A Coordination Control With Extra Active Power Exchange Way to Extend Negative Sequence Current Compensation Range for STATCOM Based on Hybrid Cascaded Converter

The star-connected Cascaded H-Bridge (CHB) converter is one of the most attractive multilevel topologies for medium/high-voltage STATCOM. However, due to the fact that CHB has separated dc-links, three-phase unbalanced active powers cannot exchange automatically, which, thus, requires extra zero sequence voltage (ZSV) to eliminate them. Especially when negative sequence current is compensated, the negative sequence current compensation range (NCCR) is seriously limited. To extend the NCCR, this article combines the CHB and the converter with a common dc-link, which is utilized to exchange and eliminate the unbalanced active power generated from CHB. Based on this new active power exchange way and conventional ZSV injection, a novel cluster voltage balancing control method is proposed to eliminate the unbalanced cluster active powers. Moreover, based on the proposed control, the NCCR is identified, which is larger than that based on the conventional control. Finally, the proposed control and the derived NCCR are verified by simulation and experimental results on a 400 V/7.5 kvar Hybrid Cascaded Converter STATCOM.