can be interpreted that dielectric loss of soybean oil

will consistently increase whenever impregnating temperature is increase, too.

91

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

50 60 70 80 90 100 110


Diss

ipat

ion

fact

or (%

)



FIGURE 4-12 Plotted dissipation factors of model capacitors impregnated with

vegetable oils for element thickness 9.0 mm

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

50 60 70 80 90 100 110


Diss

ipat

ion

fact

or (%

)

Sunflower oil Soybean oilEnvirotemp FR3 fluid

FIGURE 4-13 Plotted dissipation factors of model capacitors impregnated with

vegetable oils for element thickness 9.4 mm

92

4.4 Short circuit discharge test

In this test, the measurement has been done with the setup condition at 2.5 UN

(3.75 kV) and 25 °C, at four different impregnating temperatures with model units.

Table 4-12 and Figure 4-14 below shows the changes in capacitance of model

capacitors impregnated with four different temperatures in each impregnant.

TABLE 4-12 Summary of changes in capacitance of model capacitors impregnated

with three fluids at four different impregnating temperatures

Capacitance (Cn) (μF) Vegetable oil

impregnated type

Impregnated temperature

(°C) Before discharge

After discharge

Capacitance differences

(%)

60 1.9146 1.9259 0.59 70 1.9612 1.9690 0.40 80 1.9909 2.0001 0.46


90 2.0059 2.0125 0.33 60 1.9078 1.9185 0.56 70 1.9607 1.9693 0.44 80 1.9911 1.9992 0.41

Sunflower oil


90 1.9752 1.9842 0.46 70 1.7240 1.7455 1.25 80 1.7995 1.8223 1.27 90 1.8840 1.9014 0.92


100 1.8933 1.9086 0.81 70 1.6772 1.7011 1.42 80 1.7672 1.7900 1.29 90 1.8077 1.8270 1.07

Soy bean oil


100 1.7245 1.7491 1.43 70 1.7427 1.7691 1.51 80 1.7787 1.7981 1.09 90 1.9394 1.9556 0.84


100 1.9722 1.9846 0.63 70 1.7167 1.7469 1.76 80 1.7701 1.7830 0.73 90 1.9237 1.9406 0.88



100 1.9602 1.9708 0.54

93

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

2.20

2.40

50 60 70 80 90 100 110


Cap

acita

nce

diffe

rent

(%)

Sunflower oil Sunflower oilSoybaean oil Soybaean oilEnvirotemp FR3 Fluid Envirotemp FR3 Fluid

FIGURE 4-14 Plotted changes in capacitance of model capacitors impregnated

with three fluids at four different impregnating temperatures

From test results, it was found that the largest change in capacitance of

sunflower oil impregnated model unit is at 60°C with the element thickness of 9.0 mm

(0.59%) and the minimum change of the unit is at 90°C with the element thickness of

9.0 mm (0.33%).

For soybean oil impregnated model units, the largest change in capacitance is at

100°C with the element thickness of 9.4 mm (1.43%) and the minimum change of the

unit is at 70°C with the element thickness of 9.0 mm (0.92%).

Finally, in Envirotemp FR3 fluid impregnated model units, the highest change

in capacitance of the fluid impregnated model unit is at 70°C with element thickness

of 9.4 mm (1.76%) and the minimum change unit is impregnated at 100°C with the

element thickness of 9.4 mm (0.54%).


94

According to the overall results, the model unit with the most change in

capacitance among these three fluids is the Envirotemp FR3 impregnated capacitor,

with the element thickness of 9.4 mm and impregnated at 70°C (1.76%). This is

because of insufficient pressing of the unit. On the other hand, the model unit with the

least change in capacitance among these three fluids is the sunflower oil impregnated

one, with the element thickness of 9.0 mm and impregnating temperature at 90°C

(0.33%). The reason is that when pressing the unit to the specified thickness, the fluid

cannot penetrate into the dielectric thoroughly. Although a model unit can be

damaged more easily when its discharge difference is high, we cannot conclude that a

unit with the highest discharge difference is worst due to reasons above. Moreover, to

consider the performance of a capacitor, other electrical properties, i.e. capacitance

and dissipation factor, should be taken into account, too.

