Research On Partial Discharge Characteristics In Gas-gap Of Oil-paper Insulation Under Complex Voltage

As the core equipment of the ultra-high voltage direct current(UHVDC)transmission system,the converter transformer’s safety and reliability are particularly important.Oil-paper insulation,as the main insulation form of converter transformers,is subjected to the combined action of electricity,heat and force during long-term operation,which inevitably produces concentrated insulation defects such as air gaps and cracks.The electric field is concentrated at the defect,which causes partial discharge,and finally leads to insulation breakdown.On the other hand,the voltage conditions that the converter equipment bears are complex,not only single AC and DC voltages,but also other complex voltage waveforms such as AC and DC composite voltages,pulse voltages,harmonic voltages,etc.Different voltage conditions will affect the electric field distribution and space charge distribution at the defect,and then affect the discharge characteristics.Therefore,it is meaningful to research the partial discharge characteristics of the air gap model under different voltage waveform characteristics.In order to obtain the discharge characteristics of the oil-paper insulation air gap and its changing law under complex voltages,this paper studies the influence of the waveform characteristics of the applied voltage on the space charge distribution of the oil-paper insulation air gap,electric field changes and space charge changes during the discharge process.And then analyzes the partial discharge discharge characteristics.The main research contents of the paper are as follows.(1)Build the partial discharge test platform of the internal air gap of oil-paper insulation,obtain discharge PRPD spectrum and discharge characteristic parameters by pulse current method.Analyze the change of the air gap voltage during the discharge and various factors affecting discharge characteristics.(2)Research the partial discharge characteristics of the oil-paper insulated air gap model under different voltage change rates,repetition rates,and amplitude trapezoidal waves,analyze the difference in the air gap electric field distribution and space charge distribution under different voltage waveform characteristics,and further obtain the influence of the voltage change rate,repetition rate,amplitude on the development process of partial discharge,and analyze the influence of these parameter changes on the characteristic quantities of partial discharge(discharge amplitude,discharge repetition rate,time interval,phase,etc.)(3)Research the air gap discharge characteristics under power frequency,high frequency AC,harmonic and DC voltage.Combining the characteristics of discharge,discharge start/stop voltage,and φ-?t-V spectrum analysis,study the influence of AC voltage conditions on discharge development and analyze the mechanism.From the perspective of the comprehensive effect of the air gap voltage and the electric field of the electric charge,the influences of the voltage condition on the discharge characteristics of the internal air gap is analyzed.The voltage amplitude mainly affects the residual background charge in the first half of the week and the amount of accumulated background charge in the current half,and then affects the discharge characteristics.The voltage frequency affects the residual background charge,the amount of accumulated background charge,and the charge distribution,and causes difference in variation rate of original electric field and charge electric field,then affects the discharge characteristics.(4)Based on the microscopic particle reaction and hydrodynamic model,built a microscopic numerical model of the air gap discharge inside the oil-paper insulation,and the movement and distribution of the space charge during the discharge process are initially obtained.Based on the three-capacitor equivalent model,proposing an improved model,which fully considering the change of the air gap resistance and the influence of space charge on the development of the discharge.Calculate and analyz the voltage distribution and voltage changes during the development of the air gap discharge and the general formula of partial discharge characteristic parameters are derived and compared with the test value.The researches above helps to understand the microscopic mechanism of air-gap discharge and analyze the role of space charge in the discharge process.At the same time,it can be concluded that the change in voltage waveform characteristics affects the discharge characteristics such as the air gap discharge spectrum and discharge characteristic parameters,which provides certain guiding significance to the internal insulation of the converter equipment.