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 convtrol method for the star-connected cascaded H-bridge PWM converter in the static synchronous compensator (STATCOM) applications. The proposed control utilizes the zero-sequence voltage injection to accomplish the dc capacitor voltage balancing, and the this operation of zero-sequence voltage does not affect the original reactive power control. The proposed control method also works for the low-voltage ride-through operation. The control algorithm is verified with a 220-V 1kVA STATCOM based on star-connected cascaded PWM converter, and the test results verify that all the dc capacitor voltages are still regulated at the commanded value even as the grid voltage sag occurs.