New estimating techniques for multi-stress aging test of large generator stator winding insulation

The stator winding insulation of large generators suffers from combined stresses including thermal, mechanical, electrical and environmental stresses in service, resulting in gradual deterioration. In the present investigation, a multistress aging test model including electrical, thermal, mechanical vibration and thermal cycling factors is developed. According to the model, a multistress aging system is established and long-term aging tests of stator bar specimens in a 780 kW/6 kV motor have been carried out. Several new diagnostic parameters, such as dynamic increase rate of loss tangent, index of partial discharge magnitude, and second current rapid increase index, are put forward, which appear to be correlated with the deterioration of generator stator insulation. It has been verified that these parameters can be used to estimate the aging degree of stator winding insulation for power plants.