Effect of Thermal Stress over the Partial Discharge Inception Voltage on Twisted Pairs

Partial discharges are one of the main causes for what concerns the degradation of the electrical machines insulation system and their onset can bring the devices in question to their end of life precociously. In addition, although an electric machine does not show partial discharges when it is put into service, it is possible that this phenomenon develops over time due to the aging of the insulating material caused by the electrical stress to which it is subjected. However, depending on the application in which these machines are used, electrical aging may not be the only cause that leads to partial discharges. Some electrical machines with type I insulating system may in fact be used in more prohibitive applications that involve high temperatures, therefore in such cases a possible cause of the onset of partial discharges may be the thermal ageing. In this study, twisted pairs with thermal class W-240 °C composed of copper wires covered with a polyimide-based enamel were analyzed. Thermal ageing tests were carried out on several specimens at different temperatures in order to measure the trend of their Partial Discharge Inception Voltage (PDIV) over time. To prevent electrical ageing phenomena from modifying and influencing the results, the repeated PDIV measurements were made in a very short time. In the here considered case, the study demonstrated that at both test temperatures the specimens showed a rapid decrease in the PDIV values in the first 65 hours while subsequently there was a flatter trend. From the reported graphs it is clear how a higher temperature can lead to a more marked decrease of the PDIV values.