Study On Three-dimensional Dynamic Model Of Streamer And Lead Discharge Under Positive Polarity Lightning Impulse Voltage

With the construction of UHV AC and DC transmissio n projects,the design of insulation gap is still based on experimental methods.Understanding the physical mechanism of discharge and solving the insulation coordination are still problems and challenges in high voltage engineering.The computational high voltage engineering based on plasma physics and high voltage insulation technology has also been developing in recent years.At present,Therefore,it is urgent to carry out quantitative and physical model research of long gap discharge.Carrying out the modelling research of long air gap can not only optimize insulation design,but also lay the foundation for the research of lightning physical mechanism.Based on positive streamer and leader discharge,the model of two discharge processes and their influence factors are explored in this paper.Firstly,a 3D dynamic simulation model of the streamer discharge under positive polarity lightning impulse voltage considering humidity and pressure is established.The model is based on the calculation of geometric electric field in the whole three-dimensional space and the charge field in the streamer channel.The charge variation in the streamer branching is described by the law of charge conservation.A3 D streamer branching model considering air pressure and humidity is established,and the photoionization rate of the streamer head is used as the quantitative criterion of streamer branching.Secondly,the influence of different absolute humidity and air pressure on the characteristics of streamer discharge and its mechanism are studied.The model parameters such as streamer initial diameter,initial number,channel conductivity,reduced field strength,partial pressure of oxygen and partial pressure o f water vapor are estimated under different conditions.The 3D linear charge density distribution,electric field intensity distribution and other electrical parameters are obtained at different humidity and different humidity,which are in good agreement with the experimental results.The effects of humidity and pressure on the streamer propagation and branching characteristics are studied,and the influencing mechanism is revealed.The results show that the electric field of streamer channel with higher humidity is enhanced,but the propagation velocity is reduced,and the content of water molecules will change the electric field distribution and photoionization rate.Increasing the pressure results in increased electric field of streamer channel,and the smaller photoionization rate leads to more streamer branching number under high pressure.Thirdly,a 3D dynamic model of leader discharge under positive lightning impulse voltage is established.The model is based on the calculation of the geometric electric field in the whole three-dimensional space,the charge field of the streamer and leader channel.The strength of the electric field in space determines the propagation and branching of the streamer.The leader channel uses a simplified heating model based on Joule’s law,,in which heating coefficients and cooling coefficients are introduced to describe the transition mechanism of the discharge channel.Combined with the space electric field,the conductivity of all channels in space is calculated.Finally,the positive leader propagation is simulated,and the leader discharge characteristics under different simulation parameters are studied.The 3D distribution of conductivity,spatial charge and electric field during positive leader discharge propagation are obtained,and the discharge mechanism is analyzed.The effects of electrode parameters,conductivity(heating and cooling coefficient)and initial leader radius on the leader discharge characteristics are studied.The results show that the larger heating coefficient leads to faster streamer-leader transition and greater propagation velocity.When the cooling coefficient increases,the injected charge increasesin short time,and the leader propagation velocity also increases.With the increase of initial leader diameter,the conductivity of leader channel becomes larger,and the propagation velocity increases accordingly.