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A3-304
Session 2004
© CIGRÉ
THE STATISTICS behind the ELECTRICAL ENDURANCE TYPE TEST
for HV CIRCUIT-BREAKERS
applied by CIGRE SC A3/WG13.08 and IEC SC 17A/WG29
A.L.J. Janssen*
NUON Assetmanagement
the Netherlands
C.E. Sölver
ABB Power Technologies
Sweden
INTRODUCTION
For medium voltage circuit-breakers (≤ 52 kV) a non-mandatory electrical endurance type test has been defined
already in the old IEC-Standard 60056 as well as in the present IEC-Standard 62271-100. In addition, since
decennia, the specification of an electrical endurance type test for circuit-breakers with a rated voltage ≥ 72,5 kV
is under consideration. To study the necessity of an electrical endurance type test for modern circuit-breakers and
to specify such a type test, when necessary, IEC SC 17A has established a task force at its 1996 meeting in
Jakarta. Shortly before CIGRE SC13 had formed a new WG 13.08 to study Life Management of CircuitBreakers and this working group has been asked to collect information about short-circuit currents in service.
Although a number of publications were available about the number of short-circuit currents to be interrupted by
HV circuit-breakers, none of these papers had a worldwide scope [1][2][3][4][7][8]. Therefore CIGRE WG13.08
conducted an international enquiry among utilities from 13 countries over 4 continents. The number of responses
was 18, sometimes from several or all utilities in a country, sometimes from one utility, that might cover the
whole country. The enquiry form was kept as simple as possible in order to collect reliable information. The
questions were put forward in a certain order so that utilities could stop answering further questions when
detailed information was not available. Questions about the impedances of OH-lines, the system’s X0/X1 ratio,
the OH-line’s X0/X1 ratio, the ratio R/X, future developments, etc. have not been put forward. However, later
the IEC Task Force has investigated such parameters on a smaller scale.
The results of the international survey have been reported to CIGRE SC13 at the end of 1997: 13-97(SC)31 IWD
and was also given to the IEC Task Force. The IEC Task Force used the basic information of the document to
simulate the cumulative electrical stresses in service by means of software models and, as stated before, it used
also the input from the investigation into some specific parameters.
The IEC Task Force was later transformed into a regular Working Group, WG29. It made several proposals for
the specification of an electrical endurance type test for high-voltage circuit-breakers. As the first proposals were
too complicated, the international community asked for simplification of the proposed procedures. Furthermore
questions had been put forward about the necessity to require an electrical endurance type test for modern
circuit-breakers, as single pressure SF6-gas circuit-breakers are less vulnerable to the electrical stresses than their
predecessors in technology.
A well-known publication in this respect is [5] and the formula mentioned in this publication are used by the IEC
WG to translate the electrical wear from lower short-circuit currents to a number of tests with a higher shortcircuit current. Quite often, the formulas given in [5] are simplified to a wear proportional to the square of the
short-circuit current. Anyway, for modern circuit-breakers the electrical wear of T30 is about one tenth of that of
T100; and the wear of T10 drops even to below one percent. The formula of [5] has been derived from tests in a
anton.janssen@nuon.com
high power laboratory, but the results are in accordance with the experience as reported from service conditions.
With the exception of some very specific applications, like auxiliary breakers in high power laboratories or
generator circuit-breakers, modern circuit-breakers do not show an endurance problem when interrupting shortcircuit currents in service1. Electrical endurance limitations in service are more likely to be faced when
interrupting very frequently (thousands to tens of thousands operating cycles) the rather small currents of shuntcapacitor banks, back-to-back banks and shunt reactors, especially in case of re-ignitions and/or restrikes.2
In contrast to the IEC WG, CIGRE WG 13.08 was not convinced of the necessity to specify a designated
electrical endurance test. Based on the statistical information collected, CIGRE WG 13.08 was in favour of a
combination of the electrical endurance test with the existing test duties (for instance T10, T30, T60 and T100 to
be performed without maintenance in between) in order to keep the test simple and cost-effective. The reason
behind the different opinions of IEC WG29 and CIGRE WG13.08 was the interpretation of the statistical data.
Background information will be given about the stochastic basis for the statistical calculations and the problems
faced when combining two statistical distributions.
