Measurement of pressure wave from epoxy sheet subjected to repetitive surge voltages

This paper describes experimental study on degradation mechanism of an epoxy-based composite sheet for printed circuit in control unit of inverter-fed motors subjected to repetitive polarity-reversed voltage pulses. The polarity-reversed voltage pulses after a dc prestress are produced by direct grounding of a charged coaxial-cable. Measurements of pressure wave due to partial discharge at the polarity reversal and of breakdown time lag of the sheet subjected to repetitive pulse application are carried out. When a sphere electrode is put on the sheet without any molding, a very large pressure wave caused by discharge near the sphere-sheet contact point is detected by a piezo sensor on the back of the lower plane electrode. The time lag distribution for the sheet without molding indicates that the complete breakdown occurs after a degradation process due to repetitive partial discharges. When the sphere electrode on the sheet surface is molded by an epoxy resin, the magnitude in the pressure wave becomes small. In addition, the time lag distribution shows that the breakdown phenomenon is caused by a stochastic mechanism based on an electronic process. Degradation mechanism of the sheet with plane-plane electrode is also evaluated by the same methods.