| The problems of large-scale renewable energy integration,asynchronous AC system interconnection and shortage of transmission corridors are common phenomenon in China power system.Modular multilevel converter based multi-terminal high voltage direct current(MMC-MTDC)has become one of the effective methods to solve these problems.MMC-MTDC is taken as the research object in this thesis.The main contents include:(1)The converter valve control of MMC-MTDC system is studied.The circuit topology,operation principles and double closed-loop control strategy of MMC are introduced.Besides,the nearest level modulation and its optimized balancing strategy are studied.The simulation results verify the effectiveness of MMC control strategy.(2)In order to study the converter station control of MMC-MTDC system,the operation principles,control characteristics,advantages and disadvantages of three traditional coordination control strategies are introduced.Through MMC-MTDC simulation experiments of N-1 operation,the transient processes of three coordination strategies are compared and analyzed.(3)The small signal modeling of MMC-MTDC is studied.Based on the small signal theory of power system,MMC-MTDC is linearized from three parts: converter and AC network,controller,DC network.Then,the global small signal model of MMC-MTDC is proposed.The validity of MMC-MTDC small signal model is verified by simulation.Finally,the small signal stability of MMC-MTDC is preliminarily explored,which provides a reference for the optimal design of MTDC control parameters.(4)In order to solve the problems of traditional droop control,an improved adaptive droop control strategy for MMC-MTDC is proposed.The proposed strategy has advantages of high voltage quality and low line loss.Besides,the principle of parameter setting and the decay proof of the approximation effect are given.Finally,example analysis and simulation results of both steady and transient states verify the effectiveness of the improved strategy. |
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