A DC-Link Voltage Control Strategy for Fast Frequency Response Support

To address the decreasing power system inertia challenges due to the increasing penetration of Renewable Energy Sources (RESs), this paper proposes a Fast Frequency Response (FFR) technique for wind turbines equipped with Doubly Fed Induction Generators (DFIG). The DC-link of a DFIG unit can be controlled to partially exchange energy in case of frequency transients, thus providing FFR that in turn helps maintain grid stability. This control is achieved by levelling up or down the DC-link voltage. Supercapacitor units connected to the DC-link provide the required energy for FFR services. We perform a detailed analysis to achieve the expressions of the minimum DC-link voltage required for the proper operation of the grid side converter and the factors affecting its value. We then propose a procedure to estimate the optimal value for FFR controller gain and a DC-link protection scheme that decouples the FFR controller design from the grid characteristics. The method can be applied to any RES using converter with sufficient energy buffer in order to provide FFR service.