Condition Evaluation of Stator Winding Insulation of Vintage High Voltage Turbine Generator

Degradation of dielectric insulation system happens in turbine generators in service. Thermal, electrical, ambient and mechanical forces attack the potential barrier of power windings and reduce the electrical strength. Sudden failure and forced outage can be averted by condition assessment of generator insulation. This paper presents state-of-art diagnosis tools to evaluate the dielectric insulation properties of stator winding of generators. Degradation mechanism in insulation is dealt with physical model and electrical equivalent circuit. Test results of 25 MW, 11 KV TG of four decades old are analyzed to evaluate the insulation properties. Correlation is made between tan delta, partial discharge, corona probe data with visual examination. Corrective actions are listed to improve the insulation performance. Utility is advised to monitor the machine parameters on load and repeat dielectric tests to avoid forced outages due to insulation failure. It is suggested to plan rewinding with state of art of insulation technology to enhance the life of machine.