Stable stepwise short-term frequency support of a DFIG-based wind farm

Abstract

To arrest the frequency nadir for a frequency event, the rapid release of kinetic energy stored in a doubly fed induction generator (DFIG)-based wind farm during the initial stage of an event is needed; however, a significant output reduction of a DFIG prior to the frequency nadir can cause a late but significant frequency decline. This paper proposes a stable stepwise short-term frequency-support scheme of a DFIG-based wind farm that is capable of improving the frequency nadir while quickly recovering the rotor speed. Upon detecting a frequency event, the power reference in the proposed scheme is increased by adding constant power to the reference prior to an event and maintaining it for a preset period determined in association with the frequency nadir. The reference then linearly decreases with the rotor speed to converge the rotor speed to a preset value. To quickly recover the rotor speed, at the preset rotor speed, the reference decreases with the rotor speed and time until it intersects with the reference for maximum power point-tracking control. Simulation results clearly indicate that the proposed scheme can improve the frequency nadir while promptly recovering the rotor speed under the various wind conditions.