The international survey of CIGRE WG13.08 can be split into two categories of information collected. One
category is about the number of short-circuit currents to be interrupted and the other category is about the
amplitude of the interrupted currents. The whole population covered 70,000 circuit-breaker years and 900,000
OH-line km*years in the voltage classes of 63 kV and above.
1. NUMBER OF SHORT-CIRCUITS
Data collection
Per voltage class the number of faults (short-circuits) per year were asked, the number of circuit-breakers, the
number of OH-line circuit-breakers, the number of faults per 100 km OH-line per year, the statistical distribution
of the number of faults per OH-line, the percentages of 1/2/3 phase faults, the percentages of auto-re-closing (OC, O-CO-C, O-CO-CO), the number of faults per 100 km cable per year, per 100 transformers per year and per
busbar per year.
More than 90% of all faults occur in OH-lines, meaning that from a point of view of electrical endurance the
scope can be narrowed to OH-line breakers. For the higher voltages 90% of the faults are single-phase faults; for
the lower voltages 70%. According to [6] 60% of the single-phase faults and 66% of the two-phase faults occur
in the same phase (pole). This is caused by the statistical scattering because of the low number of faults per OHline and can easily be explained by throwing a limited number of times a dice: the average probability to hit a
certain number of the dice is one sixth, but twelve throws will for sure not result into exactly two hits per
number.
The data collected about auto-re-closing gave that 80% is successful (O-C). In 5% of the cases the second
re-closing was successful (O-CO-C) and in 15% the full sequence had to be used (O-CO-CO).
From the data collected the number of faults per 100 km OH-line shows to be approximately reverse
proportional to the voltage level. As from the information of the enquiry or by separate information collected
from most utilities the average line length could be calculated/estimated, it could also be deduced that the line
length is proportional to the voltage level. So, CIGRE WG 13.08 came to the conclusion that the number of
faults per OH-line is more or less independent from the voltage level, as will be discussed in detail in the next
chapter. This conclusion is roughly confirmed by the fault distribution for different voltages as given by a large
utility.
The most important data collected were about the average number of faults per 100 km OH-line and about the
average line lengths. These data were collected per voltage class. Not all answers were used, as several utilities
had very small sub-populations of OH-line lengths in a certain voltage class and one utility gave an extreme high
number of faults for 550 kV (11.8 per 100 km OH-line). Furthermore not all average line lengths were exactly
known. Table 1 gives the number of answers used during the evaluation.
1
Similar conclusions can be drawn for modern medium voltage technologies, where the electrical endurance also seems not
to be a real problem
2
Some circuit-breakers may face hundreds of short-circuit current interruptions, especially in regions with a high isokeraunic level (lightning) or regions often exposed to bush fires, but the related short-circuit currents are very low, about 1
kA, and do not substantially contribute to electrical wear, at least for modern circuit-breakers.
2
Table 1
Number of answers used for the evaluation
Number <
100- 200- 300- 500of
100 200
300
500
700
answers kV kV
kV
kV
kV
#/100 km 8
14
16
10
6
km real. 3
6
9
7
1
km estim. 2
4
5
3
3
>
700
kV
1
1
The number of answers is very small and therefore
the statistical basis is poor for further evaluation.
Nevertheless, the outcome of the evaluation shows
results that are not too far from figures mentioned
in the literature [1][2][3][4][7][8]. Also the
tendency that the number of faults per OH-line
seems to be independent from the voltage level, as
mentioned above, can clearly be seen.
Another item to be considered is that the answers given concern the average values for that country or utility per
voltage class. In the CIGRE evaluation the data are treated as single answers and not as averages: all the data
behind the averages are not known. Meaning that an average value to WG 13.08 is the average of the answers
given and not the average of all data behind the answers. Likewise a 90% percentile is the calculated value above
which 10% of the answers belong and below which 90% of the answers fall. It is not the 90% percentile of all
data behind the answers. Especially extreme data can show a larger variation than given in the collection of
answers.
On the other hand, it would have been very difficult if not impossible to collect all the data behind the answers,
apart from the question whether all these data should have been weighted equally or country-wise. Anyway, the
answers themselves gave already a wide variation as can be seen in table 2, where the answers as counted in
table 1 are used.
