Enhanced Electrical Properties Of Epoxy-based Spacer By Electric Field Regulation For HVDC GIL

Epoxy-based spacers are critical components in an UHV/EHV direct current gas insulated transmission line(DC-GIL),providing electrical insulation and mechanical support for the bus bar.Under a strong electric field,the surface charge accumulation on the spacer surface worsens the electric field distortion and even triggers an insulation failure,threatening the safe operation of the electric power system.Facing with the reliability problem of DC-GIL caused by the electric field distortion around the spacer,the correlation relationship between the surface charging and discharging behaviors of the spacer was investigated,the surface nonlinear conductivity(SNC)spacer and the gradient surface nonlinear conductivity(G-SNC)spacer were proposed to regulate the electric field distribution in DC-GIL,providing theoretical and experimental foundations for the research and development of the UHV/EHV DC-GIL spacer.The major works and conclusions are as follows:1)Based on a downsized sample of the HVDC GIL spacer,the surface charging and discharging characteristics of the basin-type spacer were studied in different gases under electric fields of different polarities and amplitudes.The field-dependent surface charge distribution patterns were discovered,and the discharge model of the DC-GIL spacer was established considering the interaction effect of the surface charge accumulation and the electric field distortion.By subjecting the spacer to abnormal conditions,i.e.,metal particles,temperature gradients and a polarity reversal voltage,the correlation relationship between the surface charge accumulation and the electric field distortion was revealed.2)Facing with the uncertain time-space evolutions of the surface charge accumulation and the electric field distortion of the DC-GIL spacer,the SNC spacer was proposed to adaptively regulate the surface charge and electric field distributions.Based on a ±500 kV DC-GIL simulation model,the SNC spacer exhibited the ability of adaptively regulating surface charge and electric field distributions under different conditions,i.e.,metal particles,temperature gradients and the polarity reversal voltage.Guidelines for designing the SNC spacer were proposed for balancing its field regulating effect,power loss and transient response speed.The epoxy/α12 composites of different doping contents were selected for fabricating SNC spacers by the soak coating method.The electric field regulating effects and the improved insulation performances of the SNC spacers were verified by assigning metal particles,temperature gradients and the polarity reversal voltage during tests.4)To further improve the electric field distribution and reduce the power loss,the G-SNC spacer was proposed by grading the nonlinear-conductivity layer on the spacer surface.The gradient distribution of the SNC layer on a disk-type spacer surface was optimized by the iteration method.The magnetron sputtering method was used to fabricate the SNC and the G-SNC spacers by depositing Zn O films on spacer surfaces in a uniform manner and an optimally-graded manner,respectively.Simulation and experimental results showed that the G-SNC spacer has superior electric field distributions and insulation performances than the SNC spacer under DC and polarity reversal voltages.