Assessment of insulation materials aged by surface discharges

This contribution deals with test probes for the assessment of insulation material aged by surface discharges. First the ageing of the insulation surface due to partial discharges is shown. The general procedure for the assessment of the material using pd measurement is presented. Based on this know-how a test probe is proposed which allows the generation of surface discharges in a small area without field enhancement in wedges and edges: Spherical electrodes are moulded into the material to be examined. The distance of the electrodes from each other and from the surface are optimised in order to result in a field distribution for meeting the inception conditions in a small area of the surface. Due to the small area the occurrence of parallel discharges can be excluded and the measured value of the apparent charge can be taken as a quantity reflecting the damaging energy density. Additionally the test probe is constructed in a small size in order to fit into an existing broad band measurement system (f_g=1 GHz). This enables one to monitor more closely the interactions between insulation surface and discharge channel. Furthermore, phase resolved measurements are shown and interpreted