17.1 Introduction

A partial discharge (PD) is a localised electrical discharge that only partially bridges the insulation between conductors and which may or may not occur adjacent to a conductor. This definition is given in IEC Publication IEC 60270 Ed. 2, 2000 [1], which is the revision of IEC Publication 60270 [2]. Partial discharges are in general a consequence of local electrical stress in the insulation or on the surface of the insulation. The discharges normally appear as pulses with a duration of less than 1 s. So-called continuous pulseless discharges in gaseous dielectrics also exist, but this kind of discharge will not be handled in this chapter. Furthermore, the term 'corona' is often used for the partial discharges that occur in gaseous media around conductors which are remote from solid or liquid insulation, and therefore 'corona' is a particular kind of partial discharge.

Partial discharges are a sensitive measure of local electrical stress and therefore the measurement is very often used as a quality check of the insulation. The inception of partial discharges gives information on the limit of the electrical strength of the insulating material before a complete discharge between the conductors takes place. Therefore the insulating material can be tested with high stress but without damaging or reducing the performance of the insulation. Also, for partial discharge measurements it should be taken into account that every stress of the insulation will have an influence on the life expectancy of the material, but a reasonable compromise...