Integrated Reconfigurable Converter Topology for High Voltage Battery Systems

Abstract

Traditional high-voltage battery systems are implemented using multiple battery modules connected in series, with each battery module made up of many series/parallel connected cells, to provide the required voltage and power. In the series-connected batterysystems, typically, separate converters for battery module balancing, bi-directional connection to a load and charging from an external power source are employed. In this thesis, anintegrated reconfigurable converter (IRC) topology for high-voltage series-connected battery storage systems is proposed. The main advantage of the proposed converter is thatit can be reconfigured to operate in a range of operating modes: ‘feeding a load’ from thebattery system, ‘feeding a load from a backup’ power source, ‘regenerative’ mode, ‘batterymodule balancing’ mode and ‘charging’ mode. The proposed topology shares semiconductor devices and an inductor among the operating modes which makes it compact. It alsoexhibits redundant modes which, together with a backup mode, increase its reliability.Furthermore, the proposed IRC topology minimises the batteries’ stress during chargingand discharging cycles. Operation of all modes is analysed and explained in detail.For the state of charge (SOC) ‘battery module balancing’ mode, two advanced strategies are developed. A distributed control strategy provides advantages in terms of reducedcommunication requirements and increased modularity, over a centralised battery management system. A load sharing balancing strategy distributes the load between the batterymodules based on their SOC and is useful in applications where simultaneous balancingand load supply is required.In the initial IRC configuration, the ‘charging’ mode is implemented using a unidirectional DC/DC buck converter supplied by an external DC source. To further extend theIRC flexibility, an enhanced IRC configuration with bidirectional energy transfer betweenthe battery modules and the AC grid is presented.The IRC topology can be used for hybrid energy storage systems (HESSs) as well. Theproposed integrated reconfigurable configuration for HESSs allows direct energy transferbetween the energy storage systems by using only a single converter and bypassing the DClink. This reduces stress on the DC link, reduces the DC link capacitor size and improvesoverall efficiency.All proposed IRC configurations and operating modes are experimentally verified