Research Of Modular Multilevel High-Frequency-Link Battery Energy Storage System

Modular multilevel converter(MMC)can obtain higher output voltage level,higher quality of the output waveform,and higher redundancy and scalability by increasing the number of cascaded sub-module,so this topology is widely used in medium and high power condition.Besides,there is a common dc bus in MMC,which can be directly connected to dc grid.As there exists a large amount of distribution power supply in dc grid,it needs to be access to battery energy storage system(BESS)in order to ensure the quality of transmission power and to achieve maximum utilization of rennewable energy.High frequency isolated transformer can realize electrical isolation and voltage matching between BESS and the dc grid,as well as reducing the volume and cost of transformer.This paper studies a modular multilevel high-frequency-link(MMHFL)BESS used for medium and high voltage dc grid.Firstly,a summary of construction and development of topology used for medium and high voltage dc grid BESS,modulation and control strategy of MMHFL are presented.Secondly,this paper mainly studies the principle of MMHFL under square wave modulation and an improved square wave modulation is proposed aimed at solving the dc current oscillation and increasing the stability of system by adding a new controller.Then in the case of unequal cross-arm voltage,a control method of rotating arm voltages is proposed to balance the dc current component between different phases of MMC.Besides,the carrier-phase-shift SPWM(CPS-SPWM)modulation of MMHFL is also analyzed and verified by simulation in this paper.Finally,an experimental prototype of MMHFL is built on the basis of above theoretical analysis.Three different situations are conducted to confirm the feasibility and effectiveness of the improved square wave modulation of MMHFL.