The Research on Cluster DC Voltage Balancing of Star-Connected Cascaded H-bridge STATCOM

Star-connected cascaded H-bridge STATCOMs are widely used in power systems due to their modular performance, small output harmonics, and applicability to high voltage. However, the uneven losses of three phases and the unbalanced active power generated by unbalance grid voltages lead to the problem of phase to phase DC voltage unbalance in star-connected cascaded H-bridge STATCOMs. The clustered DC voltage balancing control using zero sequence voltage injection is suitable for star-connected cascaded H-bridge STATCOMs because no zero-sequence current is generated in the three-phase three-wire system. However, the conventional zero-sequence voltage injection method contains many complex operations, such as trigonometric operations, square root and inverse trigonometric operations, etc. These complex operations increase the computational burden of the controller. In order to eliminate the complex operations of the traditional method, this paper proposes an improved phase-to-phase DC voltage equalization control strategy by deriving the active power relationship of the three phases of STATCOM, which has no complex operations such as inverse trigonometric function, and also extracts the unbalanced power directly from the grid voltage as feedforward control, which improves the control effect and dynamic performance of phase-to-phase DC voltage equalization. Finally, the effectiveness of the proposed control method is verified by simulation.