211 a3 211 summary results

SUMMARY OF RESULTS OF THE 2004 - 2007 INTERNATIONAL ENQUIRY ON RELIABILITY OF HIGH VOLTAGE EQUIPMENT

M. RUNDE1, on behalf of CIGRÉ WG A3.062

SUMMARY CIGRÉ WG A3.06 has completed a survey of reliability and failures of in-service high voltage equipment. The equipment types considered are SF6 circuit breakers, disconnectors, earthing switches, instrument transformers and gas insulated switchgear (GIS). 90 utilities from 30 countries have contributed failure and population data, making this the most comprehensive reliability survey for high voltage apparatus ever carried out. The overall major failure frequency for circuit breakers is found to be 0.30 major failures per 100 circuit breaker years of service, which is lower than in a previous survey. Shunt reactor switching is associated with substantial higher failure frequencies than other switching duties. For disconnectors and earthing switches the overall major failure frequency is determined to be 0.21 failures per 100 equipment years of service. A 3:1 ratio between the number of failures caused by the operating mechanisms and failures caused by the primary components of the disconnectors and earthing switches is observed. Instrument transformers show an overall failure frequency of about 0.053 major failures per 100 single phase instruments transformer years of service. In general, individual equipment installed in GIS appears to have lower failure frequencies than equipment in air insulated substations. The overall major failure frequency for GIS bays is about 0.37 major failures per 100 GIS circuit breaker bay years of service. (A GIS circuit breaker bay includes one circuit breaker and all associated disconnectors, instrument transformers, interconnecting busducts and/or parts of busbars and associated terminals.) Six very comprehensive CIGRÉ Technical Brochures containing all results with commentaries, information concerning how the survey was conducted, methods used for statistical analyses, recommendations for utilities and manufacturers, etc. are to be published shortly. KEYWORDS SF6 circuit breakers; disconnector switches; earthing switches, instrument transformers; gas insulated switchgear; reliability; failure rates; service experience; major failures.

1 [email protected] 2 Members of WG A3.06 are: M. Runde (NO) Convener, C. E. Sölver (SE) Past Convener, B. Bergman (CA), A. Carvalho (BR), M. L. Cormenzana (ES), H. Furuta (JP), W. Grieshaber (FR), A. Hyrczak (PL), D. Kopejtkova (CZ), J. G. Krone (NL), M. Kudoke (CH), D. Makareinis (DE), J. F. Martins (PT), K. Mestrovic (HR), I. Ohno (JP), J. Östlund (SE), K.-Y. Park (KR), J. Patel (IN), C. Protze (DE), J. Schmid (DE), J. E. Skog (US), B. Sweeney (UK), S. Tsukao (JP), F. Waite (UK).

21, rue d’Artois, F-75008 PARIS A3-211 2011 http: //www.cigre.org

2011 SC A3

Technical Colloquium

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1. INTRODUCTION

CIGRÉ considers collecting, analyzing and publishing reliability data important tasks, and in 2002 WG A3.06 “Reliability of high voltage equipment” was established. The WG was instructed to organize and carry out a worldwide enquiry on service experience with circuit breakers, disconnectors, earthing switches, instrument transformers and gas insulated switchgear (GIS). The time period covered by the survey is 2004 - 2007, and the enquiry comprises equipment rated for voltages greater than or equal to 60 kV. For circuit breakers only single pressure SF6 technology is included, therefore in practice equipment installed before around 1970 is excluded. For disconnectors, earthing switches and instrument transformers there are no limitations in age or technology. Both equipment installed in air insulated and gas insulated substations are covered. Information has been collected by asking utilities to complete and return questionnaires. 90 utilities from 30 countries have contributed, and the analyses of the results have now been completed. This paper presents a short summary, with emphasis on major failure frequencies. Far more extensive and detailed results, commentaries, comparisons with previous surveys, information concerning how the survey was conducted, statistical analyses, recommendations for utilities and manufacturers, etc. can be found in six comprehensive CIGRÉ Technical Brochures [1] - [6] that will be published shortly.

