Contribution to the Measurement of the Impulse Withstand Voltage of Oil-Paper-Insulation Systems

Impulse breakdown voltages of oil-paper-insulation systems show considerable scatter. Impulse withstand voltages associated with a certain low probability of breakdown can only be determined if the probability of breakdown as a function of the voltage is known. This probability function can be determined by means of test results. The test can be carried out on a series of about 20 simplified models by applying impulse voltages of increasing magnitude until breakdown occurs. Assuming that the variations occurring between specimens are negligible, i. e., the considerable scatter of the measured values is significantly due to the breakdown process, the probability that breakdown will occur for the first time at a particular voltage level is dependent on whether or not breakdown occurs at the lower levels. On the other hand, if the scatter is considerably caused by variations occurring between specimens rather than by the breakdown process, the influence of the lower levels prior to the breakdown voltage can be neglected. In both cases the experimental determination of the expected probability function is dealt with. Test results show that the scatter of the measured values is fundamental to the variations occurring between specimens rather than to the breakdown process.