Research On Key Issues Of Multi-conductor Transmission Line Modeling Considering Corona Effects And Calculation Of Lightning Overvoltage

Lightning accident is one of the important causes of power supply interruption in the power system.Supported by the National Natural Science Foundation project "Fast Time Domain Electromagnetic Transient Simulation Technology of the Large Scale Multi-conductors System"(Grant No 51407073),the issue of lightning overvoltage in multi-conductor transmission line with corona effect is studied.The main research achievements are presented as follows:1.A method for fast evaluation of the Cooray–Rubinstein formula for calculating horizontal electric field around lightning channel in time domain is proposed.With the help of integral form of the modified Bessel function of the first kind,the kernel function of the ground surface impedance is expressed to exponential function,and a recursive convolution is achieved,which makes the calculation of the Cooray–Rubinstein formula in time domain much more efficient.Compared with the vector fitting method,because the proposed method is completely based on the analytical formula of the impedance of the ground surface,it makes up for the deficiency of the uncertain error introduced by the vector fitting due to the truncation of the spectrum,and has the advantage of artificial controllability in time domain fitting error.What's more,the horizontal electric field at different electrical conductivity and different distances are calculated by using the proposed method.By comparing the results with the exact Sommerfeld integral method,the limitation of the C-R formula is pointed out.2.Based on the corona sheath model and combined with multi-conductor transmission line telegraph equation,a nonlinear improved telegraph equation for multi-conductor transmission lines considering corona is established.With the help of the weighted residual technique,the time domain finite element method for solving the transient response of multi-conductor transmission line considering corona is derived.The experimental example in existing literature is used to verify the proposed method,and the results show the correctness of the model and the solution method presented in this paper.3.Combined with Agrawal field-to-line coupling model,an improved Agrawal field line coupling model considering corona is proposed.Based on proposed model,the induced lightning overvoltage of three-phase overhead distribution line system considering corona effect is calculated,analyzed and discussed.In addition,this paper points out that some existing literatures are inappropriate for calculating the lightning induced overvoltage by using the full voltage as the corona onset criterion,and analyzes the different mechanisms of the corona around the lines caused by the direct lightning and the induced lightning.It is concluded that the scattered electric field should be used as the corona onset criterion.