Research On Voltage Balance Strategy Of SMs And DC Fault Clearing Strategy Based On MMC-HVDC

In the last ten years,Modular Multilevel Converter(MMC)has been widely used in the field of High Voltage Direct Current(HVDC)because of its modular structure,which can reduce the requirements for voltage withstand,improve the waveform quality of output voltage and current and ajust the number of inserted sub-modules(SMs)flexibly,MMC-HVDC has become a research hotspot of scholars in the world.The two most important factors that determine the stability of MMC-HVDC are the capacitor voltage balance problem of SMs and the DC-fault clearing problem of MMC-HVDC.The unbalance of capacitor voltage among SMs will lead to voltage fluctuation on DC side,which will affect the stability of power transmission.It will also lead to the formation of phase circulation between MMC,which will lead to the overheating of power devices,resulting in excessive loss and affecting their service life.The topology of Half Bridge Sub-modules(HBSM)adopted by MMC is simple and reliable,but it can not clear the short-circuit fault current of DC side,which seriously restricts the application of MMC.Firstly,the topology of the three-phase MMC and the HBSM is introduced,and the six working modes of the SMs and the operation principle of the three-phase MMC are discussed in detail.The mathematical model of three-phase MMC is derived,and the equivalent circuits of AC side and DC side which reflect the external nd internal characteristics of MMC are obtained.It is proved that the Double Closed-loop Control which has been widely used in two-level converter can also be transplanted into MMC control system.The five most commonly used modulation methods of MMC are listed,and the modulation principles,advantages and disadvantages as well as applicable occasions of several methods are analyzed.Secondly,Because the traditional bubble sorting algorithm has many problems,such as large amount of calculation and long time to produce results.The traditional voltage banlancing strategy will make the switching frequency of power devices too high,resulting in large switching loss.Therefore,a Binary Radix Fast Sorting Algorithm and an optimized capacitor voltage balance strategy are proposed.The Binary Radix Fast Sorting Algorithm combines the advantages of radix sorting and fast sorting,which is simple in logic,small in calculation and fast in sorting.The optimized capacitor voltage balance strategy can reduce the switching frequency flexibly,reduce the switching loss and prolong the service life of the device.The validity is verified by building a 10 level MMC-HVDC simulation model in Matlab/Simulink.Finally,because the traditional SM topology can not clear the DC fault,the topological structure which can realize the self-clearing of DC short circuit fault proposed by the existing literature still has some problems,such as complex structure,excessive power devices,excessive loss and high cost.For this reason,Forward-reverse Alternating Series Sub-module(FRASSM)topology is proposed to eliminate the DC fault by changing the traditional SM connection mode without adding power devices.A 21 level MMC-HVDC system is built In Matlab/Simulink to verify its effectiveness.,and its economy is verified by cost comparison with other topologies.