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CIGRÉ considers collecting, analyzing and publishing reliability data to be 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 into the service experience of 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 has been 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. The results of the survey are presented and discussed in six Technical Brochures, issued simultaneously.

In the past, two circuit breaker reliability surveys have been carried out. The first was performed in 1974 - 77 and concerned nearly 79 000 circuit breaker years of service. In the second surveydata was collected in the period 1988 - 91 and almost the same number of circuit breaker years were covered, but the survey was limited to single pressure SF6 technology. This report gives the results of the third international enquiry (called “present survey” in this report). The scope of this survey is mainly based on the structure and questions contained in the second survey (called “previous survey” in this report) ; therefore, in many instances trends and comparisons can be made. Again, the present survey concentrates on live tank, dead tank and GIS high voltage circuit breakers based on single pressure SF6 technology.

This document covers the results from the third international survey on the reliability of instrument transformers that was circulated in 2003 as part of a wider survey on the reliability of switchgear and related equipment. The survey collected population and failure data for instrument transformers that were in service during the years 2004 to 2007 inclusive. The scope of this survey was all instrument transformers with voltages above 60 kV, excluding AIS ring current transformers. The report analyzed the data returned to give information of the surveyed instrument transformer population and failure frequency, breaking the information down for different groups of instrument transformers (where groups could be different insulation, application, type, etc.). Additionally data regarding the failure such as cause, severity, and contributory environmental conditions was gathered and this data was analyzed. Where possible the data was compared with that of previous surveys. There have been two previous instrument transformer reliability surveys undertaken by CIGRE. The first survey was started in 1986 and was published in 1990 in Technical Brochure 057 “Paper-oil insulated measurement transformers” . The scope of the survey was limited to paper-oil insulated instrument transformers with rated voltages 72, 5 kV and above. The survey required details of each instrument transformer failure since 1970 until the end of 1986 and hence covered a 17 year period. It covered a population of 136 033 instrument transformers so the service experience was assumed by the survey to be 136 033 x 17 = 2 310 000 IT-years. As the survey was completed both by utilities and manufacturers and there was concern that the manufacturers may not have been aware of the failure of some of their instrument transformers leading to under reporting. For this reason failure frequency comparison in this report is only made with the second survey and not the first survey. The second previous survey was conducted between 1985 and 1995, a period of 10 years. The results were published in 2009 in Technical Brochure 394 “State of the art of instrument transformers”. The scope of the survey was all paper-oil, resin or SF6 gas insulated instrument transformers with rated voltages at or above 60 kV. The survey recorded a population of 131 207 instrument transformers in 1995. The survey was restricted to utility organizations only. For this third survey the failure definitions were common between all equipment types in the overall survey and the definitions were based on those definitions found in IEC 60694. There were considered to be of three main types of failure: • Failures that caused fire or explosion This failure type is a subgroup of major failures and was included to allow comparison with previous second survey. • Major Failures: 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. As fire and explosion failures is a subgroup of major failures the total number of major failures is equal to the number of fire and explosion failures and the number of major failures that did not cause fire and explosion. • Minor Failures: Failure of 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. The previous second survey did not define a failure but did split failures into three types: • Major failures, which were sudden explosive events that caused an immediate emergency outage or trip (note that the definition is different to that in this third survey). • Minor failures, which were non-violent but still required an urgent system outage within, for instance, one hour (note that the definition is different to that in this third survey). • Defects, which required a non-urgent outage (planned) outage to repair or replace the unit. The previous second survey concentrated on major failures as defined in the previous second survey and reported less on the second survey type minor failures. It is assumed that the previous second survey major failures are best compared with failures that cause fire or explosion in this third survey. The first survey defined that an instrument transformer had a failure when the instrument transformer was no longer able to perform its required function and therefore had to be removed from service. This was split into two groups: violent failures and non-violent failures. As with all previous surveys, all the values quoted in this survey are for single phase units as it is possible to have a single unit on only one phase or different designs of instrument transformer on different phases. This is different to other parts of the survey such as circuit breakers that consider one unit to cover all three phases. It should be noted that for three phase GIS instrument transformers, while the survey requested that these should be entered as three single phase units, these may have been only entered once. Where the entry in a table is blank this indicates that the result is not applicable because there is no service experience recorded for the table entry (for example the number of failures will not be applicable if there is no service experience). While the results of hypothesis tests were not in general directly reported in this document due to lack of space, hypothesis testing was used to ensure that the findings and comments made in this document were statistically significant and the confidence level used was 95%.