2. METHODS AND PROCEDURES USED IN THE SURVEY

2.1 Questionnaires

To be able to determine failure frequencies, the numbers and details of the equipment populations that are covered by the survey has to be recorded. Thus for each of the four equipment types included, the enquiry has employed two types of forms/cards/questionnaires: one for equipment populations and one for failures. Population cards were completed annually (i.e., one for each year 2004 - 2007); whereas a failure card was filled in each time a failure occurred within this time span. For GIS, there was also included a “GIS maintenance card” collecting information about different practices in GIS high voltage testing, diagnostics, monitoring, major maintenance, extension and servicing. The data collection was carried out by means of a specially developed spreadsheet tool, containing the four population cards, the four failure cards and the GIS maintenance card. All questions were of the tick box types, or they required numbers or dates as input. The tool was multi-lingual; with eleven different languages, and it was even possible to switch between them. The population cards asked for the age and number of components being covered, grouped by voltage level, application, technology, design and maintenance strategy. The failure cards requested the same type of information about the failed component, together with information describing the failure itself, such as its origin and cause, what sub-assembly failed, whether this was a minor or major failure, if environmental stress contributed, etc. The questionnaire spreadsheet tool was distributed by e-mail to utilities worldwide willing to participate in the survey. They filled in the forms, and returned a file with the answers to the working group member responsible for that country. After a quality check, the responses were forwarded and compiled in a database for subsequent statistical analysis. The information has been collected directly and solely from the utility sector, not from manufacturers or others as in some previous reliability surveys. All incoming information has been treated as confidential.

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2.2 Failure definitions

A main objective of the survey is to identify trends by comparing the findings from the present survey with those from the previous ones. Consequently, the majority of the definitions and questions are completely or nearly identical to those applied in earlier surveys. In particular, failure definitions are very important in the present context, and the terms major (MaF) and minor failures (MiF) as defined in the IEC circuit breaker standard [7] have been adopted for all the component types. A switchgear major failure is defined as “failure of a switchgear and control gear which causes the cessation of one or more of its fundamental functions. A major failure will result in an immediate change in the system operating conditions, e.g. the backup protective equipment will be required to remove the fault, or will result in mandatory removal from service within 30 minutes for unscheduled maintenance”. Correspondingly, a switchgear minor failure is “failure of an equipment other than a major failure or any failure, even complete, of a constructional element or a sub-assembly which does not cause a major failure of the equipment”. Only major failures will be dealt with in the present summary.

3. SF6 CIRCUIT BREAKERS

3.1 Participation and surveyed service experience

The survey includes 281 090 circuit breaker (CB) years of service experience. This is more than four times as much as the previous circuit breaker survey carried out around 1990 [8]. Population and failure cards were received from 30 countries, but the size of the contribution from the individual countries varies a lot. 39% of the surveyed service experience originates from one dominating country. 45% of the experience relates to live tank circuit breakers, 31% to GIS, and the remaining 24% to dead tank circuit breakers. Figure 1 shows the surveyed service experience sorted by voltage level and kind of service. Equipment rated for above 700 kV constitutes only 0.14% of the surveyed population and is not visible in the chart.

60<=U<100 kV34.2%

100<=U<200 kV36.6%

200<=U<300 kV15.3%

300< =U<500 kV11.8%

500<=U<700 kV1.9%

Overhead line56%

Transformer24%

Shunt reactor1%

Cable6%

Capacitor3%

Bus coupler10%

Figure 1. Circuit breaker service experience according to kind of service (left) and voltage (right).

There is a significant shift in operating mechanism technology in the populations in the present compared to the previous survey, see Figure 2. The fraction of the circuit breakers that are equipped with hydraulic drives is reduced from 51% to 26%, while the portion with spring drives has increased from 18% to 52%. Pneumatic drives still account for a much as 22% of the surveyed experience.

