Research On Insulation Discharge Characteristics Development Law And Mechanism Of High Frequency Power Transformer

With the development of flexible HVDC transmission and distributed power supply,High Frequency Power Transformer(HFPT)has broad application prospects due to its flexible AC and DC voltage conversion and energy transmission.In order to reduce equipment size and increase work efficiency,HFPT insulation structure uses air-solid insulation materials.The HFPT working voltage waveform has the characteristics of high frequency(above 10 kHz),large amplitude and short rise time.At the same time,its volume is much smaller than that of conventional power transformers,which makes HFPT have higher electromagnetic coupling than conventional power transformers,the skin effect is obvious and the local temperature is high,causing the high frequency insulation to be prone to premature failure.For the actual working conditions of HFPT,it is necessary to study the discharge characteristics of gas-solid insulation under high-frequency sinusoidal electrical stress.The variation law of the high-frequency discharge characteristic parameters with the development of discharge is studied.Through the theory of charge accumulation and micro-measurement technology,the discharge mechanism of gas-solid insulation under high frequency is deeply studied.In this paper,a ball-plate electrode high-frequency partial discharge experimental platform is built to simulate the Partial Discharge(PD)process of slightly inhomogeneous electric field.The partial discharge test is performed using the platform at frequency of 30 kHz and amplitudes of 1.5 kV,2 kV,2.5 kV and 3 kV.The discharge signal is collected by the high-frequency pulse current method.The discharge signal is denoised by the sym8 wavelet basis function and the threshold value,and the discharge characteristic quantities are accurately extracted by the phase opening window method.The partial discharge spectrum,discharge amplitudes,discharge times and insulation life at different voltage amplitudes are obtained.By analyzing the change rule,the result shows that:The insulation life and voltage amplitude exhibit a negative exponential relationship under high frequency sinusoidal electrical stress.In different life stages,the average discharge times of single-cycle is basically unchanged,which is approximately equal to the average discharge times number of the entire life stage.While the discharge amplitude is unchanged significantly in different life stages.Two-dimensional phase distribution of discharge times appears conical,and cone top phase moves to the left as the voltage amplitude increases.There is no significant difference in partial discharge and duration of gas-solid insulation at different damage stages under high-frequency electrical stress.A needle-plate electrode high-frequency surface discharge experimental platform was established to simulate the surface discharge process at the highly inhomogeneous electric field.The sinusoidal voltage frequency of 30 kHz and amplitudes of 8 kV,9 kV and 10 kV were used to simulate the accelerated surface discharge process.The high-speed camera was used to capture the development of surface discharge,which through corona discharge-streamer discharge-flashover.The flashover shape is linear,no spark discharge phenomenon,and the damage of the film by high-frequency surface flashover is much greater than the power frequency electrical stress.Statistical analysis of the discharge data shows that the surface discharge shows a strong polarity effect,and discharge times occurring at the polarity reversal are the highest,the discharge amplitudes are large,and the damage to the insulation is more serious.Scanning Electron Microscope(SEM)and Energy Dispersive Spectrdmeter(EDS)tests were carried out on the surface discharge areas at different damage stages.It was found that the discharge caused the surface of the material to have protrusions and"crystalline solid particles".And with the deepening of the material damage.the material surface physicochemical defects such as solid particles and protrusions increase,and the trap density and energy level increase.Those result that the discharge times and discharge amplitudes increase with the material damage deepens.