Partial discharge inception voltage of water drops on insulating surfaces stressed by electrical field

This paper reports on recent investigations, from a practical and theoretical point of view, regarding the partial discharge (PD) of water drops on an insulating surface exposed to electrical field stress. The assays are made using a sample, which consists of a cubical silicone rubber block with two embedded electrodes so that the electrical field is tangential to the surface. One or more water drops are placed on the sample between the electrodes. First, the three dimensional movement and deformation of the water drops on the insulating surface was recorded with the help of a high-speed camera. In the same time, partial discharge measurements were made using exclusively a photomultiplier module. The observed low inception voltages cannot be explained without assuming a severe deformation of the drop. Furthermore, the PD inception electrical field strength turned out to depend on absolute ambient humidity in a different way than for the general breakdown field strength: it decreases with increasing absolute humidity. Systematical investigations were performed under controlled humidity and temperature using a climate chamber. Parameter as volume of the drops was varied during the experiments. Based on the experimental findings, a correlation between PD inception and atmospheric conditions was established. The electrical field distribution was calculated using field simulation software based on the Maxwell equations. When simulation results are compared with experimental findings, a correction factor with humidity must be considered.