Thermal Cyclic Degradation of Coil Insulation for Rotating Machines

Thermal cyclic degradation of epoxy micaceous insulation for ac high voltage generators and motors was investigated. From the experimental results, it is made clear as follows. Thermal cyclic degradation begins with a separation of the innermost main insulation layer from the strand insulation at the end of the iron core. It propagates in a mode of mica delamination and the formation of cracks. This degradation condition agrees well with the measurement results of non-destructive insulation tests using divided electrodes. The compatibility between a strand insulation and a main insulation greatly affects thermal cyclic degradation. Thermal degradation cannot be ignored under certain thermal cycle condition. Thermal cyclic degradation increases with an increase in the peak conductor temperature. Even in the case of a post impregnated insulation system where the coil insulation adheres to the iron core, the thermal cyclic degradation is little for a core length below 1.4 m and a 160 °C hottest conductor temperature. Non-destructive insulation diagnoisis of hydrogen filled water-cooled turbine-driven generator insulation shows little thermal cyclic degradation after approximately 15 years normal operations. Lastly, it is proposed that a thermal cylce test under voltage application is important hereafter, to grasp the degradation of actual machines precisely.