Digital
Testing
of
High4oltage
Circuit
Breakers
circuit hreaker is a switching device that the American National Staiidarcls Institute (ANSI) defines as: "A mechanical switching device, capa- ble of making, carrying, ancl breaking currents under nor- mal circuit conditions and also making, carrying for a
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ence but also an art. Because of the complex phenomelia involved, circuit breaker prototypes have to be verified by practical tests in the laboratory. In high-pnwer labora- tories, the ability of circuit breakers to interrupt shart- circuit currents is verificd in test circuits, which are in
intended t o operate
q u e n t 1 y al t h o u g h infre- som e types are suitable for fre- quent operation."
High-voltage circuit breakers play an impartant rolc in transmission and distribution systems. They must clear faults and isolate faulted sections rapidly and reliably. In short, they must possess the followiiig qualities:
In closed position, they are good conductors
In open position, they are excellent insulators
w They can close a shorted circuit quickly and safely
without unacceptable contact erosion
They can interrupt a rated short-circuit current, or lower current, quickly without generating an abnor- mal voltage.
The only physical mechanisiii that can change in a short period of time from a conducting to an insulating state at a certain voltage is the arc. It is this principle on which
all circuit breakers are Iiased.
The first circuit breaker was developed by J.N. Kel- man in 1901. It was the predecessor of the oil circuit breaker and capable of interrupting a short-circuit cur- rent of 200 to 300 A in a 40 kV system. The circuit hreak- er was macle up of two wooden barrels containing a mixture of nil and water, in which the contacts were
immersed. Since then, circuit breaker design has uncier- gone a reinarkable dcvelopment. Nowadays. one pole of a circuit breaker is capable of interrupting ti3 kA in a 550
1 V nctwork, with SF,, gas as the arc quenching medium.
Still, the design of a circuit Iweakcr is not only a sci-
' Delft llnivcrsity of Techriolnfiy
KEMA Iii~h-I'(Jwer Laboratory
fact lumped clement representations of the power sys-
tcm. These test circuits must produce the correct wave- forms for thc (short-circuit) current as well as for the voltage that strikes thc circuit breakcr immediately after the breaker has interrupted the test current. The wave- forms of current and voltage to which the test object is subjected are laid down i n ANSI and lnternatioiial Elec- trotechnical Commission (KC) standards. These stan- dardized waveforms represent 9OX of the possible fault conditions in the real system.
Circuit
Breaker Switching
and
Arc
Modeling
The switching action, the basic function oi thc circuit breaker, refers to the change from conductor to insula- tor at a certain voltage. Be€ore interruption, the (short- circuit) current flows through the arc channet of the circuit breaker. Bccause of the nonzero resistance of the arc channel, this short-circuit current caiises a voltage across the colitacts of the circuit breaker: the arc volt- age. The arc behaves as a nonlinear resistance. Thus, both arc voltage and arc current cross the zero-value at the same time instant. If the arc is coded sufficiently at the time the current goes through zero, the circuit break- er interrupts the current, because thc electrical power input is zero. During current interruption, the arc resis- tance increases from practically zero to alinnst infinite in microseconds. Immediately aftcr current interruption, the transient recovery voltage builds up across the cir- cuit breaker. As the gas mixture in the interelectrodc space does not change to a completely insulating state instantaneously, the arc resistance is finite at that time, and a sniall current can flow: thc post-arc current,
Black-box arc models are mathematical descriptions of the electrical properties o l the arc. This type of model does not simulate the complicated physical processes inside the circuit breaker liut describes the electrical properties of the circuit breaker. Measured voltage and current traces are used to extract the parameters for the differential equations describing the nonlinear resistance of the electrical arc for that specific measurement.
Digital
Testing
The functionality of high-voltage circuit breakers is tcst- ed in high-power laboratories, Due to the ncccssary power and the physical size of the equipment, testing is rather expensive and timc consuming. In order to obtain as much information as possible about the degradation and operating limits of the circuit breaker from the cost- intensive tests, a project started with the following part- ners: KEMA High-Power Laboratory, The Netherlands; Delft University of Technology, T h e Nethcrlancls; Siemcris RG, Germany; RWE Encrgie, Germany; and Laborelec cv, Helgium. This project is sponsored by the Directoratc GeIieral XII of the ELiropcan Commission in Standards, Measurements, and Testing Prograiii under contract number SMt'4-CT96-212 1. The project is aimcd at developing digital testing of high-voltage circuit break- ers, i.e., a software product for testing of ;Imodel of such
a device, once its characteristic fingerprints are ohtaincd from rcfinecl ineasurements during standard tests. Digi- tal testing offers a wide range of new possibilities for users, manufacturers, standardizing bodies, and test lab- oratories for fine tuning circuit hreakcr abilities in rela- tion with standards and real power systems. Some developments arc:
Evaluation of th e relevance of future standards with respect to real power systems
I Evaluation of the relevance
of
future standards fordifferent circuit breaker technologies and cxtin- giiishing media
I Estimation of the circuit breaker's interrupting limit
II Reduction of full-scale testing in high-power labora- tories
w Iclentificatioii of network topologies that can pose spccial difficulties lo a circuit brealrer
I Acceleration of development of new circuit breaker
clcsigns
Monitoring the aging processes f l t circuit breakers in service
Expansion OF services for high-powcr laboratories. The steps followed so far to enable digital testing are described in the following sections. At the end of the arti- cle, examples of digital tcsting are presented.
