Combined Electrical and Thermal Stress on Twisted Pairs: Study of the Variation over Time of the Partial Discharges Inception Voltage

The electrical insulation of small power electrical machines is a critical element especially when they are powered by means of electronic power supplies using techniques such as PWM. Power supplies based upon pulsed voltage trains can introduce, under certain conditions, overvoltage and this overvoltage can produce the inception of partial discharge phenomena. Enameled wires, even in case of high insulating thermal classes, cannot resist for a long time to the erosive action of the partial discharges and therefore, inevitably, reaching the total discharge conditions. In addition, the erosion of the enamel causes a decrease of the inception voltage, aggravating the situation and quickly leading to the total breakdown. In this study enameled wires thermal class $\mathrm{W}- 240^{\circ}\mathrm{C}$ have been considered. These wires have been used to prepare twisted-pair specimen following the IEC 60851-5 standard. They have been subjected to thermo-electric ageing using a sinusoidal test voltage waveform, varying its amplitude and frequency. The obtained results show that, during the degradation, the PDIV values continuously decrease following an inverse power law relationship. Furthermore, the obtained data evidence that the PDIV derivatives do not depend on the test voltage amplitudes and frequencies but rather seems to depend on the temperature.