Overview of Bidirectional Unfolding Converters for Battery Energy Storage Systems

In order to support wider use of distributed renewable sources and mitigate the negative effects of their intermittent behavior, nowadays they are often installed together with energy storages, located on-site or nearby. Battery energy storage systems (BESS) allow not only to match consumption and generation profiles more closely, but also they provide various ancillary grid-supporting services. The enabling technology for these emerging opportunities is related to power electronic interfaces between the DC battery and the AC grid. Generally, a two-stage power conversion approach is used, where the battery-side DC-DC stage is responsible for controlling the battery current and grid-side DC-AC stage controls the current taken or injected to the grid. These two stages are connected through a bulky DC-link capacitor with stabilized voltage. Such an approach has however reached its limits in terms of efficiency, power density and cost. Therefore, a lot of research is currently focused on alternative topologies, enabling higher performances than the well-developed two stages approach. One of the promising configurations of such alternative topology includes a DC-DC converter, that forms sine voltage half-waves and an unfolding circuit, which provides the commutation of these half-waves to the grid. This paper presents a review of unfolding converters in the context of BESS. Configurations of unfolding circuits with DC-DC choppers, dual active bridge converters and multilevel converters, as well as the partial power converters are analyzed.