基于M-W极化模型的低电场极性反转下XLPE/硅橡胶空间电荷与电场分布特性研究

Study on Space Charge and Field Distribution Characteristics of XLPE/SR under Low Electric Field Polarity Reversal Based on the M-W Model

相比于单一介质的电缆绝缘,高压直流电缆附件复合绝缘因介质不连续性更易在界面处积聚空间电荷。在基于电网换相器的高压直流(LCC-HVDC)输电系统中,进行潮流反转调整时需改变电压极性,高压直流电缆附件将会承受极性反转电场,尽管是短时的,但由于空间电荷的累积效应也会在局部产生极高的电场,严重影响绝缘性能。为此,该文利用实验获得交联聚乙烯(XLPE)与硅橡胶(SR)电导率与电场强度的变化关系,基于Maxwell-Wagner(M-W)极化模型仿真研究了低电场极性反转下XLPE/SR双层介质中空间电荷与电场的分布特性。结果表明:室温下,SR的低电场电导率要高于XLPE,且SR电导率受电场强度的影响相对较小而XLPE则较大。由于界面电荷极性变化滞后于电压极性变化,极性反转期间界面处残留的负极性电荷造成SR中出现最大瞬态电场;电压极性反转越快,XLPE/SR界面残留的负极性电荷越多,SR中的最大瞬态电场强度越高;外施电压越高,由于SR与XLPE的电导率比值降低,SR中的最大瞬态电场畸变率越小。

As the key equipment of connecting the cable system,high voltage direct current(HVDC)cable accessory has become the weakest insulation point of the cable system due to the composite insulation composed of different dielectrics.Compared with cable body insulation of a single dielectric,the HVDC cable accessory of composite insulation is more easily to accumulate space charges at the interface due to the discontinuity of dielectric.In the line commutated converter high voltage direct current(LCC-HVDC)transmission system,the voltage polarity needs to be reversed when performing the power flow reversal adjustment,and cable accessories will withstand polarity reversal electric field.Although it is temporary,an extremely high electric field would be applied locally due to the cumulative effect of space charge,which threatens the safety of cable system.Firstly,the conductivities of cross-linked polyethylene(XLPE)and silicone rubber(SR)under different electric fields were measured at room temperature.Then,fitting these experimental data,the conductivity model parameters of XLPE and SR could be obtained.Finally,the finite element method was used to study the space charge and electric field distribution characteristics of XLPE/SR under polarity reversal voltage based on the Maxwell-Wagner model.Furthermore,the effects of voltage reversal time and voltage amplitude on the interface charges and the maximal transient electric field were discussed.The conductivity test results show that,the conductivity of SR is higher than XLPE within 14 kV/mm.As the field increases,the conductivity of XLPE shows an exponential growth trend and its change is obvious,while that of SR changes less and increases relatively slowly.Simulation results show that:Firstly,set the field to 5 kV/mm,voltage reversal time to 30 s and temperature to 25℃.When voltage reversal is completed,the residual negative charges at the interface enhance the field in the SR,which causes the maximal transient field reaches 8.06 kV/mm,and weaken the field in the XLPE,which reaches−1.14 kV/mm.The space charge and field distributions in the steady state before and after voltage inversion show a"mirror"distribution.Secondly,keep the temperature and field unchanged,when voltage reversal time is 5 s,30 s,90 s,the maximal transient field distortion rate in the SR is 63.82%,61.30%,55.18%,respectively.Thirdly,hold voltage reversal time and temperature unchanged,When voltage amplitude is 1.56 kV,2.60 kV,3.64 kV,the XLPE steady-state electric field distortion rate before voltage inversion is 125.55%,110.38%,90.31%,and that of SR is−85.40%,−74.28%,−59.56%,while the maximal transient field distortion rate in the SR is 71.78%,61.29%,46.59%,respectively.The results show that:(1)The conductivity of SR is higher than XLPE.There is a clear difference in the conductivity properties of XLPE and SR as a function of electric field,which is one reason for the accumulation of interface charges.(2)Since interface charge polarity change lags voltage polarity change,the residual negative charge at the interface would cause the maximal transient field in the SR.(3)The shorter the reverse period,the more residual negative charge at the XLPE/SR interface,resulting in a higher maximal transient field in the SR.(4)As the applied voltage increases,The conductivity ratio of SR to XLPE decreases gradually,so the maximal transient field distortion rate in SR decreases with increasing electric filed.