Research on surface partial discharges of oil-impregnated insulation paper at harmonic superimposed AC voltages

The valve side Wingdings of converter transformers withstand complex electric stress, and the minor insulation of converter transformers withstand combined fundamental AC and harmonic voltages. Surface partial discharge is a main form of discharge occurring in minor insulation and there is a severe lack of empirical research on surface partial discharge in oil-impregnated insulation paper at harmonic superimposed AC voltages. In this paper, the surface partial discharge experiments of oil-impregnated insulation paper were executed under different type of harmonic superimposed AC voltages. The results show that: a) when using harmonic superimposed AC voltages, PDIV values decrease with the increase of harmonic order and THD, b) PD patterns are closely related to the waveform of applied voltage, and the maximum discharge magnitude increases with the increase of harmonic order and THD, while the PD number per period doesn't change significantly with the change of order or THD of voltage under the voltage with same amplitude.