CIGRE Reliability Survey on Equipment

Summary

Reliability of substation equipment in power systems is of major concern especially for transmission and distribution system operators and asset owners. A major failure of substation equipment may result in significant system outages with the associated power restoration efforts as well as possible safety implications. There are also financial implications in case of poor reliability. In addition to the cost of a system outage and its restoration, poor reliability will contribute to higher system operating and maintenance costs to the operators and, ultimately, their customers.

For these reasons, CIGRE periodically conducts an international reliability survey on equipment in power systems that can provide good feedback on the validity of international standards. The first reliability survey was carried out in 1974-77 and covered nearly 78,000 circuit breaker years in service. The results were published in 1981 [1] and had a significant impact on testing requirements in IEC standards. In the second reliability survey, data were collected for the period 1988-91 and almost the same number of circuit breaker years was sampled, but the survey was limited to circuit breaker single pressure SF6 technology. CIGRE Technical Brochure (TB) 083 [2] remains a valuable source of information for the circuit breaker community. Later, the CIGRE TB 165 provides further considerations on lifetime management of circuit breakers. The reliability data related to GIS were also collected in 1990 (first survey [9], [10]) and in 1996 (second survey [11], [12]).

The third reliability survey included not only circuit breakers but also disconnecting switches, earthing switches, instrument transformers and gas insulated switchgear (GIS). The time period covered was 2004-2007, and the enquiry covered equipment with voltage ratings higher than or equal to 60 kV. The results from the circuit breaker part and GIS part were presented in CIGRE Technical Brochures 510 and 514 [3]-[8], respectively. Only single pressure SF6 circuit breaker technology was included in the third survey. A fourth reliability survey on substation equipment is under progress and intended to expand the scope to cover generator circuit breakers, vacuum circuit breakers, and surge arresters, in addition to SF6 circuit breakers, disconnecting switches, earthing switches, instrument transformers and GIS only focusing on major failures. The paper will present preliminary results of the fourth survey and compare them with the previous results.

1. Introduction

Reliability of power equipment such as circuit breakers, disconnecting switches, earthing switches, instrumental transformers and gas insulated switchgears (GIS) used in power systems is of major concern for transmission and distribution system operators and asset owners, since the cost of a system outage and its restoration, poor reliability will contribute to higher system operating and maintenance costs to the operators and, ultimately, their customers.

For these reasons, CIGRE periodically had conducted an international reliability survey on equipment, and updated the numbers of the equipment populations and their failure frequencies classified with the voltage ratings, the switching applications, the technologies, the designs and the maintenance strategies [1]-[12], [14]-[16]. Currently a fourth reliability survey on equipment serviced in 2014-2017 is in process, which expands the equipment scope to cover generator circuit breakers, HV and MV vacuum circuit breakers, and surge arresters. The paper presents an interim report of the results of CB and GIS in the fourth reliability survey and compares them with the results in the previous surveys.

2. Definitions of failures

Failure definitions are very important when comparing the results among different utilities. CIGRE use the terms of major failures (MaF) and minor failures (MiF) as defined in the IEC standard 62271-1 [13]. In particular, it was concluded that only major failures were analysed in the third reliability survey [4], because of difficulty in dealing with the minor failure data showing the significant differences in the definition of the minor failures and their data collections and records among the utilities. The fourth reliability survey follows the practice focusing on major failures.

According to the IEC standard, a switchgear major failure is defined as “failure of a switchgear and control gear which causes the loss 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, even complete of a constructional element or a sub-assembly which does not cause a major failure of the equipment”. CIGRE requests a participating utility to apply the IEC definitions when they evaluate the number of major failures of equipment occurred in 2014-2017 as far as possible.

3. Confidentiality Commitments in the CIGRE reliability data on equipment

The CIGRE periodically conducts a reliability survey on equipment, which can provide useful information on the validity of testing requirements for different equipment and the background and necessity of the revisions in IEC standards, in addition to some potential feedbacks how the utilities can minimize the maintenance cost, system outage and its restoration and assess the end-of-life assessment of ageing equipment. CIGRE is often requested by some stakeholders to provide useful information on equipment reliability for a certain equipment group with a similar design in order to make their important financial and technical decisions.

