Partial discharges in solid insulation

Partial discharges (PD) seem to be somewhat inevitably part of the life of most solid insulating systems for HV plant. If left undiscovered/unchecked they may well result in a significant shortening of that life. Their presence stems from a combination of poor manufacturing techniques, creating voids or inclusions in otherwise homogeneous solid insulation and/or degradation in the insulation through its in-service life, again resulting in void creation. Irrespective of the causative stress, be it electrical, mechanical, thermal or chemical, degradation of solid insulating systems will almost inevitably lead to partial discharge activity at the degradation site due to the prevalent electric field stressing and the lower breakdown strength of gaseous compared with solid media. It is therefore imperative that the mechanism(s) of PD formation, activity and degradation are clearly understood to permit the development of novel solid insulating systems robust to this stress regime. On the positive side, if there is one, the detection and measurement of partial discharge activity provides a technique for assessing degradation in a solid insulating system. The author argues that, for this reason, it is equally imperative that the relationships between PD activity and the nature, form and extent of degradation are clearly understood to condition monitor existing systems