Distributed State-of-Charge Balancing Control Strategy for Modular Super Capacitor Energy Storage System

Abstract

State-of-Charge (SOC) balancing control is one of the issues to be settled in energy storage system (ESS). In this paper, a distributed SOC balancing control strategy is proposed for the super capacitor energy storage system based on modular multilevel DC/DC converter. Independent voltage and current loops are constructed for each sub-module. In order to achieve SOC balancing control, the SOC of each super capacitor module (SCM) is introduced into the current loop, so that the sub-modules can regulate their average operating current according to their own SOC. Thus, the SOC of SCMs can be balanced during system dynamic charging and discharging process. The operating principle of the proposed method is similar with the droop method used in parallel system, therefore this strategy can keep each sub-module independent of each other during operation modes and enhance the modularity of the system. Besides, the structure of the strategy does not change with the increase of the number of series modules, so it is conducive to the extended application of the modular energy storage system. The basic principle and operating characteristics of the proposed strategy are analyzed. Finally, the proposed strategy is validated effectively by experiment results.