Optimization of Nonlinear Conductivity Characteristics of Silicone Rubber Materials for DC Cable Accessories by SiC Doping
Wu Jiale1, Xiong Peiqi1, Xing Zexi1, Gong Yangzhi2, Bian Xingming1
1. State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources North China Electric Power University Beijing 102206 China; 2. State Grid Fuzhou Electric Power Supply Company Fuzhou 350009 China
Abstract：For HVDC cables, the insulation level is the core issue, and also the key factor affecting the equipment capacity and long-term operation stability. In the cable accessories, because the electrical conductivity and dielectric constant of the insulating material cannot achieve continuous transition, the electric field distribution is highly uneven. The field strength borne by the insulating part of the accessories is several times the average value of the overall field strength, which may cause local electric field distortion inside the accessories and even directly cause insulation failure. Reasonable improvement of the electric field distribution at key positions are critical to the insulation of high-voltage cable terminal. It is an effective scheme to regulate the field distribution in the cable terminal by using nonlinear material whose electrical conductivity changes adaptively with the applied field. Silicon carbide (SiC), which has the advantages of low chemical activity, moderate cost and good thermal conductivity, is selected to modify the silicone rubber (SR) matrix. The DC breakdown and nonlinear conductivity characteristics of SiC/SR composites were experimentally investigated. The results show that although the breakdown strength of the composite material decreases to a certain extent after doping with SiC, it still meets the requirement of operation. With the increase of temperature, the SiC lattice scattering will hinder the carrier migration, resulting in the decrease of nonlinear coefficient, while the appearance of more high-energy carriers will reduce the threshold field of the conductivity. When nonlinear materials are used to regulate the electric field of the equipment, only when the conductivity characteristics of the materials match the operation environment can a better field grading effect be achieved. Under the conditions of considering the field distribution, power loss and local temperature rise, the suitable parameters range of nonlinear conductivity were determined quantitatively. The results show that when the nonlinear coefficient is fixed, the threshold field increases gradually, and the field strength at the junction and the cone surface presents completely opposite changing trends. Although doping SiC increases the power loss at the stress cone reinforced insulation, the resulting local temperature rise does not exceed 1K, which can be ignored compared with the heating on the central copper rod. The parameters range matched with 500kV DC cable terminal is: (1) For material with nonlinear coefficient of 3, the threshold field should be in the range of 2.9~4.0MV/m. (2) For material with nonlinear coefficient of 5, the threshold field should be in the range of 6.0~7.3MV/m. (3) For material with nonlinear coefficient of 10, the threshold field should be in the range of 7.0~10.2MV/m. The conductivity characteristic parameters of the prepared composites doped with 10% SiC basically meet the range (1). The composites corresponding to range (2) and range (3) are being actively explored by means of surface organic modification and surface inorganic coating. Compared with the traditional SR materials, the prepared 10% SiC/SR composite material can effectively improve the field strength distribution inside the cable terminal. The field strength at the conical surface of stress cone conductor can be reduced by 50%, and the field strength at the junction of XLPE insulation-rubber sheath-reinforced insulation increases by no more than 20%. Among them, 10% 1.5μm and 10% 10μm SiC/SR composites has better improvement effect. The findings are expected to provide ideas for improving the performance of cable accessories from the perspective of material modification.
伍珈乐, 熊沛琪, 邢泽西, 龚阳智, 卞星明. SiC掺杂对直流电缆附件用硅橡胶材料非线性电导特性的优化研究[J]. 电工技术学报, 2023, 38(5): 1129-1138.
Wu Jiale, Xiong Peiqi, Xing Zexi, Gong Yangzhi, Bian Xingming. Optimization of Nonlinear Conductivity Characteristics of Silicone Rubber Materials for DC Cable Accessories by SiC Doping. Transactions of China Electrotechnical Society, 2023, 38(5): 1129-1138.
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