In order to decarbonize the European electricity production, the European Commission set an ambitious target of 300 GW of offshore wind capacity by 2050. This large-scale offshore wind deployment and its integration in the energy system needs international coordination, long term policy targets and a robust regulatory framework. The North Sea Wind Power Hub (NSWPH) has been created in 2017 to cope with challenges related to the planning and pre-development of a series of hub-and-spoke projects based on a modular concept. It relies on the definition of elementary HVDC building blocks of bipolar 2 GW 525 kV DC MMC converters with metallic return  . In case of a contingency affecting the DC grid, part of the active power transmitted to the onshore AC grid will be lost. Depending on the type of contingency and the protection concept, this lost power can be recovered, totally or partially, after some time. This corresponds to the notions of Temporary Stop (TS) and permanent stop (PS) introduced in CENELEC standards . The amount of power that can be lost temporarily and permanently as well as the duration of the temporary stop shall then be specified to serve as input for the HVDC protection design as proposed in . From the onshore AC grid perspective, the impact of DC contingencies shall be minimized and DC faults shall never endanger the AC grid stability. In particular, the maximum RoCoF and minimum frequency (Nadir) should be limited to acceptable levels defined within AC grid codes. As a minimum, temporary and permanent stop characteristics should ensure that DC grid faults would never lead to unacceptable frequency deviations. There is currently no definition of the time frame of a temporarily stop. In guideline  only is defined when reserves should be half and fully activated. As MTDC grids can respond and recover faster than the current reserves it is important how this could benefit the design of the MTDC grid and adaption of the AC grid, to prevent over dimensioning.
Paul Verrax, ; Torwelle, Pascal; Bertinato, Alberto; Van Doorn, Joris; Kryezi, Fitim
Presented at CIGRE B4-Colloquium Vienna 2023