Effect Of The Lightning Striking To Mountain On Electromagnetic Field And Induced Overvoltages Of Multiconductor Overhead Lines

Studying the propagation characteristics and coupling effects of the lightning electromagnetic field under the mountainous complex terrain condition can provide important theoretical references for the revision of the lightning location algorithm and the lightning protection design of the power system lines.Based on the two-dimensional finite-difference time-domain method,this paper introduces conformal mesh technique to accurately simulate the mountain terrain,on this basis,firstly discusses the propagation characteristics of the lightning electromagnetic field in single mountain and multi-mountain terrain situations,and takes Kunming area of Yunnan province as an example to analyse the influences of the real terrain on the lightning electromagnetic field and the lightning location accuracy;secondly,based on the field-line coupling model,calculates the lightning induced overvoltages of the vertical and horizontal multiconductor overhead lines in the case of the lightning striking to a cone-shaped mountain,and discusses the effects of the mountain inclination and soil finite conductivity on the induced overvoltages on each phase line.The main conclusions of this paper are as follows:(1)In the case of the lightning striking to a cone-shaped mountain,the amplitude of the horizontal electric field increases with the increase of the mountain inclination,but when the inclination reaches 45°,it has no or little effect on the amplitude;the amplitudes of the vertical electric field and the horizontal magnetic field always significantly increase with the increase of the inclination.The increase of the amplitude of the vertical electric field is due to that the distortion of the electric field(electrostatic field)at the close distance(the main reason)and the reflection from the transition point between the mountain and the flat ground increase with the increase of the inclination.In addition,the vertical electric field at the transition and its vicinity is significantly smaller than that in the surrounding area due to the electrostatic shielding effect of the mountain.(2)When the lightning electromagnetic field propagates along the mountainous terrain,on the one hand,the amplitudes of the vertical electric field and the horizontal magnetic field at the observation point on the flat ground decrease significantly,the more mountains there are,the higher the mountain heights are,the more obvious the influence will be,andthe change of the amplitude is also related to the location of the observation point;on the other hand,the arrival time of the signal is also affected by the terrain,and it is found that the arrival time calculated from the path of the electromagnetic field propagating along the highest points of the terrain is smaller under the mountainous terrain condition,that is,the low mountains in the propagation path have little effect on the arrival time of the signal.In addition,taking Kunming area of Yunnan province as an example,the simulation results show that the real complex terrain will have a non-negligible influence on the propagation of the lightning electromagnetic field;further using the time of arrival(ToA)method to locate the lightning,compared with the flat ground,the arrival time delay of the signal resulting from the terrain fluctuation will cause greater positioning error.(3)In the case of the lightning striking to a cone-shaped mountain,the amplitude of the induced overvoltages at the midpoint and endpoint of each phase line in the multiconductor overhead lines of the vertical and horizontal structure increases significantly with the increase of the mountain inclination,which is mainly caused by the distortion of the electric field(electrostatic field)at the close distance resulting from the mountainous terrain.Shielding wires in multiconductor overhead lines(grounded at both endpoints with zero grounding resistance)have little effect on the amplitude of the induced overvoltages at the midpoint of each phase line,but significantly reduce the amplitude of the endpoint.The shielding effect of the shielding wires is mainly related to the structure of the multiconductor,in the vertical structure,the shielding wire has the best shielding effect on the adjacent phase line 3,and in the horizontal structure,the shielding line has the best shielding effect on the phase line 2 at the middle position.For any inclination,as the soil finite conductivity decreases,the amplitude of the lightning induced overvoltages at the midpoint and endpoint of each phase line in the vertical and horizontal structure increases significantly,and in the case of lower conductivity,the change of the inclination has more obvious effect on the amplitude of the induced overvoltages at the endpoint of each phase line.