VSC-HVDC control under unbalanced supply conditions

The Voltage Source Converter based High Voltage Direct Current (VSC-HVDC) technology has attracted much attention in recent years for its beneficial characteristics. The VSC-HVDC system is sensitive to voltage unbalance. However, voltage unbalance may occur in the three-phase power system frequently, due to unbalanced power supply, unbalanced loads and faults in ac system. It may require that the VSC-ffVDC system continues to function even under unbalanced conditions. Without dedicately designed controllers, the second harmonic voltage will appear on the dc bus. It increases the losses and may result in system instability. In the paper, a simple and effective control scheme is presented to improve the dynamic performances of VSC-HVDC system under unbalanced conditions. The controllers are implemented in two Synchronous Reference Frames (SRFs), namely the positive and negative SRFs, which rotate counterclockwise and clockwise. In addition, a new fast sequence components separation technique is also proposed. The simulation results show that the proposed control scheme performs well under both balanced and unbalanced conditions.