Table 2
Variation in the answers in the international survey
# faults
<
100- 200- 300- 500- >
per 100 km 100 200 300 500 700 700
kV
kV
kV
kV
kV kV
and year
Average 11.5 5.1
3.1
2.0
2.0 1.7
Median
10.5 4.7
2.3
2.0
1.2 1.7
Maximum 27.8 16 14.5 4.0
5.7 1.7
Minimum 1.0
0.7
0.9
0.3
0.5 1.7
Max/min
28
23
16
13
11
1
Data evaluation
The difference between the average and the median
value shows whether there is a large deviation from
a normal distribution. To calculate percentiles, one
has to be careful and for that reason CIGRE WG
13.08 exactly described the algorithms applied to
calculate the 90% and 10% percentiles. In their
approach, they defined the maximum as the 100%
percentile and the minimum as the 0% percentile,
even for the subpopulations with only a few
answers.
This means that the 90% percentile is lower than the maximum value. For instance, in case of 5 answers the
calculated 90% percentile lays between the maximum and the next value, while one could also argue that the
maximum value itself is the 100% percentile as well the 90% and the 80% percentile.3 4
Table 3
Number of faults per OH-line to CIGRE
# faults
<
100- 200- 300- 500per OH-line 100 200 300 500 700
and year
kV kV kV kV kV
Average
2.2 2.0 1.4 1.5 1.4
Median
2.3 1.6 1.1 1.3 1.1
90% percentile 3.3
3.8 3.2 2.6 2.6
>
700
kV
2.8
Table 4
Number of faults per OH-line to IEC
90%
<
100- 200- 300- 500percentiles 100 200 300 500 700
kV kV
kV
kV
kV
fault density 17.3 8.3
4.8
3.3
4.2
line length 39
76
107 125 153
# faults per 6.7 6.3
5.1
4.1
6.4
OH-line and
year
>
700
kV
1.7
268
4.6
It is IEC’s policy to cover 90% of the cases in service. Therefore, it is important to look for the accumulated
electrical stresses that covers 90% of all cases. To that purpose the number of faults per OH-line have to be
estimated. CIGRE WG 13.08 calculated per pair of answers - the number of faults per 100 km and the line length
– the (average) number of faults per OH-line. Then, with the results per voltage class, the average, median and
90% percentile were calculated. The last figure showed to be comparable for the different voltage classes and not
to deviate too much from the overall 90% percentile (3.3): see table 3. The approach of the IEC WG was
3
From a point of view of mathematics, such calculations for just a few answers is madness.
Another approach would be to assume and fit a normal distribution and to calculate the standard variance. Based on a
normal distribution the 90% percentiles show to be some tens percent higher, again with limited confidence, than the 90%
percentiles to the calculations of WG 13.08.
4
3
different, as they used per voltage class the 90% percentile of the density of faults together with the 90%
percentile of the line length, thus higher values (table 4). To the opinion of CIGRE WG 13.08, IEC’s values lead
to a 99% percentile, as will be discussed further on. IEC’s argument is that the line length is not so important as
the far end of the line will give low values of the short-circuit currents, which have almost no contribution to the
total electrical wear.
2. AMPLITUDES OF SC-CURRENTS
Data collection
Utilities have been asked to fill out a histogram of the maximum expected short-circuit current in the substations
(Ib) divided by the rated short-circuit breaking current of the circuit-breakers (Ir). Five utilities answered the
question by giving 12 histograms (for different voltages). Two other utilities gave averages and 90% percentiles
for 6 classes. As the number of answers is even smaller than in the former chapter a statistical analysis will be far
from accurate: to be recognised in the evaluation of CIGRE WG 13.08, where the overall average of Ib is stated
to be between 40% and 60% of Ir and the overall 90% percentile of Ib to be between 70% and 80% of Ir.
The large difference between Ib and Ir is caused by several margins, like the small probability that the conditions
for the maximum short-circuit power are fulfilled, the margin used by the utility for future developments, the
margin caused by the discrete steps in the standard ratings, etc.
Two utilities did a marvellous job by verifying the information collected in their protection systems and
extracting data of actual short-circuit currents in service. The actual short-circuit current is influenced by the line
impedance up to the location of the fault, the number of phases involved, X0/X1 ratio of system and line, the
actual short-circuit power at the busbar, the arc resistance of the fault, etc. From the figures given CIGRE WG
13.08 estimated that the average value of actual short-circuit currents is about 20% of Ir and that the 90%
percentile is between 30% and 40% of Ir.
