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INFLUENCE OF THE NEW K-FACTOR METHOD OF THE IEC DRAFT

60060-1 ON THE EVALUATION OF LIGHTNING IMPULSE

PARAMETERS IN RELATION TO ULTRAHIGH-VOLTAGE TESTING

Martin Hinow 1*and Thomas Steiner11HIGHVOLT Prftechnik Dresden GmbH, Germany

*Email:

Abstract: High-Voltage Impulse Generators are broadly applied for standard quality tests on high-voltage isolations such as power transformers. The wave shapes for the generated impulses arestandardized with their time parameters and tolerances in IEC 60060-1[1]. The mentioned standardwill be reviewed after nearly twenty years by the IEC where one of the most important changeswill obtain the introduction of the k-factor method in order to improve the overshoot evaluation.Thereby not only the magnitude but also the duration of an overshoot will be taken into account.Especially the application of ultrahigh-voltage testing complicates the realization of the permitted

parameters of front time T1 and overshoot . Under certain conditions the realization of bothparameters is very difficult. Therefore, the paper presents two overshoot compensation circuits and

compares theirs physical principle. It discusses the corresponding parameter ranges concerning thephysical and technological possibilities of an ultrahigh-voltage testing.

1. INTRODUCTION

The production of high voltage equipment for the usein the energy grids is vulnerable to small failures in theinsulation. An intensive quality control has been alsoestablished with the high voltage testing to ensuredefined insulation properties of the producedequipment. The insulation is stressed with gridadequate stress which includes an AC testing, a

switching and a lightning impulse testing. The latter isperformed by an impulse testing system which containsthe system components impulse generator, choppinggap, voltage divider and test object.

General testing parameters are defined in the horizontalIEC standard 60060-1. In particular the lightningimpulse is among others defined by peak value Up, the

front time T1, time to half value T2and the overshoot.Validation rules are defined in the vertical componentspecific standards.

With the goal to deliver large quantity of power over

long distances new energy grid projects e.g. in India(1200 kV AC) or China (1100 kV AC) provided a newvoltage class definition, the ultrahigh-voltage (UHV).

New testing standards have to be published for theUHV equipment. A current question is how and inwhich relation the present testing standard IEC 60060-1 can be applied to UHV equipment.

The current paper deals with the application oflightning impulse testing to UHV equipment. Itdiscusses the evaluation of overshoot and explains the

physical principle of parallel and serial overshootcompensation. Finally it identifies physical and

technological limits of UHV lightning impulse testingin relation to the existing IEC standard 60060-1.

2. STATE OF THE ART

2.1. Overshoot definition

The impulse test system can be described by theequivalent circuit, see figure 1. The current paperdistinguishes between the component generator and theadditional system components. The following elementsare considered:

Cs = charging capacitance

SG = spark gapRE = discharging resistorRD = front resistorLG = generator parasite inductanceLS = parasite inductance of the

additional system componentsCP = parasitic generator capacitanceCAFC = chopping gap capacitanceCT = test object capacitanceCU = voltage divider capacitanceRU = divider resistor.

Figure 1: equivalent circuit of an impulse testingsystem.

The impulse test system provides in the ideal case withno circuit inductance LG = 0 and LC = 0 the ideallightning impulse of two superimposed exponentialfunctions. The low but physical not complete avoidableinductances of the impulse circuit causes asuperimposed oscillation, which is called overshoot,see figure 2.

CS

RD

RE CP CAFC CT

CU

RU

LSSG LG

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Proceedings of the 16th International Symposium on High Voltage Engineering

Copyright c 2009 SAIEE, Innes House, Johannesburg

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Figure 2: Principle of an ideal lightning impulse1.2/50 with a superimposed oscillation (overshoot).

