Partial discharges measured with an automated system in epoxy resin and polyethylene

The partial-discharge (PD) characteristics of a filled epoxy casting resin were compared in long-term tests with those of polyethylene (PE). For this purpose, a newly developed automatic measuring system was used to detect and process PD parameters such as the apparent amount of charge, the PD repetition rate, the apparent PD energy and the polarity of the impulses, together with the position of the latter with respect to the phase of the applied 50-Hz AC voltage. The measurements were made on needle-plate electrode model specimens. The results recorded over long stress periods at constant voltage show that the PD impulses in epoxy resins arise discontinuously and with constantly changing amplitude. So called impulse pauses lasting several hours were recorded in these materials. PE was found to differ fundamentally from epoxy resins in their PD characteristics. In the PE material, the PD impulses occurred fairly regularly and were of almost constant amplitude. As a result of the very high impulse frequency in PE over the whole period of measurement, breakdown development was relatively rapid. Interpretation of the results is based on the concept of the origin of PD channels in the extremely inhomogeneous AC field, and, in particular, use is made of the recorded phase histograms and of the changes observed in the apparent PD energy with time