EMTP Analysis for Potential Distribution around Stator End-winding with Nonlinear Potential Grading Layer in High Voltage Rotating Machine

Partial discharge and surface discharge may occur around stator end-winding of rotating machine owing to convergent field caused by geometric configuration. In order to suppress such field, the material having nonlinear voltage - current characteristics such as SiC is painted or taped on the end-winding. This paper describes potential distribution around the end-winding of rotating machine using EMTP (Electromagnetic Transients Program) assuming RC lumped constant circuit for overvoltage tests and in operation. The result showed that painting SiC reduces the electric field along the surface due to the nonlinear resistivity. The maximum electric fields are found to exist at the point where the potential difference between neighboring coils has a maximum. As a result, the maximum field occurs at the end of SiC layer for overvoltage tests, and in the region where potential becomes almost constant after potential grading by SiC layer in operation of rotating machine. Distance difference dependence at the end of SiC layer between neighboring coils for maximum field are also discussed. The maximum field increases with the distance so that the electric field may reach the critical field of air.