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_bk), partial discharge inception voltage (PDIV), the maximum charge (ΔQ_m) and total apparent charge (Σ_jⁿΔQ_j) transfer of the partial discharges, tanδ and tanδ tip-up comprised the measured quantities statistically evaluated in the diagnostic test procedure. The AC breakdown voltage, PDIV and Δθ_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 ΔQ _m value exhibited an increase. However, no definite behavior was evinced by the (Σ_jⁿΔQ_j), tanδ and tanδ tip-up values. The significance of the diagnostic test results is discussed in terms of Weibull statistics and the partial discharge behavior with time