Modeling of stress relief materials used in HV rotating machines

The composition of semiconductor coatings, the preparation of materials, and the voltage distribution along semiconductor paints were investigated in connection with achieving proper stress relief in HV rotating machines. The voltage distribution along the semiconductive paints was obtained from an equivalent circuit. Silicon carbide and iron oxide were used as filters for an epoxy resin in the preparation of the stress relief materials. The impedances of these coatings are predominantly capacitive. An additional coating based on carbon black was also included in this study; its impedance is purely resistive. It is shown that most of the electrical stress is concentrated at only 20% of the total length of the grading coating for both iron oxide and silicon carbide composites. To obtain better results, the impedance should be at least one order of magnitude smaller than the values that were actually recorded in the laboratory. The resistive component of the iron oxide is greater than the silicon carbide one. Therefore, the iron oxide coating has a significant influence on the tan δ of the machine bar, which can represent a problem for acceptance tests and field testing of complete generators. It is concluded that modeling of stress relief materials can be used to obtain the optimum impedance of the semiconductive coatings