Study On Overvoltage Of Transmission Line Under Multi-pulse

Accidents caused by lightning occurred in transmission lines.The traditional research on transient overvoltage about transmission lines is based on single pulse lightning current source,while the percentage of multi-pulse in natural lightning is up to 70%,and the single pulse is far less than that of multi-pulse in terms of duration,energy,charge and oscillation degree.Therefore,it is necessary to study the transient overvoltage of transmission lines.The paper proposes a multi-pulse equivalent model based on five factors,multi-pulse peak lightning current and lightning current steepness,up to peak time,charge transfer of Q and the total duration of the multi-pulse,and puts forward the graphic method and numerical analysis method to calculate the parameters of the model,and finally gets the model and parameters of multi-pulse of lightning current.Because the transient overvoltage of lightning is divided into two situations:the coupling overvoltage caused by lightning induction and the overvoltage caused by direct lightning,this article through the coupling experiment of multi-pulse transmission line,obtained the actual overvoltage oscillation waveform from the experiment.The angle actually restores the coupling voltage generated by the multi-pulse to the transmission line.Through the analysis of experimental phenomena and data,it is found that the results of multi-pulse and single-pulse coupling experiments vary greatly in terms of duration,energy,charge amount,and oscillation degree.A complete simulation model is built by PSCAD/EMTDC simulation software,including five parts,the lightning current model,the tower model,the line arrester model,the insulator string flashover model and the transmission line model.On the basis of the simulation model,the single pulse and multi-pulse overvoltage under three types of lightning strike faults of lightning strike to tower,lightning strike to arrester and lightning strike to conductor are studied.By comparing the differences of overvoltage oscillation waveforms caused by different direct lightning source,this article concludes the following:whether it is bypassing or counter-attacking overvoltage,the overvoltage amplitude caused by multi-pulse lightning is greater than the single pulse lightning’s though they have the same amplitude,and the duration and degree of oscillation is much greater than the single pulse,which is consistent with the experimental results.Multi-pulse transmission line coupling experimental data and multi-pulse direct lightning over-voltage simulation of transmission line data are used in overvoltage online monitoring.Neural network algorithm is used to train and verify the experimental model data.The mean square error is only MSE=0.02496.Then the output and expectation of training and testing stage are combined to verify the stability.The results of the fitting show that both the output and the expectation similarity are both high and the stability is good.Through multiple tests in the Matlab program,the correct rate is more than 90%,and the result is basically reliable.