Design of Digital Controllers for the Reduction of Low-Frequency Harmonic Currents in Double Star Chopper Cell-based Battery Energy Storage Systems

Battery Energy Storage Systems (BESS) based on the Double-Star Chopper Cell (DSCC) allow battery packs to be connected in a modular fashion to medium and high voltage grids. However, the capacitor voltage of the module contains low-frequency oscillations, mainly at 60 and 120 Hz. These oscillations can easily propagate to the batteries' currents, thus potentially impacting their lifetime. The controller design has vital importance in alleviating this issue. In this paper, the modeling and control design of a DSCC system with batteries connected through a bidirectional (BDC) half-bridge converter are shown. The impact of the BDC current and voltage control on suppressing the low-frequency harmonics is addressed. Further, the paper explores the addition of digital filters, and their impact on the controller design. The required low-frequency harmonic attenuation estimation is described in a downscaled BESS case study. The control structures and their designs are validated using the Typhoon hardware-in-the-loop (HIL) platform.