Distributed model predictive control of all-dc offshore wind farm for short-term frequency support

From technical and economical viewpoints, an all-dc offshore wind farm (OWF) with a high-voltage direct-current (HVDC) connection, which is also known as HVDC-connected all-dc OWF, is the future trend for offshore wind energy applications. However, due to the decoupling effect of power converters, all-dc OWF cannot directly provide short-term frequency support for the onshore ac gird, that is, primary frequency response and inertia support. To address this issue, this paper presents a distributed model predictive control (DMPC) scheme for all-dc OWF. By directly suppressing the voltage deviation and the rate of change of voltage of the offshore dc collection network, this scheme indirectly decreases the frequency deviation and the rate of change of frequency (RoCoF) of the onshore ac grid. Meanwhile, the scheme ensures the stability of wind turbines in all-dc OWF during the frequency events. Considering a large number of wind turbines in the OWF, the corresponding optimization problem is solved in a distributed way using the alternating direction method of multipliers (ADMM), thus reducing the computational burden. Simulations including the performance comparison with droop control validate the effectiveness of this scheme.