Prediction of stator winding remaining life from diagnostic measurements

Diagnostic tests such as insulation resistance, polarization index, dissipation factor, capacitance and partial discharge can be useful to diagnose the condition of the stator winding insulation, and identify many problems that could lead to insulation failure. Many years of experience, however, has shown these quantities by themselves cannot predict the remaining life of a winding. However, asset managers would prefer an estimate of the remaining stator winding insulation life. Knowing the remaining life allows asset managers to obtain the maximum use from the existing winding, while avoiding in-service failures and permitting the planning of the optimal time to rewind. Over the years, various machine repair organizations in Japan, Europe and India have claimed the ability to determine the remaining life. This paper examines the literature and practical experience using these remaining life estimation tools. It is concluded that there is no evidence that the available tools provide an accurate estimate of the remaining life. A protocol is suggested for evaluating the validity of any new method to estimate remaining life.