Research On Ac Fault Ride-through Control Strategy For Sending End Of LCC-MMC Hybrid DC Transmission System

The LCC-MMC Hybrid DC transmission system with grid commutation converter on rectifier side and modular multi-level converter on inverter side can make comprehensive use of the economic advantages of LCC and the technical advantages of MMC to solve the problem of commutation failure on inverter side of traditional DC transmission.At present,it has been applied in the "Wudongde" multi-terminal Hybrid DC transmission project of China Southern Power Grid.The DC voltage at the sending end is lower than that at the receiving end owing to the drop or even interruption of DC power when an AC fault occurs at the sending end of hybrid DC transmission system.Then that will threaten safe and stable operation of the receiving end power grid.Firstly,the working principle and mathematical model of the rectifier and inverter are firstly analyzed.The components of the MMC bridge arm voltage and its quantitative relationship with the AC voltage and the DC voltage are mainly studied.According to the scenarios when it supplies power to the active network and the passive network,its basic control strategy is designed and the simulation model of the double-terminal hybrid DC transmission system is built.It lays the foundation for follow-up research.Secondly,aiming at the DC power transmission drop or even interruption caused by AC fault at the sending end of the Hybrid DC transmission system supplying power to the active network,a fault ride-through control strategy based on reducing the AC and DC components of the bridge arm voltage on inverter side is proposed.The variation law of the operating characteristic curve of the rectifier side under different drop degrees of the AC voltage at the sending end is analyzed,and a method is designed to determine the number of sub-modules on inverter side to reduce the input according to the drop degree of the AC busbar voltage on rectifier side.Reactive power constraints,set the DC voltage regulation limit of the inverter station MMC.Based on the quantitative analysis of the reduction value of DC voltage at the inverter side,the power transmission capacity of DC system after AC fault at the sending end is maintained.Finally,aiming at the problem of AC fault ride-through at the sending end of the hybrid DC system connected to passive network,the mathematical relationship between the absorbed power of different loads in passive network and the AC voltage at the inverter side is analyzed.The selection principle of relevant parameters under different severity AC faults is designed.By allowing the load to actively participate in the regulation of the unbalanced power of the converter station,the DC power transmission can be quickly restored.The load shedding can be effectively avoided,and the fault ride-through of the converter station and load at the receiving end can be realized at the same time.The designed control strategy can avoid converter station locking caused by excessive discharge of submodule capacitance,and effectively reduce the DC impulse current after fault clearing.This paper studies the AC fault ride-through capability of the sending end of LCC-MMC hybrid DC transmission system,which is of great significance for the implementation of the "WestEast electricity transmission project" in my country.And that can ensure the normal and reliable power supply to the receiving power grid and improve the security and stability of the multi-feed receiving system.