Reason Analysis Of Voltage Drop In The Process Of Charging Startup Of Nan'ao HVDC Flexible Transmission System And Suppression Measures Study

The world's first multi-terminal flexible DC transmission project was put into operation in Nanao County,Guangdong Province in December 2013,which consists of three stations.Including ± 160 kV Shuguang Converter Station,± 160 kV Jinniu Converter Station and ± 160 kV Qingao Converter Station composition.The system charging start-up process by the 220 kV plastic city substation through 110 kV plastic line to ± 160 kV plastic city converter station converter and converter valve charging completed.In the process of charging and starting,inrush current is caused by no-load input of 166 kV converter transformer of ± 160 kV plastic city converter station,which causes 110 kV busbar voltage fluctuation in 220 kV plastic city substation and affects the transporting of the wind energy of both the South Australian wind farm and the green Australian wind farm.it is of great significance to Analyse system charging start-up process of inrush causes,finding the appropriate method of inrush suppression,the stability of the system,wind energy transmission and equipment and protection devicesTo facilitate various studies,a detailed simulation model of SAFE HVDC transmission system is established.equivalent to AC grid in Sunan station,and the remaining power grids,devices and methods related to flexible DC are modeled according to the actual system parameters.The flexible DC converter station uses the simplified equivalent mode of PSCAD to accurately simulate an MMC circuit in series with up to 200 power modules while ensuring acceptable simulation time-consuming.The model simulation can relatively accurately simulate the operating characteristics of the site.Through the simulation research on the start-up process of SAFE HVDC transmission system,the change characteristics of voltage and current under both cases without considering the saturation characteristics of commutation transformer core and considering the saturation condition of commutation transformer core are analyzed.It is pointed out that at the beginning of charging,inrush current and other factors may cause the AC grid voltage drop seriously.Based on the mechanism of magnetizing inrush current generated when no-load transformer is put into operation,the reason of inrush current in charging process of flexible AC power transmission system in South Australia is analyzed.In order to further analyze the characteristics of magnetizing inrush current,a simple The simulation system analyzes the characteristics of magnetizing inrush current through simulation and finds out the restraining measures.Aiming at the characteristics of magnetizing inrush current,the method of phase selection and closing is proposed to solve the problems above,and the simulation is carried out through the converter station model.At the same time,the inrush suppression device adopting the principle of remnant magnetization and partial magnetism is tested and analyzed.The correction of the points and closing times of circuit breakers is consistent with the actual time and closing time of circuit breakers,so that the circuit breaker points and closing angles Close to the target angle of 90 degrees to achieve the purpose of residual magnetism and magnetic bias grams.The test results verify the effectiveness and feasibility of the inrush suppression method.