Study On Insulation Modification And Reliability Evaluation Of Polypropylene Film

Polypropylene(PP)films are widely used in the high energy density capacitors due to their excellent electrical properties.However,polarization and non-uniform electric field both have great impact on the dielectric reliability of the films,which affects the operating stability and reliability of capacitors.The effects of polarization/depolarization,inhomogeneous electric field and nano-MgO particles on the dielectric reliability,space charge,trap distribution and breakdown characteristics of polypropylene films are studied in this paper.The polarization/depolarization experiments of polypropylene films with different thickness were carried out by polarization/depolarization current method at the electric field strength of 20kV/mm.The space charge accumulation of polypropylene films with different polarization/depolarization degrees and different nano-MgO compositions were measured and analyzed by pulse method(PEA).The trap densities and depths of polypropylene films with different polarization/depolarization degrees and different nano-MgO compositions were measured by isothermal surface potential attenuation method.DC breakdown experiments were carried out using different electric field structures and electrode spaces.The breakdown voltage and current were recorded.The dielectric constant,dielectric loss and bulk resistivity of polypropylene films with different polarization/depolarization and nano-MgO were measured.Canny operator and gray level co-occurrence matrix were used to analyze the breakdown region quantitatively to reveal the breakdown characteristics of polypropylene film.The results show that the polarization/depolarization and inhomogeneous electric field have significant effects on the breakdown characteristics of PP films.With the increase of polarization/depolarization degree,the dielectric constant,dielectric loss and polarization current show an increasing trend,and the space charge accumulation process becomes more serious,while the density and the depth of deep traps also increase,causing the accumulation and detrapping of space charges becoming more difficult.With the increase of electrode space and electric field uniformity,breakdown voltage,breakdown area and angular second-order matrix(ASM)show an increasing trend.In addition,the breakdown voltage,breakdown current,breakdown area and angular second-order matrix decrease with the increase of polarization/depolarization degree.The dielectric constant and dielectric loss of the modified PP film increase with the increase of nano-MgO content,while the bulk resistivity increases first and then decreases.The accumulation of space charge tends to decrease first and then increase.The density and depth of deep traps increase first and then decrease.The breakdown voltage,breakdown current,breakdown area and angular second-order matrix all show the trend of increasing first and then decreasing,and the best performance is obtained when the content of nanometer magnesium oxide is 1%.The study on the reliability and insulation modification of polypropylene film is helpful to improve the dielectric stability and reliability of polypropylene film,which can improve the stability and safety of power capacitor on-line operation.