Research On Capacitor Voltage Fluctuation Characteristics And Suppression For Modular Multilevel Converters

With the advantages of flexible and expandable modular design and multi voltage-level output characteristics,modular multilevel converters(MMCs)along with its improved topologies have been widely applied in high voltage and power applications.The operating principles and mathematical models of MMCs are both based on the symmetrical MMC topology with three phases and six bridge arms.In this paper,the accurate switching-function model,average value model and coupled mathematical model under d-q synchronous rotating coordinate system are respectively built.Based on these models,different double-loop control systems and general control architecture are designed and they can be widely applied for MMCs along with its improved topologies.Submodule capacitor voltage fluctuation is an inherent characteristic of MMC type converters,which can further bring the circulation phenomenon among different phases in MMCs.The circulating current has no contribution to the AC output while increasing the effective value of arm current and adding additional losses for switching devices.This paper analyses a circulating current elimination strategy for MMCs based on single phase vector control.The proposed control strategy can effectively and independently suppress the circulating current in each phase unit of MMC,which is applicable in either single-phase or multi-phase MMC.Submodule capacitor voltage fluctuation and circulating current are tightly coupled by the instantaneous powers absorbed by bridge arms.Based on the frequency component analysis of the instantaneous powers absorbed by bridge arms of MMC,it is shown that the magnitude of capacitor voltage fluctuation is proportional to the amplitude of AC output current while inversely proportional to the operating frequency.The instantaneous powers absorbed by an bridge arm of MMC is mainly of fundamental and double-fundamental frequency under circulating current suppression control,and the double-fundamental frequency fluctuating component can be further eliminated through appropriate circulating current injection.In this way,the capacitor voltage fluctuation can be further optimized.Both simulation results based on MATLAB/Simulink software and experimental results have verified the effectiveness of the proposed circulating current suppression strategy.Besides,further simulation based on MATALB/Simulink software is carried out and verified the correctness of the optimization effect on capacitor voltage fluctuation through circulating current injection process.Since the circulating current injection process can not eliminate the instantaneous fluctuating powers of fundamental frequency absorbed by bridge arms,the optimization effect on capacitor voltage fluctuation is limited.In order to solve this problem and based on the reuse of electrical components,this paper proposes a novel modular multilevel converter based on shared submodule capacitors(SSC-MMC).The distribution relationship of instantaneous absorbed powers between arm and phase units is changed and the capacitor voltage fluctuation is effectively suppressed through the self-balancing of instantaneous absorbed powers in symmetrical three-phase units.The submodule capacitor voltage balancing is the precondition for the normal operation of MMCs along with its improved topologies.Based on the sorting control principle in MMC,a capacitor voltage balancing strategy for SSC-MMC based on multivariable sorting is proposed in this paper,which comprehensively evaluates the influences of current in each phase-unit and the switching states of each phase-unit conversion circuit on the state of charge of shared submodule capacitors.In this way,the capacitor voltage balancing in SSC-MMC is achieved.In the actual operation of SSC-MMC,the transient process of commutation is inevitable between different phase-unit conversion circuits and between different switching devices within one phase-unit conversion circuit.In order to achieve the steady operation of SSC-MMC,the control logic of switching states between different phase-unit conversion circuits in time domain as well as the “dead time” control logic of different switching devices within one phase-unit conversion circuit is designed.Based on the proposed control logic and multivariable-sorting strategy,an improved multivariable-sorting-based capacitor voltage balancing strategy for SSC-MMC is proposed,which is composed of three parts: algorithm initialization,cyclic control and switching state output process of phase-units.This paper further analyzed the capacitor voltage fluctuation and circulating current characteristics in SSC-MMC.The result shows that theoretically there is no voltage fluctuation of low-frequency component in shared submodule capacitors.Even through small asymmetry exists in three-phase AC output currents,the caused low-frequency fluctuating components in capacitor voltage can also be neglected.Simulation results based on MATLAB/Simulink software have verified the feasibility of the proposed SSC-MMC and its effectiveness on the suppression of capacitor voltage fluctuation under different operating conditions.