Variation Over Time of Partial Discharge Inception Voltage Due to Combined Electrical and Thermal Stress on Twisted Pairs

Among the several elements that are part of the electrical machines, insulation is certainly one of the most important. Consequently, the study of its behaviour in some stressful situations can be very important: a breakdown of the insulation can in fact determine the out-of-service of the device itself resulting in downtime, costly repairs or replacements. Among the reasons that often appear in the causes of aging of electrical insulation there are the Partial Discharges (PD), a phenomenon that over time tends to erode the dielectric causing the total breakdown. What makes this phenomenon even more dangerous is its evolution over time: due to the erosion of the material, the Partial Discharge Inception Voltage (PDIV) can undergo an involution and decrease continuously. It is known how an excessive lowering of this value can lead to the complete breakdown of the insulating material. However, the applied voltage is not the only element that can facilitate the inception of partial discharges: as shown in [1] the temperature can be an accelerator for the aging, as well as the frequency of the applied voltage [2]. In this study, twisted pairs made with a copper wire covered with a polyimide-based enamel and having thermal class W-240°C were examined. The samples thus made were subjected to thermo-electric aging tests at the same voltage amplitude and temperature level but with different test voltage frequencies: by doing this, lifetimes ranging from about 6 to 70 hours have been obtained. Their PDIV was measured over time in order to monitor its evolution. By calculating the PDIV values in correspondence with the total discharge, a lifetime curve was obtained as a function of the frequency of the applied sinusoidal voltage. The trend of this curve seems to have confirmed the experimentally obtained data.