4.1 Introduction

Partial discharges are localised gaseous breakdowns which can occur within any plant system provided the electric stress conditions are appropriate. Because the breakdown is only local, failing to result in a following current flow, it is described as partial.

Why are partial discharges (PD) of importance to high voltage engineers? Partial discharge activity is both a symptom of degradation in the insulating systems of power plant - irrespective of the causative stress - and a stress mechanism in itself. Wherever degradation occurs in an insulating system, be it due to electrical, mechanical, thermal or chemical/environmental conditions, it is generally accompanied by the generation of partial discharges. Once present, these then tend to dominate as the stress degradation mechanism. Irrespective of whether the insulating system is gaseous, liquid, solid or a combination of these, partial discharge activity will cause degradation. It can therefore be appreciated why understanding the processes by which partial discharges cause degradation is so important to the development of new insulating systems capable of withstanding this stress mechanism. In addition, it can also be appreciated why understanding the correlations among the measurable parameters of discharge activity and the nature, form and extent of degradation present is so important to the engineer responsible for the maintenance and asset management of existing plant systems.

4.2 Partial Discharge Degradation Mechanisms

The electrons, ions, atoms, radicals and excited molecular species produced in a partial discharge move under the influence of the following forces variously:

• thermal excitation