Analysis of in-service aged stator bars

Stator bars subjected to operating stresses for 7 and 22 year periods were retrieved from a hydro-generator to undergo short term electrical diagnostic tests in order to determine whether any significant aging of the insulation had taken place. The ac breakdown voltage (U-bk), partial discharge inception voltage (PDIV), the maximum charge (Delta Q(m)) and total apparent charge GRAPHICS transfer of the partial discharges, tan delta and tan delta tip-up comprised the measured quantities statistically evaluated in the diagnostic test procedure. The ac breakdown voltage, PDIV and Delta Q(m) were found to exhibit a systematic trend with in-service time, thereby suggesting the occurrence of aging effects. That is, both the ac breakdown voltage and the PDIV were characterized by a decrease with time in-service, whilst the Delta Q(m) value exhibited an increase. However, no definite behavior was evinced by the GRAPHICS tan delta and tan delta tip-up values. me significance of the diagnostic test results is discussed in terms of Weibull statistics and the partial discharge behavior with time.