INVESTIGATION OF ELECTRICAL BREAKDOWN OF POLYMERIC INSULATING MATERIALS USING A TECHNIQUE OF PREBREAKDOWN CURRENT MEASUREMENTS

Measurements of electrical breakdown, high-field conduction current and pre-breakdown current are made on various kinds of polymeric insulating films. The experimental results on electrical breakdown and conduction indicate that the electrical breakdown current starts to increase rapidly at about 1 ms before breakdown at 90 degrees C in PAI and PI. This observation is well explained by a thermal runaway resulting in thermal breakdown. In some cases, it constitutes direct evidence for thermal breakdown. On the other hand, no such current rise exists before breakdown observed in PE and EVA, although their electrical breakdown properties exhibit a similar tendency indicating thermal breakdown. Conditions for observing the sharp current rise just before breakdown are also studied by further experiments and numerical analysis. Investigation is made on the effects of water absorption of PI and oxidation of PE on the sharp pre-breakdown current rise. Numerical calculation of the filamentary thermal breakdown is carried out. Results show that the melting point of the sample is closely related to the probability of observing the sharp rise of pre-breakdown current. Pre-breakdown current of PI at low temperatures is also discussed. © 1990 IOP Publishing Ltd.