Integration of Energy Storage Systems within Modular Multilevel Converters for Medium-Voltage Distribution Networks

This paper presents two novel Modular Multilevel Converter (MMC) configurations for medium-voltage distribution networks, which are achieved by replacing some capacitive cells of each MMC branch with Supercapacitors (SCs) and Battery Packs (BPs), respectively. The first configuration (MMC-SC) is able to exploit SC energy content by preserving MMC branch voltage capability, thus ensuring a suitable dynamic decoupling between grid and DC-link power demands. Steady-state decoupling can be achieved by the second configuration (MMC-BP), which can charge/discharge BP as needed. MMC-SC and MMC-BP functionality is guaranteed by a multi-stage control system architecture, which has been developed in order to ensure proper MMC energy, current and voltage management at any operating conditions. The effectiveness of both MMC-SC and MMC-BP is verified through simulation studies, which regard step-changing active and reactive grid power profiles.