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JWG A2/C4.39 Electrical Transient Interaction between

Transformers and the Power System

WORKSHOPWORKSHOP

Members

A. da C. O. Rocha, Convenor (BR), A. Holdyk (DK), B. Gustavsen (NO), B. J. Jaarsveld (ZA), A.

Portillo (UY), B. Badrzadeh (AU), C. Roy (ES), E. Rahimpour (DE), G. H. da C. Oliveira (BR), H.

Motoyama (JP), M. Heindl (DE), M-O. Roux (CA), M. Popov (NL), M. Rioual (FR), P. D. Mundim

(BR), R. Degeneff (US), R. M. de Azevedo (BR), R. Saers (SE), R. Wimmer (DE), S. Mitchell (AU),

S. Okabe (JP), T. Abdulahovic (SE), T. Ngnegueu (FR), X. M. Lopez-Fernandez (ES)

Contribution have been made by

A. Troeger (CH), D. Matveev (RU), G. A. Cordero (ES), J. C. Mendes (BR), J. Leiva (AR), J.

Veens (NL), M. Reza (SE), R. Asano (ES), R. Malewski (CA), S. Yamada (JP), U.

Savadamuthu (IN), Z-J WANG (CN) , J. M. Torres (PT)

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Proposed by Mr Pierre Boss at A2 2007 Bruges Colloquium

GOAL To provide an update in the study of this broad and complex topic.

MOTIVATION

JWG A2/C4.39 Electrical Transient Interaction between Transformers

and the Power System

INTRODUCTION

Transformers suffer dielectric failure even with good insulation coordination studies and well-accepted insulation design practices.

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Survey system high frequency pulses - C4 High frequency transformers modelling and testing - C4/A2 Protection and critical connection configurations - C4 Survey utility experiences - transformer failures C4/A2 Manufacturer experience with technical specifications A2 Discussion on the possibility of pinpointing risk factors C4 Methodology for transient system studies - C4 Assess transformer voltage stress - A2 Impact of transformer insulation - A2

INTRODUCTIONSCOPE

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EXECUTIVE SUMMARY1. INTRODUCTION

2. CURRENT PRACTICE

3. OCCURRENCE OF HIGH-FREQUENCY TRANSFORMER RESONANT OVERVOLTAGES

4. TRANSFORMER MODELING

5. NETWORK MODELING

6. ASSESSMENT OF TRANSFORMER VOLTAGE STRESSES

7. IMPACT ON TRANSFORMER INSULATION

8. CASE STUDIES

9. WHITE-BOX MODEL TEST TRANSFORMER (“FICTITIOUS TRANSFORMER”)

10. RECOMENDATIONS

TECHNICAL BROCHURE CONTENT

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New approaches and challenges emerge as newtechnologies are introduced together with different powersystem scenarios.

Required knowledge of transient interactions between thetransformer and power system cannot be reached withouta close relationship between manufacturer and clients.

INTRODUCTION

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CHAPTER 2Current Practice

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Magnitude Shape Frequency Content

Transient Overvoltages on Transformers Atmospheric discharges Fault conditions Switching

Manufacturers - Users - Standards

Power system transformers are subjected to transientovervoltages from many origins, each with varying physicalcharacteristics.

Users and Manufacturers have to consider these transients inthe specifications, design, manufacturing and testing (FAT)

Technical Standards and Guides are the basis for commonlanguage and mutual understanding.

CURRENT PRACTICE

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IEC STANDARD IEEE STANDARD

Applied in countries where powersystem frequency is 50Hz

Applied in most countries wherepower system frequency is 60Hz

Transformers 60076 series60060 series

Transformers C57 series

Rated Power: input to the primaryside

Rated Power: power delivered at thesecondary terminals

The dielectric tests aim to represent the different electricstresses (transients) that might appear in a particular network.

Defining the particular network conditions and tests levels arewithin the scope of the Insulation Coordination Studies (IEC60060 Series).

Two families of standards/guides currently apply (IEEE/IEC);they do not differ for the dielectric tests.

TECHNICAL STANDARDS AND GUIDES

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Different approaches or adapted tools are used bymanufacturers to predetermine the electrical stresses on thewindings during the design process.

Users and Manufacturers evaluate the design and discussparticularities in the Design Resiews (DR)

Design and overall manufacturing process are validated byFactory Acceptance Tests (FAT); applicable tests (routine,type, …etc) being given by the standards or agreed with user.

Design / Design Review

Factory Acceptance Test(FAT)

Evaluation of withstand tospecified transientovervoltages (ElectricalStresses on the Windings)

Manufacturing Process

STANDARDS: FROM DESIGN TO FAT

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Transformers in service passed the FAT with the applicablestests and/or the special ones user/manufacturer agreed.