Measurements
and Data Analysis
I-ligh-resolution ineasurcmcnts of current and voltage in the critical period around short-circuit current zero must supply the necessary parameters, characterizing the breakers' behavior. A tailor-macle high-frequency measuring system was realized for this purpose. This system consists of a number of battery-pciwcred, single- chaniicl, 40 MHz, 12 bit AD converters, each storing the data temporarily in on-board local RAM (2Stik samples each). The concept of on-site data storage is necessary for reaching a maximum overall sysLern bandwidth, Cables to the current and voltage S ~ I I S O ~ S can t h u s be kept very short, and thc system can operate 011 floating
potential. The arc voltage is measured with standard brnad-band RCR-type voltage dividers; current is mea-
surecl with a spccial Rogowski coil. After the remote
RAM
is fillcd, data is transmitted serially through optical fibers to the proccssirig unit in the command ccntcr. The greatest challenge with respect to developing the equip- rneiit in this application design lies in the electromagnet- ic compatibility, since t h e microelectronics has t o function in an extremely hostile environment of intenseEM fields of various origin.
The system relies heavily on digital signal processing methods for reconstructing the actual voltage and cur- rent signals from the raw scnsor output. On the one h a n d , t h i s has to dc) with t h e specific frequeiicy response of the sensors and on the other hand, with cor- rections needed fnr the (reproducible) induced voltages and capacitive current that distort the measured signals. 'rests in various laboratorics have proven that thc sys- tern can measure post-arc current as small as SO mA,
microseconds after the interruption of many tens of kA. Data analysis software has been produced to carry out the signal reconstruction practically on line during the tests (Figure l), and to evaluate the performance OF
the tcst object. Even the newest profcssionat multipur- pose mathematical or laboratory software is not corn- petitive to this custom-made software considering
Short-circuit Current
I \
L\ Fails to BreakI
I
1 m sI
. . . I I .,, ...Figure 1. Vdfage and current mensurrment data
Z+gure 2. Perameter extraction software
flexibility and speed in visualizing and data processing of practically unlimited amount of measured data in a user-friendly way.
After an extensive series ol the most critical fault interruption duty for circuit breakers (the so-called “short-line fault,” see the section on “Applications of Digi- tal Testing”), a test database from various types
of
com-0.50 8 I
....
0 10 20 30 40 50 BO 70
Sequential Number of Tesl
F&ur~ 3. Degradation oPthe circuit b r e d e r poles
mercially available circuit breakers was set up. With this experimental material, an empirical arc model Iiased on classical arc models was validated that gave very good coverage of the observed processes. From the total num- ber a! ( > E O ) interruption attempts, the result of the attempt (failure/success) was predicted correctly in more than 90% of the cases by evaluating the character- istics of the arc behavior with the model.
The inodel has a set of (three) parameters, which are extracted autninatically during the cvaluation of each test (Figure 2). Automated analysis of thc collection of all the parameter sets (in nther words, the breakers’ “finger- prints“) obtained from a whole series of tests makes it possible to evaluate various physical quantities as a function of test conditions.
The aim of using this method is t o quantify the hreak- er performance (the margin M of interruption), indicat- ing how successful the breaker passed the test (M > 0) or how far off it is from passing it (M
An example is given in Figure 3, where the clegracla- tion of the three breaker poles (A, B, and C) is presented during a sequence of successive tests. It can be seen clearly that the margin of the breaker decrcascs with every test. The rate of margin decay (among others) is a Ineasure of the endurance of the breaker with respect to this type ot tests.
0).
