New VVI-VP strategy stabilizes offshore energy hubs during faults

A team from DTU and RWTH Aachen has published a paper proposing a unified current-limiting strategy for grid-forming modular multilevel converters (MMCs) used in offshore energy hubs. The strategy combines a variable virtual impedance (VVI) based on a smooth threshold function with a novel virtual-power (VP) mechanism derived from power dissipated in the virtual resistance. The VVI ensures current limitation during fault-induced overcurrents while preserving voltage-source behavior, while the VP mechanism adds a compensating power term into the synchronization loop, enabling automatic power redistribution among converters.

The paper's P-delta analysis further reveals that a more resistive VVI can improve transient stability of power-absorbing converters, and the VP mechanism further enlarges the stability margin. Electromagnetic transient (EMT) simulations validate that the combined VVI-VP strategy limits fault currents, maintains synchronism during severe faults, and achieves coordinated post-fault power sharing in fully converter-based offshore energy hubs. This addresses an understudied topic in literature, which mostly focuses on power-injecting converters rather than power-absorbing ones in HVDC-connected offshore wind farms.