Protecting Collector Lines of MMC-HVDC-Connected Offshore Wind Farms: Insights on Communication-Assisted Schemes for Inverter-Dominated Grids

(IBRs), whose fault current behaviour differs significantly from that of synchronous machines.

For onshore wind farms, communication-assisted schemes such as differential protection are widely regarded as reliable; however, the operating assumptions on which these schemes are based are being challenged in offshore wind applications.

The expansion of offshore wind plants and the use of Modular Multilevel Converter-based

High-Voltage Direct Current (MMC-HVDC) links have added further complexity to the protection of collector circuits connecting offshore converter stations to wind turbines. In these systems, IBRs are present at both ends, and their control strategies affect not only the magnitude of fault currents but also their phase angle and sequence components. To investigate the resulting protection challenges, this work evaluates different differential protection approaches using detailed EMT simulations in PSCAD of a representative offshore wind farm, considering a wide range of internal and external faults, fault types, and resistances.

The study compares conventional phase current-based differential schemes with methods that exploit negative- and zero-sequence components. The results show that, depending on the

IBRs’ control strategies, traditional differential protections can lose sensitivity, leaving the system exposed to even severe faults. These findings are also relevant to future power systems with high shares of IBRs, providing guidance for operators, protection engineers, and researchers in recognising current limitations and defining more suitable protection strategies for converter-rich environments.