Partial discharge, glass transition temperature and dissipation factor as tool to assess insulation with history of failures

Partial discharges analysis in electrical machines is an important tool that helps to diagnose health of insulation. Whether combined with historical data from factory tests, commissioning and maintenance, it can provide important information about the intrinsic characteristics of insulation and other inheritances of manufacturing and assembly process. The quality of insulation system is also verified through its manufacturing characteristics and the glass transition temperature is one of them. Manufacturers usually design this transition temperature below the temperature of insulation class. In the presented case a hydro generator had glass transition temperature verified in laboratory far above the class temperature. This high glass transition temperature means a hard and brittle insulation. The motivating fact for the laboratory analysis was eleven short circuits in AC hipot tests still during the commissioning period of generators units. It will also be shown how the partial discharges analysis became the main preventive maintenance tool for this power plant.