Space charge injection and partial discharge inception at the metal polymer interface

Injection and transport of charge carriers occurring above a distinct threshold field are shown to play a dominant role in the stability of dielectrics under long-term electrical stress. Since the required threshold fields are well above the design fields of the usual types of insulation they can only be reached very locally at field enhancement tips (conducting defects) or at interfaces in composite insulators. From such tips treeing almost inevitably develops in a series of intermediate steps when applied AC voltage leads to local transgression of the threshold field. The time to failure is found to be strongly dependent on how large the transgression is