CHAPTER 5

CONCLUSION AND RECOMMENDATIONS

5.1 Conclusion

During a few decades, polypropylene all-film power capacitors impregnated

with fluids made from biodegradable and non-toxic vegetable oils are of interest

among researchers world-wide. This study delicates to the design and test of model

capacitors impregnated with three different vegetable oils; sunflower oil, soybean oil,

and Envirotemp FR3 fluid. The main purpose is to investigate and evaluate model

units impregnated with these fluids, then find out the best circumstance in generating

a prototype of power capacitor.

Particularly in this research, there are three main processes; model design, the

production, and electrical property test of model capacitors. In our design process,

materials were well selected in order to achieve a favorable result. Polypropylene,

two-side roughened film with thickness of 17.8 μm by weight was chosen as

dielectrics in model capacitors. During the production of model units, it was necessary

to provide purifying treatments for two vegetable oils, i.e. sunflower oil and soybean

oil, before impregnating into the capacitor tank. Purification system was utilized to

remove particles and water content in fluids. Impregnation process has been done

carefully under certain vacuum condition. In the last stage, an experimental test was

carried out with thoughtful attentions. Also, measuring equipments such as the

dissipation factor tester were well standardized to obtain accurate and error-free

outcomes. All testing methods applied in this research is referred to the International

standard, IEC 60871-1 Shunt capacitor for AC power systems at a rated voltage above

1000 V.

According to the test result, it was found that impregnant types, impregnating

temperature, and space factor or thickness of the element are major factors which

affect electrical properties of model capacitors. There are four electrical properties of

model capacitors which are taken into considerations; capacitance, withstand voltage,

96

dissipation factor, and short-circuit discharge. All measured temperature was

corrected from 30 to 20 °C in accordance with standard testing methods.

In term of capacitance, model capacitors were measured by Bridge meter. The

designed units are subjected to the test with designed capacitance rated at 1.9 μF at 30

°C. From test results, sunflower oil was found to be the worst impregnant in term of

capacitance property. It impregnated not well at low temperature range, but should be

noted that model units with element thickness of 9.4 mm impregnated at 80 and 90 °C

its capacitances were higher than 2.0 μF (2.003 and 2.013 μF, respectively). The most

favorable results are from model units impregnated with Envirotemp FR3 fluid with

both element thickness of 9.0 mm (1.956 μF at 90 and 1.987 μF at 100 °C) and 9.4

mm (1.939 μF at 90 and 1.975 μF at 100 °C). For soybean oil, it was found that at 100

°C impregnating temperature their capacitances were declined in both units with

element thickness of 9.0 and 9.4 mm (1.939 μF and 1.975 μF, respectively).

In withstand voltage test between terminals intended to determine the failure of

model units, the AC test is carried out with a substantially sinusoidal voltage at 3.0

kV while the DC test is done under a direct current voltage at 6.0 kV. From test

results, it was found that all units are qualified without any problem.

For the dissipation factor or dielectric loss test, all model units are conditioned

and measured at 1500 Vrms, 50 Hz, 30 °C. The highest value was from 9.4 mm

element thick model unit impregnated with soybean oil at 100 °C (0.0793%), and the

lowest value was from 9.0 mm element thick model unit impregnated with soybean

oil at 70 °C (0.0144%). From the overview, it can be concluded that Envirotemp FR3

fluid is the most reliable impregnant which caused minimum loss to model units.

The measurement of our last investigation, short circuit discharge test, has been

done with the setup at 2.5 UN and 25 °C. In this case, we considered the difference of

capacitance between before and after discharge in each impregnant. From test results,

the lowest difference was found in 9.0 mm element thick model unit impregnated with

sunflower oil at 90 °C (0.33%) which might be caused by the poor impregnation of

the fluid itself because most of the differences in units impregnated with this fluid

were nearly be the same Though the highest difference was found in 70 °C

Envirotemp FR3 impregnated unit with element thickness of 9.4 mm (1.76%), the

97

overall value of this impregnant was in good shape when impregnating temperature

was increased and thus found to be reliable.