The CIGRE reliability data provided by the utilities in different countries has been treated as confidential and the subsequent analyses thus had to be carried out under certain restrictions that makes it impossible to identify the respondents, since they includes the detailed failure information. CIGRE keeps the confidentiality commitment on the reliability data received from different utilities, since they provided data to the survey under the explicitly stated condition that the information should be treated as confidential.

The results on the failure frequency were mostly published with the world average values that indicates some variable outcomes for lifetime assessment when a subject shares the common problems in the world. For example, frequent operations of a circuit breaker applied to reactor switching resulted in higher major failure frequency in the third survey. Reflecting the feedbacks, CIGRE SC A3 in cooperated with A2 made a quick survey on the natural frequency of HV reactors in details to evaluate the validity of TRV requirements in the IEC 62271-110. However, it often faces with difficulty to asset a meaningful lesson with the average values mixed with different designs, operating conditions and maintenance policies. It is almost impossible to estimate a clear end-of-life period on any equipment using the world average values with a variety of different designs.

In the fourth survey, CIGRE is collecting only figures of the equipment populations and the number of major failures in 2014-2017. It is not collecting the detailed service experience including consequences of the failures that were covered in the previous CIGRE reliability surveys. Therefore, the fourth survey results will not include the detailed information on each failure case.

 In the third survey, CIGRE TB 510 [4] anonymously showed the CB population and major failure frequency for each country. The values of CB major failures widely ranged from 0.06 to 2.92 % (100 CB-years). The difference of the major failure frequencies among the countries seems to be larger in the fourth survey than in the previous one. This could be due to increasing differences in the operating conditions and the maintenance policies with the different designs and the service periods. Therefore, this report attempts to show the major failure frequency as a function of the service year anonymously with a relatively similar design group.

4. Interim results of the fourth CIGRE reliability survey on equipment

The scope of the fourth CIGRE reliability survey covers HV switching equipment such as circuit breakers (CB), disconnecting switches (DS) and earthing switches (ES) with the voltage ratings higher than or equal to 60 kV. The scope of the survey on instrumental transformers (IT) and metal oxide surge arresters (MOSA) covers the applications in conjunction with HV switching equipment. Therefore they are classified with the voltage ratings of switching equipment (not the voltage ratings by themselves). The scope of the survey on gas insulated switchgear (GIS) covers HV switchgears including CB unit with the voltage rating higher than or equal to 60 kV. The GIS population is identical to the number of CB population with GIS type. The GIS with vacuum interrupters (VCB) is excluded from the scope.

The scope of the survey on VCB covers all vacuum interrupters with the voltage ratings higher than 1 kV. This is the first attempt to investigate the switching equipment at distribution voltages. Finally the scope of the survey on generator circuit breakers (GCB) covers a circuit breaker directly connected to a generator used as a base load. The voltage ratings of GCB is approximately ranged from 6 to 36 kV. The circuit breaker connected to a wind turbine are excluded from the scope.

Table 1 summarizes an interim report on the population and the world average major failure frequency for different equipment collected until October 2021. The major failure frequency for equipment excluding IT shows better than those in the third survey. The background CB populations are significantly different between the third and the fourth surveys due to new participants with large installations. The major failure rates in case of CB is the same level as those in the third survey, however, the values are continuously under evaluations, since the data collected in some countries relatively shows higher failure rates due to frequent failures of ageing CB serviced for more than 40 years. Such an old assets were not investigated in the previous surveys.

5. CIGRE reliability survey on circuit breakers (CB)

Table 2 provides an overview of the participations and time periods collected by the past three reliability surveys on circuit breaker (CB) and the on-going fourth reliability survey until the end of December 2019. Even though only 18 utilities participate in the fourth survey, the number of circuit breaker populations exceeds that of the last third survey, due to first participation of some countries with large equipment installations. CIGRE is now evaluating the reliability data carefully whether the major failures among the utilities has a common definition stipulated in IEC standard.

