Study On Grid-connected Control Strategy Of Modular Multilevel Converter Under Unbalanced Grid Condition

Modular multilevel converter(MMC)has been widely concerned because of its modularity,multiple output levels,low harmonic content of grid-connected current,and the ability to realize direct grid-connected of medium and high voltage.But because of the complexity of the topological structure of MMC,there are still many problems to be resolved,such as launch control,circulation control,and the energy balance and energy balance between upper and lower bridge arm control,energy balance in the bridge arm control,etc.,in addition,in the ac power grid failure,improve the system's ability to low voltage across(LVDC)and MMC in the application of photovoltaic system are also a problem to be solved.This paper first introduces the basic working principle of MMC,then analyzes the modulation strategies of MMC,and compares the advantages and disadvantages of different modulation strategies.Then,the starting pre-charging scheme of MMC is studied,and a new ac side pre-charging control strategy is proposed to realize the charging of the upper and lower bridge arm sub-modules together with the capacitor voltage.For the study of MMC power grid unbalanced control in passive time with submodules,the influence of power grid unbalanced on the system is analyzed and an improved low-frequency current fluctuation suppression algorithm for dc bus is proposed.Then,the corresponding solutions are proposed for the two goals of grid side current symmetrical control and active power fluctuation suppression.For child module integrated PV when MMC unbalanced grid photovoltaic(PV)grid system control research,first set up photovoltaic modules,ac side and dc side of three-terminal MMC mathematical model,analysis of the power transmission mechanism and regularity,and then put forward a kind of power mismatch algorithm of hierarchical control,mainly including alternate with,within and between upper and lower bridge arm bridge arm power mismatch control,finally solve the network side inhibition of grid current three-phase symmetrical and active power fluctuations.The simulation and experimental results verify the accuracy and reliability of the proposed scheme.