# Dynamic Frequency Support from a DFIG-Based Wind Turbine Generator via Virtual Inertia Control
## 1 Introduction  
As a renewable energy, the wind power integrations have been continuously increasing during the last few years due to air pollutants and energy shortages. Variable-speed wind turbine generators (WTGs) are the most dominating type of wind generation and replace the conventional synchronous generators due to their advantages, namely, maximum mechanical power capture, reduced acoustical noise, and reduced mechanical stresses on the turbine [1; 2]. The types of variable-speed WTGs include: doubly-fed induction generator (DFIG)-based WTG (type III) and full-scale converters-based WTGs (type IV). Compared to the full-scale converters-based WTGs, the DFIG-based WTGs are economically viable, as a partial-scale converter system is employed. Thus, as stated in [3], DFIG-based WTGs constitute more than 50% of the installed WTGs. However, large integrations of DFIG-based WTGs may bring severe challenges on electric power systems. Frequency stability, which is one type of severe challenge, must be solved. The reason is that DFIG-based WTGs decouple the rotor speed from the system frequency; thus, they are unable to provide inertia response. Furthermore, the online system inertia becomes weak [4; 5]. Such inertia issues become significant with the penetrated level of wind increase. Therefore, the worsened maximum rate of change of system frequency (_dfdf_) and system frequency nadir may trigger under frequency load shedding relays [6; 7]. Hence, DFIG-based WTGs are required to provide frequency support capability to guarantee the system frequency stability of power systems with high penetrated windenergy [8; 9]. As studied in [10], the kinetic energy available from the DFIG-based WTG is 5.25 times that of a synchronous generator when the DFIG-based WTG operates at rated wind speed. Hence, DFIG-based WTGs are better options for supporting the dynamic system frequency. In [11], DFIG-based WTGs can provide frequency support capability using two possible ways [12; 13; 14; 15; 16; 17; 18; 19; 20]. The first way injects active power to the electric power grid using reserve power from DFIG-based WTGs [12; 13]. However, to obtain reserve power, de-loading operation is needed. This means that DFIG-based WTGs deviate from maximum power point tracking (MPPT) operation using pitch-angle control or over-speed control of the DFIG-based WTG, thereby resulting in significant wind energy loss. The second way injects active power to the electric power grid by releasing the kinetic energy from DFIG-based WTGs without de-loading operation [14; 15; 16; 17; 18; 19; 20]. To provide dynamic frequency support, the authors of [14] suggest employing the _dfdf_ loop and the frequency deviation (\(\Delta f\)) loops as the additional control loops to emulate virtual inertia control response and droop control response, respectively. As the focus of this paper is investigating the effect of the _dfdf_ loop on the dynamic system frequency, only the virtual inertia control is discussed here. In [15; 16], the calculated _dfdf_ is used as the input signal to derive the additional power for virtual inertia control. In [17], the performance of virtual inertia control from the DFIG-based WTG with different control gains is investigated. To increase the frequency stability, the authors of [18] suggested using the maximum _dfdf_ for virtual inertia control. However, the use of the fixed control gain of the virtual inertia control limits the contribution to supporting the dynamic system frequency and has a possibility of resulting in stalling of the DFIG-based WTG in the low region of the rotor speed. In [19; 20], a rotor speed-dependent inertia of virtual inertia control is addressed to improve the frequency stability; however, the performance of improving the frequency stability is limited due to the small control gain, particularly for a disturbance. This paper designs a virtual inertia control strategy with purposes of increasing the system frequency stability and preventing the wind turbine from stalling.