Research On Fault-Tolerant Operation Of Modular Multilevel Converter With Battery Energy Storage System (MMC-BESS)

Battery energy storage system can realize peak load shifting,smooth new energy power fluctuations,and improve power system stability.Power conversion system(PCS)is a key component to exchange power between battery energy storage system and the grid.When modular multilevel converter is applied to a PCS to form modular multilevel converter-battery energy storage system(MMC-BESS),it has modular features and integrated distributed battery storage,and it has both DC and AC ports.In order to improve the reliability of MMC-BESS,its modularity can be used to investigate the submodule fault-tolerant operation and submodule battery-side fault-tolerant operation.Based on the MMC-BESS topology and working principle of submodule,the equivalent DC module and equivalent AC module of MMC-BESS are given.Besides,the carrier phase shifted PWM based on hot-reserved submodule and control strategies of power,capacitor voltage and circulating current are introduced,and the modulation and control strategies are verified by simulation.For the submodule fault-tolerant operation of MMC-BESS,the submodule switching function model is used to analyze its mechanism.It is concluded that when the submodule failure rates of the upper and lower bridge arms are different,the fundamental frequency circulating current will be generated;and the active component of fundamental frequency circulating current is related to the difference of submodule failure rates of the upper and lower bridge arms.In order to ensure the external characteristics of MMC-BESS remain unchanged,this paper proposes a control strategy for controlling the fundamental frequency circulating current flows within the MMC-BESS,and improves the original submodule capacitor voltage control strategy.Due to the submodule fault-tolerant operation,the circulating currents of all phases are changed.The peak value of arm current and the voltage fluctuation of submodule capacitors during fault-tolerant operation are studied,which provides a reference for the system stress design.And the effectiveness of the control strategy is verified by simulation.For the submodule battery side fault-tolerant capability of MMC-BESS,as to the working condition that the numbers of battery side fault submodules of each arm are the same,combined with the external characteristic unchanged constraint,submodule power balance constraint and modulation constraint,a method for analyzing fault tolerant ability is proposed.Namely the analytical relationship between the maximum fault-tolerant quantity of MMC-BESS and power relationship and modulation ratio.Then the method is introduced into the asymmetry condition that the number of battery side fault submodules of each arm are not completely equal,and the block diagram of solving by computer is given.The working boundaries of the symmetrical and asymmetrical conditions are verified by simulation.In order to verify the correctness of the theoretical analysis,the MMC-BESS platform is used to verify the modulation and control strategies of normal operating conditions and the mechanism analysis and control strategy during submodule fault-tolerant operation.