Partial Discharge Characteristics And Multifactor Insulation Life Evaluation Of Polyimide Under High Frequency Electrical Stress

Power electronic transformer is a new type of power transmission and transformation equipment,because of its two sides of the voltage and current controllable,can arbitrarily adjust the power factor,improve power quality characteristics,has a very broad application prospects.High frequency power transformer is the core component of PET,which is responsible for realizing voltage level transformation.Due to its excellent dielectric properties,Polyimide(PI)is widely used in the interturn insulation and ground insulation of high frequency power transformers.Due to the severe local temperature rise under high frequency voltage,polyimide will be affected by the electrothermal coupling stress,partial discharge,premature insulation failure and other problems.In this paper,the partial discharge characteristics and insulation aging failure of polyimide under high frequency electrical stress are studied deeply,which provides a theoretical basis for improving the insulation strength of high frequency power transformers and has important theoretical significance and application value.In order to simulate the partial discharge process of polyimide in a slightly inhomogeneous electric field,an experimental platform of polyimide partial discharge under high frequency sinusoidal voltage was established.In view of the high frequency partial discharge is susceptible to interference,an improved VMD algorithm is used to decompose partial discharge signals.The decomposition number K and penalty factor a are optimized based on the balance optimizer algorithm.The effective components are determined according to the kurtosis criterion,and the wavelet threshold denoising is used to further remove the residual white noise of the reconstructed signals.Based on the experimental data of partial discharge at four temperature points of 25～100℃ and five frequency points of 10～50kHz,the influence of temperature and frequency on the partial discharge of polyimide was obtained.The results show that temperature and frequency are positively correlated with the characteristic quantity of partial discharge,such as maximum discharge amplitude,total discharge amplitude and discharge times,and the two are coupled and promoted.At the same time,with the increase of temperature or frequency,the phenomenon of "cluster" and "stratification" of discharge spectrum is gradually weakened.Based on the analysis of the influence of surface charge accumulation on the distribution of air gap electric field,the influence mechanism of temperature-frequency synergism was revealed.The polyimide insulation aging experiment was carried out at 3kV,30kHz and 100℃,and 5 groups of samples at different life stages were obtained.It is found that the partial discharge "clustering" phenomenon is gradually obvious with aging,and the discharge statistical parameters increase first and then decrease,with an obvious inflection point.The infrared spectrum and nano-indentation analysis of the sample showed that the ring opening reaction of imide ring occurred during the aging,and the ether bond between benzene ring also oxidized or fractured,and the mechanical properties of the material decreased accordingly.The surface morphology of the samples at different aging stages was observed.It was found that the cross section of polyimide changed into stratified structure with aging,and the gas produced by cracking reaction precipitated into a large number of bubble-like protrusions between layers.At the moment of breakdown,a large amount of gas is generated near the breakdown point and the upper surface is broken through by the gas and torn.Based on the orthogonal experimental design method,multi-factor aging experiments were carried out in the range of amplitude 2～4kV,frequency 20～40kHz,temperature 20～100℃ and relative humidity 25～75%,and Weibull distribution was used to calculate the insulation life of polyimide under each experimental condition.Based on range analysis and variance analysis,the main and secondary effects of amplitude,frequency,temperature and humidity on insulation aging rate were determined.Based on the single factor life model and Weibull life data obtained,a multi-factor aging life model of electric-heat-humidity combined with four factors was established,and the reliability of the model was proved by verification experiments.