IEEE C57.100-2011 pdf download IEEE Standard Test Procedure for Thermal Evaluation of Insulation Systems for Liquid-Immersed Distribution and Power 1ra nsformers
6. Criteria for end of life–Distribution transformer and power transformermodel
In these tests, the life of a particular test specimen is considered to be ended when the degradation of theinsulation system has progressed to a point such that the test specimen cannot withstand any one of a seriesof tests intended to simulate the abnormal currents or voltages that are commonly experienced in actualservice. The degradation or aging is produced by a series of temperature cycles, each consisting of aspecified time at a specified hottest-spot temperature followed by a return to approximately ambientemperature. Such a series of temperature cvcles. followed by end of life tests. will hereafter be refered toas a test period. Since it is impractical to determine the exact point in the test period procedure when thesample reached the level at which it could not withstand the end-point tests, its life at the test temperatureshould be considered to be the duration of one test period multiplied by the number of periods to failureless one half of one period.
Because of the nature of this test, the word failure, as used herein. assumes a special connotation. It is usedhere to describe an insulation breakdown such that a service outage would result if it occurred in the field.Consequently. some of the criteria of failure that are commonly used in tests on new transformmers do notapply. For example, minor disturbances in the oscillograms obtained on impulse tests or increases in theleakage current on applied potential tests are not necessarily indicative of failure as defined here. Specificinstructions for the treatment of such marginal cases are given in 10.3.
Ifa test sample should fail premaurely, that is, long before its anticipated life expectancy and if subsequentcxamination conclusively shows the failure to be the result of defects in material or workmanship. ratherthan thermal degradation, failure of this sample may be ignored in determining the test results and anothersample substituted for it.
7. Test temperatures
The accuracy of an evaluation will increase as the number of test temperatures increases but its cost willalso increase. In general, tests should be made at the maximum number of temperatures that can be justifiedeconomically with the following qualifications:
a1When insuflicient previous information exists regarding the shape of the life expectancytemperature curve for the system being evaluated, tests should be made at a minimum of threetemperatures selected as recommended in IEEE Std 99. This requirement of three temperaturepoints, 20 °C intervals, which should be extrapolated no more than 20 C. would require verylong test times to permit the necessary extrapolation with confidence. As this may beimpractical, tests at additional temperatures should be made.
b)As an alternative. industry experience with testing following this standard test method hasshown that 15 °C intervals and extrapolations up to 40 °C may be suitable assuming that acomparison to the industry-proven system is made, using a relative thermal index evaluation asis described in IEEE Std 1. For this evaluation, the industry-proven system would be evaluatedusing this standard at 150 °C, 165 C, and 180 °C and then the result compared to the results ofthe candidate system at alternative temperatures. For example, for a candidate system that isexpected to have 30 C better thermal performance than the industry-proven system, the agingtemperatures would be 180 °C, 195 C, and 210 °C. Table 1 shows other possible combinations.An example of such a comparison is shown in 1l.1. part b)