The definition and possibilities for evaluationprocedures of the overshoot have been investigated [2,3] and broadly discussed [4]. In order to takeoscillation magnitude and duration into account afrequency weighted overshoot evaluation (k-factormethod) has to be applied. Its principle and advantagesare broadly documented [5]. The evaluation of the k-factor method is presented in [6, 7].

The relative frequency weighted overshoot * is givenby:

(1)*

t

bt

U

UU =

Where Ubis the maximum of an ideal impulse which isused as a reference curve and Utis the maximum of thetest voltage curve which presents at its peak theequivalent dielectric stress on the insulation as therecorded curve Ue. The equivalent dielectric stress onthe isolation for Ut can be evaluated by using the k-factor method. Therefore the following equation isused:

( ) ( ) (2)tebt

UUfkUU +=

The k-factor function has been evaluatedexperimentally with break down tests by [2, 3] and can

be expressed with:

(3)2.21

1)(

2f

fk+

=

The corresponding function is shown in figure 3. Thismeans in practice that the k-factor method reduces thevalues of overshoots with high frequencies. Overshoots

with low frequencies will be assessed with nearly thesame value.

Figure 3: Experimental results of [2, 3, 5], the k-factordescribes the relation between breakdown voltage and

overshoot frequency.

In contradiction to the frequency weighted overshoot

estimation an only magnitude related overshoot isstill in discussion [4]. This term is given by

(4)e

be

U

UU =

where Ueis the maximum of the recorded voltage. Thisevaluation procedure estimates an overshoot regardless

if the oscillation is 1ns or 1s what is physically

incorrect.

2.2. Identification of the inductance afflicted

components

As shown in figure 2 the overshoot is a physical notavoidable phenomenon. Thus the following paragraphdeals with the question which components of thetesting system are inductance afflicted and in what kindof testing respectively in which voltage class thementioned overshoot occurs dramatically. The highvoltage theory book [8] indicates the length relatedcircuit inductance of an impulse testing system for all

components with L= 1H/m. A detailed considerationshows that it has to be distinguished between thegenerator inductance LG and the inductance LSof theadditional system components. The first one dependsmainly on the quality of the front resistors andcharging capacitances and is proportional to thenumber of stages. The latter depends on the quality ofthe additional system components and the high voltageconnections between them. Higher voltage testingclasses require longer distances between the highvoltage components. Thus a impulse UHV testingsystem is naturally afflicted with a higher parasitecircuit inductance than a corresponding system of a

lower voltage testing class.

2.3. Overshoot compensation

Goal of overshoot compensation is to reduce inaddition to the implemented front resistors the

Time in s

Voltage

in

kV

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superimposed oscillation to acceptable values.Therefore the oscillating energy of the overshoot has to

be stored in an additional component, what can bemainly a resonant circuit. Such resonant circuit can berealized as parallel or as serial overshootcompensation. The first solution is shown as an

equivalent circuit in figure 4 which has beenimplemented successfully by HIGHVOLT for a1800 kV impulse testing system.

Figure 4: Impulse testing system with a parallelovershoot compensation consisting of the componentscompensation resistor, compensation capacitance andcompensation inductance.

The resonant circuit has to be adapted on the circuittypical overshoot frequency. The oscillating energy ofthe impulse front is stored in the compensationelements and is returned to the system during theimpulse tale. The serial overshoot compensation works

by the same manner. An additional serial circuitabsorbs the oscillation energy during the impulse frontand returns it to the system during the impulse tale.The principle of the equivalent circuit is shown infigure 5.

Figure 5: Impulse testing system with a serialovershoot compensation consisting on the componentscompensation resistor, compensation capacitance andcompensation inductance.

The current paper emphasises that the overshootcompensation is not the ultimate solution for overshootreduction. The overshoot compensation works togetherwith the front resistors. A detailed adaptation of thefront resistors may lead also to an effective overshootreduction.

Beside the mentioned resonance circuits also additional

overshoot reducing components e.g. Schniewindtresistor elements are existing. These low inductiveelements are installed between the generator and thetest object in order to reduce the oscillation.

