The role of space charges in PD-processes

For many years, PD-analyses have been a widely used tool in the condition monitoring of electric equipment. Over the years, many tools have been developed in the aim to characterise the empirical data. Usually frequency distributions of accumulated data are considered and displayed as purely descriptive φqn-patterns. The usually comparatively broad distributions of the phase angles of occurrence are supposed to be a result of a statistical scatter in the discharge process. The careful analysis of the sequence of discharges in different high voltage apparatuses revealed that in many cases a systematic shift of the phase angles of occurrence with a periodicity of a few cycles of the applied voltage occurs. The reason for this is a build-up or modification of local space charges by every partial discharge pulse. These space charges generate an additional local electric field that superimposes on the field generated by the externally applied voltage. The paper shows that taking these phenomena into account, the discharge process reveals to be highly deterministic and can be described by a model involving only a few physically relevant parameters