Zero-Sequence Voltage Injection for DC Capacitor Voltage Balancing Control of the Star-Connected Cascaded H-Bridge PWM Converter Under Unbalanced Grid

This paper presents a dc capacitor voltage balancing control method for the star-connected cascaded H-bridge pulsewidth modulation converter in the static synchronous compensator applications. With the zero-sequence voltage injection as a basis for dc capacitors voltage balancing, this paper investigates the detailed power flow of the converter as a whole and within individual modules under unbalanced grid voltages and then proposes a method to control the reactive power output and the dc capacitors voltage by precisely managing the power flow. This approach enhances the low-voltage ride-through capability, which is very critical as the penetration of distributed energy resources grows rapidly. The proposed control method is verified by a scaled-down prototype in the laboratory.