Implementation and analysis of multi-factor aging tests on the winding insulation of original sized generator stator bars used in large rotating machines

In standard operation, the electrical insulation system of generators, transformers, etc. used by power utilities is highly stressed with multiple simultaneous loads like electrical, thermal, mechanical, environmental or chemical stresses. Single load tests are therefore not able to fully represent the correspondent aging mechanisms. Multi-factor aging tests with electrical, thermal and mechanical loads applied at the same time have to be performed to more realistically simulate lifetime aging. This paper presents accelerated multi-factor aging tests with simultaneous electrical, thermal and mechanical loads on indirect cooled generator stator bars with original dimensions. Considering the dominant influence of thermal and mechanical stress on the chemical degradation of polymers, temperature and mechanical loads are raised above operating conditions to enable accelerated aging, while a voltage load only slightly above the rated voltage of U_r = 22 kV is used. A temperature ϑ = 155°C is applied to the insulation system, according to the respective class temperature. In addition to the high thermal stress, accelerated aging is enforced by sinusoidal (f = 25 Hz) mechanical bending stress. Thermal and mechanical load are the variable input parameters. This paper concentrates on the influence of the variation of mechanical stress on the complex system of insulation aging. The insulation is stressed until the end of lifetime occurs. Sub-periodic diagnostic tests such as the measurement of the insulations dissipation factor may allow the further development of non-destructive monitoring technologies to ensure reliable operation of large turbine generators. The obtained results will be used to analyse multi-factorial aging mechanisms in highly stressed insulation systems. This will enable further investigation on simulating the aging process by the means of physical models.