Control Strategy Design For LCC-MMC Hybrid HVDC Transmission Syetem Under Fault Conditions

At present,transmission of electricity from the west to the east dominates the power transmission modes in China,and long-distance power transmission between power systems can be achieved by HVDC transmission via overhead lines.As LCC(Line Commutated Converter)and MMC(Modular Multilevel Converter)have respective advantages and disadvantages,the combination of their advantages,i.e.the large capacity and mature technology of the LCC and the flexible manipulation of the MMC,makes it available for meeting the needs of a wide range of application environments during the transmission of electricity from the west to the east.Despite of such advantages,faults are still inevitable in actual operations.Taking the LCC-MMC hybrid HVDC system as the basic topology,this thesis analyzed the faults in each conditions in details and discussed related addressing strategies.This thesis analyzed the topological structures and working principles of LCC and MMC stations,and presented the structures and operation mechanism of various MMC sub-modules in details,based on which the mathematical models of the two types of converter stations were deduced and summarized.Then,it was determined that the constant DC voltage control strategy was adopted for the LCC,while the constant active power and constant AC voltage control strategy was adopted for the MMC.The waveform quality and the stability of the system were effectively improved by means of introducing the triple-frequency harmonic and designing the circulating current suppression controller,and the feasibility of such optimized design was approved by the simulation waveform.This thesis carried out the system steady-state simulation by combining the design of above-mentioned control strategies.The basic information mentioned above provide a theoretical basis for subsequent fault analysis,control strategy researches and other steps.In this thesis,the start-up strategy and fault detection lockout link of the system were designed.The startup strategy adopted the self-excitation method,in which the LCC terminal charged the MMC terminal at first,and the two-stage charging method was used for the pre-charging and alternatively charging to the bridge arm capacitor.Based on simulations,the system characteristics after faults at the DC side of the system were analyzed,respectively.Although this system could resist faults to a certain extent,more effective methods were still needed to address faults in some conditions.Therefore,the above needs were met by the combination of diodes added at positions near the DC side of MMC and the lockout mechanism of the half-bridge sub-module.At the detection lockout link,a control system which could detects abnormal parameters after faults was designed,and this system could shut down the AC switch of MMC station and lock allIGBTs.The feasibility of this link was proved by the simulation results.Detailed analysis on the topological structure of the two hybrid sub-modules was carried out in this thesis.This structure could reduce the number of components while allowing DC fault ride-through.By analyzing the principles of restraining DC faults and designing how to restart after faults for the two kinds of hybrid sub-modules,this hybrid HVDC transmission system could solve the faults and conduct restarting operation all by itself,which greatly improved the stability of the system.The feasibility of the topological structure of this hybrid sub-module was verified by the simulation.