Using AC current parameters to evaluate aging condition of stator insulation

The stator winding insulation is one of the most important parts in large generators and electric motors. It is always subjected to the combined thermal, mechanical, electrical and environmental stresses during long-term operation. So the gradual deterioration of mechanical and electrical properties is inevitable. Continuous deterioration of electrical insulation will result in the decreasing of breakdown voltage and failure of generator or electric motor. It is very important for the safe operation of a large generator to inspect insulation condition and evaluate the remaining life of stator winding insulation. In order to assure the validity of evaluation, it is necessary to study the characteristic parameters that can indicate the condition of insulation deterioration. In this paper, a multi-stress aging system has been developed and the accelerated aging experiment based on the stator bars of 780 kW/6 kV electric motor has been conducted in the laboratory in order to simulate the practical aging of electric motor or generator in service. The developing laws of AC current parameters with aging time are analyzed. Moreover, the relations between the breakdown voltage and them are studied. During the process of multi-stress aging of epoxy/mica insulation, the AC current parameters, including the first and second AC current rapid voltage (U_pi1, and U_pi2), equivalent impedance and current increasing factor m₂, have apparent developing laws with aging of insulation. Moreover, the second AC current rapid increase voltage U_pi2 shows close correlation with the residual breakdown. voltage, which can be used as a characteristic parameter to estimate the remaining life of epoxy/mica insulation. Therefore, the degradation of epoxy/mica insulation can be assessed efficiently by the AC testing method