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Lithium-ion Energy Storage Management for DC Nano-Grid System

Fig. 1. Lithium-ion Battery Configuration
EM: Electric Module (Monitoring and Cell Equalization)
BMM: Battery Management Module.
Making electricity grids "smarter" and facilitating them with integration of renewable energy sources (RES) are fairly accepted as the necessary steps to achieve a sustainable power industry. However, serious concerns over reliability and performance require integration of energy storage as a critical part to buffer energy or provide arbitrage. Lithium-based battery offers high specific power/energy density, and gains popularities in many applications, such as (Plug-in) hybrid electric vehicle ((P)HEV) and renewable energy system.

This paper discusses two issues: (a) integrating lithium-based battery into a multi-renewable-energy-source (MRES)-feeding DC-bus nano-grid; (b) floating charge safety for a high voltage series-connected Li-ion battery pack. Based on the analysis of this two issues, this paper then proposes an integration strategy for lithium-based energy storage in the DC-base nano-grid, and the maximum cell voltage Vcell_max controlled floating charge for long string series connected lithium-ion cells pack. An 8.4kW bi-directional multi-phase dc-dc converter is designed to validate the proposed architecture and system performance.

Fig. 2. 3-φ Interleaved Bidirectional Dc-Dc Converter interface between DC bus and Lithium-ion battery pack.
Fig. 3. Battery system operating region (V-I Curve).
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