Numerical Simulation For Surface Discharge Of Air-Polyimide Insulation And The Influencing Factors Analysis Under Pulsed Electrical Stress

Energy Internet is an important form of future power system.The focus of building an energy Internet with comprehensive state awareness,efficient information processing,and convenient and flexible application is on achieving the interconnection and interflow of energy flow and information flow the energy.The energy router can realize the grid-connected access of distributed energy,the interconnection of AC and DC grids,and the real-time regulation of load flow,and it is the key equipment for building an energy Internet.The core component of the energy router and the key to the flexible and efficient conversion of electric energy by the energy Internet is the high-frequency power transformer(HFPT),which insulation systems mainly use air and polyimide(PI).Because the high-frequency transformer is small in size,compact in structure,and it will withstand high-frequency electrical stress with short rise time and large amplitude for a long time,the operating environment is worse than that of traditional transformers.And it is easier to induce surface discharge at weak part of insulation,which will cause polyimide insulation to aging or fail prematurely under high-frequency strong electric field.With the increase of transformer capacity and voltage level,the insulation problem is more prominent,which severely restricts the development of high-frequency transformers towards high-voltage,compact,high power density and large capacity.Surface discharge has become one of the main causes of the failure of the gas-solid insulation system of high-frequency power transformers,and the research on the development mechanism and simulation model of surface discharge is still incomplete.In order to study the surface discharge development process of air-polyimide insulation,a plasma model with needle-bar electrode structure for air-polyimide surface discharge under pulse electrical stress is established in this paper.Based on the theory of fluid dynamics,this model couples particle transport equations,momentum equation,energy equation,Poisson equation and plasma chemical reaction,and fully considers the surface reaction of PI films and the secondary electron emission process caused by high-energy particles bombing the PI surface.A simplified set of reactions is used in this model,instead of hundreds of particle reactions involved in air discharge,to describe correctly the creation and destruction of charged species in a background of dry air,which greatly reduces model complexity.Accordingly,the finite element method together with log formulation is introduced to solve the model,which avoids the unreasonable situation of negative particle density in the solution process.Based on the established non-equilibrium plasma model surface discharge in needle-bar electrode structure with 1 cm gap,the dynamic evolution process of surface discharge under pulsed electric stress and the spatial and temporal distribution law of micro-parameters such as electric field and surface charge are investigated.The simulation results under negative polarity single pulse show that the process of surface discharge includes three stages:electron collapse,electron collapse to streamer and streamer development.At the initial stage of discharge,the number of electrons increases rapidly and the electrons are concentrated at the needle tip.With the development of discharge,a large number of electrons are generated at the head of the streamer,and the plasma channel extends to the ground electrode and eventually penetrates the two poles.The amount of space charge generally increases and is mainly concentrated in the head of the streamer,while the amount of charge in the streamer channel is small;Negative polarity charge accumulates on the PI surface.As the discharge develops,the distribution range of surface charge extends from the needle electrode to the ground electrode,and the average accumulated charge amount and accumulation rate increase with the development of surface discharge,which is consistent with the experimental measurement results of surface charge.Due to the existence of space charge and surface charge,electric field distribution on PI surface at the streamer development stage of surface discharge presents peak-valley-peak shape.With the increase of tangential electric field intensity in front of the discharge channel,the development rate of surface discharge increases exponentially and reaches 106m/s at the later stage of discharge development.Further study on the development process of surface discharge under repeated pulses,it is found that the development trend of surface discharge under repeated pulses is almost the same as that under single pulse.The discharge first appears near the needle electrode and then gradually develops to the ground electrode.Finally,the plasma channel penetrates the two electrodes.However,under repeated pulse,the relationship between the discharge development time and the pulse repetition frequency shows a u-shaped curve--with the increase of the pulse repetition frequency,the discharge development time first decreases and then increases,there is an inflection point,and the inflection point moves towards the high frequency direction with the increase of the peak value of the applied pulse voltage.On the basis of establishing and solving the model correctly,the influence of external environment factors and insulating material characteristics on the development process of air-polyimide surface discharge is further studied.The results indicate that the streamer propagation velocity is positively correlated with temperature,and negatively correlated with air pressure.The larger the secondary electron emission coefficient,the faster the discharge develops and the more the surface charge accumulates.Therefore,in order to effectively suppress the occurrence and development of surface discharge,it is necessary to dissipate heat in time and increase air pressure in actual operation.Accordingly,the insulation materials with small secondary electron emission coefficient should be chosen as a priority in high-frequency insulation design,or the secondary electron emission coefficient of insulation materials should be reduced through surface modification and other means to improve the insulation performance.