Distributed control structures and their comparison in cascaded hybrid battery energy storage systems

A hybrid battery system within an energy storage system is gaining interests because it provides multiple advantages compared to the traditional single type battery system in terms of cost, volume, performance, and flexibility. Moreover, motivation on using second life ex-transportation batteries on the grid applications has given a boost on using hybrid batteries because vehicle batteries could be from different manufacturer, different sizes and capacities. A modular dc-dc converter with a battery per module is a reasonable way of integrating these batteries to a grid-tie inverter. This paper addresses the control aspect of such system and discusses two distributed control strategies which independently utilises each converter module within the system according to their module characteristics. A comparison of two control strategies in terms of operational stability and dynamic response both in charging and discharging conditions is presented to understand their suitability in this application. Detailed analysis, simulation and experimental validation of the control methods are included in grid connected mode to understand both control methods.