47 - 2000 - PhD - Influence of LI to insulating systems.pdf

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Influence of lightning impulses to insulatingsystems - PhD thesis - TECHNISCHEUNIVERSITT GRAZ

THESIS MAY 2000

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1 AUTHOR:

Sonja Monica Berlijn

Statnett

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TECHNISCHE UNIVERSITT GRAZ

D I S S E R T A T I O N

INSTITUT FR HOCHSPANNUNGSTECHNIK

MIT VERSUCHSANSTALT


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I nf luence of lightning impulsesto insulating systems

Sonja Berlijn

DissertationFor the academic degree

Doctor of Technical science

Permitted by theFacul ty of Electr ical Engineering

of the

Technical University Graz

Submitted by:

1. Expert & Supervisor: O.Univ.-Prof.Dipl.-Ing.Dr. Michael Muhr

2. Expert: Univ.-Prof.Dipl-Ing.Dr. Ernst Gockenbach

Arnhem, The Netherlands May 2000


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Preface

v

Preface and acknowledgements

This doctor thesis describes the theoretical and experimentalinvestigations performed at KEMA High-Voltage Laboratory and atthe Technische Universitt Graz. The PhD project was started in

November 1997. At this time I was involved in an European projectand in two international working groups which dealt amongst otherswith the measurement and evaluation problems of lightning impulsevoltages.

The work performed in the framework of the European project was agood start for a PhD project and therefore, together with my professorM. Muhr and my colleague G. Roelofs, I decided to take my PhD inthe area of tests with lightning impulse voltages.

I would like to take this opportunity to express my gratitude to allwho were involved in my projects and my education, to all whosupported me in writing this doctor thesis and to all of my friends and

family who were there to talk or to e-mail to. Of course there aresome persons to whom I would like to express my gratitude in

particular they are listed here below in random order.

All my colleagues at KEMA High-Voltage Laboratory I would like tothank for the creation of a fine working environment, helping me withthe experimental investigations and for the interest they showed.Also other colleagues from KEMA TNK and KEMA HPL, I wouldlike to thank for their help. KEMA, in particular, G. Roelofs, H.Kempen, T. Klomp and B. Verhoeven, I would like to thank formaking this PhD project possible. G. Roelofs I would like to thankalso for his help, inspiration, support and for the comments he gave to

my work.

During the experimental investigations performed at KEMA I washelped by M. Kvarngren from KTH. I have learned quit a lot fromhim, technically and scientifically, and he became a very good friend.I would like to thank him for his criticism, for his help and support.

My friend H. Scheefhals, I would like to thank for his patience, hissupport, for listening to me all the time and of course for his advice.

The members of CIGRE 33.03, the joint task force SC12/WG33.03and all other persons involved in the European project (especiallyF. Garnacho, P. Simon, E. Gockenbach, P. Werle, K. Hackemack,

J. Blackett, M. Watts and P. Wong). I would like to thank for theirinput, their ideas and their co-operation. Without their help this

project would not have been so successful.

I would like to thank my special friend J. Benda, for his help, hissupport and his friendship during all the years.

J. Wetzer, F. Pesgens, M. Bongaerts, G. Buhre, R. Verhoeven andH. Obers I would like to thank for their contributions to this PhD.

I would like to thank Dr. Kachler from Siemens, for providingsamples. W. Lick and G. Puckel from TU Graz, I would like to thankfor performing the investigations on these samples.

Last but not least I want to thank my two dogs Romy and Orso foralways being there, keeping me company and making me laugh.

I dedicate this thesis to my parents, my

sister and her husband, my uncle Eddie,

my grandparents and my uncle Douwe.

Thanks

The EC is thanked for the financial

support of the project Digital

measurement of parameters used for

lightning impulse tests for high voltage

equipment, contract no. PL95120-SMT4-

CT96-2132.

PHans

Hauke

Ger

Patricia

Oma

Peter

Silvia

Frank

Klaus

Mats

MichaelHarrie

TonRob

Romy

Orso

Hanneke

Jean

SusanTheodor

Jean

KEMA

Bob

AndersPaolo

Herbert

Eddie

Kimlie

Trudy

Marcel

Fernando

Ernst

Jack

Malcolm

Patrick

Jeroen

Jos

Wijnand

Martijn

Godert

Sadique

Pascual

Ren

Hiske

Fred

Diane

Anouscka

Tonnie

Henk

All I forgot

Ronald

WouterDick

Cor

Leo

Pieter

Hans-Erik

Lars-Olof

Toshiji

Albert

Martti

Jari

Wolfgang Eberhart

Kurt

John

Bill

Werner George

Matthias

Herman

Terry

Siemens EC

Giuseppe

Hugh

Loes

TUEToine

Mark

Cees

Edwin

Evert

Sven-Erik

Sara

Anna

Jet

MiriamGehrardRyszard

Diny

Bruin

Wim

MartienAndr

MarjaFried

Sanne

Hannie

Robert

BerryTonnie


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Kurzfassung

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Sonja Berlijn Arnhem May 2000


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Abstract

vii

Abstract

Lightning strokes have caused a considerable number of failures ofhigh voltage equipment installed in medium and high voltage grids.In order to prevent these failures several studies have been performed

and several measures have been taken. One of these measures is totest high voltage equipment with lightning impulse voltages.

The measurement of the applied voltages and the evaluation of theparameters characterising these lightning impulse voltages have to beperformed accurate and uniform all over the world, since high-voltageequipment require large capital assets and they are exported all overthe world.

During tests of high voltage equipment with lightning impulsevoltages oscillations or/and overshoot may occur at the front or nearthe peak of a lightning impulse voltage. The measurement oflightning impulse voltages without oscillations or overshoot can

nowadays be performed with sufficient accuracy. However, problemsarise with chopped lightning impulse voltages, in case the lightningimpulse voltage has some small overshoot or oscillation and even

bigger problems arise when the lightning impulse voltage is a non-standard lightning impulse voltage. The cause for these measurement

problems is that when small oscillations or/and overshoot occur, therules for the evaluation of the parameters (as described in the presentstandard) are ambiguous. In case of non-standard lightning impulsesno evaluation rules are given in the present standard. Although the

problems existed when the evaluation was performed manually, theywere not recognised. The introduction of digital equipment and

evaluation software showed very clearly the problems and made theproblems needed to be solved.

Several solutions for the problems with the evaluation have beenproposed. Most of the solutions given up to now, based on thepresent definition in IEC 60060-1, do not taken into account thepossibilities of modern (measuring) equipment and do not considerthe introduction of new insulating materials as for instance SF 6.Besides this, the problems with the evaluation of parameters ofstandard and non-standard lightning impulse voltages are dealt withseparately.

To find an ideal solution for the evaluation problems that is based on

the breakdown physics and that is applicable to both standard andnon-standard impulses, theoretical and experimental investigationshave been performed by the author at KEMA and by others at otherinstitutes. Tests were performed on different insulating materials.

The results of these investigations showed that a new approach wasnecessary and a new evaluation method, using the k-factor function isintroduced. When using this method, lightning impulsemeasurements can be performed with a better accuracy besides this, itis possible to use the method for both standard and non-standardimpulses.

A


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Kurzfassung

viii

Keywords

lightning impulse voltage, evaluation methods, standard and non-standard lightning impulses, k-factor function, test with lightningimpulses, IEC 60060-1


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Abstract

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Kurzfassung

Blitzeinschlge in Mittel- und Hochspannungsnetze haben in derVergangenheit zu einer groen Anzahl von Fehlern anHochspannungsgerten gefhrt. Um diesen Fehlern vorzubeugen,

wurden einige Studien und verschiedene vorbeugende Manahmendurchgefhrt. Eine dieser Manahmen ist die Prfung derHochspannungsanlagen oder -gerte mit einer Blitzstospannung.

Die Messung der angelegten Spannung und die Auswertung derParameter, die die Blitzstospannung charakterisieren, mssen sehrgenau und in der ganzen Welt gleich ausgefhrt werden, damit dieErgebnisse vergleichbar sind.

