Design and control of bidirectional DC-DC converters for modular battery energy storage systems

Abstract

It has recently been shown that Distributed battery energy storage systems (BESSs) have several advantages over central battery energy storage systems. These include lightening the load on battery management systems (BMSs), reduced converter voltage ratings, increased reliability, and the ability to control individual battery modules` powers. This also gives an opportunity for flexible power sharing amongst different battery modules. However, these systems require numerous additional circuit components, including passive components, switches, and sensing and drive circuits. This dissertation focuses on creating a more efficient power sharing controller and reducing the number of components used in Distributed BESSs. Power sharing controllers, which consider the state of charge (SoC) of individual battery modules, and the system conversion efficiency, have been proposed. Initially, an adaptive power sharing controller has been presented, whose performance depends on the SoC mismatch level. An efficiency-based optimised power sharing controller is then discussed, whose performance is independent of SoC mismatch level. For this method, the system efficiency is maximised at all battery power conditions, regardless of the degree of mismatch. To demonstrate the proposed algorithms effectiveness, a six-module prototype system, with each module consisting of a half bridge DC-DC converter and a 10 Ah, 12.8 V LiFePo4 battery pack, was constructed for the experimental validation. The results show that the systems conversion efficiency has been improved up to 5.05 % in discharging mode, and 4.8 % in charging mode. In addition to the power sharing controller, a multi-port converter based Partially-Distributed BESS has also been proposed to reduce the number of circuit components in the structure. In the proposed structure, a group of battery packs is connected to their corresponding multi-port converter via enable / disable switches, with each multi-port converter connected in series. Therefore, different battery charging / discharging currents can be applied to the battery packs connected to the different modules. However, the battery packs connected to the same multiport converter are charged / discharged with the same current, and therefore, a power sharing controller with a two-level balancing feature is proposed for Partially-Distributed BESSs considering the module-level and pack-level imbalances. The SoC imbalance amongst different modules is corrected using the applied power difference based on the modules relative SoC calculated within the power sharing controller. Pack-level balancing amongst the battery packs that are controlled by the same multi-port converter is achieved using bypassing.

Experimental results for the proposed dual-port converter have been presented in both discharging and charging modes, and the results obtained validate the proposed schemes.