Research On The Insulation Properties And Creeping Discharge Evolution Process Of Polyimide Under High Frequency Stresses

Power electronic transformer(PET)present a feasible router to realize energy interconnection,dispatching and control in the future smart grids,as well as facilitate flexible voltage conversion between AC and DC,and between different DC voltage levels.PET renders a preferable application prospect in reliable integration of large-scale renewable energy,especially suitable for occasions with special requirement on volume and weight of the electric energy converter.High frequency power transformer(HFPT)is the core component for electrical isolation and voltage level conversion in a PET.Under high frequency(HF)electrical stresses,the inter-turn,inter-layer,inter-phase and ground insulation,together with the insulation along the gas-solid interfaces,will encounter a series of problems,such as hotspots caused by severe power losses,partial and creeping discharges,premature insulation failure,etc.Moreover,the insulation strength in HFPT may be affected by the coupling effects of multi-factors,including voltage parameters(waveform,frequency,amplitude,rise time,etc.)and temperature-rise.to promote the development of HFPT towards high voltage,large capacity,high frequency and miniaturization,there is a good motivation to systematically study the partial and creeping discharge characteristics,plus discharge induced insulation life behavior and the underlying physical mechanism under HF stresses,which shows prominent theoretical value and guiding significance for insulation design and operational performance evaluation of HFPT.An experimental platform for accelerated aging tests under HF stresses and a discharge signal processing unit was established,resorting to which the impact of voltage frequency on partial discharge(PD)characteristics and insulation life of polyimide films was investigated in details.The PD inception voltage(PDIV),phase resolved PD(PRPD),discharge amplitude as well as repetition rate were all recorded in accordance.It was found that,the PD number per cycle and PD amplitude both increase in the first stance and decrease afterwards as the voltage frequency increases,demonstrating a frequency induced inflection point phenomenon.Thereafter,based on correlation analyses between the space charges and the PD statistical characteristics,also combined with frequency-induced thermal effect and the frequency impact on space charge dissipation process,the influence mechanism of temperature-rise on PD behavior was thereby proposed,and a theoretical model and formation mechanism of the inflection point phenomenon were put forward.The variation of PD behavior and insulation life with regard to different HF voltage waveform,rise time and polarity were studied,in which five types of voltage waveforms in the same frequency of 1 kHz were adopted,namely sinusoidal wave,triangular wave,bipolar square wave,unipolar square wave and damped pulse wave,plus bipolar square waves with different rise times.According to the above results the equivalence between different HF waveforms and between power frequency and HF voltage were established in terms of insulation life test and PD test,including PDIV,average PD amplitude,PD number per cycle and total PD amplitude per cycle.respectively.In addition,the effect of voltage waveform parameters on the aging rate was quantified in terms of peak-peak factor,RMS factor and shape factor.Based on orthogonal experiment design theory,the insulation life of polyimide films was measured under different voltage frequencies,amplitudes and temperatures,i.e.in the range of 5.30kHz,1.5?3.0 kV and 13?180?,respectively.The impact of these three key factors as well as their inner-interactions on the aging rate were also discussed based on range analysis,variance analysis and correlation graph.According to this scheme,Both primary and secondary sequence of the three influential factors were determined,including the interactions among them.Also,the effect of the factor level variation on the tolerance coefficients to the electrical and thermal stresses was obtained,together with the accelerated aging factor under a single factor or synergistic effect of the three factors,which also demonstrated the actual interactions between the three factors.Finally,the influential coefficients of the three factors plus their interactions on insulation life were calculated using matrix method,and accordingly a generalized multi-factor model for insulation lifetime prediction was proposed,which incorporated the coupling effects between different influential factors,and was validated by designated experiments.The needle-to-bar electrode arrangement was used to study the influence of voltage frequency and temperature on the creeping discharge dynamics,namely inception voltage,flashover voltage and failure time along the gas-solid insulation surface.Further,the discharge behavior at different stages of the creeping discharge was analyzed,including the discharge pattern,phase resolved discharge and the statistical characteristics,which revealed the polarity effect on the phase resolved discharge during the positive and negative half cycle of the applied voltage.According to the scenario of the creeping discharges,some unique features were given to characterize the development stages of the creeping discharge and the imminent flashover.Apart from the discharge behavior,surface properties of the insulation material at different stages were also analyzed,including surface charge distribution,surface morphology and characteristic functional groups,based on which the evolution process of the creeping discharge along the gas-solid insulation surface was featured and revealed,together with elucidation of the interaction mechanism between creeping discharge and surface properties of the insulation material.Also,the causes for polarity effect regarding the positive and negative half cycle were clarified through influence analysis of the surface charges on electric field distribution along the gas-solid surface.A simplified but effective set of reactions was used to describe all the particle reactions in air discharges.On this basis,a numerical model for both partial discharge and creeping discharge was proposed with fluid dynamic theory,in which the transport equations of different particles,Poisson equation and plasma chemistry reactions in the discharge gap or channel were involved,along with the reaction process and accumulation of the charged particles on the dielectric surface.Then,the partial discharge process under the sphere-to-plane electrode,and the creeping discharge evolution under the needle-to-bar electrode were investigated by simulations.Distribution and variation of particle density,surface charge density and electric field versus discharge time were obtained.Accordingly,the interactions among partial discharge,surface charge accumulation and space charge distribution were discussed,together with the effects of voltage amplitude and frequency on discharge current.Also,the microscopic physical mechanism in creeping discharge evolution process was studied and revealed,as well as the influence mechanism of temperature,pressure,and the secondary electron emission on surface streamer development.Crossed verification and validation were also performed between the simulation results and the experimental ones.