###### Abstract  
The increasing level of wind power penetration is seriously threatening the frequency stability of the power system. In this article, we suggest an enhanced frequency response strategy of a doubly fed induction generator (DFIG) based on over-speed de-loaded curve using a novel power function to boost the frequency nadir and settling frequency and reduce the maximum rate of change of frequency (ROCOF) with more efficiency. To achieve this objective, the reference power increases to the torque limit at the de-load operating point and then decreases with the rotor speed toward the maximum power point tracking operating conditions. The simulation results on various wind power penetrations clearly demonstrated that the enhanced frequency response strategy is beneficial to boosting the frequency nadir and settling frequency and reduce the ROCOF. wind generation; frequency support; DFIG; low inertia power system; frequency stability +
Footnote †: journal: Applied Sciences  
**Citation**: [PERSON]; [PERSON]; [PERSON] An Enhanced Frequency Response Strategy of a DFIG Based on Over-Speed De-Loaded Curve. _Appl. Sci._**2021**, _11_, 9324. [[https://doi.org/10.3390/app11199324](https://doi.org/10.3390/app11199324)]([https://doi.org/10.3390/app11199324](https://doi.org/10.3390/app11199324))  
Academic Editor: [PERSON]  
Received: 12 September 2021  
Accepted: 4 October 2021  
Published: 8 October 2021  
**Published**: **Note**: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. **C Copyright**: (c) 2021 by the authors. License MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ([[https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/)]([https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/))). **1. Introduction**  
As one of the most popular renewable energies around the word, wind generation has developed rapidly in recent years. With the increasing level of wind power penetration, more and more doubly fed induction generators (DFIGs) replace the synchronous machines (SGs) in power systems [1, 2]. However, since DFIGs are connected to the grid employing power to electronic devices and operate in maximum power point tracking (MPPT) control, DFIGs are unable to provide frequency response service which consists of inertia response and primary frequency response [3, 4, 5]. As a result, the power system frequency control capability would decline, causing further frequency instability issues [6, 7, 8]. The frequency response functions provided by the DFIGs can be roughly divided into two types: short-term frequency support responses and long-term frequency support responses [2]. The former frequency response scheme releases the rotational kinetic energy without reserve power to participate in frequency regulation based on the rate of change of frequency (ROCOF), frequency deviation, or both [6, 7, 9, 10, 11, 12]; the latter frequency response scheme assumes that the DFIGs are operating at sub-optimal control (de-loaded operation) achieved by over-speeding and pitching, and releases the reserve power to support the system frequency [13, 14, 15, 16, 17, 18, 19]. Comparing to the former, the latter frequency response can provide continuous reserve power to supply the deficit power as the SGs. The authors of [9, 10, 11, 12] employed ROCOF and frequency deviation control loops as the supplementary control signals for participating in frequency support based on the de-loaded operation. The benefits of these schemes are mainly dependent on the setting of the control gain of the ROCOF and the frequency deviation control loops [9, 10, 11, 12]. However, the response of the previously mentioned frequency support strategies is slow, and the unchanged control coefficients are used in these control strategies so as to limit the benefit of boosting the frequency capability.