Table 3 shows a comparison of major failure frequencies on circuit breaker (CB) in the past surveys as a function of voltage ratings. The first survey indicated that the major failure frequency on CB distinctly increased with an increase of the voltage ratings. The CB reliability has been improved significantly over the past three decades and highly reliable CBs are now available for all voltage ratings up to EHV and UHV levels. However, since the CB reliability still differs among the countries, it may provide useful information if CIGRE would investigate the background information of reliability differences depending on the designs and specifications, the operating conditions, and the maintenance practices among the utilities, while keeping anonymity of the utility or the country corresponding to the data. Note that the fourth survey indicates the extremely high major failures for a specific CB group serviced with more than 40 years in some countries.

Figure 1 show a distribution of CB populations classified into the voltage rating among the surveys. The ratio of the CB installations with the voltage ratings from 60 to 100 kV (typically 72.5 kV) is decreasing, while that of higher voltages (typically 145 kV) are increasing due to expansion in transmission networks, especially in the developing and growing counties. The HV CB populations are also classified into the switching applications, the design types and the operating mechanisms.

Figure 2 show a distribution of HV CB populations classified into the switching applications among the surveys. Around one quarter of the CB populations are applied to a power transformer and about 60 percent of the CB populations are connected to a transmission line or a cable. The pie charts reveals a recent technological shift from a line to a cable due to difficulties in public acceptance. The ratio of the bus-coupler application is about 10 % and the remaining a few percent of the CB populations are a capacitor or a reactor bank switching with frequent operations.

Figure 3 show a distribution of HV CB populations classified into the design types among the surveys. The CB selection by the utilities generally keeps a similar ratio for over three decades. There is a certain trade-off relation between the price and the reliability in the selection of CBs with either a live-tank type or a metal-enclose type including GIS type. The one prevailing country in Asia mostly applies the dead tank CB in power systems, while other countries generally prefer to use the live tank CB.

 Figure 4 show a distribution of HV CB populations classified into the operating mechanisms among the surveys. The ratio of CB with the pneumatic operating mechanism is decreasing and that of CB with the spring operating mechanism is increasing due to CB development of the spring operating mechanism applied to higher voltage ratings. 420 kV, 550 kV and 800 kV double-break CBs with the spring operating mechanisms were put in service in 2008, 2012 and 2014, respectively. 420 kV and 550 kV single-break CBs with the spring operating mechanisms will be available soon. 

Figure 5 shows the world average CB major failure frequencies classified into the switching applications, the design types and the operating mechanisms. The third survey indicated that the major failure frequencies for shunt reactor switching applications were ten time larger than those for other switching applications, because the reignition overvoltage generated during frequent reactor opening operations may degrade circuit breaker components such as nozzle and contacts in some designs. In the fourth survey, the capacitor switching applications shows higher major failure frequencies so far.

Similarly to the third survey, CB with live-tank designs shows higher major failure frequencies. The major failure frequencies of the live-tank CB tends to increase with an increase of rated voltage, while that of the dead-tank relatively shows a constant frequency for all voltage ratings.

Regarding the operating mechanisms, the world-average data shows higher major frequency in hydraulic operating mechanism. However, the trends on operating mechanisms in different counties shows a certain extent of diversity depending on the differences in the design types.

6. CIGRE reliability survey on gas insulated switchgears (GIS)

Table 4 shows a summary of the populations and the major failure frequencies on gas insulated switchgears (GIS) collected by the past reliability surveys and on-going fourth reliability survey as a function of the voltage rating. The major failure frequency on GIS slightly increases with an increase of the voltage ratings especially in the second survey. In the fourth survey, one new prevailing country dominates 69 % share of the GIS populations. While In the third survey, two old prevailing countries dominate 92 % share of the GIS population. The GIS failure frequency becomes 1.46 % without two old prevailing countries in the third survey, which is the same level as the tentative world average major failure rate in the fourth survey. CIGRE is currently evaluating whether the definitions of the major failures among the countries are the same as that recommended in the IEC standards, especially for new participants. For this reason, CIGRE will not provide interim values of the major failure frequency for CB and GIS at this moment.

