Design and implementation of distributed control system for cascaded H-Bridge multilevel STATCOM

Concentrated control system (CCS) has been used for almost all reported cascaded H-Bridge (CHB) multilevel static synchronous compensators (STATCOM's). However, the CCS can not achieve “true” modular implementation. This paper presents a novel distributed control system (DCS), which combines one central controller and several cell controllers. The central controller is responsible for the upper-layer control including dc voltage loop and output current loop. The dc capacitor voltage balancing control and pulse-width modulation (PWM) algorithm are implemented by each cell controller. In DCS, each cell is treated as an independent power unit with its own controller. This makes modularization easier. Moreover, the software in the central controller and dc voltage detecting circuits become very simple compared with the traditional CCS. Some practical issues of the proposed DCS are discussed in detail. A novel dc voltage balancing method which can be implemented independently in each cell controller is proposed. A seven-level CHB STATCOM prototype is built. The validity of the proposed DCS and dc voltage balancing strategy is verified by the experimental results.