Optimal DC Current Control of Sound Pole for Suppressing Transient Overvoltage under Monopole Blocking

The monopole blocking of the LCC-HVDC system can lead to transient overvoltage at the sending-end AC system. Hence, this paper first investigates the power interaction characteristic between AC and DC systems under monopole blocking. Furthermore, by decoupling DC current from AC voltage, the sound pole commutation failure criterion after monopole blocking considering the commutation bus voltages at both ends is derived. Then, an optimal DC current control strategy for suppressing transient overvoltage is proposed. By adjusting the DC current order adaptively, the proposed strategy could have the ability to suppress the sending-end transient overvoltage and sound pole commutation failure effectively. Finally, the effectiveness of the proposed strategy is verified in the CIGRE HVDC bipolar system model in PSCAD/EMTDC.