Grid code compatibility of HVDC interfaced offshore wind farm is evaluated in this paper. DC coupling causes power system contingencies not to get reflected in the offshore wind farm. In this regard, DC link voltage level is modulated to reflect the status of the onshore power system. Accordingly, onshore power system contingencies are reflected in the offshore wind farm by means of DC link voltage measurement and appropriate exercise of the wind farm HVDC converter. It is to excite dynamic frequency response and Low Voltage Ride Through (LVRT) capabilities of the modern wind turbines. A unified control structure is also proposed for doubly fed induction generator-based offshore wind farms that can model and compare three communication based, frequency modulation based, and voltage modulation based LVRT approaches. Moreover, a hybrid frequency and voltage modulation based approach is proposed in which an appropriate discrimination is embedded between fast transients related to LVRT condition and slow transients related to frequency contingencies. Accordingly, voltage modulation and frequency modulation approaches are utilized under LVRT and frequency events respectively. Simulation results have revealed similar dynamic frequency response capability for all four approaches. Moreover, except frequency modulation approach, remaining approaches have been able to fulfill grid code requirements given agile communication speed. It is concluded that the proposed hybrid frequency and voltage modulation, as the superior approach, has advantages like minimum LVRT DC voltage profile increase and subsequent deviation from theoperating point after the fault and preservation of conventional control structure thanks to appropriate dissociation between slow and fast dynamics.

Keywords: Doubly fed induction generator, grid codes, offshore wind farm, power reduction method, VSC-HVDC transmission line.

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