Reduced Capacitor Energy Requirements in Battery Energy Storage Systems Based on Modular Multilevel Converters

Battery energy storage systems (BESS) based on modular multilevel converters (MMCs) allow battery packs to be integrated into the electrical grid in a modular fashion. Inherent to the operation of the MMC, the module's dc-link capacitor voltage experiences oscillations at grid frequency and its harmonics. This article investigates the close relation between this voltage oscillation and the battery current ripple. The precise capacitor energy requirements for various operations of BESS-MMC, which include arm/phase power balancing and state-of-charge balancing are analyzed in this article. Furthermore, this article also explores the relation between the controller design and the submodule's (SMs) capacitor sizing in terms of its energy requirements. Design guidelines for the SM level voltage control to attenuate battery ripple and detailed analysis for the capacitor energy requirement in each operating mode are presented. The control and capacitor sizing design are validated in real-time using embedded microprocessors in a Typhoon-based controller hardware-in-the-loop environment.