Meeting Great Britain’s (GB) 2030 net zero targets requires major transmission upgrades, including new onshore and offshore infrastructure to support large-scale offshore wind integration. National Energy System Operator (NESO) Pathway to 2030 Holistic Network Design (HND) outlines a coordinated plan, introducing additional HVDC links and moving toward a hybrid AC/DC grid. From a planning perspective, the shift toward converter-based resources brings challenges like low inertia, control interactions, and stability concerns. Within the HND, a radial topology has been proposed for the south cluster of wind farms on the east coast, where three offshore wind farms directly connect to an onshore AC point of common coupling (PCC) at Birkhill Wood substation via separate symmetric monopole (SM) HVDC links. Additionally, a bipolar HVDC link will be connected adjacently, at a new substation in Lincolnshire, to provide an offshore embedded HVDC link with Scotland.

In this work, undertaken by The National HVDC Centre for NESO, generic modelling of the south cluster portion of the HND is undertaken in the RTDS/RSCAD EMT simulation environment to evaluate dynamic performance and operational scenarios. The south cluster HVDC links model the onshore converters as average value MMCs, and offshore converters as two-level grid-forming VSCs. Subsea AC and DC subsea cables are modelled as frequency-dependent, and the offshore wind farms (OWFs) are aggregated type-4 turbines incorporating electrical and mechanical dynamics. Each SM HVDC connection to the offshore wind farm (OWF) is rated at 1.2 GW with a DC voltage of ±320 kV. The onshore AC system is modelled using publicly available GB network data published by NESO, incorporating AC Thevenin equivalent sources to reflect local fault level characteristics.

A comprehensive set of energisation, steady-state, and dynamic performance scenarios are assessed to support NESO with recommendations on operational requirements. Time-domain steady-state analysis is carried out to study control performance across nominal ranges of voltage and power, and to assess the coordinated response of the parallel HVDC-OWF systems to AC voltage and frequency events. The steady-state analysis is further complemented by frequency-domain studies, including AC impedance scans, to support stability assessment and resonance identification. The dynamic performance of the system was evaluated across a range of disturbance scenarios, including onshore and offshore AC faults, DC faults, HVDC converter blocking events, and wind farm disconnections. The outcomes of these studies provide NESO with operational guidance on the potential implications of such schemes for future GB system operation.

This work recommends that connection studies be approached holistically, ensuring coordinated performance and resilience under varying operating dynamics. A detailed framework for testing and verification of the performance of the south cluster as whole is recommended as opposed to distinct compliance studies to mitigate interactions and control hunting effects. Time and frequency-domain testing incorporating OEM black-box models is advised to validate harmonious response against system events.