Space Charge And Breakdown Strength Characteristics Of XLPE/BNNSs Nanocomposite Dielectric

Cross-linked polyethylene(XLPE)as the main insulation material is often affected by the combined effects of the electric field,temperature and other factors,during the operation of High Voltage Direct Current(HVDC)cables.On the one hand,space charge accumulation is easily generated inside XLPE,causing distortion and failure of the electric field,leading to breakdown damage of the cable insulation material and affecting the safe operation of the cable.On the other hand,the temperature gradient formed by the heating of conductor cores will also aggravate the electric field distortion inside XLPE,causing material aging and damage,even reducing the insulation performance of XLPE.Therefore,it is important to study the space charge and breakdown characteristics of XLPE to improve the performance of HVDC cables.Firstly,XLPE/BNNSs nanocomposite dielectric film with different BNNSs concentrations was prepared by melt blending method.The physicochemical properties of the nanofiller and nanocomposite dielectric were characterized by Fourier Transform Infrared Spectrum(FTIR),Scanning Electron Microscopy(SEM),X-ray Diffraction(XRD)and Differential Scanning Calorimetry(DSC).The thermal conductivity of the XLPE/BNNSs nanocomposite was investigated by Laser Flash Apparatus(LFA).The results of FTIR?SEM?XRD?DSC indicate that KH550 has been successfully grafted onto the surface of BNNSs,which improved the compatibility between BNNSs and XLPE matrix.BNNSs are uniformly dispersed in the XLPE matrix and exist in monolithic form without obvious agglomeration.The maximum crystallinity of the nanocomposite reaches the maximum value of 48.2%when the concentration of BNNSs was 0.5 wt%.The LFA results show that the thermal conductivity of the nanocomposite increases significantly with the addition of BNNSs,and the thermal conductivity increases gradually with the increase of BNNSs concentration.The thermal conductivity of the nanocomposite is 0.465W/(m·K)when the concentration of BNNSs was 10 wt%,which is about 1.42 times that of the pure XLPE sample.Secondly,the space charge characteristics of XLPE/BNNSs nanocomposite were investigated.The temperature and electric field dependent properties of charge conduction and accumulation in XLPE were studied by polarization current and depolarization current.The results show that the steady-state current of XLPE increases about 2 or 3 orders of magnitude with increasing temperature from 25?to 90?,and the stronger the electric field,the less the increased amplitudes.The trap energy level of XLPE varies little between 0.89 e V and 1.15 e V under different electric fields and temperatures.Besides,there exists an inflection point temperature for charge accumulation in XLPE,which is around 50?-60?.The effect of BNNSs concentration on the space charge accumulation characteristics of XLPE/BNNSs nanocomposite was investigated by the Electroacoustic pulse(PEA).The results indicate that a small amount of BNNSs(<0.5wt%)can effectively inhibit the space charge in the nanocomposite.This is mainly due to the introduction of deep traps at the interface between the electrode and the nanocomposite material,and the“charge barrier effect”has been formed to inhibit charge injection.The charge suppression effect in XLPE/BNNSs nanocomposite decreases with the increase of BNNSs concentration.When the concentration of BNNSs was 1 wt%,the accumulated charge in the nanocomposite increased to 1.08×10^-5 C,and the local electric field distortion rate was9.5%.For high concentration(>2 wt%-3 wt%),more relatively shallow traps are introduced in the nanocomposite,resulting in less accumulated charges.When the BNNSs concentration is 5 wt%,the trap energy level is 0.62 e V and the captured charge density is 5.76×10²⁰ m^-3,which improves the probability of charge transition between adjacent trap centers.Finally,the breakdown characteristics of XLPE/BNNSs nanocomposite were investigated.The effects of BNNSs concentration,space charge and temperature on the DC breakdown characteristic of XLPE/BNNSs nanocomposite were explored by the breakdown tester.The experimental results show that doping a small amount of BNNSs in the XLPE matrix can effectively improve the DC breakdown strength of XLPE/BNNSs nanocomposite dielectric.The DC breakdown strength of XLPE/BNNSs nanocomposite can be effectively improved by doping a small amount of BNNSs in the matrix.The breakdown strength of the specimen reached the maximum value of 407.52k V/mm when the BNNSs concentration was 0.5 wt%,which was about 33%higher than that of pure XLPE.The effect of space charge on the breakdown characteristics of the nanocomposite dielectric was investigated by pre-injecting charges into the sample.For specimens with different BNNSs concentrations,the breakdown strength increases when the polarity of the applied voltage is the same as that of the pre-injected charges.In addition,the breakdown strength of XLPE/BNNSs nanocomposites decreased significantly at 50?,which is due to more charge accumulation at 50?.The accumulated charge amount reaches 2.06×10^-8 C,which is 2.2 times higher than that at room temperature.