Research On Air-gap Discharge Characteristics And Space Charge Effect Under AC,DC And AC-DC Combined Voltage

Air gap discharge is a common phenomenon of air insulation defect in converter equipment.During the discharge process,charged particles are generated,and the particle microscopic process constitutes the basis of gas discharge.Meanwhile,the transient space charge and background space charge influence the characteristics of air gap discharge.On the other hand,voltage condition of converter equipment is complex,so the air gap may withstand direct-current(DC),power frequency alternating-current(AC),harmonic and AC-DC combined voltages.Different voltage conditions affect the space charge distribution and then the discharge characteristics.To obtain the characteristics and variation rules of air gap discharge under complex voltage,this paper proposes to study the charge distribution and calculate the space electric field from the microscopic perspective.Then analyze the space charge effect in the discharge process and analyze the influence of voltage conditions on the discharge characteristics.Based on the microscopic numerical model analysis,the space charge and electric field variations in air-gap discharge transient process are studied.The discharge pulse and charge characteristic parameter synchronous measurement are proposed.It is also proposed to study the interaction of space charge and discharge process,and to analyze charge electric field effect degree quantitatively by multi-parameter synchronous space charge and electric field analysis.Then on the basis,the effects of voltage conditions,such as amplitude and frequency,on discharge characteristics are studied under AC/DC and combined voltages.The main contents of this paper are as follows.(1)Micro-numerical model of needle-plate air-gap discharge based on microscopic particle reaction and hydrodynamics is proposed.The microscopic processes of positive polarity streamer discharge and negative polarity Trichel discharge are described.Based on electrostatic induction,the principle of discharge pulse is analyzed by analogy with the transport current formation.(2)The correlation between charge and discharge pulse,charge characteristic parameters are established.Temporal variation analysis based on synchronous multi-parameter is proposed to study the interaction between space charge and discharge.The quantitative effect degree of charge electric field is studied based on the multi-parameter synthetic electric field analysis.Synchronous measurement method of discharge pulse and background charge voltage is proposed.(3)Reasearch the stage characteristics of the DC voltage needle-plate air-gap discharge under transient and background charge.Research the effects of transfer charge under positive polarity and corona background charge under negative polarity.And the mechanism of charge effect in different discharge stages are analyzed.(4)Research the process of power frequency AC discharge.Based on the quantitative calculation and analysis of the needle-pole synthesis electric field,the initial discharge electric field intensity is estimated,and the degree of electric charge effect is analyzed.By pulse-charge parameter synchronous time domain analysis,the first half-cycle residual background charge affects the second half-cycle initial discharge characteristics,and accumulated background charge affects discharge characteristics of own half-cycle.On the basis,influence rules of AC voltage conditions are studied and explained based on charge characteristic quantity,discharge starting/ending voltage and φ-?t-V spectrum analysis.Voltage amplitude mainly affects the charge quantities of residual background charge quantity from the privious half cycle and the accumulated background charge of own halfcycle,and then affects the discharge characteristics.Voltage frequency affects quantities of residual background charge and accumulated background charge,affects charge distribution,and causes difference in variation rate of original electric field and charge electric field,then affects the discharge characteristics.(5)The air-gap discharge characteristics under combined voltage is studied,and effects of residual and accumulated background charges are analyzed.Then the influence rules of AC/DC proportion are obtained.Under the combined voltage,with same external voltage peak,DC component proportion affects the charge quantities of the residual and accumulated background charges,then affects the discharge characteristics such as the pulse amplitude V and discharge start/end voltages.Research results show that the frequency of AC component has little influence on the discharge characteristics.(6)The ball-plate air-gap discharge characteristics are studied,and the influences of AC voltage conditions are analyzed.Comparing the needle-plate air-gap discharge,it can be concluded that when multiple factors affect,such as the charge quantity,if the effect degree of each factor is different,the whole charge effect may be different.The influence factors of oiled-paper internal air-gap discharge are analyzed.In addition to charge effect,the variation of air-gap voltage with discharge process and voltage frequency also affects.Then,the influences of voltage conditions on internal air-gap discharge characteristics are analyzed from the perspective of synthesis effect by air-gap voltage and charge electric field.The researches above are helpful to understand the micro-mechanism of air-gap discharge,to analyze the influences of space charges in the discharge process.Meanwhile,characteristics of air-gap discharges,such as the initial voltage,are obtained under AC,DC and combined voltage,so as to provide guidance for the internal insulation design of converter equipment.