| DC gas-insulated transmission lines(GILs)show preferable potentials to meet the special needs in scenarios such as distributed energy in-situ transmission,urban power pipe gallery laying underground and power transmission crossing rivers,which also tend to be the key equipment for future development of the DC grid.The tri-post insulator plays a key role in supporting conductor and maintaining electrical insulation in a DC GIL,which renders the advantages of providing thermal expansion and/or mechanical strain compensation as an indispensable principal component.Compared with the AC application circumstances,the charge accumulation at the gas-solid interface of the tri-post insulator under DC stress is even more severe.It is also susceptible to metal particles interference and may further lead to surface discharge faults,which presents pending technological challenges for interface insulation design.Especially,the tri-post insulator has a complex surface profile,however,presently there is a lack of effective surface charge measurement methods for this case.As a result,the surface charge accumulation characteristics are unclear,and the surface discharge mechanism is unknown either.Hence,the existing interface insulation enhancement design method lacks of theoretical guidance and effective solutions.To address the above problems,this thesis comprehensively combines microscopic simulation and experimental observation methods as to systematically study the surface charge measurement method,surface charge accumulation characteristics,surface flashover characteristics induced by metal contaminants,as well as interface insulation enhancement design method for tri-post insulator,providing the theoretical basis and technical paths for the optimal design of DC GIL tri-post insulator.With regard to the complex surface profile of tri-post insulator,a multi-degreeof-freedom movement device for surface potential measurement with active electrostatic probe is designed and established.A multi-axis mechanical control device is used to adjust the five-degree-of-freedom manipulator to drive the probe movement,and accurate scanning of the insulator surface potential in all directions is thereby realized.Based on the measured surface potential by the active electrostatic probe,a three-dimensional inversion calculation model for the surface charge of tri-post insulator is further established.By adjusting the order of the boundary conditions together with Wiener filtering processing,the matrix operation is simplified,and a surface apparent charge inversion optimization algorithm suitable for asymmetric complex curved surface structure is then proposed.The effectiveness of the inversion algorithm is verified by simulation examples and the dust figure methodology.With the established surface charge measurement and inversion methodology,the surface charge accumulation characteristics of the tri-post insulator under DC voltages are experimentally studied.It is found that the surface charge accumulates by one heteropolar charge peak and two homopolar charge peaks along the bottom of the leg up to the abdomen,featuring a three-peak distribution mode.The temperature gradient will cause the amplitude and distribution range of the homopolar charge peak in the tri-post insulator leg region to increase significantly,and the surface charge density may exceed 100 μC/m2.Furthermore,the electric-thermal-flow coupled multi-field simulation model for charge accumulation of the tri-post insulator is established,and the dominant mechanism of surface charge accumulation is then revealed.It is pointed out that,the non-uniform distribution of surface conductance is the main cause for the surface charge surge at the insulator’s leg region under the temperature gradient.To explore the flashover mechanism of the tri-post insulator induced by interactions of both metal contaminants and surface charge accumulation,the adsorption behavior of metal contaminants near the tri-post insulator under DC voltage is observed.It is found that,the contaminants are mainly absorbed in the leg area of the insulator with the most severe surface charge accumulation,and the surface charge also plays a key role in the adsorption process of metal contaminants.Two types of charge spots on the insulator surface caused by metal contaminants are further defined:bipolar charge spots and unipolar charge spots,revealing the interaction mechanism between metal contaminants and surface charges.On this basis,also considering the complex conditions such as temperature gradient and lightning impulse overvoltages,the surface flashover characteristics of the tri-post insulator induced by metal contaminants are experimentally studied.The surface flashover induction mechanism of the interaction between the discharge at the terminal of the metal contaminants and the charge accumulation is proposed.The leg part tends to be the weakest insulation area of the tri-post insulator.After clarifying the mechanism of charge accumulation and flashover,a design methodology for interface insulation enhancement of DC tri-post insulators with the U-shaped gradient conductance is proposed.Through synergistic use of nonlinear gassolid interface conductivity coating and high embedded-epoxy interface conductivity coating,the body conductivity and surface conductivity of the tri-post insulator realise a U-shaped gradient distribution,which can accelerate the charge dissipation rate at the low-temperature side and reduce the accumulation of homopolar charge peaks at the legs.Furthermore,optimization of the conductivity parameters of the coatings is realized targeted at surface charge regulation.The experimental results show that,the parameter-optimized dual-interface coatings can effectively reduce the surface charge accumulation at the insulator’s legs,and also improve the surface flashover voltage under the adhesion of metal particles to a certain extent.The above research provides a theoretical basis and also effective methodology for the optimal design of the DC GIL tri-post insulator. |
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