Overview of Frequency Control Technologies for Wind Power Systems

In modern power systems, many conventional synchronous machines have been replaced by renewable energy resources, reducing the overall system inertia. The intermittent characteristics of wind power generation lead to the reduction of the frequency stability, which becomes a crucial issue. Additionally, the capacity of offshore wind farms is large; consequently, it requires that renewable energy sources like offshore wind farms support frequency regulation. In reality, wind turbines can provide frequency support by emulating the inertial control and droop characteristic of a conventional synchronous generator. The main purpose of this paper is to review and compare different control strategies of frequency regulation for offshore wind farms. Especially, the comparison between pitch angle control and over-speeding control based on the de-loading operating mode to provide power reserve is investigated in detail. In addition to wind farm itself, the transmission system can also coordinate wind farms to support frequency regulation. Thus, this paper also reviews various frequency control strategies for the VSC-HVDC connected offshore wind farm. By adjusting the DC-bus voltage of HVDC, the DC-link capacitor can absorb or release energy to provide frequency support. Furthermore, other auxiliary methods by energy storage system (ESS) or other associated elements for supporting frequency regulation are also discussed. Finally, this work provides a complete recommendation about frequency regulation techniques for offshore wind farms.