Bei Blitzstospannungsprfungen an Hochspannungsgerten knnenOszillationen und/oder berschwingungen im Anstiegsbereich odernahe des Scheitelwertes des Blitzstoimpulses entstehen. DieMessung der Blitzstoimpulse ohne Oszillationen undberschwingungen kann heute mit zufriedenstellender Genauigkeit

durchgefhrt werden. Probleme bei der Auswertung entstehen beiabgeschnittenen Blitzstoimpulsen oder wenn der Blitzstoimpulskleine berschwingungen oder Oszillationen aufweist. Nochschwieriger wird die Auswertung, wenn es sich um einen nichtstandardisierten Blitzstoimpuls handelt. Die Ursache fr dieProbleme beim Auswerten ist darin begrndet, da frBlitzstoimpulse mit berlagerten Schwingungen oder Oszillationenkeine eindeutigen Meverfahren in den Standards definiert sind. ImFall von nicht standardisierten Blitzstoimpulsen gibt es gar keineLsungsanstze in den aktuellen Standards.

In der Vergangenheit sind verschiedene Lsungsanstze fr dieseProbleme vorgeschlagen worden. Die meisten Lsungen basieren auf

den derzeitigen Definitionen in IEC 60060-1. Sie bercksichtigenaber dabei nicht die Mglichkeiten der modernen Metechnik und dieEinfhrung neuer Isoliermedien, wie zum Beispiel SF6. Auerdemwerden die Probleme bei der Bestimmung der Parameter vonstandardisierten und nicht standardisierten Blitzstoimpulsen bisherseparat behandelt.

Um einen optimierten Lsungsansatz fr die Bestimmung derParameter zu finden, der auf den physikalischenDurchschlagsmechanismen beruht und der fr beide ImpulsformenGltigkeit hat, wurden theoretische und experimentelle Untersuchen

bei der KEMA und anderen Instituten durchgefhrt. Dabei wurdendie Einfle unterschiedlicher Isoliermedien bercksichtigt. Die

Ergebnisse der Untersuchen haben gezeigt, da ein vllig neuerLsungsansatz und eine neue Berechnungsmethode, die eine k-FaktorFunktion benutzt, bentigt wird. Mit dieser Berechnungsmethode istes mglich, hhere Genauigkeiten bei der Auswertung vonBlitzstospannungsprfungen zu erzielen. Gleichzeitig kann dieseMethode sowohl fr standardisierte als auch fr nicht standardisierteBlitzstoimpulse angewendet werden.

Schlgwrter

Blitzstospannung, Auswertung, standardisierter und nichtstandardardisierter Blitzstoimpuls, k-Faktor Funktion, Nennsteh-

Blitzstospannungsprfung, IEC 60060-1

K


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Contents

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Contents

1 Introduction _______________________________________________________________________ 1

2 Lightning impulses __________________________________________________________________ 3

2.1 Introduction ______________________________________________________________________ 3

2.2 Lightning ________________________________________________________________________ 32.3 Lightning impulse tests _____________________________________________________________ 42.4 Implementation of the standards _____________________________________________________ 122.5 International work ________________________________________________________________ 182.6 Current evaluation methods _________________________________________________________ 20

3 Disruptive discharge phenomena _____________________________________________________ 25

3.1 Introduction _____________________________________________________________________ 253.2 Insulation systems ________________________________________________________________ 253.3 Breakdown processes in materials ____________________________________________________ 273.4 Influence of lightning impulse voltage to the breakdown __________________________________ 34

Breakdown statistics and statistical techniques ______________________________________________ 39

4 Goal of investigations _______________________________________________________________ 43

4.1 Introduction _____________________________________________________________________ 434.2 Goals and expectations of investigations _______________________________________________ 454.3 Insulating systems to investigate _____________________________________________________ 464.4 Tests to perform on insulating materials _______________________________________________ 464.5 Translation of results to insulating systems _____________________________________________ 474.6 Questions/hypothesis ______________________________________________________________ 48

5 Investigations; the test set-up ________________________________________________________ 49

5.1 Introduction _____________________________________________________________________ 49

5.2 Generating circuit, combined generating circuit versus modified conventional circuit ____________ 495.3 Test set-up and test method used for the tests at KEMA ___________________________________ 535.4 Test set-up used of the tests at TU Graz _______________________________________________ 78

6 Investigations; the results ___________________________________________________________ 81

6.1 Introduction _____________________________________________________________________ 816.2 Test to be performed ______________________________________________________________ 816.3 Comparison measurements in air _____________________________________________________ 846.4 Investigations performed in oil at KEMA ______________________________________________ 89Measurements performed in SF6 at NGC ___________________________________________________ 926.6 Measurements performed in PE at the Schering Institute __________________________________ 966.7 Measurements performed in air at LCOE ______________________________________________ 996.8 Measurements in oil-paper at TU Graz _______________________________________________ 1026.9 Analysis of results, general discussion ________________________________________________ 1036.10 K factor hypothesis and linearity statement; influence of oscillation frequency and amplitude __ 104

7 Proposal ________________________________________________________________________ 113

7.1 Introduction ____________________________________________________________________ 1137.2 Proposal for evaluation method _____________________________________________________ 1137.3 Proposal for algorithms ___________________________________________________________ 1197.4 Simulation with wave shapes that occur in testing _______________________________________ 1197.5 Check of the proposal with the average k-factor function _________________________________ 1217.6 K-factor in hardware versus k-factor in software ________________________________________ 125

7.7 Final proposal and discussion ______________________________________________________ 1267.8 Open questions and experiments needed ______________________________________________ 127

8 Summary and conclusions __________________________________________________________ 129


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Contents

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List of terms, symbols and abbreviations

xiii

Lists of terms, symbols and abbreviations

Symbols

A Semi-range value

B Pressure [mbar]B Confidence limit [%]C CapacitanceCe Capacitance to earthd Overshoot durationE Electric field strength [V/m]f Frequency [Hz]F Force [N]Fa Assigned scale factor [1]F(x) Distribution FunctionH Humidity [g/m3]

k Factor by which the amplitude of the applied impulse should beattenuatedK Coverage factork1 Air density correction factork2 Humidity correction factorKt Atmospheric correction factorL Inductance [H]O1 Virtual origin of a standard lightning impulse voltage

p Pressure [bar or Pa]P Power [W]P ProbabilityP(U) Distribution function

R Resistance [Ohm]R(t) Residual Curvesr Experimental standard deviationssa Standard deviation of systematic contributions that have a

rectangular distributionssg Standard deviation of systematic contributions that have Gaussian

distributiontd Time delay between the origin of the impulse and the origin of the

superimposed oscillations [s]tp Time to peak [s]T Temperature [C]

T Students t factorTc Time to chopping [s]T1 Front time of a lightning impulse voltage [s]T2 Time to half value of a lightning impulse voltage [s]T30% Time at which the lightning impulse is 30% of it peak valueT90% Time at which the lightning impulse is 90% of it peak valueU Overall uncertaintyUm Measured curve [V]Umc Mean curve [V]Up Peak value of a lightning impulse voltage [V]Urmc Reference mean curve [V]Us Systematic contributions to the overall uncertainty

Ur Random contributions to the overall uncertaintyUt Value of the test voltage [V]


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List of terms, symbols and abbreviations

xiv

U10 10% disruptive voltage [V]U20 20% disruptive voltage [V]U50 50% disruptive discharge voltage. This is the prospective voltage

value which has a 50% probability of producing a disruptivedischarge on the test object [V]

U50(peak)

50% disruptive voltage, taken from the peak voltage of the totalcurve

U50(RMC) 50% disruptive voltage, taken from the peak voltage of the mean orbase curve

dU/dt Impulse Steepness at each pointW Kinetic EnergyZ Complex Impedance [Ohm] Overshoot amplitude Wave integral above a reference voltage (x;,2) Density function(x;,2) Distribution function

Terms and abbreviationsBL Base LineChopped lightning impulse voltage a lightning impulse voltage during which a disruptive discharge

causes a rapid collapse of the voltage, which then falls to zero ornearly zero, with or without oscillations

CIGRE International Conference on Large High Voltage Electric SystemsDisruptive discharge phenoma associated with the failure of insulation under electrical

stress, in which the discharge completely bridges the insulationunder test, reducing the voltage between the electrodes practicallyto zero. It applies to electrical breakdowns in solid, liquid andgaseous dielectrics and combinations of these.

Flashover a disruptive discharge over the surface of a dielectric in a gaseousor liquid

IEC International Electrotechnical CommissionImpulse intentionally applied aperiodic transient voltage or current which

usually rises rapidly to a peak value and then falls more slowly tozero.

