## 1 Introduction
The E.ON requirement for voltage support proposes that wind turbines should provide reactive current when the voltage reaches the dead band limit, and larger reactive current injection is needed when the voltage dips go deeper, as presented in Figure 3. The reactive current support in response to severe voltage dips must be achieved within 20 ms of fault detection [12]. Figure 1: Basic structure of doubly fed induction generator (DFIG). Figure 2: Wind turbine German low-voltage ride-through (LVRT) grid criteria for voltage dips. To meet the demands of GCRs, the behavior of the DFIG during and after voltage dips should be controlled as follows: (1) overcurrent and overvoltage in stator and rotor windings should be restrained to avoid damaging the power converters; (2) speed of turbines and electromagnetic torques should be properly designed to guarantee safe operation [13]; (3) DC-link voltage should be kept constant to ensure the desirable operation of RSC and GSC [14]; (4) enough reactive power is needed to promote grid recovery [15]. To solve these problems, various methods including external retrofit techniques and internal control techniques were proposed to ameliorate the LVRT capability of DFIG-WTs. The review of LVRT techniques can serve as a reference to analyze the existing control methods and identify the prospects for further improvements in this area. In Reference [9], a comprehensive review about the grid code regulations enforced on large wind power plants was presented. However, the specific LVRT techniques which fulfill various grid code requirements were not covered in this paper. A technical review of LVRT techniques applied in DFIG-WTs was discussed in Reference [4], while a significant portion was dedicated to LVRT protection circuits and hybrid combinations. In Reference [16], relevant literature about LVRT strategies was studied regarding when DFIG-WTs are connected to an alternating current (AC) network or voltage source converter (VSC) based high voltage direct current (HVDC). However, this paper mainly focused on the power surplus of two-terminal systems and DC voltage control of multi-terminal systems. A review on LVRT techniques for improving the transient stability of DFIG-WTs was proposed in Reference [1], where LVRT solutions were divided into protection circuits, Flexible Alternating Current Transmission System (FACTS) device-based methods, traditional control methods, and advanced control techniques. However, the lack of comparative analysis results may restrict its application value. This paper gives a comprehensive review and evaluation of the proposed LVRT techniques used in DFIG-WTs to analyze the current level of LVRT techniques. The main contributions of this paper are summarized as follows: firstly, this paper offers a remarkably up-to-date review of the state of LVRT techniques applied in DFIG-WTs, especially the latest achievements in this area. Secondly, novel classifications are proposed in this paper based on internal/external retrofits and the location of modifications. The analysis results and discussions show the potential of LVRT techniques and provide future research directions based on this work. This paper is organized as follows: Section 2 presents the classification of LVRT techniques. Section 3 gives an introduction of rotor-side external retrofit techniques. Stator-side external retrofit techniques are described in Section 4. Section 5 introduces internal control techniques. Section 6 concludes this paper and discusses future research trends.