| High voltage DC transmittion technology has shown significant advtanges in areas about long distance transmittion,consumption of renewable energy,building of new effective transmittion network and construction of energy internet.In order to solve the key questions in structure design process of bushing and GIS basin-type insulator,numerical simulation and experimental testing were both applied to analyze the influence mechanism of varies parameters on insulating properties,such as temperature gradient,surface charge accumulating and metallic particles attaching.Researches results will provide theoritical and engineering guidance for the design,operating and maintanance of DC bushing and insulator.The main research work and results are following:（1）Numerical models of electrostatic,current conduction and transient electric field were built to analyze the field intensity distribution properties of DC bushing under different types of voltage.The transient electrical field in different dielectrics during the polar reversal process was discussed emphatically.Taking the influence of heating on the resistivities of solid dielectrics into consideration,a 2D axial symmetry model about coupled flow-thermal-electric field was established,which was then proved by temperature rise measuring.The temperature distribution,heat transfer of SF6 during free convection process and electric filed varying tendency under temperature gradient were analyzed with application of the coupled model on numerical simulation of high voltage DC bushing.Several advices were promoted for the structure design of bushing.Results show that,the“electric stress reversal”happens when the temperature gradient increases along with current load,which means the electric field tendency in inner side of the core is lower while the outer side is higher,which is opposite to the original desig and will exacerbate the risk in operating process.（2）A 3D numerical model about coupled flow-thermal-electric field for GIS chamber was established to analyze the temperature distribution,free convection heat transfer process of SF6 and electric filed varying tendency with temperature gradient under different current loads.The influence of metallic particle locations,size and attaching angel on surface electric field of insulator was analyzed through an electric field numerical model.A mathematic model was built to study the distribution of positive particles,negative particles and charges on the surface of insulator,considering the transport and diffusion of charged particles in gas.Furthermore,the influence of attached metallic particles on charge distribution and distortion of electric field under long time DC voltage stress was simulated.Results show that,the electric stress reversal happens when the current is 5000A.Under the combined effect of metallic particles and surface charge,the electric field intensity on surface of insulator increases significantly in areas near high voltage conductor,where the largest value is on one terminal of particles.At the same time,accumulation of surface weakens the point effect of metallic particles,which will reduce the external intensity distortion as well.（3）A DC flashover testing platform for basin type insulator was established.The insulator samples were made of epoxy resin composite material and filled with Al2O3.Flashover voltages of insulators were measured by locating metallic particles on surface of insulator manually.The attaching locations,particles size,gas pressure and voltage polarity were all taken into consideration for the testing to analyze influences of these parameters on the flashover process.Results show that the influence extend of particles depends on the location they are attached.The insulating properties will be weakened obviously only if they are in the“defeat sensitive area”.（4）DC flashover experiments were carried out by attaching metallic particles to different positions on the surface of insulators,and then the dispersion of the flashover path after multiple times of flashover were analyzed.Another set of flashover experiment were executed with finger-shaped electrode system and flat-shaped insulators to observe the microstructure of surface and change of element content.The microstructure damage mechanism of insulator surface during flashover were analyzed.Researches find that,as the particles are near the high voltage conductor,the fractal dimension of discharge trace in the grounding area decreases gradually,while that in the high voltage area increases gradually.The existence of F can be used to estimate appearance of flashover,and the content of F and Al can be used to estimate the severity of discharge burning. |
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