Accurate information about the service experience of high voltage equipment is of significant value to both electric utilities and equipment manufacturers. It helps the manufacturers improve their products and provides important knowledge for the utilities when buying equipment, organizing maintenance and benchmarking their performance. Equipment reliability data is also required when assessing the overall reliability of an electric power system, including studies into the security of supply. Furthermore, international standards applicable to high voltage equipment are improved on the basis of service experience and reliability data. CIGRÉ considers collecting, analyzing and publishing reliability data to be important tasks and in 2002, WG A3.06 “Reliability of High Voltage Equipment” was established. The WG was required to organize and carry out a worldwide enquiry into the service experience of 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 and therefore, in practice equipment installed before around 1970 has been 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. The results of the survey are presented and discussed in six Technical Brochures.

The 1st GIS international survey questionnaire was circulated in 1991 and both users and manufacturers were addressed. It collected experience up to 31.12.1990. 109 users from 34 countries and 18 manufacturers from 9 countries responded to the questionnaire. The user’s experience represented 1 817 GIS, 10 531 circuit breaker-bays (CB-bays) and 78 036 CBbay-years, the manufacturer’s experience represented 4 867 GIS, 24 870 CB-bays and 199 473 CB-bay-years. The questionnaire addressed: general data about GIS installations, factors which contributed to selection of GIS over AIS, adequacy of existing design/production/site test procedures, reliability, availability and maintainability, cause of major failures, future trends/condition monitoring /diagnostics and general open-ended questions. The survey and data analysis were published in CIGRE papers during 1992 to 1994. The 2nd GIS international survey questionnaire was circulated five years later, in 1996, with the aim of collecting service experience data available up to 31.12.1995. 80 users from 30 countries responded to the questionnaire. The collected data referred to 2115 GIS installations, 13 696 CB-bays and 118 483 CB-bay-years. In comparison to the 1st questionnaire, the 2nd questionnaire was more focused on installation overview and major failure statistics and GIS life cycle issues. Therefore only users were addressed. Besides the general data about GIS installations (similar to data sheet in the first survey) the users were asked to report the characteristics and service experience (major failures) for each individual GIS from the beginning of their putting into service and to answer more general questions concerning maintenance, lifetime and environmental aspects independently of the number of installed GIS and voltage level. The survey and data analysis were published in CIGRE papers during 1998 to 2000. The 3rd GIS international survey questionnaire was circulated in 2003 and collected population and failure data in 2004 to 2007. The structure of the questionnaire was as much as useful similar to the 2nd survey to enable a comparison.

