Improved DC voltage utilization for the star-connected cascaded H-bridges converter under unbalanced voltage sags

The static synchronous compensator (STATCOM) are installed at the medium-voltage level of power system for grid voltage regulation and power factor improvement. The star-connected cascaded multilevel converter is widely for high voltage and high rated power STATCOM application. The voltage balancing control is a fundamental issue for cascaded converter, which can be implemented by negative sequence current and zero sequence voltage injection. The zero sequence voltage injection increases the DC voltage utilization, thus the DC voltage and cascaded number should be expanded to avoid over-modulation. However, the increase of DC voltage and cascaded number result in the high cost of the system but reduce the efficiency. On the other hand, the negative sequence current injection results in significant peak current and would trigger the over-current protection. This study provides a voltage balancing strategy to improve the DC voltage utilization by regulating the effectiveness between zero sequence voltage and negative sequence current injection. In addition, the proposed method is based on power flow management. All the unbalanced components of grid voltage and converter output current are considered, thus the proposed method can satisfy for grid fault operation. Laboratory test results are for proposed method variation and validate that the converter can accomplish system operation and manage DC voltage utilization during grid fault operation.