Limits to Partial Discharge Detection: Effects of Sample and Defect Geometry

ABSTRACT A careful analysis of partial discharge detection for the case of a void within a solid dielectric casting indicates that the “apparent” PD sensitivity varies inversely with testpiece scaling and does not depend, as often stated, on testpiece capacitance. Thus the large castings used in the highest voltage class equipment and usually at the highest operating stresses are tested with the lowest “real” sensitivity. A model is developed for the maximum apparent PD magnitude from a void of given volume, shape, and position within a casting. Data supporting the validity of the model are provided over a limited range of parameters. Graphs are provided from which the “apparent” PD magnitude of a void within a coaxial casting can be estimated. These data can be extended to arbitrary geometries through the use of field plotting programs.