Nearest level modulation for high voltage transformerless gird-tied battery energy storage system

Nearest level modulation (NLM) directly using state of charge (SOC) sorting applied for cascaded H-bridge converter-based battery energy storage system (BESS) leads to battery cells overcurrent and could cause battery wear and lifetime reduction. First, the comparison of carrier phase shift (CPS) with NLM is made; and the principle of SOC sorting-based NLM and consequent occurrence of submodules (SMs) power overload are analyzed theoretically. In order to limit battery rate of dischargeability, the maximum duration time of SMs switching on for constraining power in a safe operation is calculated quantitatively. Then, this paper proposes an improved NLM algorithm that helps BESS to achieve inner-phase SOC balancing and avoid excessively high charging/discharging ratio of current at the same time. Besides, a state machine strategy is given for purpose of simpler implementing the modified NLM in digital controller. At last, effectiveness of the proposed NLM algorithm based on improved SOC sorting is thoroughly verified in BESS of 35kV/10MW/10MWh and 46 SMs per phase by MATLAB/Simulink simulations and a downscaled StarSim hard-in-loop experiment.