Aging Properties of Mica Insulation Systems Impregnated with Solventless Resin for Generator Stator Coils

In these days, the solventless resin-impregnated mica insulation systems are extensively adopted to the large powergenerator stator-coil insulation systems as a result of rapid development of synthetic resin in recent years. Various destructive and nondestructive tests with these samples were made. The electrical properties were compared between the initial coils (which were measured by the electrical properties at the acceptance test) and the used coils (which were taken out from the generator winding that was used for about ten years). It is said that the winding insulation systems will change during long running times because of a number of factors such as thermal, mechanical, and electrical. Studies by the present authors show that used coils were not deteriorated by mechanical stress because electrical breakdown voltages did not decline in comparison with those of the initial coils, and also they were not deteriorated by electrical stress because the breakdown voltages of coils at the line side were lower than at the earth side. Therefore, some changes of the electrical properties in the nondestructive test seemed to result from thermal effects. As mentioned above, it is proved that solventless resin-impregnated mica insulation systems have kept good electrical properties in service for over ten years.