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Water droplets are formed on the surface of polymeric insulators under wet conditions, which enhance the electric field especially in close vicinity of the contact line between air, water droplet and insulator surface. It is experimentally found that the maximum allowable E-field strength tangential to a polymeric insulating surface in presence of a single water droplet on the flat surface with wettability class 1 is between 6 and 7.5 kV/cm (peak). Some important parameters that have an influence on PD inception are experimentally investigated to find more reliable results. A simple prediction method of partial discharge inception E-field strength of a single water droplet on polymeric insulating surfaces based on the streamer criterion for inception is also proposed and discussed. The investigated PD inception E-fields in this contribution can be used for prediction of partial discharge inception voltage of polymeric insulators under wet conditions.Index Terms -Water droplet, electric field, polymeric insulator, streamer inception and partial discharge inception.

Water droplets on polymeric insulating surfaces provide an intensified E-field stress at the common interface between water droplet, insulating surface and surrounding air, which may lead to partial discharges, surface deterioration or even flashover. While intensive research has been performed for the AC case and also for the DC case, a new kind of stress may arise by the combination of both, which is present when a DC and an AC system are installed on the same tower. Water droplet deformations at AC, DC and combined AC-DC voltages as well as their PD characteristics are experimentally investigated and discussed in this contribution. The water droplet at AC voltage oscillates at different oscillation modes, while it elongates towards electrodes at DC voltage. The deformation under combined AC-DC voltage stress consists of an oscillation superimposed to an elongation towards the electrodes. The water droplet elongates towards the AC electrode when the polarity of DC source is negative, while it elongates towards the DC electrode in case of positive DC voltage. Oscillation modes of the water droplet at AC voltage have an influence on its PD inception E-field strength. Under DC stress the PD signals consist of two or more different PD levels which are due to the differently sharped edges of the deformed water droplet. Increasing the DC component at combined AC-DC voltage provides a lower PD inception E-field strength. Conductive water droplets have lower PD inception values in comparison with non-conductive ones under AC-DC stress.Index Terms -Water droplet, polymeric insulating surfaces, PD inception E-field strength and combined AC-DC voltage.

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