Post-Fault Power Flow Control for Multi-Terminal Bipolar HVDC Systems Connecting Offshore Wind Farms

Abstract

With increasing transport capacities required for offshore wind integration, bipolar HVDC systems are expected to replace state-of-the-art monopolar systems. Bipolar systems with a dedicated metallic return offer a 50% redundancy after single-pole DC faults and might therefore shorten outage times of offshore wind farms. To further extend this redundancy, and increase interconnection capacities, these bipolar links might to evolve towards multi-terminal HVDC networks. However, to utilize the full redundancy of bipolar systems, an adapted post-fault load flow control is required according to conceptual analyses in recent literature. AC-side coupling of the P- and N-pole offshore wind farms would allow to distribute power among the poles, but poses a challenge on the offshore wind farm control. Therefore, in this paper, a control concept is developed to redistribute power after faults in multi-terminal HVDC systems and avoid overload situations. A droop-based grid-forming control is adapted for the AC-side coupled offshore HVDC converters and validated via EMT simulations. It is shown for a 4-terminal network that the developed control concept allows to fully utilize the remaining transmission capacity up to a wind infeed of 75 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">%</sup> without curtailment of the offshore wind power. Without the developed concept, curtailment is required starting at 50%.