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

For modular multilevel DC/DC converter (MMC) applied in battery energy storage system, the input voltages and/or output references of the modules are usually unbalanced. If module fault occurs in this situation, 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. 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 keep the angles of other healthy modules unchanged. Thus, the calculation burden can be reduced, compared with existing online phase shifting method. Finally, experimental results on a five-module MMC demonstrate the proposed scheme can improve computing speed and maintain the current performance as normal operation.