Contribution of humidity to the evolution of slot partial discharges

The impact of humidity on the slot partial discharge (PD) process was evaluated in a laboratory experiment conducted in a climatic chamber under constant humidity and temperature. Two stator bars, subjected to slot PDs at two temperatures (28°C and 85°C), were aged at twice their nominal phase-to-ground voltage (16 kV) under three absolute humidity levels (5 g/m³, 9 g/³ and 13 g/³). The phase resolved partial discharge (PRPD) patterns were monitored during short aging sequences and visual observations of the surfaces were made at the beginning and end of each aging sequence. Observation confirms that humidity has a significant impact on the discharge phenomenon but other parameters, such as surface condition and temperature, affected the discharge process just as much. This paper presents the way slot PD activity evolves and affects the related PRPD pattern under different conditions of humidity, temperature and surface degradation.