MLM Multiple Level MethodNon-standard lightning impulsevoltage

lightning impulses with large overshoots or oscillations

OPV overshoot peak value [V]PE PolyethylenePSM Progressive Stress MethodPuncture Disruptive discharge in a solid mediumRMC reference mean curveSD Standard DeviationSE Single Exponential of TailSparkover disruptive discharge in a gaseous or a liquid mediumStandard lightning impulse voltage the lightning impulse voltage as defined in IEC 60060-1Surge transient occurring in electrical equipment or networks in serviceTDG Test Data GeneratorVPV virtual peak valueXLPE Cross-linked polyethylene


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Introduction

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1 Introduction

Lightning strokes have caused a considerable number of failures ofhigh voltage equipment installed in medium and high voltagenetworks. In order to prevent these failures much research has been

performed and several measures have been taken. One of thesemeasures is to test high voltage equipment with lightning impulsevoltages to verify the withstand capability.

Lightning impulse voltages represent surges i.e. transients occurringnaturally in high voltage systems under operation. A full lightningimpulse voltage (without oscillations or overshoot) is, currently inIEC-60060-1 [1], defined by its peak value Up, its virtual origin O1,and its time parameters, the front time T1and the time to half valueT2. Tests with lightning impulse voltages are made using a standardlightning impulse having a front time of 1,2 s and a time to half-value of 50 s, described as a 1,2/50 s lightning impulse voltage.

During tests of high voltage equipment with lightning impulsevoltages, oscillations and/or overshoot may occur at the front or nearthe peak of a lightning impulse voltage. It is being assumed that theseoscillations and/or overshoot can be of significance and right now amaximum amplitude of 5% of the peak value is allowed [2].

The measurement of the applied voltages and the evaluation of theparameters characterising these lightning impulse voltages have to beperformed accurate and uniform all over the world, since high-voltageequipment require large capital assets and they are exported all overthe world. That is why a lot of effort is put into generating thelightning impulse voltages as good as possible and into calibration

and maintenance of measuring systems used.

In the past the measurements of lightning impulse voltages wereperformed with the aid of analogue oscilloscopes and the evaluationof lightning impulse voltages was performed manually. Theachievable accuracy was influenced by the thickness of the trace ofthe oscilloscope and the accuracy of the test engineer [3].

Nowadays the measurement of lightning impulse voltages is mostlyperformed with digital oscilloscopes. The evaluation of theparameters is mostly done using computer programs. Themeasurement of lightning impulse voltages without oscillations or

overshoot can nowadays be performed with sufficient accuracy.However, problems arise with chopped lightning impulse voltages, incase the lightning impulse voltage has some small overshoot oroscillation [4]. Even bigger problems arise when the lightningimpulse voltage is a non-standard lightning impulse voltage. Thecause for these measurement problems is that, when small oscillationsand/or overshoot occur, the rules for the evaluation of the parameters(as described in the present standard) are ambiguous. In case of non-standard lightning impulses no evaluation rules are given in the

present standard.

Several solutions for the problems with the evaluation have beenproposed. (see paragraph 2.6). Most of the solutions given up to now,which are based on the present definition in IEC 60060-1, do nottaken into account the possibilities of modern (measuring) equipment

1


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Introduction

2

and do not consider the introduction of new insulating materials as forinstance SF6. Besides this, the problems with the evaluation of

parameters of standard and non-standard lightning impulse voltagesare dealt with separately.

The most ideal solution for the evaluation problems will be a solution,

which is applicable to both standard and non-standard lightningimpulse voltages, which uses the possibilities of modern processingtechniques and which is founded on the breakdown behaviour ofinsulating materials.

To find this most ideal solution or a solution based on the breakdownbehaviour of insulating materials or systems, theoretical andexperimental investigations were necessary.

Therefore this research was started. The main topics of the PhDstudy of which the results are presented in this thesis are:

1. study of basic literature; study towards tests with lightning impulses study and evaluation of already existing evaluation methods

2. theoretical investigation towards the breakdown behaviour ofinsulating materials and systems

3. experimental investigation towards the breakdown behaviour ofdifferent insulating materials for different lightning impulse waveshapes

4. proposal for a new evaluation method

Ad 1:

Besides a summary of the information found in the basic literature

also international developments will be explained. Because some ofthe newest information is not available yet in official publications,results will be presented of activities within CIGRE WG33.03 andTC12/WG 33.03

Ad 2:

Besides the literature investigation performed at KEMA, also theresults are presented of a literature investigation performed in theframework of the European project on Lightning Impulses.

Ad 3:

The experimental investigations on different materials were

performed at different locations and within different projects: In the framework of the European project the insulating materials,

XLPE, SF6, air and oil were investigated.

At KEMA oil was investigated

At LCOE air was investigated

At NGC SF6was invesigated

At the Schering Insitute XLPE was investigated

As part of this PhD project at the Technische Universitt Grazconfigurations representative for transformer windings with

paper-oil insulation were studied.


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Lightning Impulses

3

2 Lightning impulses

2.1 Introduction

In this chapter it is described why and how lightning impulse test are

performed. Attention is paid to the evaluation of parameterscharacterising lightning impulse voltage, to the problems that ariseduring evaluation and to the current state of the art to solve these

problems.

2.2 Lightning

It has always been the case that people are at the same time astonishedand afraid for lightning and thunder [5]. About 200 years ago (in1754) it has been proved that lightning that occur duringthunderstorms is an electrical discharge. At this moment still not allthe aspects of thunderstorms are revealed although it is known that

they are caused by the complicated process of air flows. 60%-70% ofthe lightning discharges occurs between or in clouds of differentcharge. These cloud-cloud discharges can be dangerous to airplanes.The most dangerous discharges for human beings and equipment arethe discharges between clouds and earth and earth and clouds (alsocalled lightning strokes) [6]. In Figure 1 a thundercloud is shown inthe stage before lightning occurs. In 90-95% of the cases, especiallyin the Netherlands, the cloud is negatively charged at the bottom.

A lightning stroke is an impulse current. The voltage value and thevoltage shape of the stroke are determined by the impedance thecurrent meets on its way to ground and are therefore situation

dependant. Important factors of lightning strokes are the maximumcurrent value, the charge, the energy contents and the currentsteepness (some typical values for the Netherlands are given inTable 1) [6].

Table 1 Typical stroke parameters in the Netherlands

Parameter Normal High Extreme

Max current value in kA 100 200 400

Charge in C 100 300 1000

Energy in MJ/Ohm 1 10 100Current steepness in kA/s 50 200 300

The number of lightning strokes, the number of damage cases and thelosses due to lightning strokes vary from country to country. In the

Netherlands thunderstorms occur on 107 days per year and on averageabout 2 to 3 lightning strokes occur per km2per year.

Some extreme cases of damage can be mentioned, for instance strokesthat occurred at 13 July 1997 in New York caused a damage of1 billion US Dollar and it was the reason that 10 million people didnthave electricity for 20 hours.

Detailed information about the process of lightning, atmosphericovervoltages and the influence to power grids is given in [5].

Figure 1 An example of a thundercloud

(rain is not necessarily present)

2


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Lighting Impulses

4

2.3 Lightning impulse tests

2.3.1 Background of li ghtning impulse tests

Serious studies in high voltage engineering began in 1836. In 1886the first transformer was patented. The invention of this transformer

opened the door to efficient and effective power systems working athigh voltages [2]. After 1895, when it was used in combination with

power prime movers, the use of electricity became more widespread.Within a few years the practical use of electricity increased and afterthe First World War the demand for electricity rose spectacularly [7].

The development of the equipment needed for the transportation ofelectricity was rapid up to UHV (Ultra High Voltage) levels (132 kVin the 1920s, 400 kV in the 1950s, 1100 kV in the 1970s).Unfortunately, the equipment at these higher voltages was introduced

before the principles were fully understood. The first equipmentinstalled at 132 kV was prone to fail due to lightning strokes. As

mentioned in the previous paragraph, lightning strokes can causeblack outs and considerable damage. The studies towards thelightning stroke failures in the 1920s led to a better understanding oftransient voltage distributions within the windings of transformers andto the introduction of lightning impulse tests [2].

