Design Of Sub Module Topologies And Their Control For Modular Multilevel Converter

Modular multilevel converter(MMC)has unique advantages in low harmonic content and low switching loss.However,during HVDC transmission the DC fault seriously affects the quality of power transmission.If the MMC-HVDC system can quickly and effectively clear the DC fault that is the prerequisite to ensure the reliable operation of the system.There are roughly three ways to clear the fault: AC、DC circuit breakers and sub module topology.Since AC circuit breakers have defects such as long response time and easy damage to components,they cannot be used alone in engineering;and the immature technology and high cost of high-voltage and large-capacity DC circuit breakers,so it is difficult to achieve in actual engineering.It is the best way to make MMC have DC fault blocking capability that relying on sub module topology devices to block current.Half Bridge Sub Module(HBSM)has the advantages of simple structure,easy control and low cost,but cannot block DC fault;Full Bridge Sub Module(FBSM)can block DC fault,but there is capacitive coupling that leads to large loss and high cost.So,it is a feasible solution to improve the sub module to have the ability to clear the DC fault.The article mainly expands from the following aspects:(1)Firstly,by analyzing the operation mode of a typical HBSM,I find that the anti-parallel diode cannot block the fault current.Then the full bridge sub module(FBSM),series double sub module(SDSM),clamp double sub module(CDSM),and self block sub module(SBSM)four types with fault blocking capability are analyzed.The MMC topology and the fault mechanism of the MMC-HVDC system are analyzed,and a current transfer branch structure with fault isolation is introduced.(2)Proposed an improved self block sub module(ISBSM)with the ability of blocking current.For HBSM’s anti-parallel diodes that cannot block the fault current,that improved the HBSM topology.ISBSM and HBSM are matched in a certain proportion to form different hybrid modular multilevel converters.Simulation and comparison verify that when the two sub modules each account for half of the total number of modules in the bridge arm,the fault suppression effect is the best.Finally,it analyzed the fault suppression capabilities respectively when grounding on the AC side,the valve side and the DC side.(3)Proposed self block unipolar full bridge sub module(SB-UFBSM)topology.Although IBSSM has a certain failure suppression capability,but the loss of power devices is large.It is necessary to improve the full bridge sub module(FBSM).It analyzed the working principle of SB-UFBSM and applied it to the structure of SB-MMC that failed.And it analyzed the fault clearing ability of SB-UFBSM before and after blocking in detail.In order to adapt to engineering applications,withstand voltage and control analysis of power devices are carried out.(4)It combines the SB-UFBSM and HBSM in a certain proportion to form a hybrid modular multilevel converter(HMMC).Then analyze and select the control strategy for this HMMC structure,and obtain the best matching quantity of the hybrid structure by simulation.Comparing the HMMC and SB-MMC,the simulation results verify that the former has a weaker fault blocking capability as the number of HBSM modules increases,while the SB-MMC structure has a stronger fault clearance capability.In the PSCAD/EMTDC simulation software,a model is built with a transfer branch structure that assists in blocking the fault current to simulate and verify the SB-MMC of fault clearance capability.Finally,the economics of different sub modules are compared and analyzed,and the MMC structure with current transfer branch has good economics.