Interaction between the voltage-droop and the frequency-droop control for multi-terminal HVDC systems

Abstract : Multi-terminal DC systems must provide a high level of reliability and may have to support the interconnected AC grids. This can be achieved by providing AC/DC converters with a dual controller combining both the voltage-droop and the frequency-droop control techniques. In this article, the coupling between those droops is theoretically investigated and quantified, and a proposed corrected value of the frequency-droop parameter allows the system to comply with the TSO's requirements with regards to the frequency support of the AC grids despite the unintended interaction of the two droops. A dynamic frequency-droop parameter is then proposed to also cope with the controller limits. Finally, an electromagnetic transient study of a five-terminal HVDC grid is performed to validate the theoretical results.