3. COMPARISON BETWEEN PARALLEL

AND SERIAL OVERSHOOT

COMPENSATION

Both compensation principles can be compared withvarious criterions. The most important one is the

effective reduction of overshoot. The serialcompensation is a trap circuit and the parallelcompensation is an absorption circuit. Bothcompensations use the same physical principle of aresonant circuit what leads to the consequence that

both compensation should have similar reductionbehaviours. Otherwise the parallel compensationcontains a capacitance what increase the totalcapacitance of the testing circuit. A consequence of itcould be that the load of the test object has to bereduced by the value of the parallel compensation. Amuch better solution is to use small capacitances in anorder of magnitude which is much less than CC < 1nF

to make the mentioned disadvantages negligible.

From another point of view the serial compensationrequires always a compensation resistor RC whichworks in the circuit as an additional damping resistor.This item provokes longer front times.

However the effectiveness of both circuits is primarilydetermined by the adaptation to the relevant overshootfrequency. The frequency adaptation is an engineering

process which requires detailed knowledge about thefrequency behaviour of all system components.Moreover the compensation circuit has to work

together with the generator owned damping RD.

Figure 6 shows an example comparison between aserial, parallel compensated and uncompensated1800 kV lightning impulse. Serial and parallelcompensation reduce the overshoot with the same

effectiveness from *= 10 % to *= 3.0 % (serial)

respectively *= 2.6 % (parallel) by keeping the front

time of T1= 1.3 s.

All relevant ATP-EMTP [9] simulation data aresummarized in table 1.

Figure 6: Example simulation for serial, parallelcompensated und uncompensated 1800 kV lightningimpulse with a test object capacitance of CT= 5nF.

RD

RE CP CAFC CT

CU

RU

LSSG LG

CC

RC

LC

CS

RD

RE CP CAFC CT

CU

RU

LSSG LG

CC

RCLC

8 10 12 14 16 18 20 22 24

Time in s

0

400

800

1200

1600

2000

Voltagein

kV

Uncompensated

Parallel Compensation

Serial Compensation

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Table 1:Data of the lightning impulse circuit and theapplied overshoot compensation.

Component valueTest object capacitance CT[nF] 5Voltage divider capacitance CU[nF]Voltage divider resistance RU[Ohm]

Chopping gap capacitance C AFC[nF]Parasite capacitance CP[nF]Compensation capacitance serial CC,serial[nF]

Total circuit inductance LS[H]

0.8990

0.890.474.145

Other decision criterions for installing overshootcompensation are the:

Investment cost

Suitability and

Installation size.

Serial compensation can be installed as a singlecomponent or as several to the stages dividedcomponents. Investment cost of the latter one isdetermined by cost per stages and by the number ofstages. Especially for UHV testing systems with a highnumber of stages this item is a disadvantage. If thecompensation is installed as a single component, theinvestment cost for parallel and serial compensationcan be assumed as equivalent. Only two high voltconnections have to be un-, installed in the testingsystem for a single component solution formaintenance activities. The stage related solution

requires a higher effort in this case but has theadvantage of a compact execution.

4. UHV TESTING

Testing of UHV equipment presents a new dimension(up to 3 MV) of testing systems. Because of that theissue increases of overshoot reduction by keeping acertain front time. The higher voltage level requires acorresponding higher number of generator stages whatleads consequently to a higher generator related circuitinductance LG. Moreover the UHV-level causes higherdistances between the high voltage system components

in order to keep the dielectric requirements. The longercircuit leads also to a higher circuit inductance of theadditional system components LS. The higher circuitinductance provides automatically a higher ratio ofovershoot and a lower overshoot frequency. The lattermeans that the frequency weighted overshoot

evaluation (*) will not provide lower -values byusing the k-factor method in comparison with the todayused evaluation procedure. In this case the differences

