Study on dielectric breakdown at DC polarity reversal in oil / pressboard-composite insulation system

By using Kerr electro-optic method, charge behavior in oil / pressboard (PB) composite insulation system, which is the basic insulation structure of a transformer, was investigated under DC voltage. Right after the DC voltage application, electric field became the capacitive distribution, and had a strong value which was determined by permittivity of materials. The field strength decreased gradually with time as charge in oil was accumulated on PB. When the voltage polarity was reversed, the applied electric field was superimposed by the electric field formed by the accumulated charge on PB, and very strong field appeared right after the polarity reversal. This could cause the discharge in the oil gap even when the independent applied voltage was insufficient to initiate discharge. It was found out that the critical value of the discharge initiation at the polarity reversal was determined by the breakdown field strength of the oil gap.