2.3.2 Appl icable standards

Tests with impulses are designed to demonstrate the response of theequipment to transients over a wide frequency range. Lightningimpulse voltages represent transients occurring naturally in highvoltage systems under operation.

To make sure that lightning impulse tests on high voltage equipmentare made in the most correct way and as uniform as possible, expertshave written several standards dealing with the details on lightningimpulse tests.

At least the following four general standards are applicable:

IEC 60060-1, High-voltage test techniques, part 1: Generaldefinitions and test requirements, from 1989

IEC 60060-2, High-voltage test techniques, part 2: MeasuringSystems, from 1994

IEC 61083-1, Digital recorders for measurements in high-voltage

impulse tests, part 1: requirements for digital recorders, from 1991 IEC 61083-2, Digital recorders for measurement in high voltage

impulse tests, part 2: evaluation used for the determination of theparameters of impulse waveforms, from 1996

IEC 60790, Oscilloscopes and peak voltmeters

In the next paragraphs the relevant parts of the standards applicable totests with lightning impulses will be explained in detail. A thoroughunderstanding of these subjects is necessary for understanding theremainder of this thesis.


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2.3.3 Tests with l igh tning impulse voltages

When high voltage equipment is tested with lightning impulsevoltages, a number of lightning impulse voltages of certain amplitudesand shape are applied. A lightning impulse voltage is, currently in

IEC 60060-1 [1], defined by its peak value and its time parameters.Figure 2 shows the standardised lightning impulse voltage with thepeak value Up(U=1.0), the virtual origin O1, the front time T1and thetime to half value T2.

The impulse is usually produced by an impulse generator consistingessentially of a number of capacitors that are charged in parallel from

a direct voltage source and discharged in series into a circuit thatincludes the test object.

Most lightning impulse tests are made using a standard lightningimpulse, having a front time of 1,2 s and a time to half-value of50 s, described as a 1,2/50 s lightning impulse voltage or a standardlightning impulse, that may be chopped to simulate the effect of a

protective gap flashover.

When test are done with lightning impulse voltages, transientrecordings of the applied voltage are taken and in case of testingtransformers also transient recordings of the current at the neutral endof the winding under test are taken. In general, the object passes thetest if there is no evidence of complete or incipient failure fromaudible indications or changes in the voltage or current records [2].

Of course, when applying standard lightning impulse voltages, sometolerances on the parameters are allowed. If not otherwise specified

by the relevant Technical Committee, the following differences areaccepted between specified values for the standard lightning impulseand those actually recorded:

Peak value 3% Front time 30% Time to half-value 20%

Figure 2 Standard lightning impulse

voltage


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2.3.4 Tests procedures

The lightning impulse voltage tests performed are withstand voltagetests. In IEC 60060-1, four tests procedures (A, B, C and D) aredescribed for withstand voltage tests. The recommended test

procedure depends on the nature of the test-object.

Procedure A

Three impulses of the specified shape and polarity at the ratedwithstand voltage level are applied to the test object. Therequirements of the tests are satisfied if no indication of failure isobtained, using methods of detection specified by the relevantTechnical Committee (This procedure is only recommended for testson degradable or non-self-restoring insulation.).

Procedure B

Fifteen impulses of the specified shape and polarity at the withstandvoltage level are applied to the test object. The requirements of thetests are satisfied if not more than two disruptive discharges occur inthe self-restoring part of the insulation and no indication of failure inthe non-self-restoring insulation is obtained by the detection methodsspecified by the relevant Technical Committee.

Procedure C

Three impulses of the specified shape and polarity at the withstandvoltage level are applied to the test object. If no disruptive dischargeoccurs the tests object has passed the test. If more than one disruptive

discharge occurs the tests object has failed to pass the test. If onedisruptive discharge occurs in the self-restoring part of the insulation,then nine additional impulses are applied and if no disruptivedischarge occurs the tests object has passed the test. If any detectionof failure in a non-self-restoring part of the insulation is observedwith the detection methods specified by the relevant TechnicalCommittee during any part of the test, the test object has failed to passthe test.

Procedure D

For self-restoring insulation the 10% impulse disruptive discharge

voltage, U10, may be evaluated by using statistical tests proceduresdescribed in appendix A of IEC 60060-1. These test methods permiteither direct evaluation of U10and U50or indirect evaluation of U10. Inthe latter case U10is derived from the U50value using the relationshipgiven inEquation I. The relevant Technical Committee shall specifythe value to be assumed for the conventional deviation z of thedisruptive discharge voltage. For dry tests on air insulation, withoutany other insulation involved, the per-unit value z = 0,03 can be used.The test object has passed the test if U10is not less than the specifiedimpulse withstand voltage.

Equation I Relation between U10and U50

)3,11(5010 zUU


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The following test methods can be used to evaluate U50:

the multiple-level method with n 4 voltage levels and m 10impulses per level

the up-and-down method with m = 1 impulse per group and n 20useful applications

To evaluate U10, the up-and-down method, with m = 7 impulses pergroup and at least eight useful groups can be used.

In all cases the voltage interval U between levels should beapproximately 1,5 to 3% of the estimated value of U 50.

2.3.5 Measurement of li ghtn ing impulse voltages

The measurement of the applied lightning impulse voltage is usuallydone with an Approved Measuring System. A Measuring System is acomplete set of devices suitable for performing high-voltage orimpulse-current measurements. An approved Measuring System is a

Measuring System that is shown to comply with one or more sets ofrequirements set out in IEC 60060-2 by an initial Performance Testand Acceptance Test, successive Performance Tests and Checks andinclusion of these results in a Record of Performance.

General requirements

The general requirements for an Approved Measuring System aregiven on the next page, more detailed requirements and the methodsfor Performance Checks and Tests can be found in IEC 60060-2:

to measure the peak value of full impulses with an overalluncertainty within 3%

to measure the peak value of chopped impulses with on overalluncertainty which is depending on the time to chopping as follows: for front chopped impulses (0,5 s Tc2 s) within 5% for tail-chopped impulses (Tc2s) within 3%

to measure the time parameters which define the waveform with anoverall uncertainty within 10%

to measure oscillations which may be superimposed on an impulseto ensure that they do not exceed the permitted levels given inIEC 60060-1

the scale factor shall remain constant over long periods. Morespecific, the scale factor of the converting device and the

transmission system shall not vary by more than 1% for the rangesof the ambient temperature and clearances given in the Record ofPerformance

the measuring instruments shall for oscilloscopes and peakvoltmeters comply with IEC 60790 and for digital recorders withIEC 61083-1.

Evidence that the Measuring System meets the requirements is givenin its Record of Performance.


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Measuring System for lightning impulse voltages

A Measuring System for the measurement of lightning impulsevoltages comprises the following components:

converting device with the leads required for connecting this

device to the test object and the connections to earth transmission system connecting the output terminals of the device

to the indicating or recording instrument with its attenuating,terminating and adapting impedances or networks

indicating or recording instrument together with any connectionsto the power supply

software or the procedure for the evaluation of the parameters

Indicating or recording instruments

In high-voltage laboratories world-wide different indicating orrecording instruments for the measurement of lightning impulse

voltage are being used, distinction can be made between the followingthree types of instruments

peak voltmeters analogue oscilloscopes digital oscilloscopes or digital recorders

The evaluation of the parameters is done either manually orautomatic, depending on the type of indicating or recordinginstrument and the availability of software. How the evaluation of thethree parameters (Up, T1, T2) is done precisely is explained in the next

paragraph.

2.3.6

Evaluation of the parameters character ising l ightn ing

impulse voltages

When the applied lightning impulse voltage has been measured, theparameters characterising this lightning impulse voltage have to beevaluated according to the evaluation guidelines given inIEC 60060-1 [1]. The evaluation of parameters is different for fullstandard lightning impulse voltage, for lightning impulse voltageschopped on the tail or on the front for non-standard lightning impulsevoltages. In the next three paragraphs the definitions in IEC 60060-1will be explained in detail.

2.3.6.1

Evaluation of the parameters of standard l ightn ing

impulse voltages

For full standard lightning impulse voltages the following parametershave to be evaluated:

value of the test voltage, U t (in some cases Ut is equal to Up, inother cases not, see the explanation below)

front time, T1 time to half value, T2

Value of the test voltage

For a lighting impulse without oscillations, the value of the testvoltage is its peak value, seeFigure 3 orFigure 2.

