Residual voltage endurance of generator insulation systems

The insulation system of a generator is stressed by electrical, mechanical and thermal loads during the whole lifetime. By the means of diagnostic measurements the worsening of the condition within the dielectric system can be evaluated. To ensure the operational safety for the next maintenance period a hipot test with raised level can be applied. In spite of all carefulness it can happen that a breakdown of the insulation system happens. Beside the costs and required time for machine repair contractual damages can result in high costs for the operator. The idea of these investigations was to examine the residual voltage endurance of aged generators. For this reason operational aged generator bars were dismantled carefully from two stators of same type which were set out of operation for renewal. The generator bars were tested by the means of following dielectric methods: Insulation Resistance, Dielectric Dissipation Factor and Partial Discharge as well as accompanying material tests. The test objects were electrically aged with a three time raised nominal voltage with an expected lifetime between 100 and 200 hours. In regular intervals the dielectric tests were repeated periodically. In this paper elected results were presented with the focus on the diagnostic expressiveness of the investigated dielectric parameters. Statistic methods were used to describe and display the aging behavior of the investigated insulation system.