Research On Photovoltaic-energy Storage System Based On Modular Multilevel Converters

Photovoltaic-energy storage system is a power compensator between distributed photovoltaic and energy storage system,with the ability to keep safety and stable condition of the grid.The photovoltaic-energy storage system based on Modular Multilevel Converter(MMC)integrates photovoltaic grid-connected inverters and energy storage converters,with high output voltage,small stress of power semiconductor devices,low switching frequency,power quality,etc.The thesis deals with several technical challenges associated with the photovoltaic-energy storage system based on MMC,as well as the control strategy with MMC.Initially,the working principle of photovoltaic-energy storage system based on MMC is researched.The topology and operation principle of MMC is introduced.The circulating current ripple of system is derived analytically.The modulation strategy for the photovoltaic-energy storage system is discussed.The dc voltage control strategies are proposed from two aspects of modulation wave compensation and modulation strategy,and the control strategies were compared and analyzed.The control strategy of grid-connected and off grid-connected under asymmetric system is introduced.Subsequently,the photovoltaic-energy storage system based on MMC is researched.The photovoltaic-energy storage system is equivalent to a three-port converter(TPC)from,the perspective of power flow.And the working mode of system is introduced.The working principle of PV array and MPPT algorithm are introduced.The influence of asymmetric ac system on MMC energy storage system is analyzed.The interphase power balance control strategy based on dc circulating current injection is proposed.And influence of dc circulating current controller on the stability of system is studied.The variation of arm currents is also analyzed,this is beneficial for power device re-selection.The validity of the interphase power balance control strategy is.verified by the simulation and experiment.Lastly,the SOC balance strategy in the photovoltaic-energy storage system based on MMC is researched.To solve the imbalance of SOC in MMC-ESS,two kinds of control strategies were proposed.One is differential charging and discharging control based on the droop control.Another is the SOC balance control based on the dc circulating current control.While the second control strategy has been improved,to ensure the speed of SOC adjust and safe operation of system.The validity of control strategy is also verified by simulations and experiments.