Potential distribution on stress grading system of coil end under operating temperature of rotating machines

Stator coil end of a rotating machine has a structure of electric field grading system (SG system) to relieve the high electric field. Although a rotating machine is operated at higher than $100^{\circ}\mathrm{C}$, the effect of temperature on SG system is not well understood. To investigate the performance of SG system under high temperature, experiments are conducted using a model bar simulating the end of a stator coil. AC 50 Hz voltage of 10kVp is applied to a virgin model bar at room temperature (RT) and then is heated up to $100^{\circ}\mathrm{C}$, and cooled to RT. This heating and cooling procedure is conducted for four times and potential distribution of the model bar at each stage was measured using a Pockels sensor. It was observed that the thermal history affects the electric field distribution formed on the SG system. The maximum value of the electric field strength on SG system, Emax, at RT gradually increased with the number heating cycle. On the other hand, the electric field strength at high temperature increased, but the effect of the thermal history was not as pronounced as at RT.