From experience, transformers in service may still fail; Reasonsare usually not straightforward but rather associated tocombination of events over time, leading to final failure.

The brochure has collected, various cases, covering numerousapplications, all over the world, where the failure root causeanalysis (RCA) pointed to “transient over voltage” in the grid.

SERVICE EXPERIENCE

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Brief Approximative Historical Review

Improving standards/guides and common practices is acontinuous process; adapting to the power grid infrastructuredevelopment and operation.

JWG A2 C4.39 has tried to lean on and capitalize from the contributions of previous Cigre/IEEE related Working Groups.

STANDARDS/GUIDE: A LIVING PROCESS1900 2000

First long distance HVlines interconnectingcustomers andgeneration centers

Development of ZNO

Full Wave LI

Gapped Horns and Chopped Wave LI

unecessarySpecial

tests. IEC 60076-3

2000

Development of GIS => VFTO

Faults

Chopped Wave LI is

routine test again

Um>170kVIEC 60076-3 2013

1960’s

Development of750 kV Networks

Chopped Wave LI

Use of grounding wires on the lines

Arcinig horn type surge arrestors to

protect transformers

Switching Surge Impulse Um>300kV

Evaluation of the effects of lightning on

the lines

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CHAPTER 3Occurence of high-frequency

transformer resonant overvoltages

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A transformer is essentially a large RLC circuit equivalent.

Many internal resonances, weakly damped

An oscillating overvoltage occurring at transformer terminals

High internal overvoltages by resonant voltage build-up.

TRANSFORMER RESONANT OVERVOLTAGES

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Example: Voltage magnification at tap point (200 MVA transformer)

Tap voltage vs.of applied voltage [%]

TRANSFORMER RESONANT OVERVOLTAGES

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Switching operations in cable networks often leads to voltages withoscillating frequency components

TRANSFORMER RESONANT OVERVOLTAGES

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Overhead line

Overhead line

T1Cable

G1

T2Cable

G2

Closing circuit breaker

Overhead line

Overhead line

T1Cable

G1

T2Cable

G2

Closing circuit breaker

JWG A2/C4.39

Example:Transferred overvoltage from HV winding to LV winding (410 MVAgenerator step-up transformer)

TRANSFORMER RESONANT OVERVOLTAGES

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Example: Transferred overvoltage from HV winding to LV winding (410 MVA generator step-up transformer)

Overhead line

Overhead line

T1Cable

G1

T2Cable

G2

Closing circuit breaker

Overhead line

Overhead line

T1Cable

G1

T2Cable

G2

Closing circuit breaker

TRANSFORMER RESONANT OVERVOLTAGES

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Generation of oscillating overvoltage on transformer terminals

Energizing cables and short transmission lines from remote end (closing circuit breaker)

Energizing capacitor bank

Ground fault initiation

Operation of disconnectors

Frequent switchings are more likely to cause transformer failures.

TRANSFORMER RESONANT OVERVOLTAGES

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CHAPTER 4Transformer modeling

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An overview of alternatives and comparison:

4.1 Simplified procedures 4.2 White box appoach4.3 Black box appoach4.4 Grey box appoach4.5 Comparison

JWG A2/C4.39

TRANSFORMERS MODELING

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Three types of simplified transformer modelling approaches.

A common idea is that the model parameters should be easily obtained compared to Black-box or White-box representations.

1. Power frequency standard model with external capacitance

2. Concentrated Capacitance Model for Fast Transients

3. Frequency dependent model for fast transients

4.1 SIMPLIFIED PROCEDURES

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Why use a white box model? How is it implemented How is it computed Describe the Lossy Lumped Parameter Model widely

used by manufactures Example

4.2 WHITE BOX APPOACH

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What is the advantage of Black box model used in EMTP-Type simulations

How is Characterization done

How is Model extraction done

01

( )N

m

m ms a

RY R

Jumpersto ground

Source Ref. Input

A

Currentsensor

Attenuator

2

3

1

4

56

From: Measurements White box model

4.3 BLACK BOX APPOACH

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What are the grey box model characteristics

How is built

How are the parameter obtained

4.4 GREY BOX APPOACH

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Typical applicationsTypical Model BandwidthVery Fast Transients, above 2MHzData BasisModel ExtractionModel ComplexitySimulation TimeIntegration with EMTP type software

4.5 COMPARISON TABLE

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CHAPTER 5Network modeling

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Network modelling is important for a correctrepresentation of transient oscillations

It is important to use appropriate models in termsof the frequency range (slow- , fast- or very fast-transients)

The phenomena of prestrike and restrike in thecircuit breakers is the source of transients withbroad frequency range

This is one example where broad frequency rangenetwork modelling is needed

NETWORK MODELING

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Slow frequency studies like transformer energizingrequire low frequency transformer models and thesurrounding network representation is generally simpler.

