The concept of hybrid wind power plants (HWPPs) that consist of wind, solar and batteries has received a lot of attention, since HWPPs provide a number of advantages thanks to the complementary nature of wind and solar energy and the flexibility of batteries. Nevertheless, converter-based technologies, as interfaces of HWPPs to the utility grid, contribute to the reduction of total system inertia, making the system more volatile and creating additional threats to frequency stability. To address these operational challenges, the capability of supercapacitors (SCs) to provide fast frequency reserve (FFR) is explored in this paper to enhance the frequency response of the HWPP. Two topologies for integrating SCs into the HWPP are proposed: (1) connecting SC to the DC link of wind turbine (WT) via a DC-DC converter interface, (2) directly connecting SC to the DC link of WT without converter interface. Frequency controllers at the asset level are proposed for these two topologies accordingly. The idea of the proposed frequency controller is to provide frequency response by varying SC voltage in proportion to frequency deviation, namely droop-based FFR. A practical SC sizing method for FFR provision is also discussed. The simulation results have shown, that in the case of frequency event, the proposed frequency controllers for SCs in both topologies positively contribute to the frequency of the system by reducing the rate of change of frequency by at least 5% and improving frequency nadir by at least 10%, compared to the case where the SC has no contribution to FFR. However, the capacitor size requirement for directly connected SC is more demanding in order to achieve the same level of improvement. The performance of frequency support has been highly related to total system inertia and control parameters. Therefore, any change to the severity of frequency events or control parameters calls for the reevaluation of the capacitance.
Operation and control of a Current Source Converter series tapping of an LCC-HVDC link for integration of Offshore Wind Power Plants