## 3 Fault Ride-Through
An anti-parallel diode is also utilised to protect the chopper switch from voltage spike when the semiconductor switch goes to the off state. However, the DC braking chopper does not effectively restrict the rotor transient over-current. Therefore, high-rated anti-parallel diodes should be employed in the RSC [9]. Similar to the crowbar scheme, when the DC chopper is activated, the RSC switching is ceased. A static synchronous compensator (STATCOM), as the voltage sag compensation device, has been discussed in [20]. Figure 4 shows the location and simple configuration of the STATCOM. Due to the low capacity of back-to-back converters, in case of the fault in the weak utility, there is a possibility of a voltage instability. Accordingly, voltage sag compensation devices, such as the STATCOM, have been proposed to support the voltage profile during the fault condition. There are  
Figure 4: The doubly-fed induction generator (DFIG) configuration with the most well-known fault ride-through (FRT) enhancement approaches. DVR: Dynamic voltage restorers; STATCOM: Static synchronous compensator. coordinated and independent reactive power controls between the STATCOM and the DFIG [20; 21]. In the independent control method, the voltage control is only done by the STATCOM. The STATCOM should be incorporated with a fault current limiting method because it is not solely able to restrict the fault current level in the RSC. Also, the dynamic voltage restorer (DVR) is another voltage sag compensation device, which have been utilised to overcome voltage sag during the fault. In Figure 4, the simple configuration of the DVR is presented. Similar to the STATCOM, auxiliary methods should be incorporated with the DVR, such as changing the control method of the back-to-back converters or the pitch angle control [22]. It should be noted that these mentioned devices require large storage capacity and a high number of semiconductor switches. From a fault current limiting point of view, the FCLs and the SDBRs are employed to restrict the fault current level in the DFIG. Up until now, many configurations of FCLs have been proposed and studied. The FCL structures can be divided into two categories, with regard to impedance type and components. Considering the impedance type, they are resistive, inductive, resistive-inductive, and resonance type FCLs. Meanwhile, whether a superconductor is utilised or not, there is another category [23]. Recently, a new approach of applying the FCLs has been proposed that has the capability of producing variable resistance [24]. Considering the component type, the FCLs are either a solid state or saturated core transformer [14]. Despite the current limiting characteristics of the FCLs, they have been employed for different applications. From a transient stability point of view, the application of variable and controllable resistance FCLs has been proved [24]. Regarding the location of the FCLs, they are also employed to compensate the voltage sag in the PCC [25]. In order to improve the FRT capability of the DFIG, the FCLs are placed in different locations of the DFIG. Figure 3A shows the studied locations in the literature. In each location, with respect to the FCL impedance type, the impacts on the key parameters of the DFIG are different during the fault condition. From the FCL's location point of view, the FCLs are mostly located at the terminal side and the stator side of the DFIG. Some of the FCLs are placed in the rotor side and a few of the configurations are in the DC link. Meanwhile, from an impedance point of view, the FCLs are mostly the resistive type, as it is clear from Figure 2. In the following section, it will be discussed how the impedance type and the FCL's location could affect the FRT capability of the DFIG based wind turbine. The potential best choice of location and impedance type, in order to obtain the most favourable FRT capability of the DFIG, will be mentioned. In the following section, the FCLs' structures are categorized based on the impedance type.