Effect of HV Impulses on Partial Discharge Activity in Oil-Impregnated Paper Insulation

Sammanfattning: High voltage (HV) transients in electrical power systems are mainly caused by lightning strikes and switching operations.  The high voltage transient from its point of origin can propagate in either direction in the electrical power transmission line, and if its magnitude would be below the Basic Insulation Level (BIL) of the equipment for impulse voltage, it can reach the terminals of the equipment without being dissipated by surge arresters.  The dielectric properties of a particular insulation at the instant of interaction with a high voltage transient would determine the level of the degradation of the electrical insulation caused by the high voltage transient. This thesis suggests on-line diagnostic techniques for monitoring the oil-impregnated paper condenser bushings by utilizing the naturally occurring HV transients in electrical power systems.The aim of the first experiment was to investigate the partial discharge (PD) characteristics of the PD defects which are likely to be found in oil-imregnated paper condenser bushings. In order to understand the condition at which the HV impulses would cause the change in phase resolved partial discharge (PRPD) patterns, four more experiments were performed. The second experiment was set in such a way that the effect of HV impulses alone could be investigated. On the other hand, the third experiment was set to examine the effect of PD-ageing at elevated AC voltage. The fourth experiment aimed in investigating the effect of combined PD-ageing at elevated AC voltage and the HV impulses. In order to investigate how different electrical stresses affect the dielectric properties of the oil-impreganted paper insulation, the fifth experiment about the dielectric spectroscopy (DS) in frequency domain was performed.The measurement results presented in this thesis show that high voltage impulses can have a large impact on the behavior of PRPD patterns if the test sample is aged by partial discharges, but a smaller impact when the test sample is unaged. The measurement results also show that the degradation of the oil-impregnated papers is much faster when the oil-impregnated papers are suffered from the combination of the HV impulses and the PD activity at elevated AC voltage than when they are suffered from the PD activity at elevated AC voltage alone.