Analysis of in-service aged stator bars

Stator bars subjected to operating stresses for 7 and 22 year periods were retrieved from a hydrogenerator 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/sub bk/), partial discharge inception voltage (PDIV), the maximum charge (/spl Delta/Q/sub m/) and total apparent charge (/spl Sigma//sub j//sup n//spl Delta/Q/sub j/) transfer of the partial discharges, tan/spl delta/ and tan/spl delta/ tip-up comprised the measured quantities statistically evaluated in the diagnostic test procedure. The AC breakdown voltage, PDIV and /spl Delta//spl theta//sub 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 /spl Delta/Q/sub m/ value exhibited an increase. However, no definite behavior was evinced by the (/spl Sigma//sub j//sup n//spl Delta/Q/sub j/), tan/spl delta/ and tan/spl delta/ tip-up values. The significance of the diagnostic test results is discussed in terms of Weibull statistics and the partial discharge behavior with time.