Study of elastic waves of acoustic frequencies generated by surface partial discharges of solid insulators in vacuum

Abstract

The paper presents studies on the electrical properties of cylindrical shape samples of insulators made from polyamide, epoxy resin, polymethyl methacrylate, and electrical porcelain, located in vacuum at a pressure of 0.1 mPa. The insulator samples had a diameter of 20 mm and a height of 1–10 mm. The study was performed with an alternating voltage of 50 Hz in the range up to 70 kV (peak). The examination has shown that partial discharges on the surface of the insulator generate elastic waves of acoustic frequency. The waves may be used to locate the position of the partial discharges by means of suitably sensitive equipment. During the measurements the following parameters of the acoustic signals have been recorded: the largest amplitude of signal, the number of signal amplitudes exceeding the discrimination level, the rate of signals, the number of events the envelopes of which exceed the discrimination level, the rate of events and the root mean square value of signal. It was found that for purposes of determining the voltage threshold necessary for occurrence of acoustic activity, a measurement of the largest amplitude of a given acoustic signal is the most appropriate. Nevertheless, for forecasting a flashover occurrence of examined insulators a measurement of the root mean square value of acoustic emission signal may be also effectively used. Values of both of these parameters significantly increased just before an instance of surface flashover.