The long-term accelerated aging tests in laboratory environment have been conducted on actual rotating coils subjected to combined stresses including electrical, thermal, mechanical vibration and thermal cycling factors under simulated conditions. It is to identify the meaningful diagnostic measures for the assessment of degradation and the identification of aging mechanisms. Experimental techniques for acquisition and analysis of failure data have been investigated. Partial discharge (PD) measurements have been performed using TE571 digital PD analyzer in different aging periods and a combination of statistical and basic parameters has been studied to evaluate the degradation state of stator winding insulation along with destructive measurements. It has been found that the statistical parameters such as the skewness of PD distributions appear to be correlated with the delamination in the epoxy-mica insulation system. The results provide a basis for an on-line interpretation method of stator winding insulation.
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