Modelling of streamer breakdown under positive polarity HVDC in subtropical conditions.

Abstract

Atmospheric air is an essential component in high voltage power systems as it is used to provide electrical insulation. Therefore, it is required that research is extensively performed on the gaseous insulator to ensure reliability of the system. This dissertation presents the research that has been performed on the streamer breakdown of needle-plane air gaps that were less than 15 cm in length. An experiment was conducted to investigate the streamer breakdown in air gaps. The investigation also included determining the influence of humidity and pressure on the breakdown voltage when HVDC is applied to the high voltage electrode in subtropical conditions. The results indicated that humidity did not influence the breakdown voltage for small gap lengths but both humidity and pressure have a directly proportional relationship with the breakdown voltage of air gaps longer in length. It was observed in the results that the breakdown voltage reduced in magnitude when the humidity increased, and this could have been a result of the breakdown developing out of the glow discharge without impulses when the HVDC was applied. It was observed that the atmospheric pressure did not influence the breakdown voltage for the smaller gap lengths, but the breakdown voltage was directly proportional to the pressure for larger gap lengths. High-speed photography was implemented in the experiment to capture the streamer breakdown mechanism that occurred in these air gaps. An analysis of the results obtained from the experiment showed that the needle-plane electrode combination created single, straight streamers whose stem increased in length when the gap length increased. Branches would also occur from the streamer when the gap length was greater than or equal to 12 cm and the streamer would occasionally take a bent path for gap lengths greater than 10 cm when propagating towards the cathode. Streamers would take this bent path because of space charge being present along the normal path when propagating towards the cathode. This caused the streamer to propagate around the space charge as it is unable to propagate through regions of high concentration. The dissertation also consisted of a numerical simulation on COMSOL Multiphysics which modelled the initiation of a cathode directed streamer and the propagation of it in an air gap. The necessary steps that were taken for the implementation and simulation of the electrical streamer in a 0.1 cm needle-plane gap is also included. The results of the numerical simulation were presented and analysed. The model was used to investigate the influence of both humidity and pressure on the breakdown voltage. The results indicated that the breakdown voltage of an air gap increased when humidity increased in the system as the atmospheric pressure was controlled. The breakdown voltage is also directly proportional to the atmospheric pressure when the humidity in the system was controlled. The experimental results and the numerical results obtained indicate that the breakdown voltage of an air gap is dependent on the humidity and pressure of the system and that space charge influences the path a streamer takes when propagating to the cathode.