Field-enhancing defects in polymeric insulators causing dielectric aging

Defects that produce strong local field enhancement are discussed. They are especially harmful because they do not produce partial discharges detectable in usual factory control tests. Typical examples for such defects are electrode protrusions, floating conducting particles, and the tips of electrical trees. Charge injection from field enhancement tips is shown to be the first step in a series of pretreeing events. In model experiments using a needle/plate arrangement, a field threshold for injection is observed, separating a low-field regime of `infinite´ lifetime for the dielectric from a high-field regime where treeing will almost inevitably set in after some incubation time. This incubation time is shown to depend very sensitively on how much the applied voltage exceeds the value at which the critical field is reached locally. The value of this threshold field is in the range of megavolts per centimeter for pure or filled homogeneous insulators and about two orders of magnitude lower in compound insulators containing (bad) interfaces