Research On MMC-HVDC Submodule Topology And Converter Control Strateg

Modular multilevel converter is widely used in HVDC and offshore wind power transmission due to its advantages of high flexibility of modular structure expansion,low harmonic content of AC output voltage,simple redundancy configuration,etc.The modular multilevel converter based high voltage direct current has a wide range of application prospects,and has become one of the research hotspots in the world.The quick clearing of short-circuit fault on the DC side of MMC-HVDC,the voltage equalization control of MMC sub-module and the suppression of circulating current are all important factors to ensure the stable operation of MMC-HVDC system.In this paper,aiming at these problems,the following aspects are studied.Firstly,the topology and working principle of the MMC sub-module are analyzed.Based on the working principle of the sub-module,the overall topology structure of the three-phase MMC is analyzed,and the equivalent mathematical models of the DC side and the AC side of the MMC are given.In order to effectively solve the problem of DC side short-circuit fault of MMC-HVDC system,a new sub-module topology with DC fault self-clearing ability is proposed,and the self-clearing principle of DC fault when the new sub-module topology is used to construct three-phase MMC is analyzed.The simulation results verify the effectiveness of the proposed new sub-module topology with DC fault self-clearing capability.Secondly,the mathematical model of the capacitance voltage fluctuation of sub-module is analyzed,and the necessity of capacitance voltage equalization control of sub-module is explained.Four indexes of evaluating the method of capacitance voltage equalization control of sub-module are introduced.Aiming at the disadvantages of traditional bubble sorting algorithm,such as large amount of calculation,long time needed for calculation etc,proposed a voltage equalization control method for sub-modules capacitance voltage with sub-modules capacitance voltage subsection and random input sub-module.The sub-module capacitance voltage subsection mode and input sequence of each submodule are introduced.The influence of each parameter on the capacitance voltage equalization effect of the sub-module was explained by taking different control parameters and four indexes of evaluating the capacitance voltage equalization control method,and the comparison with the bubble sorting method was made to verify the effectiveness of the proposed sub-module capacitance voltage subsection and random input sub-module voltage equalization control method.Finally,the steady-state mathematical model of the circulating current is presented to illustrate the harm caused by the existence of the circulating current,and the principle of the level-increased nearest level modulation method is analyzed and it shows that the method can increase the peak-to-peak value of the circulating current.On this basis,a new method of circulating current suppressing based on deadbeat and proportional integral(PI)composite control is proposed,which not only retains the advantages of the level-increased nearest level modulation method,but also inhibits the MMC circulating current.The effectiveness of the proposed circulating current suppression strategy was verified by simulation results.