Modified Hybrid Modulation Technique for Cascaded H-Bridge Converter with DC Bus Oscillation

Cascaded H-bridge converter (CHBC) has emerged as a popular multi-level converter (MLC) topology for interfacing a utility scale battery energy storage (BES) with the medium/high voltage grid. Conventionally, interface has been implemented by connecting battery packs (BP) to Single-Phase H-Bridge through bidirectional dc-dc converter (BDC) to form a battery module (BM) which then connected in series to form a leg of CHBC. This single-phase H-bridge interface employed in CHBC draws the ripple power from DC bus at twice the mains frequency. It results in oscillation on the DC bus voltage which adversely affects the quality of the output voltage. Another challenge with the grid connected BES control is the state of charge (SoC) balancing among the BP. To address either or both of this issue, additional intermediate converter interface is needed which affects overall system cost, efficiency, and reliability. This paper proposes volt-second (V-s) balance-based modified hybrid modulation technique (HMT) for CHBC that achieves both SoC balancing and reduction in harmonic from the output voltage despite the oscillations in the DC bus voltage of individual BMs. Proposed methodology works with existing CHBC based system and sensors with reduced dc bus capacitor. The proposed modulation technique is validated on 5-level CHBC on the Typhoon Real Time Simulator.