| With the increasing demand of renewable energy grid-connection,the system intermittent oscillation problems affected by the environment becomes more and more prominent,which seriously affects the safe and reliable operation of power grids.In order to suppress the power fluctuation of renewable energy grid-connection,integrated energy storage has become an effective solution.Modular multilevel converter(MMC)has been recognized as one of the most attractive technologies for energy storage integration in high voltage and large capacity application,due to its superior advantages such as higher modular design,scalability,low output harmonics,etc.Traditional MMC based centralized energy storage system shows simple structure and convenient control,but its energy storage capacity can not be flexibly changed.Traditional MMC based distributed energy storage system is flexible in energy storage configuration due to its modular structure.However,the decentralized connection between energy storage modules and submodules greatly increases the system control complexity,which makes it difficult to improve the utilization ratio of energy storage capacity.Considering advantages of the above-mentioned two systems,this paper proposes an improved MMC based symmetrical super capacitor energy storage system(IMMC-SSCESS).In view of its operating principle,modulation and control strategies as well as specific application,the following research is carried out.Different energy storage interface into MMC topologies are summarized firstly,and the operating principles and power transmission of IMMC-SSCESS are analyzed.Based on mathematical modeling analysis,the submodule capacitor voltage fluctuation with different frequency is derived,and then the phase disposition sinusoidal pulse width modulation(PD-SPWM)strategy and selective sorting-based submodule capacitor voltage balancing bottom layer control strategy are designed.Furthermore,the essential circulating current suppressing mechanism is analyzed,and a universal circulating current suppressing(UCCS)bottom layer control strategy is adopted.Moreover,aiming at two goals of stabilizing DC voltage and controlling AC and DC power,the research on middle system layer control strategies and corresponding top layer control strategies are carried out.Under the DC voltage system layer control,the state of charge(SOC)power correction balancing control strategy and the Buck-Boost power control strategy are proposed.Meanwhile,under the AC and DC power system layer control,the SOC balancing control based on virtual potential feedforward strategy and Buck-Boost voltage and current control strategy are proposed.Furthermore,the system simulation model is established,and compared with the traditional distributed global energy storage system under the AC and DC power control,the advantages of the proposed system in stability,dynamic response and anti-interference are verified.In addition,under the DC voltage control,aiming at two operating conditions of rectifier and inverter,the function of energy storage peak load shifting is verified.Finally,a down-scale experimental prototype of three-phase five-level MMC based super capacitor energy storage system is developed,and the theoretical analysis and simulation results are verified by experimental results.It has been demonstrated that the effectiveness of the proposed control strategy and the necessity of integrated energy storage to the system.Additionally,an IMMC-SSCESS based electrical energy router platform is constructed,considering the power requirements of different terminals,and the feasibility of the proposed power coordinatied control strategy is verified. |
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