Stress Grading Performance Analysis of Stator Coil Considering the Manufacturing Method and the Insulation Design

In rotating machines, the approach of applying nonlinear conductive stress grading tape (SGT) has been applied to reduce the electric field stress in the coil’s overhang region. To characterize and verify the electric field grading effect of SGT in electric machines, electric field analysis using the nonlinear conductivity of SGT as a boundary condition must be considered. However, previous studies have shown limitations in the characterization and application of SGT materials in insulation system design for various reasons, such as lack of conductivity measurement standards for nonlinear materials or not considering possible changes in material properties during the manufacturing procedure. In this study, the nonlinear conductivity of SGT was measured by considering the rotating machine’s manufacturing procedure. A finite element method (FEM) electric field analysis was performed using the measured nonlinear conductivity. Based on the improved conductivity measurement method of SGT and FEM analysis, the effects of changing several insulation design parameters in the field grading performance were studied on a 13.8 kV stator coil. Moreover, the effect of preheating on the SGT conductivity was also examined. Based on analysis results, we realized the possibility of more precise insulation design of the stator coil. Our findings supplement the insufficient insulation design guide on the SGT part and contribute to establishing a systematic approach for insulation design. Furthermore, the proposed methods enable design optimization and detection of the cause of insulation failure.