Flexible Power Distribution Control in an Asymmetrical-Cascaded-Multilevel-Converter-Based Hybrid Energy Storage System

This paper proposes a novel control method for an asymmetrical-cascaded-multilevel-converter-based hybrid energy storage system (HESS), which includes one battery and several electrical double-layer capacitors (EDLCs). Traditionally, it is challenging to control the power distribution between different cascaded H-bridges by using different energy storage components as dc sources. In this paper, a feedforward space-vector-modulation (FFSVM) technique is used to distribute the active power between the battery and EDLCs. With the proposed voltage feedforward mechanism, the HESS can flexibly operate in the normal operation mode, the high-power output mode, and the reverse power absorption mode. The principle and effectiveness of the improved FFSVM technique are analyzed. Meanwhile, the state-of-charge-balancing mechanisms are introduced. The simulation results obtained in MATLAB/Simulink as well as the experimental results of a hybrid energy storage test bed based on the proposed system are presented to verify its performance.

I. Introduction

The cascaded multilevel converter (CMC) has been widely employed in applications as static synchronous compensators and motor drivers [1]– [7]. Since the CMC topology can reduce the voltage of an individual dc source, it is also suitable for an energy storage system (ESS). Meanwhile, its modular structure allows the usage of low-cost switching devices with low electromagnetic interference, low total harmonic distortion at low frequency, and high fault-tolerant capability [8]– [13]. Moreover, high energy efficiency is achieved via its transformerless single-stage configuration. These merits make the CMC suitable for medium- and high-voltage applications. The CMC can be classified into symmetric and asymmetric [14]. The latter uses unequal dc voltage sources to create more output voltage levels with the same number of power devices [15], [16].