This paper presents measurements of partial discharge (PD) from water droplets on a silicone rubber insulating surface in an ac field. Tests are done by placing droplets on the surface using a micro-pipette, and by condensing water in a controlled humidity chamber. The field enhancement factor due to droplets placed on the insulation surface is experimentally and numerically determined. Values in the range of 3.8 to 6.3 are obtained depending on the size and number of droplets. The time resolved analysis of PD from water condensation indicates a threshold field, between 3.0 and 3.5 kVlcm, above which the PD activity increases with time, and brings the insulation to a surface breakdown. The corresponding field enhancement factor obtained for the condensation experiments is between 5.8 and 6.8. The results therefore suggest that the PD activity on the hydrophobic surface can be used effectively to learn about the insulation surface conditions.

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Concerns about corona and its harmful effects, particularly in polymer insulators, have increased in recent years. Research involving corona identification, quantification, and formation mechanism is important to understand the aging process and to avoid premature failures. This paper investigates the corona inception and its relation with polymer surface conditions. Experiments of visual corona identification and RadioInterference Voltage (RIV) measurements were performed. Corona onset voltage was investigated in arrangements simulating different service conditions. Wet and contaminated insulating surfaces were reproduced. An RTV coated insulating sample and a polymer insulator were used. The electric field (E-field) at corona onset was numerically evaluated through computational simulations using the Finite Element Method. Corona onset voltage, E-field and RIV were analyzed according to the contamination level. The results clearly show how certain service conditions, particularly the association water droplets and pollution, enhance the E-field and can lead to corona onset.

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