Improved Phase-Shifting Algorithm of Modular Multilevel DC/DC Converter for BESS Under Module Fault Conditions

For a modular multilevel dc/dc converter (MMC) applied in a battery energy storage system (BESS), the input voltages and/or output references of the modules are usually unbalanced. If module fault occurs in this situation, the existing modulation strategies will lead a heavy calculation burden to mitigate the degraded current ripple. To address this issue, a low-computational phase-shifting algorithm is proposed to suppress the ripple when module fails. A harmonic model in phasor diagram is first given, which indicates that a certain harmonic can be eliminated if a closed polygon of those harmonic vectors is reconstructed. On this basis, the closed polygon at carrier frequency can be reconstructed by only adjusting part of phase-shifted angles of the healthy modules, while keeping the angles of other healthy modules unchanged. Thus, the calculation burden can be reduced, compared with the existing online phase-shifting method. Finally, experimental results on a five-module MMC demonstrate that the proposed scheme can improve computing speed and maintain the current performance as normal operation.