Study On Topology Of High-frequency-link Isolated Modular Multilevel Converter

Solid state transformer(SST)is the key equipment for flexible grid integration of renewable energy systems and future hybirde AC/DC power system.Although Modular Multilevel Converter(MMC)based SST has significant advantages in power and voltage scalability and maintenance,as well as in fault-tolerance strategy implementation,it has several drawbacks such as bulky capacitors and complicated sub moduler(SM)voltage balance control strategy.At the same time,because of the structure of MMC puls isolated DC-DC converter,the power conversion stage is increased and the efficiency is reduced.This paper introduces the concept of High-Frequency Link(HFL)in to the MMC,proposes a novel high-frequency-link isolated modular multilevel converter(HFL-MMC)topology.The topology and modulation strategy of high-frequency-link SM(HFL-SM),the topology structure,modulation strategy and working principle of the HFL-MMC are studied.Firstly,base on the half-bridge cell(HBC)of conventional MMC,this paper comes up with a HFL-SM topology and the corresponding unified modulation strategy.The proposed SM can realize high-frequency-isolated the bidiercitonal power conversion.the operation principles of the two work modes are anaylsis.And the reasons of the secondary voltage spike and duty loss are described.A RCD clamping circuit is design for the secondary voltage spike.Secondly,this paper describes the topology structure and the operation principle of the MMC.Moreover,the novel HFL-MMC topology and the Phase Shift Pulse Width Modulation(PSPWM)modulation stragegy is proposed.Power transfer steady analysis among multi-ports is provided too.The comparative analysis between MMC based SST and HFL-MMCbased SST is anakyzed.Finally,to verify the operating principle and the theoretical analysis,a single-phase HFL-MMC prototype and a three-phaseHFL-MMC system have been implemented.The control system of core controller puls distributed controller is design.The experimental results of the HFL-SM,single-phase HFL-MMC and three-phase HFL-MMC verify the feasibility of the novel HFL-MMC topology.