The influence of defects on the short-term breakdown characteristics and long-term dc performance of LDPE insulation

Extruded polyethylene is used as the bulk insulation for ac HV cables becau se of its high electric resistivity and breakdown strength. Although the ma terial at present has limited use in dc power cables, it is used extensivel y in submarine optical communication cable systems. This paper reports on t he study of the short-term characteristics and long-term performance of low -density polyethylene (LDPE) insulation under de electric stress. The resul ts are presented in which controlled defects as found in practical systems (voids, metallic and non-metallic particles) were introduced into well char acterized polymer material so that their influence on electrical strength a nd breakdown mechanisms could be determined. Samples were compression-molde d under laboratory conditions and subjected to ramp-to-failure and electric al aging tests at various stresses. Weibull statistics are used to analyze the results. Examination of electrically aged samples indicates an oxidatio n degradation around the defects due to stress enhancement. From the failed samples a value of n in the inverse power model was estimated. The residua l life of aged samples also was examined.