Figure 3 Evaluation rules for a Full

lightning impulse according to

IEC 60060-1

Figure 4 Evaluation rules for lightning

impulses with overshoot or oscillation


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With some test circuits, oscillations or overshoot may occur at thepeak of the impulse, seeFigure 4 a) to d). If the frequency of suchoscillations is not less than 0,5 MHz or the duration of overshoot notmore than 1 s, a mean curve should be drawn as inFigure 4 a) and

b). For the purpose of measurement, the maximum amplitude of this

mean curve is chosen as the peak value defining the value of the testvoltage.

Overshoot or oscillations in the neighbourhood of the peak, measuredby a system according to IEC Publication 60060-2, are toleratedprovided their single peak amplitude is not larger than 5% of the peakvalue.

Front Time T1

The front time T1of a lighting impulse is a virtual parameter definedas 1,67 times the interval T between the instants when the impulse is30% and 90% of the peak value Up, (points A and B, Figure 3).

Time to half-value T2

The time to half-value T2of a lighting impulse is a virtual parameterdefined as the time interval between the virtual origin O 1 and theinstant when the voltage has decreased to half the peak value.

2.3.6.2 Evaluation of the parameters of chopped lightning

impulse voltages

For chopped lightning impulse voltages the following parametershave to be evaluated:

value of the test voltage Ut front time T1 time to chopping Tc

The value of the test voltage and the front time have to be determinedusing the definitions given in the previous paragraph.

Time to chopping

The time to chopping Tc is a virtual parameter defined as the timeinterval between the virtual origin O1and the instant of chopping.

The instant of chopping is that at which the rapid collapse of voltage

which characterises the chopping first occurs.The instant of chopping is different if the impulse is chopped in thefront or in the tail, seeFigure 6 andFigure 5.

2.3.6.3 Evaluation of the parameters of non-standard

li ghtn ing impulse voltages

For other impulse shapes then the ones explained in the previous twoparagraphs (see for example Figure 7 e)-g)) the relevant TechnicalCommittee shall define the value of the test voltage taking intoaccount the type of the test and test object.

Figure 5 Evaluation rules for a lightning

impulse chopped on the tail

Figure 6 Evaluation rules for a lightning

impulse chopped on the front

Figure 7 Examples of non-standard

lighting impulse voltages


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2.3.7 Validation of software, IEC 61083-2

When the parameters characterising lightning impulse voltages areevaluated automatically with the aid of software, this software has to

be validated according to IEC 61083-2. In IEC 61083-2 the test

procedures to be applied to assess the accuracy of software used toprocess and read the records of impulses and calibration signals arespecified. More specific, it:

defines the terms specifically related to digital processing establishes the tests which are necessary to show that the software

is compatible with the requirements of IEC 61083-1 andIEC 60060-1

specifies limits on estimated values of parameters of the referencewaveforms

gives the requirements for the record of performance

The compatibility with the standard can be demonstrated with

software validation. The software is validated using the TDG (testdata generator). The TDG is a computer program that generatesreference waveforms with specified parameters.

Software may be validated for the evaluation of one or more sets ofimpulse parameters (e.g. T1, Up). Any parameter for which thesoftware is validated shall be evaluated for all reference waveforms ineach selected waveform group. The values of the parametersdetermined by the software under test are compared with the limitsgiven in IEC 81083-2, table 2. The software under test passes the testfor each set, if all values of that parameter set are within the specifiedlimits. For a full lightning impulse a deviation of around 1% is

allowed.In the Record of Performance information about the software should

be given as well as the list of groups of impulses for which the testswere performed and the list of parameters for which the software wastested and passed.

2.3.8 Measurement uncertain ty

The measurement of the parameters characterising a lightning impulsevoltage using approved Measuring Systems, have to be performedwithin the uncertainty limits mentioned in IEC 60060-1 (3% or 5%for the peak value and 10% for the time parameters).

In this paragraph it is explained how the measurement uncertainty of ameasurement made with a Measuring System should be calculated.

2.3.8.1 General

Uncertainty is a statement of [9]:

the limits (U) of the range of values within which the true valueof a measurement is expected to lie in relation to the recordedresult and

the probability of the true value lying within these limits; thisprobability is expressed as the confidence level (B)

An example of a measurement with its

uncertainty is:

1080 kV 20 kV

Estimated confidence level not less

than 95%


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In most measurement, the overall measurement uncertainty will resultfrom a combination of several contributions that are classified intotwo categories according to the method used to evaluate theirnumerical values [8]:

systematic contributions

random contributionsThe overall uncertainty is calculated from these contributions.

As a high-voltage laboratory, one has to comply with severalstandards dealing with measurement uncertainty. These standards are:

IEC 60060-2 annex H [9] STL-guide [10] EAL-2

These standards are in general identical, however they differ on a fewpoints, mainly the calculation of the random contributions. At KEMAHigh-Voltage laboratory the measurement uncertainty is calculated

with the method described in IEC 60060-2, since this is the mostprecise. Therefore only this standard is explained here.

2.3.8.2 I EC 60060-2 annex H

In this standard the two categories of contributions to the uncertaintyare also:

systematic contributions to the overall uncertainty, Us random contributions to the overall uncertainty, Ur

The systematic and random contributions to the uncertainty arecalculated separately. The overall uncertainty is calculated fromthese two contributions by the formula given inEquation IV.

2.3.8.3 Systematic contr ibutions

According to IEC 60060-2 annex H, systematic contributions arethose that are not evaluated statistically but are estimated by othermeans. Two examples are the uncertainty of a calibration and theresolution of an instrument.

There are two types of systematic contributions:

rectangular (ssa); these systematic contributions have a rectangulardistribution, that is, any measured value between the estimatedlimits (a, were a is the semi-range value) is assumed to be equally

probable. Gaussian (ssg); it is assumed that these systematic contributions

have a Gaussian distribution

The standard deviation for all systematic contributions (s s) iscalculated from these two systematic contributions according toEquation V.

The systematic uncertainty is calculated from this standard deviationby Equation III. In which k is the coverage factor. The coveragefactor k is 2 for a confidence level B of 95%.

Equation II standard deviation of a

rectangular distribution

Equation III systematic contribution to the

uncertainty

Equation IV Calculation of the overall

uncertainty out of the two contributions,

when Usand Urare calculated at the same

confidence level

Equation V standard deviation of the

systematic contribution

Equation VI experimental standard

deviation, where:

n is the number of measurements

xiare the measured values for i = 1 to n

xmis the mean of the measured values

22rs UUU

22

sgsas sss

22

sgsass sskskU

n

i

mir xxn

s1

2)(

1

1

3

assa


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2.3.8.4 Random contri butions

Random contributions are those that are derived statistically from arepetitive measurement and, being random, will usually be found bymeasurement to tend a Gaussian distribution. Each random

contribution is characterised by the experimental standard deviation(sr) of the sample of measured values, see Equation VI. Thecontribution to the uncertainty is then given by Equation VIII. Thestudents t factor t is 2,26 for 10 measurements.

If a value of sr has been established from a large number ofmeasurements (e.g. n 20) for a measuring system to which nosignificant change is made, then the uncertainty in a subsequent singleor repeated measurement (n2) is given inEquation VII.

2.3.8.5 Overal l uncertainty

The overall uncertainty is calculated from the random and systematic

contributions to the uncertainty using Equation IV. A more generalform for the overall uncertainty U is given inEquation IX.

This implies that the overall measurement for one measurement canbe calculated withEquation X.

2.4 Implementation of the standards

2.4.1 Generation of li ghtn ing impul se voltages

The generation of a lightning impulse voltage with high amplitude isgenerally done with a lightning impulse generator, also known as a

Marx generator. The principle of such a generator is parallel chargingand series discharging of its capacitors, see Figure 8. The peak value,the front time and the time to half value of the generated lightningimpulse voltages are a function of the impedance of the load (objectunder test), the inductance of the whole test circuit and the resistancesand capacitances of the generator [2].

For some arrangements, e.g. large generators, high inductive load orlow resistance of the test circuit, oscillations or/and overshoot occurat the front or near the peak of an lightning impulse voltage. Anexample of such a case is given inFigure 10. It is being assumed thatthese oscillations and/or overshoot can be of significance and a

maximum amplitude of 5% of the peak value is allowed. Thereforeeffort is put into generating lightning impulse voltages withoutovershoot and/or oscillations [11].

