| Lightning is a dangerous natural disaster,which will pose a threat to our personal and property safety in serious cases.With the increasing social demand for electricity,the number and coverage of transmission line are also increasing,which is vulnerable to the damage of lightning,which leads to the power system paralysis.Therefore,it is particularly important to determine the location of lightning location quickly and accurately.Lightning burst is accompanied by abundant very low frequency（VLF）signals.Locating the location of lightning can be regarded as locating the location of VLF signal source.In two different VLF signal propagation paths,the phase and phase velocity of VLF signal propagation are different even if the propagation distance is the same,because VLF signal is affected by ionospheric anisotropy and ground conductivity.The phase and phase velocity of VLF vary greatly at different times of every day.In addition,different seasons,even at the same time,phase and phase velocity are not the same.If only the speed of light is used as the propagation speed of VLF signal,lightning location will produce errors,especially when the propagation distance is long.Therefore,according to the propagation model of VLF signal in the ground ionosphere waveguide space,this thesis proposes a lightning location correction technology based on the "full-wave" waveguide mode theory and the improved fruit fly optimization algorithm（MFOA）.The main work of this thesis is as follows:（1）Aiming at the problem of how to modify the propagation speed of VLF signal effectively,a waveguide model suitable for VLF signal propagation is constructed.By analyzing the characteristics and propagation features of VLF signal,the propagation type of VLF signal in the ground ionospheric waveguide is determined.Then the waveguide model is constructed,and the propagation speed of VLF signal is modified.（2）Aiming at the problem that VLF phase velocity is not calculated in WKB approximate solution of VLF waveguide mode field,a phase velocity calculation method based on the "full-wave" waveguide mode theory is proposed.By the method of piecewise numerical integration,the function of the whole propagation path is derived,and then the phase velocity formula of near-field region,far-field region and wave guide region is derived.After that,the numerical calculation program written in Visual Studio is used to calculate the accurate value of the average phase velocity.The results show that the corrected phase velocity is more accurate.（3）Aiming at the problem that lightning location algorithm is easy to fall into local optimum and has more iterations,an improved fruit fly optimization algorithm（MFOA）is proposed.The shortcomings of FOA algorithm are improved by adding adaptive step size and relaxation factor.The simulation results show that MFOA algorithm can effectively solve the problem of falling into local optimum and has many iterations.The simulation and analysis of the example show that after the propagation speed of lightning pulse signal is modified by VLF "full-wave" waveguide mode method and substituted into MFOA algorithm,the distance between the lightning position coordinates and the real lightning position coordinates can be within the error range,and the error is relatively small,which proves the effectiveness and practicability of the correction method in this thesis. |