Figure 6 show a distribution of HV GIS populations classified into the voltage rating among the surveys. The ratio of the GIS installations with the voltage rating from 60 to 100 kV (typically 72.5 kV) is decreasing, while that of higher voltages (typically 145 kV and 245 kV) are increasing due to expansion in networks, similarly in case of CB populations.

7. Conclusions

The paper provides a preliminary report of the fourth CIGRE reliability survey on substation equipment focusing on CB and GIS and compares the results with the past surveys. The world average major failure frequency for different equipment generally shows good service experience. The major failure frequency of CB and GIS shows slightly higher rates than those in the third survey. However, the background CB populations are significantly different between the third and fourth surveys due to new participants with large installations. The major failure rates in case of CB and GIS are still under further evaluations.

The reliability on switching equipment seems to differ among the utilities in different countries, it may provide useful information if CIGRE would investigate the background information of the reliability and its dependence on the designs, the specifications, the maintenance practices and operating conditions, while keeping anonymity of the utility or the country corresponding to the data. Note that the fourth survey indicates the extremely high major failures for a specific CB group serviced with more than 40 years in some countries.

The on-going fourth surveys is also collecting the information on vacuum circuit breakers (VCB) and metal oxide surge arresters (MOSA) for the first time. VCBs applied at both MV distribution and HV transmission levels generally show excellent service experience. MOSAs are installed to protect various substation equipment and they generally show excellent service experience. Most of the MOSA major failures are limited to the MOSA itself not affecting a neighbouring equipment, which means that MOSA successfully protects the equipment from the excessive overvoltages.

The fourth CIGRE survey will update the reliability performance of various substation equipment and provide useful information on lifetime management and end-of-life assessment of equipment, which is expected to keep the specified performance during the lifetime.

References

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2. CIGRE WG 13.06, “Final report of the second international enquiry on high voltage circuit-breaker failures and defects in service”, CIGRE Technical Brochure 083, 1994
3. CIGRE 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 509, 2012
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5. CIGRE WG A3.06, “Final Report of the 2004-2007 International Enquiry on Reliability of High Voltage Equipment, Part 3: Disconnectors and Earthing Switches”, CIGRE Technical Brochure 511, 2012
6. CIGRE WG A3.06, “Final Report of the 2004-2007 International Enquiry on Reliability of High Voltage Equipment, Part 4: Instrument transformer”, CIGRE Technical Brochure 512, 2012
7. CIGRE WG A3.06, “Final Report of the 2004-2007 International Enquiry on Reliability of High Voltage Equipment, Part 5: Gas Insulated Switchgears”, CIGRE Technical Brochure 513, 2012
8. CIGRE WG A3.06, “Final Report of the 2004-2007 International Enquiry on Reliability of High Voltage Equipment, Part 6: Gas Insulated Switchgears (GIS) Practices”, CIGRE Technical Brochure 514, 2012
9. I. M. Welch, “GIS Experience Survey and Database“, ELECTRA No. 157, page 81-83, December 1994
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12. CIGRE WG 23.02, “Report on the Second International Survey on High Voltage Gas Insulated Substations (GIS) Service Experience”, CIGRE Technical Brochure 150, 2000
13. IEC Standard 62271-1, “High-voltage switchgear and controlgear, Part 1: Common Specifications”
14. CIGRE WG A3.08, “Life Management of Circuit Breakers”, CIGRE Technical Brochure 165, 2000
15. CIGRE WG 13.09, “User Guide for the Application of Monitoring and Diagnostic Techniques for Switching Equipment for Rated Voltages of 72.5 kV and above”, CIGRE Technical Brochure 167, 2000
16. CIGRE WG A3.29, “Deterioration & ageing phenomena of HV substation equipment”, CIGRE Technical Brochure 725, 2018

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• Contact Author: H. ITO