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High-voltage direct-current (HVDC) grids may provide fast frequency support to ac grids with the aid of supplementary control algorithms and synthetic inertia contribution from offshore wind farms. However, when all converters within the HVDC grid are fitted with these supplementary controllers, undesirable power flows and reduced power transfers may occur during a power imbalance. This is due to simultaneous frequency oscillations on the different ac systems connected to the HVDC grid arising during the support operation. To prevent this adverse effect, an auxiliary dead-band controller (ADC) is proposed in this study. The ADC modifies the dead-band set-point of the fast frequency controllers using measurements of the rate of change of frequency and frequency deviation. A four-terminal HVDC integrated with an offshore wind farm is modelled to analyse and study the effectiveness of three different supplementary fast frequency control algorithms. Results show that the proposed ADC scheme improves the performance of fast frequency control algorithms. For completeness, a small-signal stability analysis is carried out to confirm that a stable system operation is maintained.

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Conventional power system studies are normally performed using offline analytical tools and computer-based simulations. Hardware-in-the-loop (HiL) configurations are very helpful to carry out experimental tests in a safe environment without affecting the operation of a practical power system. Following this line, a real-time HiL (RT-HiL) platform suitable for a wide range of power system studies is presented in this paper. Emphasis is placed on the individual components of the platform. To demonstrate the capabilities of the RT-HiL configuration, two case studies are presented: subsynchronous resonance damping and fast frequency support provision. The ac grids are modeled in the RT simulator, while dc networks are implemented in a physical test-bed. The ability of the RT-HiL platform to emulate ac/dc interactions and grid support services is shown. For completeness, experimental results are compared against PSCAD simulation results-showing a good agreement. It should be emphasized that the presented experimental platform goes beyond conventional HiL setups instead representing a hybrid hardware-based emulator with RT-HiL simulation.