Degradation and breakdown of mica under partial discharge stressing

Given the increasingly sophisticated tools available to acquire, store and process partial discharge measurements, it is imperative that the mechanisms of partial discharge degradation and the factors influencing the nature, form, extent and rate of degradation are elucidated. This will permit correlation among the measurements of partial discharge activity and the prevalent degradation in the practical situation. Only by these means will the ultimate peal, i.e. the prediction of remnant life of plant insulating systems through partial discharge measurement, be realised. The paper reports the results from the initial stage of an extensive investigation of the partial discharge degradation of mica. Ruby-muscovite and amber-phlogopite have been subjected to axial partial discharge stressing under AC, both polarities of DC and both polarities of impulse electrical stress. Two gaseous atmospheres, air and nitrogen, have been utilised to determine the degradation mechanism(s) involved. A mechanism of discharge degradation is hypothesised, based on the effects of bombardment by energetic particles and chemical interaction with gaseous discharge by-products.