Study On Lightning Overvoltage And Protection Of UHVDC Transmission System

With the rapid development of China’s national economy,China urgently needs to vigorously develop high-capacity,long-distance UHVDC transmission technology.At present,with the increasing height of the towers,the growing transmission distance and the positive and negative bipolar erection of the same tower,the UHVDC system is facing serious lightning disasters,and the proportion of tripping accidents caused by lightning strikes to the total number of accidents is increasing every year,which seriously threatens the safety and reliability of the power system.This is a serious threat to the safe and reliable operation of the power system.Therefore,it is of academic significance and practical application to analyze and protect against lightning overvoltage in UHVDC transmission.The main research contents of this paper are as follows:For DC transmission system based on the lightning overvoltage model and simulation research,this paper establishes an electromagnetic transient model for the AC side and DC side of converter stations under lightning intrusion waves for±800k V DC transmission projects,including the lightning current model,the tower model,insulator flashover model,converter valve model,the DC side equipment（including flat wave reactor,filter,voltage divider,the isolation knife switches,wear casing wall,etc.）and a variety of lightning arrester model,etc;determines the lightning current amplitude,the mode of operation and the level of insulation of electrical equipment to lay the foundation for carrying out lightning overvoltage and protection.This paper conducts a computational study of lightning intrusion waves on the DC side of the converter station,including the study of the lightning overvoltage generated by each equipment in the station under the winding DC pole line,grounding pole line and metal return line,the study of its maximum voltage amplitude and distribution,and the analysis of the degree of impact on overvoltage after changing the lightning strike location,operating polarity,and bypassing the lightning current amplitude.This paper studies the overvoltage amplitude and its voltage distribution characteristics generated by each device in the station under the counterattack DC pole line and metal pole line,and on this basis analyzes the impact of the counterattack location and impact grounding resistance on the degree of lightning overvoltage of each device on the DC side of the converter station.The study shows that the Fdc1 type surge arrester under the counterattack pole line exceeds its nominal current and needs to increase the number of parallel branches of the surge arrester;the polarity of the pole line working voltage has a greater impact on the overvoltage of the DC side equipment.In this paper,the AC side lightning intrusion wave of the converter station is calculated and studied,and an AC side electromagnetic simulation model is established according to the AC side layout and electrical main wiring of the converter station,to analyse the overvoltage level generated by each equipment in the station under various operation modes of the bypass transmission line;the impact of different inputs of AC filters and AC PLC on the equipment lightning overvoltage is studied.It can be found that the input of AC filter and AC PLC can improve the lightning protection capability on the AC side.This paper conducts research on lightning overvoltage protection for DC transmission systems,studying the effects of different arrangements of flat wave reactors,the number of groups and parameters of DC filters in operation on equipment overvoltage,finding that unbalanced flat wave reactor arrangements are more conducive to lightning protection,suggesting that in lightning protection calculations,a single filter group in operation is used for calculations,and that the main capacitance C₁of the DC filter has the greatest effect on lightning overvoltage magnitude.