| Investigation to numerical simulation models of streamer discharges in natural ester insulating oils is of great significance to reveal influencing factors of streamer discharges in insulating oils and to improve electrical performances of natural ester insulating oils.However,streamer discharges in natural ester insulating oils present characteristics of stochastic multi-branches,differing significantly from the simulative results of the current numerical simulation models of streamer discharges.The mechanisms,numerical modeling,and simulation of multi-branch streamer discharges in natural ester insulating oils are thus investigated in the dissertation,which includes main works as follows.(1)Characteristics and modeling of surge in carrier density during streamer discharges in natural ester insulating oils were studied.Phenomenon of the surge in carrier density in natural ester insulating oils was analyzed by utilizing a high-speed observation platform for streamer discharges.The characteristics and influencing factors of the local surge in carrier density during streamer discharges were explored.A group of discharge experiments with a double-needle plate electrode structure were designed to investigate the characteristics of the electric field influencing on the surge in carrier density and streamer branches,and to obtain the mapping relation between the surge in carrier density and the streamer branching behaviors.The numerical simulation model of the source term of the surge in carrier density was constructed based on the above analysis.A novel idea was thus proposed by introducing the source term of surge in carrier density into simulating streamer branching behaviors.(2)Numerical modeling and simulations of the multi-branch streamer discharge in natural ester insulating oils were investigated.The source term of surge in carrier density was considered in numerical simulation model of streamer discharges on base of the Poisson equation and current continuity equations for constructing a numerical simulation model of multi-branch streamer discharges.The simulations of streamer discharges in natural ester insulating oils were carried out to investigate the influences of external voltage,voltage rise time,and random fluctuation in carrier densities on streamer branching behaviors,using the finite element simulation.Influences of random fluctuation in the carrier density on the branching characteristics of positive and negative streamers were analyzed by simulation and compared with experimental results,which indicated that the multi-branch streamer discharge model generating simulation results in consistance with the experimental results of positive and negative streamers.(3)Numerical simulations of multi-branch streamer discharges considering polarization effect of nanoparticles in natural ester insulating oils were investigated.The source term of nanoparticles polarization was considered in the multi-branch streamer discharge model to constructe a numerical simulation model of multi-branch streamer discharges,considering the polarization effect of nanoparticles.With considering the polarization effect of nanoparticles in natural ester insulating oils,the simulation of streamer discharges was carried out to investigate the polarization effect of nanoparticles influencing on the branching morphologies and distributions of electric field and space charges of the positive and negative streamers,by using the finite element simulation.Influences of nanoparticles polarization time and quantity on the branching and propagation characteristics of streamers were analyzed by simulation and compared with experimental results.The comparison indicated that the multi-branch streamer discharge model in consideration of the polarization effect of nanoparticles generating simulation results in consistance with the experimental results of positive and negative streamers.(4)Numerical simulations of multi-branch streamer discharges considering paperboard barrier effect in natural ester insulating oils were investigated.A multi-branch streamer discharge model considering barrier effect of paperboards was proposed with a paperboard barrier model introduced into the multi-branch streamer discharge simulation.The simulations of streamer discharges in natural ester insulating oil considering paperboard barrier effect were carried out by using the finite element simulation,to obtain the characteristics of paperboard barriers influencing on the propagation of streamers,streamer branching,and distributions of electric field and space charges.The simulation results were compared with experimental and theoretical analysis result.The comparison indicated that the multi-branch streamer discharge model in consideration of the paperboard barrier effect generating the simulation results in consistance with the experimental and theoretical analysis results.The research results above indicate that the branching and propagation behavior of streamer discharges are well demonstrated by the multi-branch streamer discharge model,which contributes to reveal the generation mechanisms and development characteristics of the streamer branching,and to provide model support for researching improvement approaches of breakdown performances of liquid dielectrics. |
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