Partial discharges in cylindrical cavities at variable frequency of the applied voltage

Measurements of partial discharges are commonly used to diagnose the insulation system in high voltage components. Traditionally a single fixed frequency of the applied voltage is used for such measurements as in the Phase Resolved Partial Discharge Analysis (PRPDA) technique. With the Variable Frequency Phase Resolved Partial Discharge Analysis (VF-PRPDA) technique the frequency of the applied voltage is instead variable. This technique provides more information about the condition of the insulation than the PRPDA technique. To extract the extra information a physical understanding of the frequency dependence of partial discharges is necessary. In this thesis partial discharges in cylindrical cavities in polycarbonate are measured using the VF-PRPDA technique in the frequency range 10 mHz – 100 Hz. It is studied how the cavity diameter and height influence the frequency dependence of partial discharges. Insulated cavities are compared with cavities bounded by an electrode. It is shown that from measurements at variable applied frequency it is possible to distinguish between cavities of different dimensions and between insulated and metal bounded cavities. A two-dimensional field model of partial discharges in a cylindrical cavity is developed. The sequence of discharges in the cavity is simulated by use of the field computation program FEMLAB R . Discharges are modeled with a voltage and current dependent streamer conductivity and are simulated dynamically to obtain charge and current consistency. It is shown that the frequency dependence of partial discharges is significantly influenced by the statistical time lag and by the two dielectric time constants related to charge movements on the cavity surface and in the bulk insulation. Simulation results are used to interpret the frequency dependent partial discharge activity in a cylindrical cavity.