Analysis Of Discharge Characteristics And Influencing Factors Of Negative Streamer In Non-uniform Field Short Air Gap

With the vigorous development of UHV transmission technology in China,the stable and safe operation of power grid has been paid more and more attention.The safe operation of the power grid depends largely on the operation state of the transmission and distribution equipment.The operation safety and insulation characteristics of the transmission and distribution equipment exposed to the air for a long time are very important to the safe and stable operation of the power system.Under the influence of the external environment,transmission lines and electrical equipment will have insulation defects,which will cause local electric field distortion to form uneven electric field,thus causing local short gap gas discharge,and even the breakdown of the entire insulation medium,affecting the normal operation of the power system.The causes of gas discharge are complex.Streamer discharge is an important initial stage of air gap discharge,which has a significant impact on the subsequent discharge development process.In order to study the development process of streamer discharge,a simulation model of short air gap negative streamer discharge under non-uniform electric field is established by using fluid mechanics method.The development characteristics of negative streamer discharge under different influencing factors are compared and analyzed,and the breakdown characteristics of streamer discharge are studied by negative streamer discharge test.The main work of this paper is as follows :(1)The process of electron generation and disappearance in negative streamer discharge is described.The electron transport parameters such as impact ionization coefficient,attachment coefficient and electron mobility in streamer discharge are calculated by Boltzmann two-term approximation theory and compared with the existing research results.The results show that the electron transport parameters are mainly affected by the electric field strength.With the increase of electric field strength,the impact ionization coefficient increases,the attachment coefficient increases first and then decreases,the electron mobility decreases,and the diffusion coefficient increases.(2)The hydrodynamic simulation model of negative streamer discharge in 1cm’needle-plate’ non-uniform field air gap under step voltage is established,and the development process of negative streamer discharge is analyzed.The simulation model of this paper is effectively verified by comparing with the research results of existing literature and carrying out negative streamer discharge test under the same conditions.During the development of negative streamer discharge,the electric field intensity,electron concentration and positive ion concentration of streamer head decrease first and then increase with the development of streamer.The negative ion concentration continues to increase with the development of streamer.The average velocity of streamer development is about 2.09mm/ns,and the radius of streamer discharge channel is about 3mm.(3)The effects of applied voltage,needle electrode curvature radius and air relative humidity on the development characteristics of negative streamer are calculated and analyzed.With the decrease of the rise time of the applied voltage and the increase of the applied voltage amplitude,the electric field intensity and electron concentration of the streamer head increase,and the average velocity of the streamer development and the amplitude of the streamer discharge current increase.The smaller the radius of curvature of the needle electrode,the larger the electric field intensity and electron concentration of the streamer head,the smaller the amplitude of the streamer discharge current,and the larger the average velocity of the streamer development.The influence of air relative humidity on negative streamer discharge is complex.When the air relative humidity is small,the influence of air relative humidity on streamer discharge is small.When the air relative humidity exceeds 60%,the influence of air relative humidity on streamer discharge characteristics is intensified.With the increase of air relative humidity,the streamer discharge current increases,but the development speed of streamer discharge changes little.(4)A photoelectric joint observation platform for negative streamer discharge test in’needle-plate’ air gap is built to realize high-precision measurement of streamer discharge current.The effects of needle electrode shape and ’needle-plate ’air gap distance on negative streamer discharge characteristics are studied experimentally.It is found that the shape of needle electrode has a great influence on the breakdown voltage and current of negative streamer discharge.The breakdown voltage of cone electrode is the highest,and the current of hemisphere electrode is the highest.As the ’needle-plate’ air gap distance increases,the breakdown voltage of the negative streamer discharge increases,and the amplitude of the streamer discharge current increases.