For some test arrangements the oscillations and/or overshoot can bereduced by adapting the test circuit. When adapting the test circuit, acompromise have to be made between the amplitude of theoscillations and/or overshoot and the permissible front time (tolerance30%) and time to half value (tolerance 20%). For some testarrangements a standard lightning impulse can not be generated [2].

Equation VII calculation of the uncertainty

for n220 with the uncertainty obtained

for n20

Equation VIII random contribution to the

uncertainty, where:t the students t factor

Equation IX general formula for the

calculation of the overall uncertainty

Equation X measurement uncertainty for

one measurement

Figure 8 Schematic diagram of an impulse

generator

22

2

3

1

nk

st

k

UakU r

i

ii

n

stU rr

22

2

3

1

k

st

k

UakU r

i

i

i

2n

skU rr


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2.4.2 Analogue practise of evaluating parameters

In the past, but also in some cases nowadays, the measurement oflightning impulse voltages is performed using analogue oscilloscopes.The evaluation of lightning impulse voltages was performed manuallyusing pencil and ruler. The achievable accuracy was influenced by

the thickness of the trace of the oscilloscope and the accuracy of thetest engineer and is limited [3], see also Figure 9. Since theevaluation had to be performed manually, also the number of

parameters that can be evaluated is limited. The parameters describedin IEC 60060-1, are the most practical ones for manual evaluation.According to IEC60060-1, the definitions of the parameter applyeither to impulses without oscillations or overshoot or to the meancurve drawn trough the oscillations and overshoot. Because of thenatural filtering of the measurement system, only oscillationand/overshoot of higher amplitude and low frequency could be seen.Besides this, the thickness of the oscilloscope trace in conjunctionwith the thickness of the line drawn as the mean curve makes italmost impossible to discriminate between different possible meancurves [12] in case of oscillations or overshoot within the tolerancelimits. In case of large oscillations or overshoot or non-standard waveshapes (e.g. transformer tests) discussing took place between the testengineers to draw the best mean curve. Over a period of sixty years

this manual procedure was the best way to evaluate the parameters ofimpulses measured with an analogue oscilloscope.

2.4.3 Present-day methods of evaluating parameters

Nowadays the measurement of lightning impulse voltages is mostlyperformed using digital oscilloscopes and measuring systems with ahigher bandwidth. Disadvantage of this is that small overshoot andoscillations can easily be made visible. Advantage of this is that theevaluation of the parameters can be done using computer programs.

The advantage of using computer programs for the evaluation ofparameters is that the evaluation is performed more uniformly andfaster. However, software packages have difficulties with theimplementation of evaluation rules made for manual evaluation, sincethey need more precise instructions. Because of that in some cases,for instance non-standard lightning impulse voltage, still a manualevaluation is used.

2.4.4

Resul ts of recent in tercomparisons

To maintain international traceability, from time to time internationalintercomparisons are organised. A recent internationalintercomparison (1995-1996) [4] showed that the requirements of IEC60060-2 can be met for smooth standard lightning impulse voltages.The values of all participating laboratories but one agreed to eachother within 1% for the peak value and within 5% for the time

parameters. Slightly higher deviations of comparison results,especially for T1 were obtained when short front (0,84 0,95 s)impulses were compared. Although an attempt was made to definethe chopped impulses clearly, these comparison results were heavilyinfluenced by different impulse shapes used in different laboratories.

Figure 9 Example of an analogue

oscillogram and manual evaluation

Figure 10 Typical lightning impulses in

transformer testing


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For front chopped impulses, measurement uncertainties in the peakvalue and the time to chopping of larger than 6% were seen.

A comparison of software showed that, by using different methods alarge spread of some parameters determined from different meancurves has been obtained [13]. In paper [17] different methods, all

acceptable to the standard, have been compared and differences fromeach other up to 1,7% for the peak value, 18% for the front time and3% for the time to half value have been obtained when case no 11 ofthe TDG of IEC 61083-2 (seeFigure 11)was analysed.

Both comparisons showed that there are problems with the evaluationmethods given in IEC 60060-1. It seems right now that the hardwareis good enough for the accuracy required nowadays, but that thesoftware or evaluation methods are the major contribution to themeasurement uncertainty due to the ambiguous definitions

2.4.5 Present-day evaluation problems

The procedure for the evaluation of parameters as described in IEC60060-1 is given inFigure 12. As can been seen fromFigure 12 it is avery complex and incomplete procedure. The evaluation guidelines,are ambiguous for standard lightning impulse voltage or not given atall for non-standard lightning impulse voltages, for both manual andautomatic evaluation.

Figure 11 Case no. 11 of the TDG of

IEC 61083-2

Figure 12 Flow diagram of evaluation

procedureFull Lightning Impulse

Determination of Mean Curve

Oscillations orOvershoot ?

Specia l Case ?

Amplitude> 5%?

Freq > 0,5 MHzDur. 1 s?

Test voltage = Peak voltage Evaluate T 1 and T 2

Test voltage = Peak voltageof mean curve

Determine T 1 and T 2

Standard Lightning Impulse

TC determines Tes t voltage

Determine T 1 and T 2

Non StandardLightningImpulse

T 1=1,2 s30%T 2=50 s20%?

Yes

Yes

Yes

Yes

No

No

No

No

Yes

No


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One of the main problems with the evaluation of parametersnowadays is the determination of the mean curve. The standard

allows different mean curves, resulting in different values for the

parameters characterising the same lightning impulse voltage [12].

The reason for the ambiguousness in the standard is that the

evaluation guidelines given in IEC 60060-1 were introduced in thetime there were mainly analogue oscilloscopes were available and theevaluation of the parameters had to be done manually using pencil,ruler and engineering judgement.

By eye it was decided if overshoot or oscillation appeared and in caseof overshoot and oscillation appeared outside the allowed limits, amean curve was drawn. The thickness of the oscilloscope trace in

conjunction with the thickness of the line drawn as the mean curve

made it impossible to discriminate between different mean curvesand therefore the problems with the evaluation guidelines did notarise. Since then a lot has changed.

in 1994 a new standard, IEC 60060-2 ed. 1994, was introduced inwhich the requirements towards measurement uncertainty for themeasurement of lightning impulse voltages are more strict thanthey were before. The cause for this is that the interest ofmanufacturers and purchasers of high voltage equipment arehigher and because the measuring technique has been improved,

the need for the reproducibility and traceability of the test resultshas increased, because of the interest of manufacturers and

purchasers of high voltage equipment and because of theintroduction of EN 45001. The results of the evaluation of the

parameters should be person independent, it should not be possible

that the results are influenced by clients and one should be able tocalibrate the evaluation methods used, digital recording instruments (digitizers and digital oscilloscopes)

have been introduced and nowadays the evaluation of theparameters is mainly done by software. The software thatevaluates the parameters is used to save on the one hand time andon the other hand to be sure that the evaluation of the parameters isreproducible and traceable. A negative, but also positive side ofusing software is that this software needs clear algorithms for theevaluation of parameters. An advantage of using software for theevaluation of parameters is that the engineering judgement and theexperience of engineers can be put into the computer (expert

systems) and that it permits a wider range of parameters than waspreviously practical.

The three changes mentioned above, are the reason that therequirements to the evaluation guidelines for standard lightningimpulse voltages have increased and that the need for evaluationguidelines for non-standard lightning impulse voltages arose. Theyare the reason that the problems with the interpretation of IEC 60060-1 have increased.


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2.4.6 Problems with the interpretation of I EC 60060-1

Independent whether manual or digital evaluation methods are used,there are some problems with the interpretation of IEC 60060-1. Thefollowing list presents some of the problems associated with the

definitions and interpretation of the standard. This list certainly notcomprises all of the questions to be resolved, but most of them areprobably included [14].

What is the definition of the mean curve, especially for impulseswith both overshoot and oscillations?

How to determine the T30% and T90%when there are oscillationsclose to these times (see case no. 11 in IEC 61083-2)?