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Hydraulic51%

Pneumatic30%

Spring18%

Hydraulic26%

Pneumati c22%

Spring52%

Other1%

Figure 2. Comparison between the previous and present surveys with regard to type of operation mechanism used.

3.2 Failure frequencies and characteristics

The overall major failure frequency is calculated to be 0.30 MaF per 100 CB-years of service. In the previous survey the corresponding number was 0.67, indicating that SF6 circuit breaker reliability has improved significantly during the some 16 years between the surveys. The failure frequency, still however, increases substantially with increasing voltage level, see Figure 3.

60<

=U

<10

0kV

100<

=U

<20

0kV

200<

=U

<30

0kV

300<

=U

<50

0kV

500<

=U

<70

0kV

>=

700

kV

Tot

al

0

1

2

3

4

5

Ma

Ffr

eq

uen

cy[p

er1

00C

B-y

ears

]

Previous survey

Present survey

Figure 3. Major failure frequency as a function of voltage level for the present and the previous survey.

Live tank SF6 circuit breakers show an approximately three times higher major failure frequency as GIS or metal enclosed units; 0.48 and 0.14 MaF per 100 CB-years of service, respectively. This is approximately the same ratio as in the previous survey. For the first time failure frequencies have been correlated to the breaker’s kind of service; i.e., whether it switches overhead lines, cables, transformers, reactors, etc. The results are shown in Figure 4. Large differences are observed. The major failure frequencies for circuit breakers operating on shunt reactors are around one order of magnitude higher than for line and transformer breakers. It is assumed that this partly, but not entirely, attributed to the former ones in general are operated more frequently. Figure 5 shows major failure frequency as a function of year of installation. Hence, general aging effects but also changes in design and quality are expected to influence this relationship.

5

Ove

rhea

dlin

e

Tra

nsfo

rmer

Cab

le

Shu

ntre

acto

r

Cap

acito

r

Bus

coup

ler

Oth

er

Tot

al

0

1

2

3

4

5

6

7

MaF

fre

qu

en

cy[p

er

100

CB

-ye

ars

]

Live tank

Dead tank

GIS

Figure 4. Circuit breaker major failure frequencies for different kinds of service.

Bef

ore

1979

1979

-198

3

198

4-19

88

1989

-199

3

1994

-199

8

1999

-200

3

2004

-200

7

0

0.2

0.4

0.6

0.8

1

Ma

Ffr

eq

uen

cy[p

er1

00C

B-y

ears

]

Live tank

Dead tank

GIS

Figure 5. Major failure frequencies as function of year of installation for the different SF6 circuit breaker technologies.

For AIS and GIS there is a tendency that newer equipment has less major failures than older equipment, and the difference is substantial. For the surveyed dead tank population no such relation-ship is visible. The major failure mode distribution is shown in the left part of Figure 6. “Does not close on command” and “Locking in open or closed position” are the most common major failure modes.

Does not close on command28%

Does not open on command16%

Opens without command5%

Locking in open or close position25%

Loss of mechanical integrity8%

Other17%

Components at service voltage20%

Electrical control and aux. circuits30%

Operating mechanism50%

Figure 6. Distribution of circuit breaker major failure modes (left) and sub components responsible for the major failure (right).

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When identifying the sub component responsible for the major failures the distribution becomes as shown in the right part of Figure 6. Roughly half of the major failures occur in the operating mechanism, and a large portion is caused by malfunctions in the electrical control and auxiliary circuit. These findings are approximately the same as in the previous survey.

4. DISCONNECTORS AND EARTHING SWITCHES


Large international service experience enquiries have not previously been performed for disconnectors and earthing switches, so there is no earlier results for comparison. The survey includes 935 204 years of disconnector and earthing switch (DE) years of service collected from 25 countries. Of this, 77% relate to disconnectors (DC) and 23% to earthing switches (ES). AIS equipment constitutes approximately 67%, the rest being GIS equipment. Figure 7 and Figure 8 show the distribution of the surveyed service experience.