Data evaluation
Unlike with the density of faults, for the ratio Ib/Ir CIGRE WG 13.08 has made no differentiation to the voltage
classes. All figures from the histograms have been put together and from there the percentiles have been
calculated. Table 5 gives the percentiles.
Table 5
Estimated percentiles of Ib/Ir to CIGRE
50% percentile
60% percentile
70% percentile
80% percentile
90% percentile
0.49
0.54
0.60
0.70
0.80
Table 6
Estimated 90% percentiles of Ib/Ir to IEC
90%
percentile
Ib/Ir
<
100
kV
0.62
100200
kV
0.73
200300
kV
0.76
300500
kV
0.78
500700
kV
0.74
>
700
kV
0.66
IEC WG29 only calculated and used the 90% percentile of the ratio Ib/Ir, but on the other hand made a
differentiation between the voltage classes, based on the data from two large utilities: table 6.
3. COMBINATION OF TWO DISTRIBUTIONS
Figure 1
Cumulative distributions for 245 kV circuit-breakers
4
procent
procent
100
90
90
80
80
70
70
60
60
50
50
40
40
30
30
20
0,8
0,7
0
0,6
Figure 2
Cumulative distribution of fault density with Isc
90%
10%
100%
70%
Percentiles of nr. faults per 100 km
80%
30%
50%
Isc at
busbar/Irated
60%
Number of faults per km
50%
0
0,5
5
30%
4
40%
3
90%
70%
10
10%
2
20
20%
10
percentil. max. Isc
Figure 3
Cumulative distribution of fault density with Isc
Cumulative distributions
For 245 kV the cumulative distribution of the density of faults and the maximum expected short-circuit power on
the busbars, calculated by CIGRE WG13.08, is given in figure 1. Both distributions are regarded as independent,
so that they can be put in a three dimensional cumulative distribution figure with on the x-axis the density, on the yaxis the severity and on the z-axis the probability, as outlined in figure 2.
“Chess-board” approach
It has been decided to choose a slightly different approach, by putting the probability for the density (percentiles of
number of faults per 100 km OH-line) on the x-axis and the probability of the severity (percentiles of Ib/Ir) on the yaxis. The two dimensional cumulative distribution can be calculated from the percentiles, as will be discussed further
on, and is outlined in figure 3. The pattern of probabilities along the x-axis and the y-axis has been called the “chessboard”: figure 4.
Point X represents the combination of the 90% percentile of the severity and the 90% percentile of the density. To
the upper row belong the circuit-breakers facing more faults per 100 km per year than the 90% percentile; i.e. 10% of
the circuit-breakers. To the utmost right column belong the circuit-breakers installed in situations with a higher
maximum expected short-circuit current than the 90% percentile; again 10% of all breakers. Square A represents all
circuit-breakers with a number of faults higher than the 90% percentile and a higher short-circuit power than the 90%
percentile; therefore square A contains 1% of the whole population.
Figure 4
Figure 5 245 kV/40 kA
5
Table 7
Reduction from the density percentiles
Density perc. #/100 km Reduction
90% percentile
4.8
80% percentile
3.5
-27%
70% percentile
3.2
-33%
60% percentile
2.4
-50%
50% percentile
2.3
-52%
81% of the total number of circuit-breakers face a density lower than the 90% percentile and a short-circuit power
lower than the 90% percentile; i.e. the 81 squares below and left from point X. The right hand column up to square A
(9 squares) are occupied by the circuit-breakers facing a short-circuit power larger than the 90% percentile, but less
faults than belonging to the 90% percentile of the number of faults per 100 km. The upper row up to square A (9
squares) are occupied by the 9% circuit-breakers that face a larger number of faults than the 90% percentile, but a
lower maximum expected short-circuit current at the busbar than the 90% percentile.
Each square represents 1% of the whole population. Point Y represents the combination of the 80% percentile for the
density of faults and the 90% percentile for the severity. 2% of all circuit-breakers face a density between the 80%
percentile and the 100% percentile together with a severity between the 90% percentile and the 100% percentile.