between* and can be considered as small becauseof the low overshoot frequencies at approximately f=250 kHz. A second simulation clarifies the relation

between front time T1 and overshoot compensation in

the UHV range, see figure 7. An uncompensated 3 MVlightning impulse for a test object with a capacitance ofCT =5 nF has an overshoot of more than 16 % by

keeping a front time of T1= 1.56 s. The total circuitinductance for the entire testing system has been

assumed with L = 70 H. An overshoot compensationcan reduce the oscillation to a certain level. A completeabsorption is not possible. The simulation example hadthe goal to reduce the overshoot to 5% by increasingthe front resistor RD. Both compensation principles

provide equivalent results, the front time increases up

to 1.8 s what is not conform to the IEC 60060-1. Itshould be left to the relevant technical committee todefine which parameter is more important and to allowthe corresponding tolerances.

Figure 7: Example simulation for serial, parallel

compensated und uncompensated 3000 kV lightningimpulse with a test object capacitance of CT= 5 nF

The data of the simulation example is summarized inTable 2. The marginal difference between bothevaluation procedures can be seen as well as thedifference between a lightning impulse with andwithout compensation.

Table 2:Simulation data of a UHV lightning impulse

ParameterUncom-

pensatedParallel

compensatedSerial

compensated

T1[s] 1.5 1.8 1.8* [%] 16.1 5.0 5.0

[%] 16.7 5.4 5.4

5. CONCLUSION

Testing of UHV equipment permits not an identicalapplication of the existing IEC 60060-1. Especially theevaluation of overshoot and its permitted value has to

be reviewed. As the simulation showed the permitted

maximal front time of T1=1.56 s and the allowedmaximal overshoot of 5% are in a discrepancy. The

current paper shows that serial and parallel overshootcompensation has an equivalent efficiency foroscillation reduction. Moreover it has been showed thatthe k-factor method reduces the overshoot values onlymarginal.

8 10 12 14 16 18 20 22 24

Time in s

0

500

1000

1500

2000

2500

3000

3500

Voltage

in

kV

Uncompensated

Serial Compensation

Parallel Compensation

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6. ACKNOWLEDGMENTS

Authors would like to express their thanks to MatthiasBirle from Technical University Ilmenau (Germany)for the support with the system simulation.

7.

REFERENCES

[1]

IEC 60060-1 (1989-11) High-Voltage TestTechniques Part 1: General Definitions and TestRequirements.

[2] Pascual Simon Comin: Research of thecharacteristic parameters of the behavior ofdielectric media under non standard lightningimpulse in high voltage; PHD thesis University ofZaragoza Spain 2004.

[3]

Sonja Berlijn, "Influence of lightning impulses toinsulating systems, PhD thesis TU Graz Austria,2000.

[4]

IEC TC 42 Maintenance Team: Committee Draftfor Voting 42/236 CDV: High-voltage TestTechniques, Part 1: General definitions and testrequirements, 2008

[5]

S. Berlijn, F. Garnacho, F. Simon, E. Gockenbach,P. Werle, K. Hackemack, M. Watts "Final report:Digital measurement of parameters used forlightning impulse tests for high voltageequipment", EU contract no. PL- 951210-SMT-CT96-2132, 1999

[6] Anne Pfeffer: Analysis of lightning impulsevoltages using the k-factor; diploma thesisUniversity of Stuttgart, Germany, 2007.

[7]

F J. K. Hllstrm et al.: Applicability of differentimplementations of k-factor filtering schemes forthe revision of IEC60060-1 and -2, Proceedingsof the XIVth International Symposium on High-Voltage Engineering, Beijing, 2005, paper B-32, p92..

[8]

A. Kchler: Hochspannungstechnik SpringerBerlin Heidelberg 2005; ISBN: 3-540-21411-9

[9]

EEUG: ATP EMTP realise 2008; AlternativeTransients Program (Simulation Software)

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