60<

=U

<10

0kV

100<

=U

<20

0kV

200<

=U

<30

0kV

300

<=

U<

500

kV

500<

=U

<70

0kV

>=

700

kV

0

50000

100000

150000

200000

250000

AIS

se

rvic

eex

pe

rie

nce

[DE

-yea

rs]

Centre break

Double break

Knee type

Vert ical break

Semi-pantograph

Pantograph

Earthing switch

Figure 7. Distribution of AIS earthing switch and disconnector designs at the different voltage ranges.

60<

=U

<10

0kV

100<

=U

<20

0kV

200<

=U

<30

0kV

300

<=

U<

500

kV

500<

=U

<70

0kV

>=

700

kV

0

40000

80000

120000

160000

GIS

serv

ice

exp

eri

en

ce

[DE

-ye

ars

]

Earthing switches

Other

Disconnectors

Combined DS ES

Figure 8. GIS disconnector and earthing switch surveyed service experience.

As can be seen in the graphs above, the AIS disconnector designs used at the different voltage levels differ substantially. The double break technology dominates below 100 kV.

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The overall major failure frequency is found to be 0.29 and 0.05 MaF per 100 DE years of service for AIS and GIS equipment, respectively. As opposed to for circuit breakers, there is no obvious influence of voltage level, see Figure 9. Equipment rated for greater than 700 kV is not included in this figure, as the surveyed population is very small.

60<

=U

<10

0kV

100<

=U

<20

0kV

200<

=U

<30

0kV

300<

=U

<50

0kV

500<

=U

<70

0kV

Tot

al

0

0.1

0.2

0.3

0.4

MaF

fre

qu

en

cy[p

er

100

DE

-yea

rs]

AIS

GIS

Figure 9. Major failure frequency for disconnectors and earthing switches as a function of voltage.

The age of the equipment, or more precisely the year of installation seems to have some impact on the failure frequencies, see Figure 10. In general, older disconnectors and earthing switches have more failures than new ones, but the observed difference is small. Much of the equipment that has been in service for 40 or more years shows a very modest failure frequency.

Bef

ore

1974

1974

-19

83

1984

-19

93

1994

-20

03

2004

-20

07

Tot

al

0

0.1

0.2

0.3

0.4

MaF

fre

qu

en

cy[p

er

100

DE

-yea

rs]

AIS

GIS

Figure 10. Major failure frequency as a function of disconnectors and earthing switches year of installation.

When disregarding the (rather small) population of knee type devices, the many different designs used for AIS disconnectors show failure frequencies that are within a factor of two, see Figure 11. As for circuit breakers the most common failure mode for disconnectors and earthing switches is “Does not operate on command”, accounting for roughly three out of four major failures, see Figure 12. Moreover, as could be expected, “Dielectric breakdown” constitutes a larger portion of the failures in GIS than in AIS and for “Loss of mechanical integrity” the opposite is seen.

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Cen

tre

brea

k

Dou

ble

brea

k

Kne

ety

pe

Ver

tical

brea

k

Sem

i-pa

ntog

raph

Pan

togr

aph

Ear

thin

gsw

itche

s

0

0.4

0.8

1.2

MaF

fre

qu

en

cy[p

er

10

0D

E-y

ears

]

Figure 11. Major failure frequency for the different AIS disconnector designs.

Does not operate on command72%


Dielectrical breakdown2%

Other6%

Loss of mechancial integrity13%

Does not operate on command79%


Dielectrical breakdown13%

Other4%Loss of mechancial integrity

1%

Figure 12. Distribution of major failure modes for AIS (left) and GIS (right) disconnectors and earthing switches.

