半导电层的高频特性对电缆附件内电场分布的影响

Effect of High Frequency Characteristics of Semiconducting Lay on Electric Field Distribution in Cable Accessories

电缆附件常常在系统电压开合闸过程中发生故障,除了高频暂态过电压的原因外,电缆附件材料的高频特性也值得关注。本文基于35 kV冷缩式电缆附件及电缆,测量了附件用绝缘料三元乙丙橡胶(EPDM)、硅橡胶(SIR)及其半导电料与电缆用绝缘料交联聚乙烯(XLPE)及其外半导电料在MHz～GHz的介电频谱。根据测量结果,仿真计算了35 kV电缆中间接头在直流电压、交流叠加暂态(MHz～GHz)、直流叠加暂态(MHz～GHz)过电压下的内电场分布。结果表明:对于EPDM、SIR附件绝缘料及XLPE电缆绝缘料,当频率从MHz升高至GHz时,介电常数几乎不变,但电导率上升了2～3个数量级;对于电缆及附件用半导电料,当频率从MHz升高至GHz时,电导率增大,介电常数减小,其中电缆外半导电料的电导率和介电常数变化更大。仿真结果显示,在高频暂态电压作用下,电缆本体与附件界面处场强将超出设计安全值,在施加频率为1 MHz的电压时,电缆附件电场分布符合设计安全值,当施加频率达到100 MHz和1 000 MHz的电压时,附件橡胶绝缘与电缆绝缘界面电场分别达到2.765 kV/mm和5.613 kV/mm,有可能造成界面击穿故障,从而影响电缆附件的运行可靠性。

Cable accessories always break down in the switching process of system voltage. Besides the reason of high frequency transient overvoltage, the high frequency characteristics of cable accessory deserves attention. In this paper, based on 35 kV cold shrinkage cable and accessories, the dielectric frequency spectra of ethylene propylene diene monomer(EPDM), silicon rubber(SIR) insulating materials and their semiconducting materials for accessories and XLPE insulating materials and their semiconducting materials for cable were tested ranging from 1 MHz to 1 GHz. According to the test results, the electric field distribution in 35 kV cable intermediate joint were simulated under DC voltage, AC superposition transient overvoltage, and DC superposition transient overvoltage, respectively. The results show that for the EPDM, SIR, and XLPE insulating material, when the frequency increases from 1 MHz to 1 GHz, the conductivity increases 2~3 orders of magnitude, while the permittivity is almost the same. For the semiconducting materials, when the frequency increases from 1 MHz to 1 GHz, the conductivity increases and the permittivity decreases, and the change of conductivity and permittivity is bigger for the outer semiconducting material of cable. The simulation results show that under high frequency transient voltage, the electric field strength at the interface between cable and accessory would exceed the designed safety threshold. When the voltage frequency is 1 MHz, the electric field distribution in cable accessories is within the safety threshold. However, when the voltage frequency increases to 100 MHz and 1 000 MHz, the electric field strength at the interface between cable and cable accessories attains 2.765 kV/mm and 5.613 kV/mm, respectively, which may cause breakdown of interface and affect the operation reliability of cable accessory.