## 1 Introduction  
With issues regarding the energy crisis and climate degradation becoming increasingly prominent [1; 2], developing renewable energy such as wind power has become a major demand for many countries to implement national energy security and low-carbon strategies [3; 4]. For example, in order to cater to the augmenting energy needs, the government of India has established a target of installing 175 GW of renewable sources, which includes 65 GW of wind energy [5]. Additionally, the European Commission has taken the lead regarding energy targets by publishing the ambitious plan \"A Clean Planet for All\" and setting the target producing 300 GW of wind power by 2050 [6]. Due to wind power's excellent characteristics, such as being cost-effective and the small capacity of the power converter used [7], DFIGs have been widely popular in wind power applications [8; 9]. However, the stator winding of a DFIG is directly connected to the power grid [10], so it is easy for it to be affected by voltage fluctuations in the power grid [11; 12]. Therefore, it is very challenging to assess how to complete the fault ride through of DFIG and reduce its negative impact on the power grid, and how to analyze the fault characteristics of DFIG during the faults. In [13], an asymmetrical fault ride-throughcontrol strategy was proposed by considering the voltage capacity limitation of a rotor side converter (RSC), and this strategy could fulfill the fault ride through of DFIG in an severe asymmetrical fault. In [14], an air-gap flux feedback control strategy was designed and applied to a VSG-controlled DFIG to accelerate the decay of transient components. When using this method, DFIG can behave like a synchronous generator and stabilize the voltage and frequency during faults. In [15], a scaled current tracking control was proposed for DFIG to enhance the low-voltage ride-through capacity without flux observation, which can suppress the overcurrent and overvoltage of DFIG rotor. However, during severe faults, enhancing hardware measures need to be taken for fault ride through. In [16], an internal model control controller was proposed to improve the fault ride-through capabilities, which takes into account the power limitations of DFIG converters and the DC link voltage during the faults and consequent crowbar activation. In [17,18], the coordinated control of crowbar and chopper protection was considered to achieve the fault ride through of DFIG. In [19], the crowbar circuit was proposed to assist the RSC to improve fault response, which can divert the transient overcurrent of both fault initiation and clearance. In [20], the crowbar was utilized to suppress the rotor overcurrent and the releasing time of the crowbar was analyzed. In [21], an active crowbar circuit composed of silicon-controlled rectifiers was presented to achieve the fault ride through, and this design could strengthen the reliability of the circuit due to the high surge capability of silicon-controlled rectifiers. In [22,23], a novel protection scheme was proposed in which a superconducting fault current limiter is connected in series with the rotor winding of the DFIG or the DFIG outlet, which can effectively limit the maximum value of the fault rotor current or improve the fault voltage at the outlet of the DFIG. In [24], a modular multilevel converter was employed for the DFIG to suppress the fault current, which can provide the transient damping of the DFIG rotor and stator currents during severe grid faults to achieve the fault ride through. In [25], a modulated series dynamic braking resistor control strategy was presented to enhance the capacity of DFIG fault ride through. This scheme can offer fault voltage compensation and provide power evacuation to reduce the power imbalance during the fault periods. Analyzing the fault characteristics of DFIGs during fault ride through is of great significance for power system protection.