5. INSTRUMENT TRANSFORMERS


The survey includes 1 290 335 instrument transformers (IT) years of service experience. As opposed to the other component types, instrument transformers are reported as single phase units. The reason for this is that it is possible to have an instrument transformer on one phase only or to have different designs on different phases. Population and failure cards were received from 73 utilities in 25 countries. Figure 13 shows how the surveyed experience distributes with regard to voltage and kind. As can be seen, the majority of the information is obtained from voltages below 200 kV. Around 40% come from GIS installations. The majority of AIS instrument transformers service experience reported is for sealed oil impregnated paper instrument transformers. Very few optical and electronic instrument transformers were reported in the survey, and it was not possible to do any detailed analysis on these kinds of instrument transformers.

9

60<

=U

<10

0kV

100<

=U

<20

0kV

200<

=U

<30

0kV

300

<=

U<

500

kV

500<

=U

<70

0kV

>=

700

kV

0

100000

200000

300000

400000

500000

ITs

erv

ice

exp

erie

nc

e[I

T-y

ears

]GIS Current transformers

GIS Combined current and voltage transformers

GIS Voltage transformers

AIS Current transformers

AIS Combined current and voltage transformer

AIS Capacitive voltage transformers

AIS Magnetic voltage transformers

Figure 13. Instrument transformer surveyed service experience (counted as single phase unit years).


The overall major failure frequency is found to be 0.053 failures per 100 IT-years of service. As can be seen from Figure 14, GIS instrument transformers are found to be significantly more reliable than those installed in AIS.

60<

=U

<10

0kV

100<

=U

<20

0kV

200<

=U

<30

0kV

300<

=U

<50

0kV

500<

=U

<70

0kV

Tot

al

0

0.04

0.08

0.12

0.16

Ma

Ffr

equ

enc

y[p

er10

0IT

-ye

ars

]

AIS

GIS

Figure 14. Major failure frequencies for AIS and GIS instrument transformers.

The failure frequencies are less than the failure frequencies reported in a previous survey carried out around 1990 [9]. The previous study had a significantly different population that was surveyed (for example the previous survey did not include many GIS instrument transformers and the countries where the instrument transformers were installed were different). It is likely these differences in population have a significant affect and would have caused the previous survey to report a higher failure frequency. Figure 15 shows the major failure frequencies for the different kinds of instrument transformers. For GIS, current transformers appear more reliable compared with voltage transformers. This is anticipated as voltage transformers for GIS are generally more complex than current transformers. For AIS, the failure frequencies calculated seem to be a bit mixed, with current transformers and combined current and voltage transformers having a lower major failure frequency than voltage transformers.

10

MVT CVT CCVT CT VT CT0

0.04

0.08

0.12M

aFfr

eq

ue

nc

y[p

er

100

IT-y

ear

s]

MVT: Magnetic voltage transformerCVT: Capacitive voltage transformerCCVT: Combined current and voltage transformerCT: Current transformerVT: Voltage transformer

GISAIS

Figure 15. Major failure frequencies grouped after kind of instrument transformer for AIS equipment (green) and GIS equipment (pink).

Major failure frequency as a function of year of manufacture is shown in Figure 16. There is indication that instrument transformers manufactured prior to 1988 have a higher failure frequency than those manufactured after 1988. However there is no dramatic increase, and the increase in failure frequency reported is unlikely in general to require the user to significantly change maintenance or consider replacement.

Bef

ore

1979

1979

-198

3

198

4-19

88

1989

-199

3

1994

-199

8

1999

-200

3

2004

-200

7

0

0.04

0.08

0.12

0.16

Ma

Ffr

equ

enc

y[p

er10

0IT

-ye

ars

]

AIS

GIS

Figure 16. Instrument transformer major failure rate as a function of year of manufacture.

For AIS oil impregnated paper instrument transformers, there are a number of reports of failures that caused fire and explosion. These failures are generally reported as internal dielectric failure. Additionally, the largest proportion of major failures for oil impregnated paper instrument transformers is due to internal dielectric failure. This suggests that manufacturers and users should concentrate on improving the dielectric performance of oil impregnated paper instrument transformers. For SF6 insulated instrument transformers the major failures reported were generally leaks.

