Current Ripple Mitigation Strategy of Modular Multilevel DCDC Converter for Battery Energy Storage System

For modular multilevel dc/dc converter (MDC) with conventional modulation strategies, the inductor current ripple will increase if dc/dc units' input voltages and/or output references are unbalanced. In this article, a current ripple mitigation strategy is proposed for MDC battery energy storage system, which is based on harmonic model for ripple analysis using the Fourier series. By varying the duties and phase-shifted angles simultaneously, multiple current harmonics are eliminated. Moreover, in order to freely adjust batteries' current under the condition of duties is calculated by ripple suppressing method in every modulation process, the control period is divided into several intervals and variables in each interval are separate. Through constraining every unit's average duty of all intervals, battery current is dominated under distribution scheme while achieving the goal of ripple mitigation. Experimental result on a four-unit MDC is presented to verify the feasibility and effectivity of the proposed modulation method.