| From the appearance of high voltage transmission line in the nineteenth century, ceramic is the main insulating material of high voltage outdoor insulation all along. With the development of power system and the increase of voltage level, the mechanical and electrical load added on insulators has been increased. It already couldn't reach the requirements of insulators for power system. Then people had developed some polymeric insulating materials such as epoxy resin, ethylene propylene rubber, room temperature vulcanized (RTV) silicone rubber and high temperature vulcanized (HTV) silicone rubber, etc. Taking all things considered, HTV silicone rubber is the best choice for outdoor insulation. Since the discovery of HTV silicone rubber in the 1970s, its percentage in the world power system's outdoor insulation increased quickly. Now it has been one of main choices for outdoor insulation. However, it also appeared some problems with the increase of usage, instantaneous loss or weakening of hydrophobicity in the running, which couldn't be ignored. Therefore, that the paper studied the discharge characteristics of discrete water drops on silicone rubber's surface and the effect of discharge on hydrophobicity has important academic value and scientific meaning.Water drops can only accrete on the surface of silicone rubber at discrete form because of its hydrophobicity. The paper mainly studied the effect of the corona and partial discharge (PD) around water drops and the flashover between water drops on hydrophobicity. Based on the simulating tests on the sample model and chemical analysis, it obtained the discharge law of discrete water drops and analyzed the chemical changes of the surface in the process of discharge. The main works and conclusions of the paper are as follows,(1) According to simulating tests it obtained the discharge characteristics of discrete water drops. Discrete water drops on the surface greatly reduced the flashover voltage of test sample. The extent of decrease related to the volume, number, conductivity, distribution and location of water drop. In the early stage of discharge, the strengthened electrical field area around water drop produced intense PD. The quantity of PD sharply increased with the electrical field. Sometimes it even could up to hundreds of pC.(2) It recorded the motion and distortion of water drop in the process of discharge by high-speed camera. In the electrical field force, water drop undulated and vibrated at the frequency of electrical field. The extent of undulation and vibration related to the volume and location of water drop. Once the electrical field exceeded the range, Water drop would be distorted and elongated. The adjacent water drops would connect with each other and become one bigger water drop, while the bigger water drop would divide into two small ones probably.(3) The distortion and elongate of water drop resulted to flashover with a few water drops on the surface. However, when it has many water drops, it firstly became water film. Then that the electrical field of dry bands between water drops, water drop and water film were intensified would produce small arcs. The flashover owed to the development of small arcs.(4) It gained the chemical changes happened in the process of discharge at the surface of silicone rubber by the Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR). In the process of chemical change, a lot of factors against hydrophobicity came into being at the surface, for example hydrophilic byproduct (silanol), the increase of oxygen content and structure change of part of backbone chain. These led to the loss or weakening of hydrophobicity instantaneously. But the basis of silicone rubber didn't change. Once little molecular weight (LMW) transferred from inner to surface and covered the face, the hydrophobicity could recover after a period of time.
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