ANALYSIS AND DESIGN OF MODULAR MULTILEVEL CONVERTERS WITH PARTIAL INTEGRATION OF ENERGY STORAGE SYSTEMS

Abstract

With the massive integration of renewable energies, power converters will become increasingly important in the electrical grid of the future. One of the expected consequences is the reduction of system inertia. A promising alternative for this problem is the integration of energy storage elements (ESEs) into modular multilevel converters (MMCs). This paper presents a novel design methodology for a 3-phase MMC with partial integration of energy storage (MMC-PIES). The arms of the proposed converter contain standard submodules without energy storage (SMs) and submodules with an integrated ESE (ES-SMs). ESSMs with half-bridge (HB) and full-bridge (FB) topologies are explored, while an HB configuration is kept for standard SMs. The methodology includes a minimisation of the amplitude of the circulating current that needs to be injected and a method to calculate the specifications of the converter. Like the results found in the literature, the proposed approach shows that using FB cells demands less ES-SMs. However, it is identified that the capacitances and ESEs must be remarkably bulkier, rendering this topology less attractive. The results also show that the partial integration with HB ES-SMs is a viable solution, causing only a small impact on the SM capacitance and on the ESEs rating.