Study of the Partial Discharge Inception Voltage Variation Over Time on Enameled Wires Subjected to Thermal-Electrical Stress

The electrical insulation is a crucial element in electrical machines. The performance of the insulation system can be affected by the operating conditions that cause ageing of the insulation. The evaluation of the effects of the environment stresses is a key in the prediction of the electrical insulation breakdown. Being capable of forecasting the maintenance avoiding the out of service due to insulation breakdown is certainly an objective. It is proven that partial discharges (PD) are one of the causes of electrical insulation breakdown. The evolution of PD is an indication of the condition of the electrical insulating system. The severe lowering of the Partial Discharge Inception Voltage (PDIV) value can be a signal of the approaching to the insulation total breakdown. The continuous lowering of the PDIV is due to the degradation of the insulation properties caused by the stresses applied during the operation time. The aim of this study is to evidence and compare the effects of thermal stress on different kind of enamelled wires analysing the PDIV because is one of the accelerating factors together with relative humidity and frequency of the applied voltage [1]-[7]. In this study, twisted pairs made with copper wires covered with different enamels and having thermal class 200°C were assessed. The samples were subjected to thermo-electrical aging tests at the same voltage amplitude and frequency while different temperatures have been applied. The PDIV relevant to the samples was measured regularly to monitor its evolution over time.