Control Strategy Of Battery Energy Storage System Based On MMC

With the continuous expansion of new energy grid-connected scale,the random fluctuation of wind farms output power and the instability of grid-connected voltage is becoming more and more prominent.In order to adjust the power quality of grid-connected points,it also has the capacity of active and reactive power regulation.Power electronic converters have received extensive attention.In this type of converter,the Modular Multilevel-converter-based Battery Energy Storage System(MMC-BESS)has become the research hotspots in the field of energy storage converters.However,the current research on energy storage management in MMC-BESS is not enough,and the expansion and application of its active and reactive power compensation capabilities need further study.In this paper,the application of MMC-BESS in the power quality adjustment of wind farms grid-connected points are taken as the research background,and the following work is completed:Based on the MMC-BESS topology,the mathematical model of the stationary coordinated system is established,and the relationship between the voltage and current of the bridge arm and the grid side and the circulation is given.The analysis shows the influence of the internal circulation of MMC-BESS on the power of the upper and lower arms of the same phase.The Carrier Phase-Shifted PWM(CPS-PWM)strategy is analyzed.An improved CPS-PWM is proposed,which can realize the power difference control of the sub-module in the same bridge arm of increasing the output harmonic content.The power increment is derived and calculated.The overall control strategy of the system is designed for the active and reactive control of the grid.Aiming at the imbalance of state of charge(SOC)of each energy storage unit inside MMC-BESS,the principle of SOC balances with circulating current control is analyzed,and the SOC balance of DC loops control phase is obtained.The DC and fundamental loops are controlled together.The bridge arm SOC balance is concluded,and the corresponding controller is designed.Combined with the improved CPS-PWM design logic judgment program,the sub-module SOC balance strategy is proposed.In view of the fluctuation of output power and voltage instability at grid-connected point of wind farm,the existing strategies for suppressing fluctuation power are compared,and the suppression scheme based on filtering principle is selected.The relationship between reactive power and voltage at point of common coupling(PCC)is analyzed,and the PCC point voltage regulation scheme is designed.Finally,a three-phase MMC-BESS prototype with 12 sub-modules is built,and the effectiveness of SOC balance control is verified by experiments.An experimental environment is designed to simulate the power fluctuation and voltage imbalance of wind farms.A series of experimental results show that MMC-BESS has the ability to suppress fluctuation power and regulate the voltage of PCC points.The research in this paper will promote the application of MMC-BESS in practical engineering.