The effects of square wave voltage rise time on PD statistics in time and frequency domain

IEC standard for the design and qualification of the insulation system of inverter-fed motors controlled by power electronics refers to the measurement of partial discharge inception voltage (PDIV) or repetitive partial discharge inception voltage (RPDIV). This is, however, not a simple testing procedure when carried out under repetitive impulse voltage. Indeed, impulse voltages cause strong switch-on/off disturbance, which make much more difficult to distinguish PD signals from noise. To overcome this problem, the effects of impulse rise time on PD pulse signals should be investigated in order to design appropriate PD sensors with high signal-to-noise ratio and to choose suitable digital signal processing methods after the high-speed analog to digital conversion. This work investigates the influence of repetitive impulsive rise time on PD statistics focusing on experiments performed on crossed enameled wires under square wave impulsive voltages with different rise times, from 50 ns (the minimum) to sinusoidal. The dependence of PD magnitudes and voltages (firing voltages), as well as PD pulse energy distribution maps (in the time/frequency domain), on square wave voltage rise time, are analyzed. It is shown that the rise times of the square wave voltage can have significant influence on PD statistics both in time and frequency domains; impulse voltages with shorter rise time are likely to induce PD events with relatively higher energy in 1.0 to 1.5 GHz domain, which will be beneficial for increasing signal-to-noise ratio when using ultra-high frequency antenna; PD repetition rate decreases with rise times.