Flashover behaviour of converter transformer insulation subjected to superimposed ac and dc voltages

In HVDC application, the interdisc winding insulation of a converter transformer is subjected to superimposed AC and DC voltages. Therefore, a knowledge of the behaviour of such an insulation system subjected to AC, DC and superimposed AC and DC voltages is essential. In order to simulate similar conditions in the interdisc insulation system, experiments are conducted with both oil and oil-spacer insulation systems. In general, the important factors, which influence the breakdown phenomena with these voltages are, field enhancement, charged particles, dielectric mismatch, space charges and surface charging of the interface