直流电缆用交联聚乙烯绝缘的击穿概率及其尺度效应仿真

Breakdown Probability and Size Effect Simulation of XLPE Insulation for DC Power Cables

电力电缆在远距离大容量电力传输和海上风电并网中起着极其重要的作用。受直流电场影响,交联聚乙烯(XLPE)电缆存在着空间电荷积聚和电荷能量积累现象,导致绝缘材料被击穿而引发故障。该文建立电荷输运与分子链位移击穿模型,对XLPE介质中载流子输运和能量积累过程进行了仿真,得到XLPE特征击穿场强-试样厚度依赖特性以及击穿概率分布。仿真结果表明,XLPE直流击穿场强受尺度效应影响明显,击穿场强随着试样厚度的增加而减少,且击穿强度概率计算结果服从威布尔分布。建立电荷输运随机变量参数分散性与击穿概率分布的定量关系。随着试样厚度的增加,绝缘材料内部的缺陷分散性增加,电荷输运随机变量参数高斯分布中的方差增加,击穿威布尔分布形状参数减小。陷阱能级是影响击穿威布尔分布形状参数的最关键因素。该研究为电力电缆的优化设计和可靠性评估提供了仿真技术支撑。

Power cables play an extremely important role in long-distance high-capacity power transmission and offshore wind power grid connection.Affected by DC electric field,cross-linked polyethylene(XLPE)cables are subject to space charge accumulation and charge energy accumulation phenomena,leading to insulation material breakdown and causing faults.According to the solid dielectric scale effect principle,increasing the thickness or area of the specimen will result in an increase in volume and an increase in the probability of local defects,causing the breakdown field strength to decrease.However,the microscopic defect parameter variation of insulation materials with specimen thickness and its quantitative relationship with the breakdown probability have yet to be clarified.Therefore,this paper proposes a charge transport and molecular displacement modulated model(CTMD),and studies the characteristic breakdown field-specimen thickness dependence of XLPE and the probability distribution of the breakdown field by the carrier transport and energy accumulation processes in the dielectrics.The electrodes inject holes and electrons into the bulk of materials when a ramp voltage is applied to the insulating materials.Under the influence of the electric field,these carriers move into the insulating materials.Deep traps are generated at the interfaces of the crystalline/amorphous zone(the interface between spherical crystals)and formed polar groups in XLPE.The deep traps capture holes and electrons,causing distortions in the electric field due to space charge accumulation.Also,as the trapped charges are being affected by Coulomb forces,the expansion of the free volume due to molecular chain displacement raises the charge energy that is traveling inside it.The local current surges and the insulating material is broken through when the charge energy builds up to the point of crossing the trap potential barrier.The findings demonstrate that the scale effect has a substantial impact on the DC breakdown field of XLPE.The breakdown field decreases with increasing specimen thickness,and the breakdown strength probability follows the Weibull distribution.Additionally,the breakdown probability distribution of XLPE can be modeled by adding random variables to CTMD.Altering the variance of the charge transport random variable controls the breakdown Weibull distribution's shape parameter.The shape parameter of the breakdown Weibull distribution decreases to 10.12,5.76,5.49,5.36,and 4.34 as the specimen thickness increases from 50mm to 250mm.The trap level most significantly influences the Weibull distribution,and the value of the shape distribution parameter varies the fastest with the variation of the trap level.The following conclusions are obtained from the simulations:(1)A correlation is established between the long-range motion of molecular chains under the action of Coulomb forces and the thickness dependence of the breakdown field of XLPE.The molecular chains containing trap charges have more time to undergo long-range motion under the electric field as the specimen thickens and the free volume increases.At a lower electric field,the charges gain enough energy in the free volume to cause insulation material breakdown.(2)In the CTMD model,the probability distribution of the breakdown field obtained from the simulation obeys the Weibull distribution.It is found that the trap level is the most critical influencing factor of the shape parameter of the breakdown Weibull distribution.(3)As the specimen thickness increases,the variance in the Gaussian distribution of the charge transport random variable parameters gradually increases,the shape parameter of the breakdown Weibull distribution gradually becomes smaller,and the dispersion of defects inside the insulating material increases substantially.