The ability of diagnostic tests to estimate the remaining life of stator insulation

Stator windings from three generators and one motor were subjected to a wide variety of diagnostic tests. Diagnostic tests included insulation resistance, polarization index, capacitance, dissipation factor tip-up, partial discharge magnitude, and discharge inception voltage. The destructive tests included breaking down individual coils with either AC, DC, or impulse voltages. Since the results of the diagnostic tests did not correlate with the breakdown voltages, it is concluded that remaining life cannot be predicted on the basis of diagnostic tests alone. Equations and other relationships recently developed in Japan to predict the remaining life of the stator groundwall insulation systems could not be confirmed. Diagnostic tests are thought to be most useful for indicating the trend in insulation aging in a particular machine.>