6. GAS INSULATED SWITCHGEAR


Failure information from a total of 88 971 GIS circuit breaker bay years of service experience have been collected from 55 utilities in 24 countries. The contribution from the different countries is quite

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uneven, as two dominating countries account for as much as 92% of the surveyed experience. The results are compared with a previous GIS survey [10] covering service experience up to 1995. The portion of hybrid GIS installations is still relatively small, representing only about 8% of the collected GIS service experience. Outdoor installations slightly prevail over indoor at rated voltages of 300 kV and above, and are used more frequently than in the past. The utilities in countries other than the two dominant ones prefer indoor installations at all voltage classes, except for 700 kV and above. 6.2 Failure frequencies and characteristics

Figure 17 shows a comparison between the failure frequencies determined in the present and previous survey. As for the circuit breakers there seems to be an increased failure frequency with increasing voltage. Moreover, reliability seems to have improved a little, with the overall failure frequency reduced from 0.53 to 0.37 major failures per 100 circuit breaker bay years of service. However, it should be kept in mind that the populations considered in the two surveys differ significantly, and that the number of failures is low in some of the categories (e.g., only four major failures are recorded for GIS rated for 700 kV and above).

60<

=U

<10

0kV

100<

=U

<20

0kV

200<

=U

<30

0kV

300<

=U

<50

0kV

500<

=U

<70

0kV

>=

700

kV

Tot

al

0

2

4

6

MaF

fre

qu

en

cy[p

er

100

CB

-ba

y-y

ear

s]

Previous survey

Present survey

Figure 17. GIS major failure frequencies as a function of voltage for the present and a previous survey.

Figure 18 reveals the influences of age on GIS reliability. For most of the voltage levels a shape resembling the well-known bathtub curve appears. The failure frequencies increase (with different steepness) the older the GIS are, and in the first years after installation some infant mortality failures appear. As expected, the switching equipment are responsible for most of the major failure in the GIS, see Figure 19. GIS instrument transformers are only responsible for some 5%.

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Bef

ore

1979

1979

-198

3

1984

-198

8

1989

-199

3

1994

-199

8

1999

-200

3

2004

-200

7

0

1

2

3

4

Ma

Ffr

equ

en

cy

[pe

r10

0C

B-b

ay-

ye

ars

]

60 U < 100 kV

100 U < 200 kV

200 U < 300 kV

300 U < 500 kV

500 U < 700 kV

Figure 18. GIS major failure frequency for the different voltage levels, sorted after year of installation.

60<

=U

<10

0kV

100<

=U

<20

0kV

200<

=U

<3

00kV

300<

=U

<50

0kV

500<

=U

<70

0kV

>=

700

kV All

0

20

40

60

80

100

GIS

Ma

Fd

istr

ibu

tio

n[%

]

Other

Instrument transformers

Disconnectors and earthing switches

Circuit breakers

Figure 19. GIS major failures sorted after voltage level and after the component that failed.

The prevailing GIS major failure modes are “Failing to perform requested operation or function” (63%) and “Dielectric breakdown” (23%). The portion of the former rises with increasing age of the GIS and reaches a maximum at an age of about 15 to 20 years old. This most probably illustrates that most overhauls of operating mechanisms are scheduled to this age span, and that this timing is sometimes too late. With regard to sub assemblies responsible for major failures, “Component in primary circuit”, “Component in control, auxiliary or monitoring circuit” and “Component in operating mechanism” share the responsibility by about one third each.

