It is necessary to estimate precisely the breakdown strength of coil insulation if utilities intend to continue using stator coils which have been used for a long time. For this purpose, monitoring of the maximum discharge magnitude (qm) of partial discharges (PDs) in coil insulation has been found to be useful. A monitoring method has been developed using the resistance temperature detector (RTD) laid in the stator coil slot of a hydro power generator as a PD current pulse detector. For the purpose of deciding diagnosis criteria based on the PD monitoring, long term PD characteristics should be clarified. We set monitoring systems in 4 hydro power stations. Over a period of one year, we have monitored several combinations of PD, line voltage, coil temperature and output current every hour. We analyzed these data and found strong correlation between qm and coil temperature.
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