Active and Reactive Power Allocation Strategy to Maximize the Range of Negative-Sequence Current Compensation Based on Hybrid Cascaded STATCOM

Negative-sequence current compensation (NSCC) is essential to support the imbalanced grid and improve the grid power quality. In medium-/high-voltage applications, the cascaded H-bridge (CHB) static synchronous compensator (STATCOM) is advantageous due to its low voltage stress. However, the injection of negative-sequence current causes three-phase imbalanced active power. Although the introduction of zero-sequence voltage can transfer this part of active power, CHB's output voltage will increase accordingly, which restricts the NSCC. To increase the output capacity, the converter with a common dc bus, called power balance unit (PBU), is integrated into the CHB to obtain a hybrid cascaded (HC) structure to freely exchange the imbalanced active power via the common dc bus. In order to achieve the maximum NSCC, it is necessary to minimize CHB's output voltage in this HC STATCOM. This article reveals the mathematical relationship between active power, reactive power transferred by PBU, and the CHB's output voltage for the HC STATCOM. On this basis, a novel control strategy is proposed to minimize the CHB's output voltage by transferring both active and reactive power from PBU. Thus, the range of NSCC is maximized. Finally, the proposed strategy is experimentally verified on a 400-V/7.5-kVar HC STATCOM platform.