Optimization of Multiple-Layer End-Turn Stress Grading System for High Voltage Turbogenerators

SiC-based nonlinear resistive material has been used for end-turn stress grading (SG) of turbogenerators over decades. In order to reduce power dissipations and avoid thermal breakdown of the SG material, high-voltage class generators adopt multiple-layer SG system, at which two SG materials with different nonlinear resistivity are used. With increasing capacity and efficiency of the apparatus, electrical, and especially thermal stresses at the system may become problematic, hence higher reliable SG system is required. For suppressing the local heating effectively, an optimization method of the multiple-layer system was investigated, where a new combination of the two SG materials was proposed and length of the SG layer in the longitudinal direction along a coil was optimized by using analytical approximations of the power dissipation. The optimized SG system showed 20% lower power dissipation and also 15% higher flashover voltage than those of the conventional one. Consequently, the effectiveness of the optimization method proposed was successfully confirmed.