7. GIS PRACTICES

Completed “GIS maintenance cards” were received from 20 utilities from 12 countries. Among the findings from analyzing the various topics addressed are:

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Commissioning: Power frequency voltage testing with simultaneous partial discharge (PD) measure-ment is the most frequently used high voltage test during GIS commissioning. Impulse voltage testing, as a complimentary method to power frequency high voltage test, is not widely used. Condition monitoring and diagnostics: There is still little confidence in GIS continuous monitoring techniques. This also concerns PD measurement that is widely used for diagnostic testing. Various other diagnostic test methods commonly used on AIS are commonly used on GIS as well. Major maintenance (overhaul) experience: Only a third of the received cards indicate any experience with major maintenance. The prevailing reasons for performing major maintenance are that a fixed time has elapsed and that corrective maintenance is required. Predictive major maintenance based on equipment condition or on reliability centered studies is mentioned in only about one fifth of the responses. Extensions: Most responders indicated experience with GIS extensions. All reported GIS extensions were made by their original manufacturers, and in one third of cases there was not even a need for an adapter. Application of new technologies: Network operating requirements is the dominating factor when deciding on the single line diagram for new GIS. The responses indicate that most utilities do not apply different approaches for GIS and AIS. The experience with combined function GIS apparatus is small. The experience with turnkey projects is positive in more than a half of the responses. In general utilities still do not have much confidence in functional specifications.

8. CONCLUSIONS

Analyses of population of failure data covering the time period 2004 - 2007 and collected from 90 utilities in 30 countries yielded the following overall major failure frequencies: SF6 circuit breakers: 0.30 MaF per 100 CB years of service; Disconnectors and earthing switches: 0.21 MaF per 100 DE years of service; Instrument transformers: 0.053 MaF per 100 IT years of service (single phase units); Gas insulated switchgear: 0.37 MaF per 100 GIS CB bay years of service. All results and findings from the WG A3.06 survey, including commentaries, information about how the survey was conducted, methods used for statistical analyses, recommendations for utilities and manufacturers, are presented in six comprehensive CIGRÉ Technical Brochures that will be published shortly.

9. BIBLIOGRAPHY

[1] CIGRÉ WG A3.06: Final Report of the 2004 - 2007 International Enquiry on Reliability of High Voltage Equipment, Part 1 - Summary and General Matters, CIGRÉ Technical Brochure, 2011. (to be published)

[2] CIGRÉ WG A3.06: Final Report of the 2004 - 2007 International Enquiry on Reliability of High Voltage Equipment, Part 2 - SF6 Circuit Breakers, CIGRÉ Technical Brochure, 2011. (to be published)

[3] CIGRÉ WG A3.06: Final Report of the 2004 - 2007 International Enquiry on Reliability of High Voltage Equipment, Part 3 - Disconnectors and Earthing Switches, CIGRÉ Technical Brochure, 2011. (to be published)

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[4] CIGRÉ WG A3.06: Final Report of the 2004 - 2007 International Enquiry on Reliability of High Voltage Equipment, Part 4 - Instrument Transformers, CIGRÉ Technical Brochure, 2011. (to be published)

[5] CIGRÉ WG A3.06: Final Report of the 2004 - 2007 International Enquiry on Reliability of High Voltage Equipment, Part 5 - Gas Insulated Switchgear CIGRÉ Technical Brochure, 2011. (to be published)

[6] CIGRÉ WG A3.06: Final Report of the 2004 - 2007 International Enquiry on Reliability of High Voltage Equipment, Part 6 - GIS practices, CIGRÉ Technical Brochure, 2011. (to be published)

[7] IEC 60694:2002: Common specifications for high-voltage switchgear and control gear standards.

[8] CIGRÉ WG 13.06: Final report of the second international enquiry on high voltage circuit-breaker failures and defects in service, CIGRÉ Technical Brochure no. 83, 1994.

[9] CIGRÉ SC A3: State of the art of instrument transformers, CIGRÉ Technical Brochure no. 394, 2009.

[10] CIGRÉ WG 23.02: Report on the second international survey on high voltage gas insulated substations (GIS) service experience, CIGRÉ Technical Brochure no. 150, 2000.