A Review of Results From Thermal Cycling Tests of Hydrogenerator Stator Windings

Thermal cycling of generators, occurring during intermittent operation of power stations, increases the thermomechanical stress on the stator groundwall insulation and can, therefore, contribute to failure of the generator. Thermal cycling stresses can, to some extent, be replicated and accelerated in laboratory tests. Such tests are performed to ascertain whether a new stator winding insulation system can handle intermittent operation, to evaluate the relative difference between different types of insulations or to investigate degradation processes and failure mechanisms. Results from thermal cycling tests of hydrogenerator stator windings are reviewed. Only stator windings of air-cooled hydrogenerators with thermoset groundwall insulation of mica-epoxy or mica-polyester are considered. The published literature of laboratory experiments performed in order to study effects of thermal cycling on groundwall insulation through different types of diagnostic measurements is described and discussed. Partial discharge (PD) and tan delta measurements gave, in some cases, an indication of a trend in the condition during thermal cycling, while in other cases it was difficult to interpret the test results. Many agree that measurement of breakdown voltage of stator windings can give an indication of the aging and remaining lifetime of the insulation. The ability of other measurements such as tan delta, PD, and voltage endurance to assess the remaining lifetime of the stator winding is less convincing since the results vary greatly.