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CIGRÉ Technical Brochure No. 512: Final Report of the 2004 – 2007 International Enquiry on Reliability of High Voltage Equipment, Part 4 - Instrument Transformers

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%.

high voltage equipment; reliability; maintenance; enquiry; instrument transformers; major failures; minor failures; major failure frequencies; failure characteristics

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