Even if there was no failure happened in this study, the purification and element

pressing process were found to be one of the most important factor affect with test

results. Insufficient fluid filtration can lead to non-throughout penetration of a fluid to

a dielectric inside, which consequently brings to unsatisfied test result, also. Though

model units can achieve better performance in the element with lower thickness, over-

pressing to obtain thinner element leads to inferior impregnation by reducing the

penetration rate of a fluid. Also, It should be noted that under-pressing of the element

can cause unfavorable result in short-circuit discharge as shown in chapter 4. When

pressed strength of the element is not sufficiently enough, the penetration of

impregnant through a dielectric cannot be done effectively. This phenomenon leads to

larger change in capacitance, the difference of before and after discharge, which can

be brought to the damage of our model capacitor.

When compared between these fluids, there are conclusions that, firstly, all

fluids can achieve designed capacitance, thus can be considered as suitable

impregnants along with proper element thickness. Secondly, the lower the space

factor, the better performance of fluid, however, a fluid cannot penetrate well in

extremely low space factor (lower than 9.0 mm thick in this case). Thirdly, sunflower

oil can be impregnated at lower temperature (70 ºC), thus might be considered as

superior impregnant to other two fluids. Finally, higher space factor may cause a

decrease in capacitance and an increase in dissipation factor, leads to unexpected

failure

5.2 Recommendations for future work

From the experience in our research, there are several recommendations which

are expected to be useful in any area of study involved with a power capacitor

impregnated with vegetable oil as following;

5.2.1 It is recommended to future research on including the aging test to

investigate electrical properties; capacitance, dissipation factor, withstand voltage,

and short circuit discharge of model units impregnated with the similar vegetable oils.

98

Aging test can be used as a shortcut in accelerating the life of capacitors, which is

useful when consider about their life cycle in continuous or heavy-duty applications.

5.2.2 It is also encouraged to future research on shunt capacitors. This type of

capacitor is widely used in medium voltage applications for the purpose of power

factor correction. Any study in the future could apply similar design and test of this

research to investigate the function of the capacitor in term of electrical properties of

interest.

5.2.3 There are many natural vegetable oils which have appropriate attribute

for capacitance applications, castor oil for example. It is a sensation to investigate

these fluids with some interesting application such as in storage or impulse capacitor.

In addition, a comparison between the measured results of vegetable oils impregnated

in such capacitor can leads to other brand new contents.

5.2.4 Dissipation factor is a key parameter in evaluating performance of

capacitors. Future research should consider arranging an investigation to develop

better quality of any vegetable oil as an impregnant in order to reduce this dissipation

factor so that loss in a capacitor can be preserved effectively.

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101

BIOGRAPHY

Name : Mr. Boonchoo Somboonpen

Thesis Title : Design and Test of Vegetable Oil Impregnated Polypropylene

Film Capacitors

Major Field : Electrical Power Engineering

Biography

I was born on August 28, 1963 . First, I graduated Vocational Education

Certificate in Electricity from Lopburi Technical College, Lopburi in 1983. Second I

got Diploma from Electrical Technology, Faculty of Technical Education and

Science, King Mongkut’s Institute of Technology North Bangkok Campus, Bangkok

in 1985. Third I graduated Bachelor degree from Faculty of Technical Education

and Science, King Mongkut’s Institute of Technology North Bangkok Campus,

Bangkok in 1987. Fourth I graduated Master from Science Technical Education

(Electrical Technology) major, King Mongkut’s Institute of Technology North

Bangkok, Bangkok in 1995, and I graduated Bachelor from Engineering (Electrical

Engineering), Faculty of Engineering, Rajamangala Institute of Technology,

Pathumthani in 2003.

My Workplace address is : Electrical Engineering Department, Engineering

Faculty, Pathumwan Institute of Technology, 833 Rama 1 rd. Wangmai Pathumwan

Bangkok 11000

My Address is 833/9 Rama 1 rd. Wangmai Pathumwan Bangkok 11000,

and my email address is [email protected]

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