Simulation Study On Mechanism Of Induced Streamer Branching In Suspended Nanoparticles In Transformer Oil

As one of the important power equipment in the power system,the oil-immersed transformer directly affect the stable operation of the power system.Transformer oil is an insulating and heat dissipating material in oil-immersed transformers.Under high electric field stresses,shrub-like or filament-like discharge channels,which are named as "streamers",can be formed in oil.Once the streamer connects across the electrodes,a high-conductivity arc is generated,resulting in a short circuit between the electrodes and a breakdown fault of transformers.The breakdown performance of transformer oil can be improved significantly by adding a certain number of TiO2 nanoparticles into the transformer oil.It was found that the addition of nanoparticles made the positive streamer pattern of transformer oil change in the previous research of the research group.The channel diameter of the positive streamer become thicker,the number of channels is increased,and a large number of branches appear.Some explanations have been put forward for the breakdown mechanism of nanofluid,but these explanations mainly explain the breakdown performance of nanofluid by changing the space charge of nanoparticles.However,it is still unclear how the nanoparticles change the morphology of the streamer.Firstly,a 2-D needle-sphere electrode configuration streamer simulation model based on needle-ball electrode was established.The trap characteristics of nanofluid was measured by thermal stimulation current method and the development model of streamer was established in nanofluild.The correctness of the simulation model was verified by comparing the simulation results with the experimental results.The results show that the streamer development model base on field ionization can better reflect the development process of streamer.A large number of shallow traps are generated in the nanofluid which are one of the reasons for the decrease of the streamer rate in the transformer oil.Secondly,in order to analyze the mechanism of nanoparticle induced streamer branching,the positive streamer simulation model in solid-liquid composite media was established based on field dependent ionization theory.The influence of suspended particles on the development of streamer in transformer oil was studied.The simulation results show that the polarization of the suspended particles causes the electric field to be distorted and increase the local electric field strength.If the electric field distortion region can cause enough oil molecules to be ionized,it may induce the generation of streamer branch.The degree of influence of suspended particles is related to the particle size of the suspended particles,the dielectric constant and the position of the relative streamer.The larger the particle size of the suspended particles,the greater the dielectric constant and the closer the particles are to the edge of the stream,it is easier to generate branch.Finally,the phenomenon that the nano-sized particles can not change the development characteristics of the streamer.This paper analyzes the structure and thickness of the electric double layer between nanoparticle and transformer oil molecules.The trap distribution in the electric double layer was studied and the migration model of electrons in the electric double layer structure is established and explore the nanoparticle induced streamer branch model.The results show the thickness of the electric double layer is 22 microns;Since the charge distribution in the electric double layer obeys the Born distribution,the closer to the surface of the nanoparticle,the more dipole polarization and electron polarization of the transformer oil,the greater the local trap density.When the electrons generated by ionization pass through the electric double layer,the mobility is lower.The mobility will be reduced,resulting in the change of charge density distribution at the end of the streamer,and the streamer branch is induced.