Switching studies (circuit-breakers operations,transients due to faults) generally imply therepresentation of propagation phenomena in thenetwork.

Lightning studies require high frequency representationof the components

Very fast transients studies like disconnectorsswitchings in GIS require high frequency representationof all components; even very short busbars should betaken into account

NETWORK MODELING

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To simulate multiple restrikes, withstand voltage characteristics, critical quenching capability and chopping currents are needed

Increased time scale of the first restrike

Test circuit for restrikes modelling

Accurate simulation of multiple restrikes [ref 5.44]

CIRCUIT BREAKER MODELING

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Surge arresters should be represented by a model that take intoaccount the frequency dependence of the surge arrester; this isverified if the simulated arrester residual voltages (for standardimpulse shapes 8/20 us, 30/60/us and 0.5us front-of-wave) areclose to those provided by the manufacturer (ANSI/IEEE StdC62.11-1993).

Surge arrester model according to Schmidt [ref 5.31]

Response of the surge arrester when excited with current impulse 10 kA, 0.5 us [5.44]

Response of the surge arrester when excited with current impulse 10 kA, 30/60 us [5.44]

SURGE ARRESTER MODELING

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CHAPTER 6Assessment of transformer

voltage stresses

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Overview of current practice and introduction tonew methods

Conventional approach by manufacturer

Time Domain Severity Factor

Frequency Domain Severity Factor

Conclusions

ASSESSMENT OF TRANSFORMES VOLTAGE STRESSES

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Standards Insulation co-ordination studies Non standard impulses Design

TIME DOMAIN: CONVENTIONAL APPROACH

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The Severity factor is a ratio between measuredsignals and acceptance test

Introduced to compare measured transient to whatthe transformer was tested for

Highlights additional required tests

For design and monitoring

SEVERITY FACTOR

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Visualization of turn-turnover voltage compared tothe acceptance tests

TIME DOMAIN SEVERITY FACTOR

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Ratio in Frequency domain between transient and acceptancetest

Trends

FREQUENCY DOMAIN SEVERITY FACTOR

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CHAPTER 7Impact on Transformer Insulation

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Transformer insulationDesigned to withstand the normal service voltages but alsothe effects of transient overvoltages like lightning impulse.

Insulation designTransient voltage distribution inside transformer should becalculate under different types of applied voltages

Transformer insulation can be divided to two groups:1. Insulation inside windings - which is called internalinsulation2. Insulation between windings and between windings andgrounded parts such as core and tank, i.e. main insulation

INTRODUCTION

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• Breakdown voltage of a dielectric material is a function ofits physical and chemical properties, as well, impuritiespresent in it.

• Evaluation of insulation strength is a difficult issue.Full understanding of the impact of steep-front waves on the

dielectric strength of insulation material is still underinvestigation.

• Following parameters should be considered in analysing the insulation strength:

Moisture ImpuritiesInsulation thickness Oil velocity Temperature

Duration of voltage application Frequency of voltage applicationPressureAging

INSULATION STRENGTH

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• The most determinant aging factor of insulation is thedegradation of paper due to thermal stress.

• Although it is less likely to cause a problem earlier,dielectric degradation should be taken into account.(around 12% maximum).

.

INSULATION AGING

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A drop in dielectric strength is expected to repetitivevoltage stresses.

Sufficient consideration must be taken intoconsideration when designing transformers

V-N characteristic up to 500 kV class

EFFECT OF REPETITIVE IMPULSES

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CHAPTER 8Case studies

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13 Case studies cover the models presented in thebrochure.

From theoretical to practical cases

From academic studies to failure investigations

All cases are summarized in a table

CASE STUDIES

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Case study number 1 2 3 4 5 6 7 8 9 10 11 12 13

