## 3 Fault Ride-Through  
As mentioned in the introduction, there are different methods to improve the FRT capability of the DFIG. Advanced control methods are discussed mostly in the research papers. These methods control active and reactive power injections, of the DFIG, to restore the PCC voltage during the fault. Most of these methods are complicated and thus they are not so interesting for the industry [16]. Furthermore, during the zero-voltage sag in the PCC, the terminal voltage, as a reference, will be lost and the control method might be inapplicable [17]. Another method is to change the configuration of the DFIG. In this situation, the changes happen in the back-to-back converters. A fault-tolerant DFIG-based wind turbine has been proposed to enhance the FRT capability [10; 18]. The configuration of the proposed DFIG is shown in Figure 4. Instead of utilising a general, six-semiconductor-switch converter, a nine-switch GSC has been used. In normal operation, three lower switches are in the on state. When a fault occurs, three lower switches operate to compensate the voltage on the neutral side of the winding in the stator. Another change in the configuration of the conventional DFIG has been studied in [11; 19]. The proposed scheme is shown in Figure 4, in grey. Similar to the conventional DFIG, the stator is directly linked to the grid through the terminal. However, an extra converter, which is connected to the star point of the stator winding, is employed to provide an effective active power transfer to the grid, and a better power  
Figure 3: **(A)** The doubly-fed induction generator (DFIG) configuration, with all possible highlighted locations for the fault current limiters and series dynamic braking resistors, in the fault ride-through capability improvement. RSC: Rotor side converter; GSC: Grid side converter; PCC: Point of the common coupling. **(B)** The schematic of control circuits for: **(a)** The rotor side converter; and **(b)** the grid side converter [15]. Adapted with permission from [15], Copyright Publisher, 2017.  
system disturbance ride-through during the low voltage. As it is clear from Figure 4, these changes in the configuration of the conventional DFIG are probably not of interest to industry. However, to be practically implemented, they should have economical justification. Adding extra hardware is another scheme to improve the FRT capability of the DFIG. Crowbar protection, DC braking chopper, voltage sag compensation devices, and fault current limiting devices are employed as extra hardware in the DFIG to enhance the ride-through capability during the fault condition. The crowbar protection is the most well-known scheme, which is practically employed and utilised in the FRT improvement of the DFIG. The configuration of the crowbar is shown in Figure 4. The crowbar includes a three-phase diode bridge rectifier, a bypass resistance, and a switch (either a semiconductor switch, such as an isolated gate bipolar transistor in the active crowbar, or a switch such as a thyristor in the conventional crowbar). The active crowbar has been introduced to overcome the continuous absorption of the reactive power by the squirrel-cage induction generator in the conventional crowbar [7]. To operate the active crowbar by a semiconductor switch and cease the RSC switching, a threshold current or a threshold voltage is defined regarding the transient over-current in the rotor side or the over-voltage in the DC link, respectively. Whenever the measured current voltage is less than the threshold current voltage, the DFIG converters resume to normal operation [8]. The RSC stop and the reactive power absorption by the active crowbar are the main disadvantages of this method. DC braking chopper is another scheme to keep the DC-link voltage value in the safe area of operation during the fault condition. The schematic of the DC chopper is depicted in Figure 4. The parallel connected configuration of the DC chopper consists of a resistor in series with a semiconductor switch.