工频叠加操作冲击电压下预制式电缆终端破坏机理研究

Failure Mechanism of Prefabricated Cable Terminal Under Power Frequency Voltage Superimposed Switching Impulse Voltage

工频叠加操作冲击电压是电缆运行中时常面临的情况,在该情况下高频谐波电压会使电缆终端材料介电常数发生变化,进而造成电缆终端所承受的电应力、热应力和机械应力发生改变,最终会对电缆终端的绝缘造成破坏。文中,首先建立了工频叠加操作冲击电压的数学模型,同时分析了不同冲击电压叠加相位对叠加后电压波形的影响;其次,利用有限元软件建立了35 kV预制式电缆终端的仿真模型,对仿真模型参数设置、边界条件设置以及网格剖分设置进行了详细说明;最后,仿真计算了工频叠加操作冲击电压下电缆终端应力情况,包含电应力、热应力以及机械应力变化情况。结果表明:工频叠加操作冲击电压后,电缆终端材料的介电常数频率特性使得电场强度有所减小,起到了均匀电场的作用,且电场强度的整体变化率较低(0.05%以内),不足以造成电缆终端的破坏;电缆终端温度最大值位于铜芯内部,基本接近初始温度,电压作用引起的电缆终端温差变化较小(0.004 K内),基本可以忽略;操作冲击电压叠加后高频谐波电压分量使得电缆终端半导电层电致伸缩系数发生变化,这导致半导电层径向应变(1.35 mm)大于轴向应变(0.05 mm),最终使得半导电层和绝缘之间容易出现间隙,是造成电缆终端的绝缘破坏的主要原因。本研究可为电缆终端结构设计和优化以及材料选择与改性提供参考。

The power frequency superimposed switching impulse voltage is often faced in the operation of the cable.In this case,the high-frequency harmonic voltage will change the dielectric constant of the cable terminal material,thereby causing changes in the electrical stress,thermal stress and mechanical stress on the cable terminal,which will eventually damage the insulation of the cable terminal.Firstly,the mathematical model of the power frequency superimposed switching impulse voltage is established,and the influence of different impulse voltage superposition phases on the superimposed voltage waveform is analyzed.Secondly,the simulation model of the 35 kV prefabricated cable terminal is established using finite element software,and the parameter settings,boundary conditions and meshing settings of the simulation model are described in detail.Finally,the stress of the cable terminal under the superimposed switching impulse voltage of power frequency is simulated and calculated,including the changes of electrical stress,thermal stress and mechanical stress.The results show that the frequency characteristics of the dielectric constant of the cable terminal material reduce the electric field strength,which plays a role in making the electric field uniform,and the overall change rate of the electric field strength is low(within 0.05%),which is not enough to cause damage to the cable termination.The maximum temperature of the cable terminal is located inside the copper core,which is basically close to the initial temperature.The temperature difference of the cable terminal caused by the voltage is small(within 0.004 K),which can be ignored.After the switching impulse voltage is superimposed,the high-frequency harmonic voltage component causes the electrostrictive coefficient of the semiconducting layer of the cable terminal to change.This causes the radial strain(1.35 mm)to be larger than the axial strain(0.05 mm),and makes a gap between the outer semiconducting layer and the insulation,which is the main reason for the insulation failure of the cable terminal.This research can provide a reference for the design and optimization of cable termination structures,as well as material selection and modification.