Research On Coordination Control Strategy Of LCC-MMC Hybrid HVDC Transmission System Under Fault

China's energy resources are characterized by “the poor in the east,the rich in the west” This is the exact opposite of the demand for electricity.The west-to-east power transmission project and the deployment of the national interconnected power grid make DC transmission technology play a pivotal role in the future development of the power grid.The hybrid multi-terminal DC transmission system combines the advantages of traditional DC transmission and flexible DC transmission,and has obvious advantages for solving the problems of power transmission and grid connection of new energy in China.In this thesis,based on the in-depth study of the topology and working principle of a hybrid multi-terminal DC transmission system,the basic control strategy and protection control strategy are studied.Firstly,the advantages and disadvantages of traditional HVDC transmission and flexible HVDC transmission are described.The hybrid HVDC transmission system is introduced based on their respective shortcomings,and its research significance and current research status are analyzed.The feasibility of the proposed hybrid HVDC transmission system topology is analyzed analysis.Secondly,the hybrid multi-terminal DC power transmission system designed in this thesis is a line commutated converter(LCC)at the transmitting end and a modular multilevel converter(MMC)based at the receiving end.The topology and working mechanism of the sender-side LCC converter,the topology of the receiver-side MMC converter,the working mechanism of the SM sub-module,and the operating principle of the MMC are described.By comparing different modulation methods of MMC converters,it was finally determined that the nearest level approximation modulation strategy has more advantages.After analyzing the topologies of several common hybrid DC transmission systems,this thesis proposes a three-terminal hybrid DC transmission system topology based on LCC-MMC.Thirdly,the mathematical models of the three converter stations at the sending end and the receiving end under different coordinate systems are established,and the station-level control strategy of the LCC-MMC hybrid transmission system is designed.LCC inverter is controlled by constant DC voltage,MMC converter station adopts constant DC voltage control at one end and active power control at another end.At the same time,the loop-current suppression link based on DFI-CCSC is added,and simulation verification is carried out.Besides,in order to ensure the safe and stable operation of the system and the simulation and verification of the fault protectionstrategy described later,a simulation model of the hybrid DC transmission system was built on the PSCAD / EMTDC platform,and the main parameter selection principles and controller design of the system were detailed set forth.Finally,a fault coordination control strategy is designed for the AC-side fault in the hybrid DC transmission system.The LCC rectifier station uses the minimum trigger angle control,and the MMC converter station uses the maximum modulation ratio control.This strategy can be used when the AC side fails to ensure the effective transmission of active power and reduce the fall of DC voltage.In addition,the fault characteristics of the DC line of the hybrid DC transmission system and the restart process of the system are analyzed.Then,the effectiveness and feasibility of the proposed strategy are verified by simulation.