Abstract
Ultracapacitors have recently received great attention for energy storage due to their small pollution, high power density, and long lifetime. In many applications, ultracapacitors need to be charged with a high current, where a multi-module charging system is typically adopted. Although the classical decentralized control method can control the charging process of ultracapacitors, there exists a problem that the charging current may be imbalanced among charging modules. In this paper, a cooperative cascade charging method is proposed for the multi-module charging system to reduce the current imbalance among charging modules. First, the state-space averaging method and graph theory are used to model the multiple-module charging system. Second, an effective cooperative cascade control is proposed, where the outer voltage loop stabilizes the output voltage to the desired voltage and the inner current loop guarantees the current of each charger to follow the target current. The block diagram is used to establish the closed-loop model of the charging system. In order to evaluate the proposed charging method, a laboratory prototype was established. Compared with the classical decentralized method, this method can effectively suppress the current imbalance, which is proved by simulation and experimental results.