Arc-Circuit Interaction Software
At the final stage of the realization of digital testing, mea- sured arc model parameters will be used as input for the arc model. Of course, this arc model behaves a a nonlin- ear element in the electrical circuit ancl must therefore be analyzed with a dedicated cnmputer program. The analysis of arc-circuit interaction iiivolving nonlinear clc- ments in relation to stiff differential equations makes it necessary to perform the calculations with a variable step size and adjustable accuracy of the computed cur-
rents, voltages, and canductaiices. Because they have fixed step-size solvers, EMTP ancl comparable programs are less suitable for this purpose and thercfore a new
approach, the integration of differential algebraic cqua- tions (DAE) by means of the backward differentiation for- mulas
@DO
method, has been chnsen in clevelopinga
new softwarc package lor electrical transients computa- tion. This new transient program, XTrans, has been developed at the Delft University of Technology especial- ly lor arc-circuit interaction studies. The program runs on a PC with the MS-Windows operating system and works fully graphical, as shown in Figure 4. The program is in use at several high-power and high-voltage laborato-
ries in the world.
The program makes use of libraries that contain infor- mation about the behavior of element models. The pro- gram structure is depicted in Figure 5 . This structure has been realized with object-oriented programming. The compiled code of t h e element models is placed i n dynamic link libraries
@Us).
The models are, thereforc, separate from the main program, which makes it easy tocreate new models and use them in the main program. A full working demonstration version of thc XTrans program can he downloaded from the homepage of the electrical power systems group at the Delft University of Technology, http://eps.et.tuclelft.nl.
Applications of Digital Testing
Influence of Parallel Capacitance
Powerful possibilities with digital testing are created when the arc model, validated as described in the sec- tion on Measurements and Data Analysis, is coupled with a circuit analysis packagc. Then, the performance of a circuit breaker, the fingerprints of which were obtained from real tests, can be estimated in circuits other than the test circuit.
For examptc, the influence of various standard substa- tion components on the breakers' capaliilities can be estimated through digital testing.
Here the influence of a parallel capacitance is calculat- ed (for example, the parasitic capacitance at a current transformer, CT,) in the substation. In Table 1, the perfor-
mance of a short-line fault interruption i s compared in the presence of two types OE CTs: CT 1, having 200 pF of parasitic capacitancc, and C1' 2, having 400 pP. These CTs can be located near the circuit breaker and remote (the latter implying a n additional 50 pH of busbar between CT and breaker). As a reference, the case with-
out CT has a performance o f 1 .O.
Table 1 shows that the difference between the two types of CTs is rather sinall when compared to the gain obtained by the CT that was installed to the breaker as
closely as possible.
Critical Line Length Determination
One of the most severe currents for a circuit breaker to interrupt is the short-line fault (SLF). In the case of a short-line fault, the short-circuit point is on a high-volt- age transmission line a few kilometers away from the breaker terminals. After current interruption, a very steep, triangular-shaped waveform (with a rate of rise of 5-10 kV/microsecond) stresses the extinguishing medi-
um between the contacts. The percentage SLF indicates to what extent the short-circuit current is reduced by the transmission line, e.g., a short-circuit current of 40 kA is reduced to 36 kA in case of a 90% SLF. In the IEC stan- dard, 75% and 90% SLF tests are prescribed.
As an example of digital testing, the critical line length, the short-line fault percentage that stresses the
circuit breaker most, will be determined for a 145 kV,
Figure 4. XTrans transient program
I Libraty 1 1 Library I
L ----_J L I_-_- I__I _-__
Artificial Lino r - - - 1 _. - - - I
r
I F 3 -ModelI'
F-*
w
Figure 6. A direct SLF test circuit in the XTrans program
31.5 kA, SF, circuit breaker. A direct SLF test circuit is shown in Figure 6, Thrce different indicators, active at different time intervals (before current zero, at current zero, and after current zero) are used to quantify the
slress on the circuit breaker model.
Before ciirrent zero: the timc before current zero where the arc resistance equals thc surge imped- ance of thc transmission line t(H =
3,
The closer thc value is to current zero, the more severe the breaker is stressed by the test circuit.At current zero: the arc resistance XO. The lower the arc resistance value at the current zero cross- ing, the stronger the breaker is stressed by the test circuit.
After current zero: thc post-arc ener.gy Epa. This value is t h e integral nf th e multiplication of the small post-arc current and the recovery voltage. It is clear that only for successful interruptions an Epa value can be calculated. The higher t h e Epa value is, the more severe the breaker is stressed by the test circuit.
The actual computation is based on 75 current zero recordings of the circuit breaker of which t h e circuit breaker model parameters have been determined. For cwh sct of parameters, the stress at the various short- h i e fault percentages is computed. At last, the overall stress is visualized, which is shown in Figure 7.
All three indicators show that the circuit breaker model is stressed most severely at a 93% SI ,F, whereas a 90% SLF is prescribed in the IEC standard. This shows that digital testing can he applicd to use the information o1)tainecl from laboratory tests for the development nf
fuuture stantlards.
