Study on Construction of a Electrical Discharge Model for Developing Optimum Insulation Design of Compact UHV Transmission Line

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A model of electrical discharge development by which electrical insulation strength can be predicted is required for realizing a high reliability of insulation design of a compact UHV transmission line system expected as a future promising system. To make such a model, the precise observation of the discharge development is strongly needed. Some factors affecting the discharge development was examined successfully by using a laser induced plasma which can control the initiation and the path of the discharge. A Pockels sensor was developed and used to measure electric field in the discharge space because the discharge development was controlled mainly by the electric field.A 3.5J CO2 laser beam was transmitted into a rod-to-plane gap which was dc electrically stressed. Trigger and guide effects of the discharge were usually observed simultaneously. The occurrence of the discharge was triggered by the irradiation of the laser and subsequently the path of it was guided along the laser induced plasma. It was found that the breakdown voltage of the gap was decreased by the existence of the plasma.In this study, only the trigger effect was observed under such a condition that a relatively low positive voltage was applied to the rod electrode. The discharge path did not lie along the laser induced plasma. The delay time between the irradiation of laser and the breakdown of a gap varied from several hundreds mus to several ms. From the measurement of the electric field it was found that negative charge appeared at the positive polarity side of the laser induced plasma and enhanced the electric field near the plasma. The trigger effect can be caused due to the drift of negative ions produced in the laser induced plasma.