Discharge mechanism at hvdc polarity reversal in oil / pressboard composite insulation system

At the instant of polarity reversal of HVDC, very high electric field was generated in oil because the applied electric field was superimposed by the electric field of accumulated charges on pressboard (PB). When the total electric field exceeded the breakdown electric field strength, breakdown discharge in oil gap and surface discharge on the PB occurred. In this paper, by electric field measurement using Kerr electro-optic technique and optical observation, breakdown discharge characteristics and mechanisms in oil/PB composite insulation system were investigated at polarity reversal voltage. We revealed that the surface discharge neutralizes the accumulated charges on PB. The above experiments were carried out for different PB arrangements and discussed the discharge characteristics under HVDC polarity reversal conditions were discussed.