ELECTRICAL AGING AND ELECTROLUMINESCENCE IN EPOXY UNDER REPETITIVE VOLTAGE SURGES

An extensive statistical accelerated aging experiment, involving a total of 180 unfilled type epoxy specimens, has been conducted to determine the number of voltage surges required to initiate electrical trees under various voltage magnitudes, surge repetition rates and polarity conditions. The specimens were tested one at a time according to a full factorial experimental design. During many of the aging tests, the electroluminescence (EL) from the epoxy specimens was monitored. EL has been reported to be both a symptom and/or a cause of tree initiation in polymers. Conventional Weibull analysis techniques were used to investigate the experimental results in addition to a powerful new commercial computer program; the latter was capable of estimating the multifactor life model with both the Weibull and log-normal distributions, allowing for censoring. The analysis showed that life decreased with increasing applied voltage. The life was longer for negative polarity surges. As the surge repetition rate increased, the number of surges to initiate a tree increased, an effect which is believed to be related to the time required for injected space charge to dissipate. A modified inverse power model, with a voltage power law constant similar to that found in ac aging, was determined to be appropriate. No correlation was observed between either the trend in EL over time or the total amount of EL and the number of surges required to initiate a tree.