Research On High Field Charge Transport Characteristics Of Polypropylene For Metalized Film Capacitor

Metallized polypropylene film capacitors are widely used as support capacitors for converter circuits,and are also one of the key equipment in converter valve halls of DC transmission systems.However,in the process of its service,it must withstand the combined effects of a high DC electric field and a high-temperature thermal field for a long time,which leads to the accumulation of space charge in the polypropylene film medium,accelerating the local field strength distortion and aging and even breakdown of the insulation,degradation of electrical insulation performance of polypropylene film and failure ultimately.The performance of the insulating dielectric film plays an important role in the use of capacitors.At the present,there are few studies on the characteristics of charge transport in polypropylene films.Under the electro-thermal coupling field,the regulation of charge transport in polypropylene films and the improvement of related performance needs further study.A polypropylene film with a thickness of 9?m was selected as the research object to test the conductance current characteristics and DC breakdown field strength characteristics under different electric fields and temperatures.The MATLAB model was used to establish the bipolar carrier transport model.Based on the model,the charge transport process and electric field distribution characteristics of polypropylene film under the electro-thermal coupling field were simulated.The contributions of the microcosmic parameters such as trap characteristics,injection barrier and the macroscopic parameters,such as temperature,applied electric field,to the spatial charge distribution and electric field distribution in polypropylene thin film.Through experiments,it is found that for the conductance test,after applying the electric field and temperature,the conductance current gradually decayed from a higher value with the passage of the pressurization time,and basically reached a steady state after one hour.When the current temperature is 20?and the electric field strength is less than 150k V/mm,the steady-state conductance current value increases linearly with the increase of the field strength.When the electric field strength reaches above150k V/mm,the material's strong field conductance current value increases significantly to 10^-8A magnitude,no longer maintain the original linear relationship.When the electric field strength is 70k V/mm and the temperature changes from 20?to 120?,the steady-state conductance current increases exponentially with the temperature.The conductance current at 120?is three orders of magnitude higher than that at room temperature.For the DC breakdown characteristics test,at room temperature,the DC breakdown field strength of the polypropylene film material can reach about 497k V/mm.Through the simulation of charge and electric field distribution of polypropylene film by bipolar carriers transport model,it is found that when the electric field strength is less than 150k V/mm,the trapped charge plays a leading role in charge distribution and electric field distribution,the process of charge trapping and detrapping mainly affects the transmission of carriers.At the lower trap level depth and density,charge density and electric field distortion rate in the medium are small,and space charge is relaxed mainly through bulk conduction quickly,which makes the materials have a higher bulk conductance current.At the higher trap level depth and density,internal charge density and electric field distortion rate of the medium are also small,and the injected charge is limited to the electrode-medium boundary at a few microns from the dielectric surface,and the depth of migration into the medium becomes shallow.When the electric field reaches 150k V/mm and above,the trap depth and density play a key role in a short time.With the increase of pressurization time,the injection and migration of charge into the medium increases significantly,and the degree of charge accumulation and electric field distortion increases gradually.Increasing a operation temperature will increase the carrier mobility,thereby reducing the charge density and electric field distortion rate,but will cause the material conductivity to increase slightly.Increasing a certain injection barrier will suppress the injection of charge,and at the same time reduce the conductivity of the materials.Through the simulation of DC breakdown characteristics,it is found that the increasing temperature and voltage gradually,resulting in a large increase in the injected charge density and migration depth.The combined effect of high temperature and high field for a long time can make the trapped charge in the medium more likely to fall out,which can participate in the carrier migration process and increase the leakage current density of the materials.At the same time,the collision and ionization process of a large number of space charges inside polypropylene can be intensified and the DC breakdown performance of polypropylene can be reduced significantly.