Dielectric Response and Partial Discharge Characteristics of Stator Winding Insulation System with SiC Stress Grading

Abstract

The typical construction of a stator coil includes the use of end corona protection (ECP)coating, which is made of semi-conductive materials like silicon carbide (SiC). The purpose of ECP is to smooth the electric stress distribution near the slot exit, limiting the electric field and partial discharge (PD) activity within the insulation system. This thesis investigates how ECP affects the dielectric response of a stator coil in high voltage measurements, both in the time-domain and frequency-domain. It also studies how well time-domain results transformed to the frequency-domain correspond to direct measurements. As a further point, the effect of the ECP on PD activity was demonstrated. Measurements of dielectric response and PD were made on new coil-halves produced in the usual way in a production run for a motor. The results show that applying the ECP design significantly reduces the PD activity and increases the inception voltage. In addition, the dielectric response of the coil with ECP tape has shown to have a nonlinear voltage dependent characteristic due to the presence of ECP. As the voltage goes higher, this causes a shift of the loss peak toward higher frequency. A distributed-element model of the end section of a stator coil was used to model the nonlinear effect due to ECP, and its results are compared to the measured results. Lastly, FEM simulation of a stator coil end section is presented, showing the contribution of ECP tape in evenly distributing the electric stress near the slot exit.