For the calculation of the front time, T 1, the peak value should beused as reference for the 100% value of the impulse. When youhave overshoots or oscillations, which peak value shall be used,the peak value of the original impulse or the peak value of the

mean curve? How should the amplitude and duration or frequency of

overshoots and oscillations be calculated? How should a non-standard wave shape, for instance those that

occur during real testing,Figure 7 orFigure 10,be evaluated? How to know if they are inside the tolerances of IEC 60060-1? What is the physical background that supports the magical

0,5 MHz value for the frequency of oscillations and the 1svalue for the duration of the overshoot?

Are the parameters described in IEC 60060-1 really theparameters that characterise the breakdown process? Could other

parameters describe this process better? Should the same parameters be used for all materials (XLPE, SF6,oil, vacuum, etc.)?

2.4.7 Current practise in laboratories

In order to know how the high-voltage laboratories around the worldare facing the evaluation problems and to note which evaluationmethods are used, two different questionnaires (one for manufacturersand testing laboratories and another one for calibration laboratories)were prepared in the framework of the European project.

In the questionnaire questions are present about details of the test

circuit that are relevant when oscillations and overshoot are present.Besides this, questions were asked how these laboratories evaluatevoltage and time parameters in the cases of full and chopped impulseswith oscillations superimposed (f 0,5 MHz) and/or overshoot(d 1 s).

Only manufacturers and testing laboratories were asked about thepossible influence on the breakdown behaviour of dielectric materialsof these overshoots and/or high frequency oscillations with smallamplitude superimposed to the lighting impulse. In the following

paragraph the results of the questionnaire concerning the evaluationare presented. In [14]more detailed information can be found.


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2.4.7.1 Resul ts of the questionnair es

23 manufacturer laboratories, 8 testing laboratories and 12 calibrationlaboratories all over the world answered the questionnaires. The mainconclusions are summarised in the next three paragraphs, detailedinformation can be found in [15]. In general it can be stated that the

problems experienced in testing laboratories, laboratories ofmanufacturers and calibration laboratories are greater than expected.

2.4.7.1.1 Manufacturers laboratories

For full impulses:

the majority of manufacturers of power transformers and cablesuse the maximum value as the test voltage (not in accordance withIEC 60060-1), while the majority of manufacturers of other highvoltage equipment considers the peak value of the mean curve

the majority of the laboratories determine T1 from the originalcurve, using the peak value of the mean curve as the 100% value

only some manufacturers of transformers determine T1 from theoriginal curve, using the peak value of the original curve as the100% value

For chopped impulses:

for the two methods to determine the test voltage (maximumvalue, or the peak value of the mean curve) the majority oflaboratories uses the maximum value

For the questions to the possible influence of oscillations andovershoot:

about 30% of the manufacturer laboratories supplied information

of the possible influence on the breakdown behaviour of differentdielectric materials, when overshoots and/or high frequencyoscillations are superimposed on the lightning impulse. There wasno agreement between the received answers.

2.4.7.1.2 Testing laborator ies

For full impulses:

all laboratories consider the peak value of the mean curve as thetest voltage, which is in accordance to the standard IEC 60-1

the majority of the laboratories use a mean curve that removes theoscillations to determine the time parameters

For chopped impulses: almost all laboratories consider the maximum value as the test

voltage the majority of the laboratories consider the time to the point

when the voltage decreases suddenly as the time to chopping

The answers from testing laboratories are in a better agreement thanthe ones of manufacturer laboratories, and are also in betteragreement with IEC 60060-1.


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2.4.7.1.3 Calibration laboratori es

For full impulses:

half of the laboratories consider the peak value of the mean curveas the test voltage and the other half considers the maximum

value as the test voltage the majority of the laboratories use a mean curve that removes

oscillations to determine the time parameters

Calibration laboratories propose also some following alternativemethods for evaluating lightning impulses:

Method 1: Comparison of the wave shapes of the referencemeasuring system and the approved measuring system beingcalibrated.Method 2: Use of a tolerance band. The tolerance band isdetermined taking into account the uncertainty of the referencemeasuring system.

For chopped impulses:

the majority of the laboratories considers the maximum value asthe test voltage

there is no general agreement on how to determine the time tochopping, although in the majority of the answers the time tochopping is considered to be the time when the voltage decreasessuddenly

2.5 International work

In this paragraph the state of the art will be given of the internationalwork that is being carried out at this moment on the field of theevaluation of parameters characterising lightning impulse voltages.Basically there were two approaches, one approach tried to solve the

problems in the evaluation of the standard lightning impulse voltagesand the other one tried to solve the problems with the evaluation ofthe non-standard lightning impulse voltages. The author introduced athird one.

2.5.1 Current status of evaluation guidelines for standard

li ghtn ing impulse voltages

The problems with the evaluation of parameters have been recognisedby CIGRE WG 33.03 and within CIGRE WG 33.03 the evaluationguidelines for parameters characterising standardised lightningimpulse voltages as given in IEC 60060-1 are being discussed forseveral years now. Within the CIGRE WG 33.03, even a special taskforce has been given the assignment to investigate the evaluation of

parameters.

The main problem is nowadays considered to be the definition of themean curve. When using manual evaluation the mean curve isdrawn by hand, when using software for the evaluation the mean

curve is some mathematical function.

One thought that when one was able to find an unambiguous

mathematical function for this mean curve the evaluation problemsof the parameters characterising standardised lightning impulse


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voltages would be solved. The definition of the mathematical functionof the mean curve is however not so simple and people have

proposed also different solutions (see Chapter 2.6). Besides newmethods for calculating the mean curve also other methods like thetolerance band have been introduced. The disadvantage of most ofthe proposals explained in Chapter2.6 is that these do not solve the

problem for non-standard lightning impulse voltages.

2.5.2 Current status of evaluation guidelines for non-

standard lightn ing impulse voltages

In September 1996 CIGRE WG33.03 and IEC SC 12 proposed toformate a joint task force with SC 12. The objectives of this joint taskforce are:

to set the parameters to be measured and the limits of uncertaintywhich should be achieved

to develop and evaluate a Test Data Generator, similar to the one

used in IEC 1083-2, but for lightning impulse voltages thatrepresent those which can occur in transformer tests

preparation of a report from CIGRE 33.03 to IEC TC42, providingtechnical basis for a new IEC publication

At the first meeting of the joint task force a start was made to list theparameters which are considered to be either essential, or useful, todetermine the result of a test with lightning impulse voltages on atransformer (see Chapter2.6). The consideration of these parameterswas based on the breakdown processes due to the effect of the appliedlightning impulse voltage. It was taken into account that the use ofdigital recorders and the processing facilities now available permits

the measurement of a wider range of parameters than has beenpractical before [16].

This proposal has been discussed in the relevant committees and aTest Data Generator is being made. The join task force has been seton hold, because it was found best to wait for the results from theEuropean research project, that is discussed in the next paragraph.

2.5.3 PhD and Eu ropean research project

According to the author the two approaches were too much based onmathematical models and assumptions. When maintaining these twoapproaches a solution or two solutions that would be accepted by the

many parties would never be found. To solve the problems with theevaluation for both standard and non-standard lightning impulsevoltages as discussed in CIGRE WG 33.03 and the joint taskforceTC12/WG33.03, the relevancy of the parameters had to be proven.This required research. Therefore the author, as an employee ofKEMA Nederland BV, initiated to propose, together with FFII/LCOE,the Schering Institute and NGC, to the European community DG XIIin the SMT fourth framework, a research project. The title of thisproject is Digital measurement of parameters used for lightning

impulse test for high voltage equipment. The project has started atJanuary 1 1997 and the author was the project leader and thought

about what to do in the project. The duration of the project was twoand a half years.


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The aim of this project was to:

define one or more sets of parameters to characterise lightningimpulse voltages

write a report in which the relevancy of the proposed parametersare explained

establish unambiguous algorithms to calculate these parameters write a proposal for the relevant parts of IEC 60060-1 and

IEC 61083-2

At KEMA the author was responsible for carrying out the work, shewas the supervisor of the students involved in the project, built thetest set-up together with M. Kvarngren and carried out most of theinvestigations and tests herself.

Besides the work performed in the framework of this Europeanproject, more work has been performed. The author made anadditional literature investigation and was responsible for andevaluated the measurement results of the investigations on paper-oil

samples provided by Siemens made at TU-Graz.

All the work explained before in this paragraph is presented in thisthesis.

