An Improved Frequency Support Algorithm for MT-HVDC Systems

This paper develops a new control strategy for voltage source converter (VSC) based multi-terminal high voltage dc (MT-HVDC) grids for enhancing the mutual frequency support between MT-HVDC interconnected asynchronous ac systems. The existing droop control structure of VSCs is augmented with an additional feedback voltage-based loop that generates a voltage shifting term to be added to the original dc voltage reference. The addition of this integrator-based controller nullifies the difference between voltage errors seen at adjacent converter stations. This action helps to overcome inaccuracies in the mutual frequency support caused by the uneven voltages across a dc system. The proposed strategy uses only local variables for the continuous-time control; thus, it does not rely on a high-bandwidth communication network, and its performance is not affected by the change of ac systems' parameters. Modal analysis is carried out to investigate the stability of the proposed controller. Finally, comprehensive simulation studies are used to verify the proposed strategy's performance and compare it with recently reported alternative controllers.

Automated summary

This paper proposes a new control strategy to enhance the mutual frequency support between multi-terminal high voltage dc grids. By adding a feedback voltage-based loop and a voltage shifting term to the existing droop control structure, the proposed strategy nullifies the voltage error difference between adjacent converter stations. This strategy uses only local variables for control, does not rely on a high-bandwidth communication network, and is not affected by the change of ac systems' parameters.