Stability of DC voltage droop controllers in VSC HVDC systems

Future multi-terminal HVDC systems are expected to utilize dc voltage droop controllers and several control implementations have been proposed in literature. This paper first classifies possible dc droop implementations in a simple framework. Then, the small-signal stability of a VSC-based converter station is analyzed for all the identified droop control schemes. The stability range for the system is determined as a function of the droop gain and is used to compare the flexibility and robustness of the implementations. The comparisons reveal that the droop implementations based on the ac current and dc voltage achieve a wider stability region than the other schemes and a limited sensitivity to the droop gain.