Negative Sequence Voltage Control under Unbalanced Grid for Star-connected Cascaded H-bridge STATCOM

Under unbalanced grid, the active power among three clusters is distributed unequally for the star-connected cascaded H-bridge static synchronous compensator (CHB STATCOM), which will result in three unbalanced cluster voltages and affect the safety operation of STATCOM. Negative-Sequence-Voltage-Based cluster voltage balancing control has a large capability in regulating active power. To improve the dynamic response of cluster voltages under unbalanced grid conditions, cluster voltage balancing feedforward control is usually introduced. The negative sequence voltage in the conventional feedforward control is derived in $\alpha \beta$ frame, which introduces anti-trigonometric functions and makes the calculation very complex. In addition, the relation between positive sequence current control and negative sequence voltage is neglected, which makes the negative sequence from the feedforward control inaccurate. In this paper, a novel feedforward control algorithm is proposed. The novel algorithm uses the dc form representation in dq frame and takes the relationship between positive sequence current and negative sequence voltage into consideration. Hence, the proposed feedforward control is more straightforward and accurate. The proposed control scheme is verified by the experimental results from a 400 V/ ±5 kVar CHB STATCOM.