Research Of The Control Strategy On Modular Multilevel Converter Based SOP

With the continuous advancement of energy transformation in our country,a large number of distributed generation will be widely connected to the distribution network,which can bring voltage over-limit,bi-directional power flow and feeder power imbalance to the distribution network.Due to the weak grid of our country's distribution networks,most of them adopt closed-loop design and open-loop operation,which leads to the traditional control methods can't effectively solve the above problems.As a result,the application of power electronics technology to upgrade the distribution network has become an important trend.As a new device based on power electronic technology,SOP(soft open point,SOP)is used to replace the conventional tie switch.It can realize flexible regulation of feeder flow,improve voltage distribution level,and promote the ability of distribution network to consume distributed generation.Modular multilevel converter(MMC)has been widely used in high voltage and high power because of its excellent output voltage characteristics,easy expansion and redundant control.Considering the application of SOP in medium voltage distribution networks,combined with economic costs and technical difficulties,modular multilevel topologies can meet the requirements well.Due to the control strategy has an important influence on the stable operation of the system,this paper will carry out research on the SOP control strategy based on MMC topology.In this paper,the three-port SOP of MMC topology is studied.Firstly,the basic principles of MMC are studied,including the working principle of MMC sub-module and three-phase MMC;the mathematical models of three-port SOP based on MMC are established;the commonly used basic control strategies are introduced.Secondly,for the modular multi-level topology,its modulation methods are introduced,and focus on the carrier phase shift modulation method used in this paper.For the internal circulating currents problem,the mechanism of circulation currents generation is analyzed.According to the feature of circulation currents,the double fundamental frequency negative sequence coordinate transformation method is used to suppress the circulation currents.For the problem of sub-module capacitor voltage equalization control,the causes of sub-module capacitor voltage imbalance are analyzed,and then combined with the carrier phase shift modulation strategy,the sub-module capacitor voltage equalization control strategy is improved.The effectiveness of the proposed strategy of circulating current suppression and sub-module capacitor voltage equalization control is verified by building a simulation model.Aiming at the problem of many PI parameters in multi-port SOP,the robustness and poor dynamic performance of PI control,this paper adopts a nonlinear current control strategy combining feedback linearization and sliding mode control,and uses this composite control to replace the current inner loop of PI double-loop control.Besides,the problems of this control are improved in this paper.Then combined with the three operating modes of SOP,the current control scheme combining feedback linearization and sliding mode control in different modes is realized respectively.Finally,the system controller is designed based on the current control strategy,the sub-module capacitor voltage equalization control strategy and the circulating currents suppression strategy.The simulation model of MMC based three-port SOP is built in MATLAB/simulink.Besides,the simulation of the actual operation mode of the SOP further validates the effectiveness of the proposed control strategy.