Interface Electrical Performance Enhancement Design Method Of DC Basin Insulator Composite Gradient Insulation

The construction of the "new power system with alternative energy as the main body" has become the inevitable choice to achieve the national strategic goal of"carbon-peaking and carbon-neutrality",and the high voltage direct current transmission technology is an important physical support for the realization of alternative energy grid connection and the safe and stable operation of the large power grid.The gas insulated transmission line(GIL),as the key equipment of the future DC power grid,is able to meet the special transmission requirements of offshore wind power delivery and offshore isolated islands power supply,across rivers and lakes and urban power corridors.Compared with AC GIL equipment,the gas-solid interface charge accumulation problem of basin insulator surface under DC voltage is more serious,and the harm of metal contaminants,such as metal particles and dust,is more prominent.The interweave of these two factors is easy to cause surface insulation failure and surface discharge ablation.Interface insulation has always been the vulnerable spot of DC GIL composite insulation system.At present,there is no quantitative optimization design method for the geometric structure of DC insulators,and the surface coating technology cannot effectively adapt to the complex curved surface structure of actual basin insulators,so it is difficult to realize the integrated design of normal and tangential gradient electric fields.Therefore,it is necessary to carry out innovative research on the design theory and method of interface electrical performance enhancement of composite gradient insulation.This research comprehensively combines multi-scale simulation and experimental observation methods to carry out systematic research on construction methods of composite gradient interface insulation,normal electric field regulation of insulator surface,tangential gradient insulation optimization and service performance evaluation of interface insulation,which can provide theoretical basis and technical approach for the optimal design of DC GIL basin insulators.Through establishing a multi-field simulation model for surface charge accumulation on DC insulators,the surface electric field distribution mode and surface charge accumulation characteristics of insulators with different structures are obtained.It is pointed out that the low-taper basin insulators can control the normal field strength at a lower level,which is more suitable for DC operating conditions.According to the distribution characteristics of the electric field strength,a zonal coating scheme of the nonlinear conductivity coating on the insulator surface was proposed to realize the optimal control of the local tangential electric field and optimize the coating mode and coating parameters suitable for the convex and concave sides of the basin insulator.Based on the above,the construction method for the composite gradient interface insulation of the DC basin insulator was formed,and the conjunctive adjust and control strategy for tangential and normal electric fields combining geometric structure optimization and zoning coating with nonlinear conductivity material was established.The observation platform for metal dust adsorption and discharge on the insulator surface was established to research and obtain the common characteristics of distribution patterns on the surface of basin insulators with different inclination angles.It is found that insulators with large inclination angles are beneficial to reduce the influence of dust on the surface insulation.Based on the force analysis,the kinetic model of dust adsorption process was built and the mechanism of dust adsorption was thus clarified.According to the mechanism,the geometric structure optimization design criterion considering the influence of dust was put forward.and the inclination angle of DC insulator should be between 60° and 90°.Furthermore,the design method of basin insulator based on surface contour curve optimization was proposed.Six geometric coefficients,which could describe the inclination angle and curve profile of the insulator,were used as control parameters to reconstruct the insulator.The optimization was carried out by using particle swarm optimization algorithm to obtain the geometric structure parameters of the novel-structure DC basin insulator,and the normal electric field is improved.By proposing the molecular simulation prediction method for dielectric properties and corona resistance of coating materials for insulators,the influence of introducing fluorine-containing groups on the microstructure,trap characteristics and corona resistance of epoxy insulation system was analyzed,and the fluorine-containing epoxy molecular structure with application potential was selected.Furthermore,the fluorinecontaining epoxy monomer substituted by the side-chain-CF3 group was synthesized by an effective technical pathway,and the SiC doped fluorine-containing epoxy composite coating with both metal dust adsorption inhibition and viscosity resistance was designed and prepared.Moreover,based on the geometric parameters of the novelstructure DC basin insulator,a composite gradient basin insulator model was constructed by applying the zoning gradient coating mode with nonlinear conductivity conposites.The regulating effect of the surface composite gradient electric field strength was analyzed.Thus,The results showed that the maximum tangential field strength was reduced by 12%,and the insulation strength along the surface was further improved.In order to evaluate the long-term service performance of the composite coating.a corona erosion simulation platform and a thermal aging test platform for the coating were built.The macro-micro morphology,surface insulation strength and adhesion tensile strength of the coating under long-term electrical stress and thermal stress were obtained,and the multivariate evaluation index system and the evaluation method for the long-term service performance of the insulator surface coating were established.Furthermore,a combination of the spraying method and impregnation method was adopted to develop the composite gradient basin insulator prototype.The surface charge distribution characteristics,surface insulation strength and mechanical strength of the novel DC insulator were measured by building an electrical and mechanical performance test platform.The experiment results showed that the composite gradient basin insulator can effectively reduce the surface charge accumulation,and greatly improve the gas-solid interface insulation strength.The flashover voltage can be enhanced by more than 20%,and the influence of particles and dust on the interface insulation is alleviated.The above research provides the theoretical basis and the effective method for electrical performance enhancement design of DC basin insulator composite gradient insulation interface.