Control of a 6.6-kV Transformerless STATCOM Based on the MMCC-SDBC using SiC MOSFETs

This paper describes a transformerless STATCOM (static synchronous compensator) intended for 6.6-kV power distribution system in Japan. The STATCOM is characterized by the use of MMCC-SDBC (modular multilevel cascade converter - single-delta bridge-cells) topology and SiC MOSFETs. The paper discusses a control method developed for the 6.6-kV system for reactive-power and negative-sequence compensation with focus on dc-capacitor voltage control. The dc-capacitor voltage control is comprised of (a) inter-phase balancing control and (b) intra-phase balancing control. The latter uses a new technique that is based on the control of dead times of bride cells. A three-phase 200-V 6-kVA experimental system is designed, constructed and tested to verify the effectiveness of the control method.