Partial Discharge Patterns of Oil-Paper Insulation under AC Voltage Superimposed Harmonics

There are plenty of factors that influence partial discharge (PD), one significant factor is the harmonic issue. The minor insulation of valve side windings within converter transformers withstands complex electrical stress, which is fundamental AC voltage superimposed with harmonics. In this paper, partial discharge tests were performed on needle-plane electrode. Single harmonics with 3rd, 5th, 7th and 9th order, and fundamental AC voltage superimposed harmonics with different harmonic frequency and component, were applied on the defect model. Statistical phase distributions and typical parameters of PDs were acquired by detector using pulse current method. Test results showed that harmonic has a significant impact on PD phenomenon. PD inception voltage has no direct relationship with harmonic order and component. Pulse repetition rate per cycle decreases while that of per second increases with the increase of single harmonic order. In addition, PD magnitude grows bigger and PD phase core moves left toward 0° and 180°. As harmonic order and its component increase, both maximum and mean PD amplitude, as well as pulse repetition rate increase. The applied voltage waveform and its change rate dU/dt both indicate the PD distribution in the case of phase-resolved measurements. These results will assist the understanding of partial discharge behaviors under complex voltage waveforms within converter transformers.