Low-Frequency Current Ripple Minimization of Single-Star Bridge Cells-Based Battery Energy Storage System Using Optimal Third Harmonic Voltage Injection

Single-star bridge cell (SSBC) based multilevel converters are a promising solution for constructing high-voltage and large-capacity battery energy storage systems (BESSs) in power systems. Nevertheless, an undesirable second harmonic current (SHC) will be generated in the battery units since the single-phase instantaneous power of the SSBC pulsates at twice the line frequency. Such SHC-dominated low-frequency current ripple not only shortens the lifespan of the BUs but also degrades the system's efficiency. However, this intrinsic issue is overlooked in recent studies. To this end, an SHC suppression scheme that makes full use of the residual modulation index of the system to inject an optimal third harmonic voltage (THV) is proposed. The phase of the THV is determined first, followed by a closed-loop control strategy to regulate its magnitude dynamically for achieving a minimum SHC within the modulation constraints. The scheme is implemented with the variables that already existed in the main control system, resulting in a low increase in cost. The adverse impact of the technique on the AC-side output characteristics is investigated quantitatively, and a modified design procedure for the DC filter network is presented to address this issue. Hardware-in-the-loop experiments are carried out to validate the technique.