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08_efield_hydrophobic - IEEE Transactions on Dielectrics...

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Effect of Hydrophobicity on Electric Field Distribution and Discharges along Various Wetted Hydrophobic Surfaces Haifeng Gao, Zhidong Jia, Yingke Mao, Zhicheng Guan and Liming Wang Laboratory of Advanced Technology of Electrical Engineering and Energy, Graduate School at Shenzhen, Tsinghua University, City of Shenzhen, 518055, P. R. China ABSTRACT According to hydrophobicity and hydrophobicity transfer studies on silicone rubber surfaces, although the silicone rubber can transfer its hydrophobicity to various contaminants deposited on its surface, the hydrophobic states of water droplets on clean silicone rubber surfaces and on various contaminated silicone rubber surfaces were found to be different based on evaluation by contact angle measurement. Furthermore, surface discharge and aging of materials decreases the hydrophobicity of the surface. Focused on the difference of hydrophobicity and the effect of water droplets on the hydrophobic contaminated surfaces, some contrastive computations and tests of water droplets on various surfaces were carried out in this study. The results showed that the water droplets existing on the surface of silicone rubber materials might change the electric field and voltage distribution along the surface, and even reduced the initial voltage of the corona discharge, which was verified with a comparison test on a high voltage side of polymer insulator monitored by a photomultiplier tube (PMT) system. Since the leakage current provides much useful information to diagnose the state of outdoor insulation, this study investigates the time and frequency distributions of surface discharges on ceramic insulators, new room temperature vulcanized (RTV) silicone rubber coated insulators and aging RTV insulators with various hydrophobic states with the short time Fourier transform and Gabor transform methods. Then a correlation was found between the distribution of the spectrum of surface discharges and hydrophobicity via the joint time and frequency analysis of discharges on various hydrophobic surfaces. Index Terms Hydrophobicity, silicone rubber, discharge, leakage current, electric field distribution 1 INTRODUCTION S ILICONE rubber insulating materials are known to resist water filming and hence surface resistance for hydrophobicity [1, 2]. It is known that intense discharge will reduce surface hydrophobicity and cause some chemical changes on the surface of materials [3, 4]. Although silicone rubber insulating materials can recover their hydrophobicity during dry periods [5], along with the increase of the energized years, the surface hydrophobicity of silicone rubber will decrease [6, 7], which means that aging reduces the surface’s hydrophobicity. According to hydrophobicity and hydrophobicity transfer studies on RTV silicone rubber coated samples with two kinds of artificial pollutants, the contact angle of water droplets on various contaminated RTV coating surfaces were found to be different with the same transfer time, as shown in Figures 1.
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