This report gives the results of the 1st international enquiry on reliability of high voltage disconnectors and earthing switches that was circulated in 2003 as part of a wider survey on the reliability of switchgear and related equipment. The survey collected population and failure data for disconnectors and earthing switches that were in service during the years 2004 to 2007 inclusive. The scope of this survey was all disconnectors and earthing switches from air insulated switchgear (AIS) and gas insulated switchgear (GIS) substations with voltages above 60 kV. All dana in this report are for 3-phase equipment. The report analyzed the returned survey response data for disconnectors and earthing switches providing service experience and failure frequency information. The information is organized in two main sections: AIS disconnectors and earthing switches and GIS disconnectors and earthing switches. Concerning title designation used in tables and graphs, two common expressions are used: AIS DE (+ title of table or graph) and GIS DE (+ title of table or graph). This format explicitly informs the reader as to whether the table/graph refers to air insulated disconnectors and earthing switches (AIS DE) or gas insulated disconnectors and earthing switches (GIS DE). Within each section, the results summarized in tables and graphs are commented with findings and conclusions concerning the following issues: service experience, major failures and minor failures, major failure frequencies and failure characteristics. Furthermore, customized analyses, such as analysis by design of the equipment (which includes the component at service voltage and the kinematic chain) or drive (which includes operating mechanism as well as electrical control and auxiliary circuits), were performed for disconnectors and earthing switches. Additionally, the AIS disconnectors and earthing switches section includes a short analysis on maintenance. The calculation of major failure frequencies for AIS and GIS is based on the matching of service experience data with major failures only. For service experience distributions, all available (matched and mismatched) data were used. Therefore, it is not possible to calculate or recalculate any major failure frequency from the published total service experience and total failure data. Because of the assumed underreporting of minor failures there is no calculation of minor failure frequencies in this survey. Because of two prevailing countries, it was necessary to carry out two separate analyses: one analysis with all data and another analysis with all data excluding the two prevailing countries. In order to compare the results of these analyses hypothesis tests were performed. The tests show if the subpopulation (all data without two prevailing countries) has an equal behavior as the total population (all data). The major results, conclusions and recommendations resulted from the analyses are presented in the last three sections of this report.

In 2002 CIGRÉ established working group A3.06 “Reliability of High Voltage Equipment” which was instructed to organize and carry out a worldwide enquiry into the service experience of 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 has been 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. The surveyed service experience includes 281 090 circuit breaker years, 935 204 disconnector and earthing switch years, 1 290 335 instrument transformer years, and 88 971 GIS circuit breaker bay years. Service experience is counted in three phase units multiplied by time, except for instrument transformers which are counted as single phase units multiplied by time. The number of failure cards received for all components types combined amounts to 15 834, of which 3 847 concerned major failures. This makes this survey by far 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 (three phase units). This is significantly lower than in a previous survey, and the improvement is observed for all types of enclosures. The failure frequency is found to increase with increasing rated voltage. For the first time the dependency of the failure frequencies to the kind of service is investigated. It is found that for all ages and kinds of circuit breakers shunt reactor switching results in the highest failure frequencies. The operating mechanisms account for the majority of all failures, which was also observed in a previous survey. Concerning the influence of age, younger circuit breakers tend to show lower failure frequencies than older ones. Wear and aging are the most frequently reported causes for minor as well as for major failures. For disconnectors and earthing switches the overall major failure frequency is determined to 0.21 failures per 100 equipment years of service (three phase units). The failure frequencies are significantly higher for disconnectors and earthing switches installed in air insulated substations than in gas insulated substations. One interesting result of the analysis is the overall 3:1-ratio between failures caused by the operating mechanisms and failures caused by the primary components of the equipment. Instrument transformers show an overall failure frequency of about 0.05 major failures per 100 single phase instruments transformer years of service. The major failure frequency for gas insulated substation instrument transformers is less than for those installed in air insulated substations. Instrument transformers with voltages between 60 and 100 kV have a lower major failure frequency also. The overall major failure frequency for GIS bays is about 0.4 major failures per 100 GIS circuit breaker bay years of service. This is lower than in the previous survey, and the difference is greater for indoor GIS. 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. The major failure frequency for voltages up to 300 kV is found to be almost independent on voltage rating and lower than for GIS operating at higher voltages. Plots of failure frequency vs. GIS manufacturing year exhibit the bath tube curve shape. In general, individual equipment installed in GIS appears to have lower failure frequencies than equipment in air insulated substations. The evaluation of GIS maintenance and diagnostic practices shows that there is room for future improvement in efforts for preventing major failures and for initiating major maintenance at the right time. These efforts are mainly concerned with on-site GIS high voltage dielectric testing after erection and extension, and with diagnostic measurements and monitoring methods and their interpretation.