A Study on the Control Design of Nonisolated Converter Configurations for Hybrid Energy Storage Systems

This paper is focused on the control strategies for nonisolated hybrid energy storage systems in power support applications for microgrids and nanogrids. The hybrid storage system under analysis consists of two storage units with significantly different ratings and characteristics. These units are interfaced to the main system through dedicated power converters with coordinated control schemes to manage the power flows according to the system requirements. First, the most relevant candidate options for the power electronics topology configurations in nanogrid applications are selected. These target solutions are reduced to two possibilities: the first one based on independent bidirectional boost converters for each device, and the second one based on the series-parallel connection (SPC) of the storage units. Then, the basic control scheme is implemented at each of these topologies. This allows us to compare the performance of each solution not only considering static aspects, such as gain limitations, efficiency, or power density, but also considering the dynamic behavior. Finally, a strategy to design the control loops of the resulting HESS is proposed, studied, simulated, and experimentally implemented on a 2-kW demonstrator. The reported results show how the proposed control strategy applied to the SPC presents better performance in terms of dynamic behavior and steady-state operation.