Topic Sub Topic

Cable-transform

er high-frequency interaction and transferred overvoltages on a generator step-up transform

er

High-frequency interaction betw

een a wind turbine

transformer and the pow

er system

Laboratory tests on the interaction between a w

ind power

transformer, cables, and a vacuum

circuit breaker

Failure Analysis of tw

o step-up transformers at Três M

arias H

ydroelectric Pow

er Plant

Analysis of dry-type transform

er failure caused by SF6

switching operation

Analysis of dry-type transform

er failure caused by VC

B

switching operation

The effect of the connecting cable on the transformer

transients

Itaipu hydro-electric plant: comparison betw

een black-box and grey-box m

odeling approaches

500kV transform

er failure case caused by resonance overvoltage due to lightning surge intrusion from

a transm

ission line

Distribution transform

er failure caused by lightning backflashoverand resonant voltage buildup

500/275kV transform

er failure caused by resonance overvoltage due to the closing surge of the breaker in a cable-system

substation –C

ombined phenom

ena of switching and

transferred overvoltages

High-frequency m

odeling of a 500 kV transform

er –D

etailed m

odel vs. simplified m

odel

Frequency Dependent Transform

er model for sw

itching sim

ulations by using FRA

and transient measurem

ents

Origin of overvoltage Circuit breaker x x x x x x xDisconnector x x xLightning x x x xFault initiation x x x x

Model Type Black box x x x x xWhite box x x x x x x x xGrey box xSimplified model x x x x x x x

Field measurement(s) x x x x xSimulation(s) x x x x x x x x x x x x xInsulation stress assessment FDSF x x x x x x x x

Internal stresses x x x x x x x xBIL or SIL v.s peak voltage x x x x x x

CASE STUDIES

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CHAPTER 9White-Box Models Test

Transformer (“Fictitious Transformer”)

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Excitation with different waves shapes indifferent terminal points.

Valuable contribution to the transformerindustry about the state-of-the-art of thecomputing of voltage transients insidetransformers.

A simple transformer able to test different transformer “white-box”mathematical models designed to compute the distribution ofinternal transient voltages.

“Fictitious Transformer”, two winding, 100 MVA, 230/69 kV,transformer geometry was created.

INTRODUCTION

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Relative good agreement inmaximum node voltageresponses (in the nodemaximum voltage value, notin the time in which thismaximum occurs)

Poor agreement in maximumbranch voltage responses

These voltages aredifferences between nodevoltage of similar values andwave shapes

This fact increase the errorsin the numerical calculations

LIGTHING IMPULSE: MAXIMUM VOLTAGE VALUES

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Similar tendencies in all sofwares for voltage wave.

Relative good agreement in node voltage.

Poorer agreement in branch voltages.

Poorer agreement in HV-LV transferred voltage.

LIGTHING IMPULSE: TEMPORAL WAVE SHAPES

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Transformer model without taking into account internal damping effectsTransformer model without taking into account internal damping effects

Response to damped oscillatory wave shape with D = 0.9

Transformer model without taking into account internal damping effects

Response to damped oscillatory wave shape with D = 0.9

Transformer model taking into account internal damping effects

Response in the center of the lower part of the HV windingDAMPED OSCILLATORY WAVE SHAPES

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• There is a high fault probability when we apply to thetransformer a oscillatory wave shapes with frequency equal toone of the resonance frequencies of the transformer

• In the frequency domain all the waves present values higherthan the reference envelope (impulse test).

• In the time domain, the more damped wave is safe.• This proves that the FDSF is more conservative than TDSF

due to design safety margins.

102

103

104

105

106

10-6

10-4

10-2

100

102

104

106

108

Energy Spectral Density in HV winding

Frequency (Hz)

Ene

rgy

Spe

ctra

l Den

sity

(V.s

)2

D = 0.6, f = 14.9 khzD = 0.7, f = 14.9 khzD = 0.8, f = 14.9 khzD = 0.9, f = 14.9 khzEnvelope

485052545658606264666870

0.8

1

1.2

1.4

1.6

1.8

2

2.2Time Domain Severity Factor in HV winding

Nodes of the HV winding

TDS

F

D = 0.6, f = 14.9 khzD = 0.7, f = 14.9 khzD = 0.8, f = 14.9 khzD = 0.9, f = 14.9 khzEnvelope

TIME DOMAIN & FREQUENCY DOMAIN SEVERITY FACTORS

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CHAPTER 10Recommendations

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• Interleaving disc windings and in references for intershielding disc windings

• Internal ZnO surge arrester

DESIGN PRACTICES

DESIGN PRACTICES

GOAL

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It is recommended that utilities design their network to protect thetransformers from the system transients as much as possible:

Transients caused by the energization of capacitor banks caneasily be controlled by using closing resistors, series reactors orcontrolled switching;

Reduction of probability of re-strikes due to circuit-breakerswitching

Minimize disconnector operation close to transformers

Transients caused by the energization of transmission lines canbe controlled or reduced by using closing resistors or controlledswitching.

Remote energization should be avoided by installing an extracircuit breaker on the transformer side of the cable/line.

