An improved control method for limiting current in VSC-HVDC system under unbalanced conditions

An increase in the current of the converter's switching valves beyond their rated value and fluctuations in the DC-link voltage during unbalanced fault conditions are among the most significant challenges in voltage source converter-based high voltage direct current (VSC-HVDC) systems. These issues can significantly reduce the reliability of the system. Existing methods to address these problems tend to increase the complexity of the control system and involve high computational demands. Additionally, their performance may not be suitable for all common transformer configurations in VSC-HVDC systems. In unbalanced fault scenarios, this work proposes a straightforward way to manage the current and minimize DC-link voltage variations. The suggested approach successfully regulates the current across all typical transformer winding connections to the appropriate value. The studied system is simulated under double line-to-ground and single line-to-ground fault conditions in PSCAD/EMTDC. The results confirm the effectiveness of the proposed control method across all common transformer configurations in both two-terminal and multi-terminal VSC-HVDC systems.