Abstract
The development of efficient and reliable offshore electrical transmission infrastructure is a key factor in the proliferation of offshore wind farms (OWFs). Traditionally, high-voltage AC (HVAC) transmission has been used for OWFs. Recently, voltage-source-converter-based (VSC-based) high-voltage DC (VSC-HVDC) transmission technologies have also been considered due to their grid-forming capabilities. Diode-rectifier-based (DR-based) HVDC (DR-HVDC) transmission is also getting attention due to its increased reliability and reduced offshore platform footprint. Parallel operation of transmission systems using such technologies can be expected in the near future as new OWFs are planned in the vicinity of existing ones, with connections to more than one onshore AC system. This work addresses the control and parallel operation of three transmission links: VSC-HVDC, DR-HVDC, and HVAC, connecting a large OWF (cluster) to three different onshore AC systems. The HVAC link forms the offshore AC grid, while the diode rectifier and the wind farm are synchronized to this grid voltage. The offshore HVDC converter can operate in grid-following or grid-forming mode, depending on the requirement. The contributions of this paper are threefold. (1) Novel DR- and VSC-HVDC control methods are proposed for the parallel operation of the three transmission systems. (2) An effective control method for the offshore converter of VSC-HVDC is proposed such that it can effectively operate as either a grid-following or a grid-forming converter. (3) A novel phase-locked loop (PLL) control for VSC-HVDC is proposed for the easy transition from the grid-following to the grid-forming converter in case the HVAC link trips. Dynamic simulations in PSCAD validate the ability of the proposed controllers to ride through faults and transition between grid-following and grid-forming operation.
Highlights
Offshore wind is known as a reliable source of energy
The applicability of high voltage AC (HVAC) is limited to the offshore wind farms (OWFs) that are relatively close to the shore
The operation of HVAC, voltage source converters (VSCs)-High voltage DC (HVDC), and diode rectifiers (DRs)-HVDC transmission systems integrating a large offshore wind farm with three different onshore power systems was investigated in this paper
Summary
Offshore wind is known as a reliable source of energy. Compared to onshore wind, it has higher full-load hours and more constant power generation capacity across the year. Energies 2019, 12, 3435 based on voltage source converters (VSCs), which provide advantages such as smaller footprints and grid-forming capability The latter implies that the offshore HVDC terminal can generate (form) the AC voltage for the corresponding offshore AC network, which makes it possible to use the prevailing (grid-following) approach to controlling wind turbines. DR-HVDC transmission systems offer advantages such as smaller footprints, lower costs, higher efficiency, and higher reliability [5,7,9] Due to their lack of grid-forming capabilities, their use requires delegating such responsibility to, e.g., the wind turbines (WTs) [10] or their parallel operation with VSC-HVDC and/or HVAC links, which take the responsibility of forming the offshore.
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