Twisted pairs specimens subjected to several waveform voltages in presence of partial discharges

Considering the importance of the degradation phenomena acting in the insulation of induction motor windings supplied by inverters or converters (modern IGBT drives), a research program to study such a degradation in the presence of partial discharges (PD) has been planned. Special tests were performed on suitable specimens, i.e. on twisted pairs of polyimide-imide enamelled wires. Three different voltage waveforms (peak-to-peak voltage $Vpp - levels higher than the PD inception voltage and fundamental frequency equal to 300 Hz) were selected for the tests: a PWM-like one, a train of triangular voltages and a sinusoidal shape voltage. The specimens were exposed to PD while hosted in an oven at 100°C, at 5 different Vpp per waveform. The relevant PD patterns have been measured by means of a digital system, while the five times-to-breakdown (lifetimes) were recorded. The latter curves of lifetimes vs Vpp did fit an inverse power law relation, showing a slope practically independent of the applied voltage waveform.