| The service experiences of transmission line shows that lightning trip-out accident share a large portion of accident of transmission line.However,limited by the experimental conditions and electromagnetic field theory development level,it is difficult to realize accurate study.In order to investigate the electromagnetic transient characteristics of transmission line more accurately,an accurate calculation model must be proposed.It is also the foundation of lightning protection theory.The research of line-tower-grounding system model provides the theoretical basis for electromagnetic transient characteristics research under lightning strike.The accurate calculation model is good for improving the stability of grounding system.And it also can reduce the waste of investment for lightning protection project.After the electromagnetic transient characteristics of transmission line were analyzed,the way of decreasing lightning overvoltage was put forward.This will provides a certain theoretical basis for the future lightning protection works.To analyze the electromagnetic transient characteristics of transmission line,a line-tower-grounding system model was established.The model based on the theory of moment method and electromagnetic field integral equation.It can take the wave process on transmission tower and current distribution characteristics of grounding system into consideration.Based on this new model,the paper also proposed an iteration method to calculate impulse grounding resistance.Research at the electromagnetic transient characteristics of 500kV transmission with cup-type tower indicates that soil resistivity has a big effect on impact response.Where the soil resistivity is high,the lightning current flows slowly in grounding system,current is more likely to flow into adjacent towers.The amplitude of lightning current does not change current distribution in line-tower-grounding system.Furthermore,front time of impulse current is primarily influence impact response of tower and grounding system,it has a very low impact on the overhead ground wire.High frequency components have a large proportion of front time of impulse current,and conductor inductance hinder the spread of high frequency current.Therefore,the rising rate of impulse voltage waveform at cross arm is fast and amplitude is very large.By analyzing the lightning overvoltage result at a 500kV transmission line with cup-type tower,lightning resistance level calculated by standard method was higher than actual value.Influence factors such as front time and amplitude of impulse current and soil resistivity were selected and the lightning overvoltage fitting formula was given.Based on the same parameter,a circuit model with wave impedance was built by ATP.Overvoltage at cross arm was 16.75%higher in CDEGS while current in grounding system was 8.3%lower.By contrasting ATP and CDEGS simulation results,circuit model would cause equivalent error.By using power-frequency grounding resistance as a guide to lightning protection reconstruction is incomplete.The design of grounding system was optimized,aiming at minimize the max value of lightning overvoltage.The material of grounding system was replaced by flexible graphite composite grounding material.And short grounding electrodes were added to grounding system.After reformation,the impact grounding resistance of a 500kV transmission line which located in high soil resistivity was reduced from 25.58? to 12.02?.At the same time,the lightning overvoltage was reduced from 2375.37kV to 2166.645kV.The measures to reduce lightning overvoltage have some particular consulting significance to actual project. |
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