Frequency-response Analysis and Compensator Enhancement of Droop Control for VSC-HVDC

DC voltage droop control is suggested by a number of authors for multi-terminal VSC-HVDC systems, due to its power sharing capability and distributed control characteristics. However, the stability and robustness of such a closed-loop droop controlled system have not been addressed in detail. In this paper, analytical models capturing the key dynamics of the droop dc voltage loop are presented. Transient performance and stability margins of such models are assessed utilising frequency-response techniques. The effect of operating point, ac system strength, and other control loop designs are examined. Design considerations for the droop controller are also indicated by frequency-domain analysis. A more advanced compensator-based droop controller is introduced to enhance robust stability without degrading performance.