Research On Battery Energy Storage System Based On Modular Multilevel Converter

The rapid development of renewable energy has put challenges to the power quality,stability and capacity reserve of grid.With the characteristic of quick response,higher efficiency and proven technique,battery energy storage system(BESS)has became the key point of the steady operation for grid with high percentage renewable energy.BESS based on modular multilevel converter(MMC)has several significant advantages,such as lower THD of output voltage,smaller voltage stress of power electronics devices,fault tolerance ability,which make MMC-BESS alternative scheme for high-voltage and large-capacity energy storage situation.The modular structure of MMC-BESS contributes to lowering the battery pack voltage,and then weaken the "cask effect" caused by inconsistencies among battery units,which is one of the bottleneck factors of BESS stability and capacity utilization.What's more,the battery module can be balanced via controlling the submodule power.Aiming at MMC-BESS and its control strategy,this thesis conducts some researchs as follows:First of all,the topology and operation principle of MMC-BESS are analyzed,and mathematic model is built.This thesis derives the relationship among battery port,DC port and AC port.Meanwhile,the interaction between battery power and circulating currents is analyzed.The theory lays a foundation for the control strategy.Secondly,a power control strategy based on the power flow analysis of MMC-BESS is proposed.The strategy contains DC,AC port control and individual battery module power control.Phase battery power is controlled by injecting zero sequence voltage or adjusting DC circulating current.Difference power between upper and lower arms within the same phase is controlled by the 1st order circulating current among three phases.The power of individual submodule within the same arm is redistributed via adjusting the submodule output voltages.The derivation of zero sequence voltage and the 1st order currents without zero sequence are presented.Simulation results verify the validity of the power control strategy.Thirdly,to address the inconsistencies among the battery modules SOCs,a 3-level SOC balancing control scheme based on the individual battery module power control is raised,in which the power orders are modified by SOCs difference.The scheme is compromised of inter-phases balancing,inter-arrms balancing and submodules within arm balancing.The paper also analysis the interaction among the three level of SOC.balancing.A SOC balancing control method under fault-tolerant operation mode is proposed.Simulation results verify the validity of the SOC balancing control strategy.Finally,a experimental prototype with 24 submodules is conducted.The power control strategy and multiple operation mode of the MMC-BESS is implemented on the prototype.Experiment results verify the validity of the control scheme and the operation modes.