Research On Lightning Protection Characteristics And Measures Of Distribution Lines Based On Fractal Theory

Upon occurs the lightning,lightning induced overvoltage is the main cause of insulation flashover in transmission lines,and the power grid is becoming more and more complicated,which leads to more serious disaster.Therefore,it's essential to focuses on the research of lightning protection characteristics of distribution lines,in order to play a guiding role in the lightning protection design and lightning accident research of the initial stage and operation stage of distribution lines.With the study of domestic scholars,although there are many guiding models for the analysis of lightning and lightning protection assessment of the transmission lines,but how to get a more accurate description of the probability and statistics of the development of the lightning leader still needs further study.Since the reason above,this paper begins with the stochastic characteristics of lightning to explore a lightning stroke model that meets the physical process and develop a new lightning impulse withstand evaluation method provide theoretical basis for the lightning risk assessment and planning.This paper mainly consist the following three aspects:First,analyze and compare several lightning base current models,such as double exponential model,Heidler model and DU model.Double exponential is too simple,the Heidler model requires much when turns to computing and Du models has too many adjustable parameters to get the suitable parameters.Account for these deficiencies,this paper established a new base current model namely PM based current model which is simple and convenient to calculate,and besides,due to the adjustable parameters of the PM based current model are independent from each other,it is very convenient to debug and get the ideal waveform.Set the parameters using the lightning current measurement data of many scholars recorded,and the simulation results were in agreement with the measurement data.Second,the characteristics of the lightning discharge channel are studied,and the fractal characteristics of the discharge channel are confirmed.The law of the development of lightning leader is the result of uncertainty and random factors,so accurate discharge channel model must describe this two factors well at the same time,make this a criterion when establishing the fractal lightning discharge channel model.Fractal dimension can be used to distinguish different fractals,to ensure the simulation fractal model can simulatethe natural lightning discharge channel correctly,use box counting method to calculate the fractal dimension of fractal channel and natural lightning discharge channel.Test different probability developing index to make this two dimension index equal,the model is the lightning discharge channel model based on fractal theory.The lightning fractal channel model is established,based on this model the electromagnetic field around the lightning channel is calculated and verified.Third,the solution of the differential equation of electro-magnet field and induced voltage is studied,to avoid solving in the frequency domain and the large calculations.Using the finite difference method to discrete the Maxwell equations and Agrawal field line coupling equation in the times domain to get the solution,that's the numerical solution of induced voltage in transmission line.In order to make a further calculation and analysis of lightning induced flashover rate,some corresponding measures are put forward.