| The establishment of modern energy system with clean and low-carbon,and the realization of the large-scale grid connection for renewable energy and the interconnection of AC and DC systems are the major national strategies.Their safe operations and rapid developments are the strong guarantee for the sustained growth of national economy and the improvement of national comprehensive strength.A new type of power electronic transformer that realizes the grid connection for renewable energy and the interconnection of AC and DC systems is proposed,which is suitable for occasions with special requirements for the weight and volume of electric energy conversion equipment.High-frequency transformer is the core component of power electronic transformer to realize electrical isolation and voltage conversion.It is connected with high-power electronic converter and the winding is subjected to the repetitive square wave voltage with short rise time,large amplitude and high frequency(400 Hz-20 k Hz)for a long time.The frequent high-frequency stresses causes insulation problems such as partial discharge,severe heating,and premature insulation failure.At the same time,the damage degree of solid insulation under high-frequency is much higher than that of oil-paper insulation.It restricts the development of high-frequency transformer to high voltage,high power,small size,and high frequency.In summary,high-frequency discharge has become a key scientific problem that needs to be solved urgently.And also,the discharge characteristics and mechanisms of oil-paper insulation under high-frequency stress are still unclear,and the related theories are also lacking.It is difficult to achieve the quantitative calculation and analysis of discharge under high-frequency stress for oil-paper insulation,which brings severe challenges to the insulation design and development of high-frequency transformers.Therefore,combined with the experiment and theoretical analysis,this paper attempts to clarify the discharge characteristics and mechanism of oil-paper insulation,explore the influence of charge transportation on the discharge of oil-paper insulation,and reveal the inherent nature of the deterioration of oil-paper insulation and solid insulation under highfrequency stress based on the theories of hot electron and charge trapping and de-trapping.Firstly,the methods of pulse current and ultra-high frequency current transformer were utilized to carry out the partial discharge test of oil-paper insulation under high-frequency stresses.The partial characteristics and influence mechanism were explored.It can be found from the experiments that a frequency-induced inflection point effect is existed.In other words,the max PD charge,the average PD charge,PD current,PD power and PD amplitude first increase and then decrease with the increase of frequency.These results might contribute to the combined effect of the heating effect caused by frequency and the charge transport.An extreme point is existed in discharge repetition rate with the increase of the amplitude of applied voltage,which is related to charge transport of high-field strength.Besides,the discharge voltage of oil-paper insulation under high-frequency stresses is increased by about2.6 times that of solid insulation.The differences in carrier mobility caused by the change of trap distribution are the fundamental reason for the different discharge characteristics of oilpaper insulation and solid insulation under high-frequency stresses.It is known from the above experiments,the discharge characteristics of oil-paper insulation under high-frequency stress are related to the charge transport.Therefore,surface charge tests of oil-paper insulation under high frequency stresses are carried out and the characteristics of surface charge distribution and trap distribution are studied.It can be found that the relationship of initial surface charge and frequency follows inverse power relations and the decay rate of surface potential shows Gaussian distribution.On that basis,the number of traps in oil-paper insulation is quantitatively calculated and the changing rules between the number of traps and the frequency is summarized.The changing rules of trap between oilpaper insulation and solid insulation under high frequency stresses are compared and analyzed.The influence of charge transport on the discharge characteristics of oil-paper insulation was explored from the mechanism of initial electron generation and the migration of carrier.The influence of shallow trap with high frequency on the mechanism of initial electron generation of oil-paper insulation was revealed and the concrete analytical expression of initial electron generation probability is derived.Besides,the influence of deep trap with high frequency on carrier mobility was clarified and concrete analytical expression of carrier mobility is obtained.Finally,different testing methods including optical microscope,scanning electron microscope,fourier infrared spectrometer,energy dispersive spectrometer and X-ray diffractometer,are utilized.The surface morphology,bond broken types of molecular chain and products types formed for oil-immersed Nomex paper after breakdown were studied.Then,the damage mechanism of oil-immersed Nomex paper between high-frequency stress and AC stress is compared and analyzed.It can be found that the branches of creepage are not existed in the oil-paper insulation during the entire discharge process under high-frequency stresses and their damage degree is higher than that AC stress,but lower than that solid insulation under high-frequency stresses.On this basis,the damage mechanism of high-frequency discharge for oil-impregnated Nomex paper was clarified,and the different mechanism of deterioration between oil-paper insulation and solid insulation under high-frequency stress was revealed.It can be found that the difference in carrier mobility caused by the change of trap density is the essential reason for the different mechanism of deterioration between oilpaper insulation and solid insulation under high-frequency stress. |
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