Research On The Suppressing Strategy For Submodule Capacitor Voltage Fluctuation Of MMC Based On CCVDB

Modular Multilevel Converter(MMC)plays an important role in many fields,such as HVDC and Energy Internet,because of its excellent harmonic performance and strong scalability.At the same time,it also gets more and more in-depth research.In the future,in order to play a greater advantage for MMC,this paper does an in-depth research on control strategies of MMC.1)The topologies of MMC and half-bridge submodule(SM)are introduced,the working status of the SM are determined,and the mathematical relations among the various electrical quantities of MMC are deduced,and the mathematical model is obtained.The layered control theory of MMC is explained,the relationship between layers is expounded,the control strategy required by each layer is determined,and the overall control strategy of MMC is constructed.2)Aiming at the problem of the increased loss caused by MMC circulating current(CC),the generation of circulating current is analyzed,and the principles of the complete CC suppressing strategy and universal CC suppressing strategy are studied.In the simulation,different working conditions are formed by the changes of DC bus voltage and active power,and they are compared with the uncontrolled CC strategy.The simulation results show that when the uncontrolled CC strategy is applied,the fluctuations of CC are very wide,the arm current and the capacitor voltage have a lot of harmonics.When adopting the complete CC suppressing strategy and the universal CC suppressing strategy,it can achieve a good effect on suppressing CC,reduce the fluctuation of the arm current and reduce its harmonics,and have the same impact on the SM capacitor voltage.3)Aiming at the problem that excessive fluctuation of the SM capacitor voltage of the MMC leads to an increase in the capacitance demand,the forming process of SM capacitor voltage fluctuation is deduced,and a new strategy based on Circulating Current Voltage Drop Balance(CCVDB)is proposed,which can achieve the suppressing effect by controlling the second-order component of CC.In simulation,it is compared with the traditional capacitor voltage fluctuation suppressing strategy.Under two different conditions: the changes of active power and DC bus voltage,the analysis shows that compared with the traditional strategy,the strategy based on CCVDB suppresses the fluctuation of capacitor voltage to a greater extent,reduces the demand of capacitance,and reduces the THD of capacitor voltage.The paper has 55 figures,21 tables and 64 references.