| As a key equipment in the DC transmission system,high-voltage DC cable should have sufficient and reliable insulation performance.However,due to the composite insulation structure composed of different media between the main insulation of the cable and the accessory insulation.the phenomenon of interface charge accumulation is prone to occur under the action of DC voltage,which causes distortion of the electric field inside the insulation material and accelerates the aging of insulation or even breakdown,threatening the safety of cable transmission system.In the LCC-HVDC transmission system,the insulation of high-voltage DC cables will experience a short-term voltage polarity reversal process when the system adjusts the power flow,and the distortion of the electric field under this condition may be more severe.Therefore,improving the insulation performance of the composite insulation structure in the cable by regulating the interface charge has become an urgent problem to be solved.In this study,SiC-SR and TiO2-SR nanocomposite materials suitable for high-voltage DC cable accessories were prepared using solution blending method.The space charge characteristics and electric field distribution of XLPE and double-layer dielectric composed of different concentration nanocomposite SRs during the voltage polarity reversal process were tested and analyzed using the electric pulse method.At the same time,a breakdown platform suitable for sheet-shaped samples was independently designed and constructed to test the influence of polarity reversal and doping concentration on the breakdown field strength of the double-layer dielectric.The research results show that:(1)At steady state before and after voltage reversal,the distribution of space charge and electric field roughly exhibits a "mirror-symmetric" relationship.The maximum transient electric field appears in the SR sample at the end of the polarity reversal,and its value is significantly higher than the applied average field.Analysis suggests that this is due to the "lag of interface charge polarity change behind the voltage polarity change".Moreover,the degree of electric field distortion in the SR sample increases as the voltage reversal time shortens.(2)The doping of a small amount of nano-SiC and TiO2 will introduce deep traps in the SR matrix and trap free electrons near the lower electrode to form a negative charge layer.By increasing the Schottky barrier of charge injection,the charge injection is hindered,which has a significant inhibiting and improving effect on interfacial charge accumulation and electric field distortion.As the doping concentration further increases,the amount of interfacial charge accumulation increases,and the inhibition effect on the maximum distorted electric field appearing during the inversion process is also weakened.(3)The polarity reversal breakdown field strength of XLPE/SR double layer samples is significantly lower than the DC breakdown field strength,and the corresponding applied breakdown field strength decreases with the shortening of the polarity reversal time.The analysis shows that the electric field distortion caused by the accumulation of interfacial charges is the main reason for the composite medium to be more prone to breakdown during the polarity reversal process.(4)Doping nano-SiC and TiO2 particles with appropriate concentrations can effectively enhance the polarity reversal breakdown field strength of XLPE/SR bilayer media by "suppressing the maximum electric field distortion that occurs during the polarity reversal process on the SR sample side". |
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