Modelling of cavity partial discharges at variable applied frequency

Sammanfattning: The presence of partial discharges (PD) in high voltage components is generally a sign of defects and degradation in the electrical insulation. To diagnose the condition of high voltage insulation, PD measurements is commonly used. The Variable Frequency Phase Resolved PD Analysis (VF-PRPDA) technique measures PD at variable frequency of the applied voltage. With this technique, the frequency dependence of PD can be utilized to extract more information about the insulation defects than is possible from traditional PD measurements at a single applied frequency.In this thesis the PD process in a disc-shaped cavity is measured and modelled at variable frequency (0.01 - 100 Hz) of the applied voltage. The aim is to interpret the PD frequency dependence in terms of physical conditions at the cavity. The measurements show that the PD process in the cavity is frequency dependent. The PD phase and magnitude distributions, as well as the number of PDs per voltage cycle, change with the varying frequency. Moreover, the PD frequency dependence changes with the applied voltage amplitude, the size of the cavity and the location of the cavity (insulated or electrode bounded).A physical model is presented and used to dynamically simulate the sequence of PDs in the cavity at different applied frequencies. The simulations show that essential features in the measured PD patterns can be reproduced. The PD frequency dependence is interpreted as a variation in influence on the PD activity from the statistical time lag of PD and the charge transport in the cavity surface, at different applied frequencies. The simulation results also show that certain cavity parameters, like the cavity surface conductivity and the rate of electron emission from the cavity surface, change with the time between consecutive PDs, and accordingly with the applied frequency. This effect also contributes to the PD frequency dependence.