Control Scheme for Second Harmonic Current Elimination in Single-Star Configuration Based Cascade Multilevel Energy Storage System

Single-star configuration-based cascade multilevel energy storage system is among the most promising solution for high-voltage and large-capacity battery energy storage systems. However, such a solution has inherent second harmonic current (SHC) pulsing in each cluster, which requires a huge passive filter network to maintain the battery current ripple and the capacitor voltage ripple below a tolerable value. To address this issue, unlike the existing method using additional passive and active circuits, this letter proposes a novel SHC elimination method which is achieved by moving it to a higher order using the third harmonic voltage injection. The technique can achieve a 75% reduction of the battery current ripple and a 50% reduction of the capacitor voltage ripple, allowing the system to attain small harmonic currents or reduce the size of the passive filter. The hardware-in-the-loop results verify the effectiveness and performance of the technique under both normal and faulty grid conditions.