SYSTEM ASPECTS

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Transformers energization should be done using circuit-breakers with pre-insertion resistances. Controlled switchingshould be evaluated in relation to the inrush current.

If possible, operating the disconnectors in a GIS should beavoided or forbidden.

Good practices of transmission line design and substationcoordination insulation including proper shielding, surge arresterlocation, line surge arrester, etc must be applied.

SYSTEM ASPECTS

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Specifying Lighting Impulse Chopped on the Tail (LIC)

The frequency spectrum of the LIC waveforms exceed thefrequency spectrum of the Lighting Impulse (LI) waveform from30 kHz up to 1 MHz.

The LIC dielectric test was proposed in IEC as a special testand now it is standardized as in IEEE. It is a current practice ofmany utilities but not all.

Upgrading Test Levels

Utilities with low voltage levels should consider upgrading thelevels for the LI and the LIC dielectric tests.

Such a practice will increase the probability that transientsapplied during a transformer life will be covered by thestandard wave tests.

SPECIFICATIONS: DIELECTRIC TESTS

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Adding Unloaded Dielectric Tests to Specification

LI and LIC dielectric tests perform with all the untested lineterminal earthed as recommended in IEC 60076-3 isinsufficient to test appropriately all possible high-frequencyresonances that could be excited in the service life of atransformer.

It is recommended that transformers be tested (LI and LIC)with untested line terminals unloaded and protected byarresters or connected to a typical line impedance value andprotected by arresters.

Specifying Additional Dielectric Tests

Additional dielectric tests can be specified. For example, a FastFront Switching Impulse (FFSI) can be specified to simulatefrequent energization.

SPECIFICATIONS: DIELECTRIC TESTS

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Overvoltage phenomena depends on systemconfigurations and transformer frequency response:

Difficult to determine in the specification stage

Provide the manufacturer with representativeimpulses

User’s experiences with similar electricaltransformer environment: Transient measurement;failure analysis

Strategic importance of the transformer and thesystem’s past operation experience

SPECIFICATIONS: SYSTEM STUDIES

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Transformer high frequency modelling

Simplified to more complex models

Insulation stress assessment

A simulation of the voltage distribution in theinternal parts of the transformer for any givenvoltage impulse can be performed by themanufacturer

Design review

SPECIFICATIONS

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Interaction between manufacturer and user

SPECIFICATIONS

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Transformer Failure Analysis

Transient as a probable cause of failure

Actions should be taken, involving the manufacturer responsible for the repair or refurbishment

Examples of failures analysis in the brochure

SPECIFICATIONS

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TRANSIENT MEASUREMENTS

Important part of the process of analyzing interactions between the transformer and the power grid.

Temporary setup : Post mortem analysis

Permanent setup : On-line measurement system

• good coupling device • no risk of interruption of the normal

operation

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CONCLUSIONS

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CIGRE WG A2/C4.39 was formed to clarify/explore reasons fordielectric failures attributed to transient voltage excitation – evenwhen good insulation design and coordination practice had beenfollowed.

1. Current factory insulation proof tests do not completelyaddress all types of transient events or terminations.

2. Transformer manufacturers and purchasers assume the issueof transient voltage design and verification is adequatelyaddressed by current impulse standards – this is not true forall the cases

3. Other working groups have addressed this issue but muchwork remains to be done.

CONCLUSIONS

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4. There is a high probability that system-initiated transientvoltages contain oscillatory wave forms. These may produceinternal winding voltages that exceed the transformerinsulation withstand capability.

5. The combination of system and transformer impedanceaffects the wave shape (and frequency) of the transientovervoltage. There are several different modellingapproaches (white, black, grey) used to examine thesystem/transformer response.

6. Standard simulation tools provide sufficient transformer and system modelling capability to examine transient performance.

CONCLUSIONS

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7. The standard approach to assess internal transformervoltage stress considers tools and design information onlyavailable to manufactures.

8. The utility can make an initial evaluation using the frequencydomain severity factor (FDSF).

9. Repetitive overvoltages and aging reduces insulationwithstand capability and should be recognized in thetransformer design.

10. High frequency breakdown characteristics of solid materialsare not well known and deserve future work.

11. Thirteen case studies are presented demonstrating situations where system transient lead to excessive internal overvoltages.

CONCLUSIONS

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12. A fictitious transformer design was used to evaluate the whitebox performance of eleven different participants(manufactures, universities, and consultants).

13. The transformer insulation design must recognize the presence of a broad spectrum of system transients (aperiodic and oscillatory).

14. It is desirable that a transformer terminal equivalent model be available so that system studies could be performed

15. This requires close cooperation between the manufacturer and purchaser in the design stage.

CONCLUSIONS