Individual Phase Current Control Based on Optimal Zero-Sequence Current Separation for a Star-Connected Cascade STATCOM Under Unbalanced Conditions

This paper proposes an individual phase current control (IPCC) method based on optimal zero-sequence current separation for star-connected cascade STATCOMs under unbalanced grid voltage. The IPCC is adopted to get better adaptation to unbalanced conditions. Each cluster of the cascade STATCOM has its own phase current control loop and dc-voltage feedback loop, and it can generate the necessary negative-sequence current automatically to rebalance power. However, the zero-sequence current which is unnecessary and harmful in star configuration is introduced in the process. An optimal zero-sequence current separation is presented to remove the zero-sequence current by reconstructing the reactive current command. This improved IPCC enables star-connected STATCOMs adapt to unbalanced conditions without the need for any power balancing algorithms. Also, the operation range of the proposed control method is analyzed. Simulation research studies are performed to verify the operation range analysis, and the experimental operation is tested on a 25-level +/- 10 Mvar/10-kV STATCOM product installed at a wind farm.