2.6 Current evaluation methods

In this paragraph an overview is given from the currentautomatic/mathematical evaluation methods. The currentmathematical evaluation methods and the developments in these can

be divided into two categories:

evaluation methods using IEC 60060-1 parameters and its

definitions evaluation methods using other parameters and definitions

In [17] an overview is given from the methods available, which willbe summarised here and is supplemented there where necessary. Atthe end of this paragraph some conclusions are drawn.

2.6.1 Methods based on I EC 60060-1

The definitions in IEC 60060-1 apply to impulses without oscillationsor overshoot and to the mean curve drawn through the oscillations orovershoot. To decide whether or not the parameters have to beevaluated from the original curve or the mean curve, a mean curve has

to be drawn and the frequency or the duration of the overshoot have tobe determined. In other words, the frequency or the duration of theresidual curve, R(t), has to be analysed. R(t) is the difference betweenthe original curve W(t) and the mean curve M(t). Another possibilityis to determine the main frequency directly by means of FFT.

The first step in the analysis is to detect and to remove oscillations,second step in the evaluation is the detection and removal of theovershoot. There are different methods available for determiningoscillations and overshoot. They will be explained separately.


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2.6.1.1 Methods for determin ing oscill ations

There are two types of mean curves that can be used for determiningwhether or not the parameters shall be calculated from the meancurve:

Global mean curves

Local mean curvesRegardless of the criterion applied it is necessary to use a numericalmethod to calculate the mean curve. Many options have beendeveloped, the majority on the basis of fitting to mathematical modelsand others by means of digital filtering.

2.6.1.1.1 F itti ng to mathematical models

Mathematical models most frequently used for determining meancurves and some newly introduced ones are:

straight line

circumference arcs [18] third degree polynomials, seeEquation XI [19], [20], [21] double exponential functions, see Equation XII [22], [23], [24],

[25] quadruple exponential functions, [26], [27], [28] double exponential functions with sin and/or cos terms, see

Equation XIV [29] tension splines [30] wavelet packett transform [31]

2.6.1.1.2 Digital fi lteri ng

There are two methods of digital filtering which have been applied forthe evaluation of parameters of impulses:

moving window filtering [32], [33] low pass FIR filtering [34]

2.6.1.1.3 Choosing the reference mean curve

Before analysing the R(t) it is clear that the best reference mean curveshould be chosen, preferably by a mathematical procedure.

There are several methods for determining the best RMC (referencemean curve), this is to minimise the:

absolute mean value of the residual curve within a specific timeinterval (MP)

root mean square of the residual curve within a specific timeinterval (RMSP)

least mean square error between the measured impulse and theRMC (LMS), seeEquation XIII [29]

Also generetic algorithm models have been used to select thereference mean curve [35].

Equation XI third degree polynomial

Equation XII double exponential function

Equation XIII Least mean square error of

the measured impulse Um and the mean

curve Umc

Equation XIV double exponential function

with sin and cos terms

ttmc eeAU

ttttmc etCetBeeAU sincos1

n

i

cm iUiUF1

2

432

2

3

1 ptptptpUmc


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2.6.1.2 Methods for determining overshoot

Before overshoot can be determined, it is useful or even necessary toremove in advance any oscillation or superimposed noise. Anovershoot can be detected analysing the peak zone of the lightning

impulse voltage. There are two methods for detecting overshoot: fit the tail with a single exponential function. If the impulse is

above the single exponential function that fits the tail of the wavethere is an overshoot, other wise there is no overshoot

compare the lightning impulse voltage with a specific mean curve.If the lightning impulse voltage is above the reference mean curve,there is an overshoot, otherwise there is no overshoot

Both method have some disadvantages, it is said that with the firstmethod an overshoot can exist below the single exponential function,with the second method, the result depends on the specific meancurve used.

The two methods result into three possibilities [36], as shown inFigure 13. In [17] some alternative methods for determining theduration of an amplitude of the overshoot are given, using the virtual

peak value.

2.6.2 Other methods

Since the methods investigated do not show acceptable agreements,two different methods have been proposed:

the method using a tolerance band introducing new parameters

2.6.2.1 Tolerance band

The major problem with the method described in the previousparagraph is, that in any case a mean curve has to be drawn and thatdifferent mean curves can result in different values for the parameters.A method that avoids the use of a mean curve is the tolerance bandmethod. Inside this tolerance band any real standard lightningimpulse voltage should be found. This band of a 1,2/50 impulse must

be obtained from the values for the tolerances given in IEC 60060-1.

In Figure 14 an example is given of a tolerance band for a 1,2/50lightning impulse voltage. This tolerance band method can easily be

Figure 13 Three different possibilities to

determine the overshoot duration


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calculated with a computer and requires less computing time than thecalculation of reference mean curves.

2.6.2.2 New parameters

During lightning impulse tests with transformer non-standard waveshapes are measured, see Figure 10. For these wave shapes noevaluation methods are given. Therefore within the joint taskforceSC12/WG33.03, a possible set of parameters to be measured duringimpulse test on transformers were specified and defined. This set of

parameters has to be proven relevant by the transformer industry andby experimental investigations.

The set of parameters consists of parameters that are needed for testswith other types of apparatus and parameters that are required fortransformer tests only [37].

parameters needed for tests on other types of apparatus base level origin virtual origin

peak value time to chopping average rate of rise (front time)

parameters required for test with transformers extreme value of a full impulse extreme value of the undershoot of a chopped impulse time to extreme value time interval Tk wave integral average rate of voltage collapse during chopping

2.6.3

ConclusionsThe present rules indicated in IEC 60060-1 are insufficient and manydifferent interpretations are possible. In addition, very high accuracyis required for the present definition and for the set of the limit of theovershoot or oscillation. Therefore there are two possibilities, eitherto give better and more useful definitions of the parameters of interestor to completely change the approach [17]. In case of the first option

better and clearer requirements should be included in the standard todetermine the parameters of an impulse. But independent of themethod chosen, more extensive discussion and experimental evidenceis needed to establish the requirements for the evaluation of

parameters characterising lightning impulse voltages to be included inthe revision of IEC 60060-1.

Figure 14 Tolerance band for a 1,2/50 lightning impulse voltage (on the left the zoom of the full tolerance band on the right)


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3 Disruptive discharge phenomena

3.1 Introduction

Before the goal of the investigations will be explained and the tests

performed and their results will be described, a theoreticalinvestigation towards the disruptive discharge behaviour of insulatingsystems, or better materials, will be presented.

Disruptive discharges (also called breakdowns) are phenomenaassociated with the failure of insulation under electrical stress, inwhich the discharge completely bridges the insulation under test,reducing the voltage between the electrodes practically to zero. Itapplies to electrical breakdowns in solid, liquid and gaseousdielectrics and combinations of these. The theoretical investigationwas also focussed mainly on the influence of the wave shape of alightning impulse to the discharge. At the end of this chapter

attention is paid to statistical techniques used when performingbreakdown tests.

3.2 Insulation systems

3.2.1 General

Dimensioning an insulation system requires exact knowledge of thetype, magnitude and the duration of the electric stress whilesimultaneously considering the ambient conditions. The design andconstruction of high-voltage equipment requires also experience inthe laws of electric fields.

Insulation arrangements for high voltage usually contain liquid, solidor gaseous insulating materials which strength is many times that ofatmospheric air. For the practical application of these materials notonly their physical properties, but also their technological andconstruction features must be taken into account [38]. The propertiesof the insulating materials must be know, so that the optimum designof the insulating system can be chosen. A particular problem in thisrespect is that the determination of the properties of the insulatingmaterials is done with model samples under standardised conditions.Extrapolation of these models to real insulating systems is notunconditionally possible. For instance in case of transformers (which

will be explained in more detail in the next paragraph), the stress isnot evenly distributed and the samples used for testing are relativelysimple models. In any case, before the breakdown behaviour ofinsulating systems or samples can be fully understood, the breakdown

behaviour of insulating materials should be known.

3.2.2 Transformers

3.2.2.1 L igh tning impulse tests on transformers

One apparatus that is difficult to test with lightning impulse voltagesbecause of the superimposed oscillations and overshoot that occur is atransformer. For the dimensioning of a transformer, the magnetic,electric and thermal fields are important.

3


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Particularly in the region of the windings, strong electric fields occurin transformers, were high tempera

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