Partial Discharge Characteristics caused by Metallic Particles under DC voltage on Oil Immersed Insulation

Development of direct current (DC) power equipment is necessary for spreading DC power transmission systems in order to increase the amount of renewable energy. The insulation performance of power equipment deteriorates by various defects. In particular, when a metallic particle is present inside the power equipment, such as oil immersed transformer, the partial discharge inception voltage (PDIV) decreases greatly. For AC transformers, many investigations have been made on partial discharge (PD) caused by metallic particles. On the other hand, there are few reports on PD under DC voltage. In this paper, we have investigated PD characteristics caused by metallic particles under DC voltage in the oil / pressboard (PB) composite insulation system. Three results were obtained from our experiments. First, PDIV under positive DC voltage was lower than under negative DC voltage due to the difference of the discharge formations, etc. On the other hand, PDIV under positive DC voltage was the same as PDIV under AC voltage in the oil insulation setup. Secondly, PDIV was almost constant if the curvature radius was 10 $\mu$m or less due to the electric field relaxation effect caused by space charges in the vicinity of the needle tip. Finally, the electric field in the vicinity of the needle tip at the PDIV in oil was in the range of 200~400 kV/mm, regardless of the gap distance, the type of the applied voltage: AC or positive DC voltage, and the insulation system: only oil system or oil / PB composite system. That is, the condition for starting PD in the presence of metallic particle is determined by the electric field in the vicinity of the tip of the metallic particle regardless of the gap distance, the type of the applied voltage, and the insulation system.