Half of these circuit-breakers occupy square A (between 90% percentiles and 100% percentiles) and the other 1%
occupies square B (80%-90% percentile and 90%-100% percentile).
In each square the electrical wear can be given belonging to the combination of percentiles (in square A for point X,
in square B for point Y, etc.). In square A the 90%/90% case will be shown and this is referred to as the reference
case, as the IEC WG used this combination to specify the electrical endurance type test (using somewhat different
values for the 90% percentiles of Ib/Ir; compare Tables 5 and 6). In the other squares the reduction in electrical wear
in comparison to the reference case will be given.
Examples
By a special software, for each voltage class and each short-circuit current rating the distribution of faults along the
line and the amplitude of the short-circuit currents can be calculated, taking into account a number of parameters like
line length, line impedances, number of phases involved, auto-re-closing sequence, system parameters, etc. The
distribution of calculated short-circuit currents is converted to an equivalent number of T100 tests (single “O”operations) by applying the formula of [5].
The outcome of this exercise shows that the number of T100 tests is proportional to the assumed density of faults, as
may be expected. For 245 kV the reduction from row to row is given in table 7. The effects of the percentiles for the
short-circuit power are very complicated and cannot be extrapolated: the software is needed. For a 245/40 kA circuitbreaker the right upper corner of the chessboard has been calculated (fig. 5).
Figure 6 550 kV/50 kA
Figure 7 72.5 kV/25 kA
6
The next step is to determine which 10% of the circuit-breakers (which 10 squares) represent the highest electrical
stress. Or, in other words, 90% of the circuit-breakers will face an electrical wear less than given in those 10 squares
(will face a higher reduction than those 10 squares). In the example given a reduction of -47% would fulfil this
requirement: the shaded squares in figure 5.
Other classes of rated voltage and short-circuit current give different patterns and reductions. Two examples are
elaborated in the figures 6 and 7 for a 550 kV/50 kA circuit-breaker resp. a 72.5 kV/25 kA circuit-breaker.
4. FORMULATION OF ELECTRICAL ENDURANCE TESTS
Activity within IEC SC17A
After many years of preparation, including many discussions and drafts, on March 15, 2002, IEC SC 17A has
distributed a Draft Technical Report 17A/629/DTR for voting. At the IEC SC 17A meeting in Beijing (October
2002) some questions have been raised with respect of possible mathematical mistakes or misinterpretations in this
document, that through positive voting by the IEC National Committees had been accepted. Based on statistical
considerations, described in former chapters, Sweden later put forward an official appeal against publication. At
voting a majority of the IEC National Committees were in favour of the appeal; their comments have been compiled
in IEC 17/972/INF. However IEC’s Standardisation Management Board had to decide on the subsequent request
from SC 17A for a derogation to circulate a second draft technical report and voted not to grant derogation (IEC
SMB/2637/RV). As a result a Technical Report, based on document 17A/629/DTR, is now due to be published [9].
Future perspective
The IEC discussions and voting results illustrate the ambiguity related since decades with an electrical endurance
type test for circuit-breakers rated 72,5 kV and above. Some utilities claim the usefulness of such tests [10], other
utilities doubt whether the tests are representative for real service life, as they cannot imagine that large numbers of
interruptions do correspond to high short-circuit currents. And again other experts state that a few interruptions of
asymmetrical short-circuit currents (T100A) is far more severe than a relatively high number of symmetrical current
interruptions. The topic of electrical endurance type testing is for sure not yet solved in a definitive way.
IEC Technical Report 62271-310 offers a large flexibility in the specification of the electrical endurance type
test, including a default definition. The type test is non-mandatory and consists of two parts: the aging part and
the verification part. The flexibility is in the aging part, while the verification part is obligatory and precisely
prescribed. Aging may be covered (partly) by the “normal” short-circuit type tests. As usual with IEC Technical
Reports, the coming years will be used to evaluate the degree of application of the type test and the gained
experience with the specification of the electrical endurance type test for circuit-breakers with a rated voltage of
72.5 kV and above.