Acknowledgments
The authors are indebted to the Ih-ectornte Gencral X1I of the Eurc-
penn Cominissiori i n Stiuidartls, Measurements, aiitl l'esting Program for sponsoring. The effort nf the other partners involvcrl in the project is greatly appreciated.
For Further Reuding
K.P.P. Sinccts, V. Kertesz, "Lvaluatinn of high-vollage circuit Imakcr performance with a ncw arc model," IEE Pmreediqs ort Gerierorion
1.5 1,4 -5 1,2 g 1,3 5 1 , l
5 1
,$
0,9 .-5
0,8 a,7 W 0,5 85 86 07 00 09 $0 9 1 92 93 94 95 % SLF -I----_ -+- RO + t(R=Z) EpaFigure Z Criticai h e length determination by meum flfdig!td testhi
Tmrisrriission, orid Uislribuliori,
N.I).H. Hijl, L. van tlcr Sluis. "Ncw approach to t h e calculatinn of el ectri cnl transients ," Ewopetm Trmmtuhms on .E!clcctrictd Po u;w h g i - m e r i n g (ETEO, Volume 8. Niiinlicr 3. pp. 175-179 arid 181-IR6.
May/June 1998.
Biographies
Pictcr H. Scliavemaker ohtaiiietl his MSc in electrical engineering from
tlw Delft Univcrsity of Technulqy i n 1994. Alter gratluntion, he per- Iorinccl rcscnrch OIL puwer system state estimation with the Power Sys- t e m s 1.aboratory. In 1995, lic s t a r t e d as an application engineer programming Stilistatlon Control Systems with ABU in the Netherlands. Since 1996, he hns bccn with thc Power Systems I.ahoratnry, where he is currently assistant prnfessor. He is working 011 PhD rcscai-cli 011 cligi-
tal testing of high-voltage circuit tireakers within the framework of a Luropenn project. His rnniii rcscarcli interests include power system transients antl power system calculations. H e is a member 01 KEE. He can be readied by E-mail, P.H.Schavcinakc~its.tudelft,r~l.
Lou van der Sluis obtained his MSc i I i electrical engineering
from t h e Delft Ilniversity of Technology in 1974. Hc julrrcd thc KEMA High-Powcr Labriratory in 1977 as a test enginccr and was irivolvcd in t h e development of ;t data acquisition system fur thc Higti-Puwer Laboratory, cornputer calculations of test circuits. ancl the analysis of test data by digita1 cornputer. In 1990, l i e became a part-lime pro- fessor, and, sincc 1992, tic has beeri employed as n full-time profes-
sor a t t h e llelft Unlvcrsity 01 Tccti~iulugy in the Pnwer Systems Department. He is a senior member of IKEE arid corivc~icr of WG CC-03 of Cigrb and Circd to study the transient recovery voltages in metliutn and high voltage networks. IIe can he reached by Bmail, L.vanderSluisQi ts.tiidellt.nl.
R e d P.P, Smeets ohtninetl his MSc in physics trorl1 tlie Eiridhrwen University of Technology in 1981. Hc ohtairwtl the PtiD degree in 1987
for research work on vaciiiiin arcs at tlic sninc university. From 1983 tc
1995, tic was a staIf rriember of the Riiergy Systems Ulvision of thc Fac- ulty of Electrical Engineering, Einrlhoven Unlvcrsity. During tlie ycar
1991, hc spcrit a sddiatical leave at 'I'oshilm Corporation's Heavy Appa- ratus EIigiricerinR Laboratory i n Japan. 111 1995, 11c joined KEMA for R8rD 01 thc High-Power Lilboi-atory. I IC: is n memticr of Cigrd WG 13.04, the Current Zero Club, Cigr6, and the IEEE, I le can he reached hy 1; niail, r.~~.~~.smeetsQkemn.ril.
Viktor Kcrt6az otitained his MSc in electrlcal engincerlng irotn Butlapcst University nf TechndoLy iii 19G6, Phi) in 1976, and nSc in 1989. Hc jclirlccl the High-Puwer Laboratory of the Electrical Power Research Institute, Uutlapcst, as a rcscarcher arid t a t erigirieer i n 1966. He ihecainc a professor of mathematics at the Budapest Ilniversity ol 'I'ctchiiology I n I979 arid has hcld thc chair of full professor since 1989.
He has had n scicntiflc contact with KEMR in the iield of circuit breaker measuring prolilenis antl the analysis of arc pticnnmcna slncc 1978. He contributed as a m e m l w tn Cigrt W(; 13.01 (arc niotlclirig). He can tle reached by L m a i l , kertes7.vQelender.liu.
