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Internal Discharges in Cavities in Solid Dielectric Materials

Internal discharges in cavities in solid dielectrics lead to the degradation, deterioration and complete failure of insulating materials. This would shorten the life time of the electrical equipments. This may affect the reliability of the power supply. A new technique for detecting and measuring internal discharges in cavities in solid dielectrics under alternating voltage conditions has been developed by using an Electrical research association detector (ERA detector model 5) and a multi channel pulse height analyzer. The system was calibrated by feeding voltage pulses of known magnitude into the circuit from a pulse function generator. The technique enabled the study of the magnitude and the repetition rate, of discharges in artificial air filled cavities in polythene samples. The loss tangent was measured using a H.V Schering Bridge. The inception and extinction voltages for samples were determined before each experiment. The measurements were compared and showed a high consisted. The results obtained showed that for all cavity locations the discharge inception and extinction voltages decreased as the cavity area was increased, this was found in both cylindrical and spherical cavities. Again this can be attributed to the increase in the number of discharging sites on the surfaces of the cavity. In polythene a sample with cavity facing the high voltage electrode the discharges behaved in a different manner from that facing earthed electrode. The general theme that the repetition rate and magnitude of discharges decrease with the time of voltage application, this is due to the semi conducting deposits accumulated on the cavity surfaces.