5. CONCLUSIONS
1. IEC SC 17A is about to publish a Technical Report on Electrical Endurance Type Tests for circuit-breakers
with a rated voltage of 72.5 kV and above. Opposite to the ANSI-Standards (C37.04, 1999, cl.5.8.2.5.b, where
a 800% Service Capability Duty is required in combination with the type tests to verify the circuit-breaker
performance) in IEC Publication 62271-310 a dedicated type test sequence will be defined, based on data
collected by CIGRE WG 13.08 and IEC 17A/WG29.
2. Although in the CIGRE survey the populations of circuit-breaker-years and OH-line-km-years are quite large,
the number of answers used for the statistical evaluation is rather small. Nevertheless the average values and
90% percentiles for the number of faults per OH-line show some consistency and are in line with publications
on much smaller populations. Furthermore the average values and 90% percentiles are in line with the detailed
statistical data from two large utilities.
3. The input data for the statistical analyses were average values for the number of faults on OH-lines. This may
cause a filtering effect with respect to extreme values, but again the results from the analyses are in line with
other reports.
4. The information collected about the maximum expected short-circuit currents in the substations shows that
these are rather low in comparison to the rated short-circuit current of circuit-breakers. The real value of the
short-circuit current level in service is even lower.
5. On a statistical basis the accumulated interrupted short-circuit current of a line breaker can be calculated for a
certain level of assumed short-circuit power in the substation and a given density of faults on the OH-line.
7
6. Given the statistical distribution of the number of faults per 100 km OH-line and given the statistical
distribution of the short-circuit powers, a chessboard can be built with the percentiles of both distributions.
Each square of a 10 by 10 chessboard thus represents 1% of the whole population of circuit-breakers.
7. As for each square (combination of probabilities) the total electrical wear can be calculated and translated into
T100 operations, a complete distribution of electrical stresses can be achieved. The distribution of electrical
stresses can be split into 90%-10%: 90 squares with lower total wear and 10 squares with higher total wear
than the borderline (i.e. the 90% percentile of electrical wear).
8. The exercise results into a substantial reduction in electrical wear (90% percentile) in comparison to the values
used by the IEC SC 17A/WG29.
9. The topic of electrical endurance type tests for circuit-breakers rated 72,5 kV and above still requires further
consideration.
The “chess-board” is not the proper metaphor, as a ten by ten board refers to “checkers” rather than “chess”.
LITERATURE
[1] Analysis of Transient Recovery Voltage Rating Concepts
C.L. Wagner, H.M. Smith;
IEEE/PAS-103, No.11, Nov.’84, pp.3354-3362
[2] Three-phase Interruption of Single and Two-Phase Faults: Breaking Stresses in the Healthy Phases
R. Eriksson, V.S. Rashkes
Electra No.77, pp. 77-92
[3] Schalterbeanspruchung im Hochspannungsnetzen
Noack
Book from former DDR, chapter 6
[4] Survey of TRV conditions on the CGEB 400 kV system
P.K.Basak, P.G. Parrott;
IEE Proc., Vol.128, Pt. C, No.6, Nov.’81, pp.342-350
[5] Electrical Endurance and Reliability of Circuit-Breakers
A. Pons, A. Sabot, G. Babusci
IEEE/PD, Vol.8, No.1, Jan’93, pp.168-174
[6] Test Requirements and Facilities for HV Circuit Breakers
S. Rovelli; G.P. Mazza;
Instit. of Engineers, Australia, Nat. Conf. Publ. 79/3
[7] Stress of HV Circuit-Breakers during Operation in the Networks – German Utilities’ Experience
C. Neumann, e.a.
CIGRE SC13 Session 2002, report 13-304
[8] Beanspruchungen von Hochspannungs-Leistungsschaltern am Beispiel zweier EVU
J. Becker, G. Balzer, R. Meister, C. Neumann
Elektrizitätswirtschaft, Jg.’99 (2000), Heft 11, pp. 27-34
[9] IEC Publication 62271-310, High-voltage switchgear and controlgear
Part 310: Electrical endurance testing for circuit-breakers rated 72,5 kV and above
[10] Electrical endurance tests for HV circuit-breakers: EDF experience
R. Jeanjean, C. Salzard, P. Migaud
IEEE/PES SM 2002 (to be published): 0-7803-7322-7/02
[11] Life Management of Circuit-breakers
CIGRE WG 13.08
CIGRE Technical Brochure 165, August 2000
8