Interaction between the Voltage-Droop and the Frequency-Droop Control for Multi-Terminal HVDC Systems

MTDC systems need to provide a high level of reliability as well as efficient support to the interconnected AC grids. This can be achieved by equipping each converter with a dual controller combining both the voltage-droop and the frequency-droop control techniques. In this article, the theory behind the coupling between the two droops is investigated and an electromagnetic transient study of a 5-terminal HVDC grid is discussed to validate the theoretical results. The use of the dual controller allows the whole system to be more flexible. However, the frequency droop coefficient must be corrected to comply with the TSO´s requirements.