# Fast Frequency Response of a Doubly-Fed Induction Generator for System Inertia Support
## 1 Introduction  
Wind power has rapidly grown during recent decades because of the concerns about CO2 emissions and a lack of fossil fuel [1]. As conventional thermal generators are gradually replaced by converter-interfaced variable-speed wind turbine generators (WTGs), the system inertia of all online synchronous generator fleets is reduced [2]. This is because WTGs are unable to respond to the frequency change following a disturbance [3]. Such reduced inertia issues become significant as the penetration level of wind increases. Accordingly, to lessen these impacts, some countries specify needs for the short-term frequency support (STFS) of WTGs to release the rotating kinetic energy of WTGs to support the system inertia [4, 5].  
To arrest the frequency change, STFS methods switch the power reference from the maximum power tracking (MPT) operation power reference to a novel reference function following a disturbance [6, 7, 8, 9, 10, 11, 12]. The authors of [6] defined the active power reference as a function of time to support the system frequency. However, the same incremental power is used for any wind speed condition; furthermore, a severe second frequency drop (SFD) is caused because the output power is instantly reduced torestore the rotor speed. To lessen the size of an SFD, the output power is reduced in a ramp manner when recovering the rotor speed [7; 8]. Nevertheless, parameters of the power reference are mutually interdependent, raising difficulties of setting parameters in the power reference for STFS. Besides this, the short-term overproduction capability and its behavior were analyzed in [9; 10]; however, a severe SFD would be caused when the power reference is changed to the deceleration period because of a large power reduction [9]. In addition, the small difference between the mechanical and electrical powers of WTGs delays the rotor speed recovery [10], therefore, the WTG harvests less mechanical power.  
To heighten the frequency stability and avoid the stall of the rotor speed, the active power reference is defined in the rotor speed domain [11; 12]. [PERSON] et al. [11] suggested an STFS method considering the torque limit. Nevertheless, the increased power is set as an inverse proportional function of the rotor speed. A stable adaptive STFS is addressed that adds a constant power to the reference for MPT operation [12]. Unlike the scheme in [11], the incremental power is set as a proportional function of the rotor speed. However, the rotor speed convergence is slow, which thereby releases more rotating kinetic energy when the system frequency is rebounded, and further delays the rotor speed recovery. Furthermore, a small power reduction is used for recovering the rotor speed; the slow rotor speed recovery is inevitable, which is adverse to capture more mechanical input power by WTGs.  
This paper addresses an enhanced STFS method of doubly-fed induction generators (DFIGs) to preserve better performance of the system frequency nadir with less released rotating kinetic energy and speed up the rotor speed recovery. To preserve better performance of the system frequency nadir with less released rotating kinetic energy, a rotor speed dependent incremental power is used. For speeding up the rotor speed recovery with a small SFD, the proposed reference smoothly reduces with the rotor speed and time. In addition, this study assumes that DFIG operates in MPT operation mode without preset power reserve. The performance of STFS based on an electromagnetic transient program restructured version (EMTP-RV) simulator is validated under various wind speed conditions.