## 6 Conclusions and Future Trend Exploration  
This paper gave a comprehensive review of the proposed LVRT solutions used in DFIG-WTs, including the mature techniques applied in engineering practice and the latest theoretical achievements. All LVRT solutions were classified into three categories according to their location and the ways they work. Both advantages and disadvantages were discussed in this paper, and the modified topologies or methods were also given. External device installations inject specific circuits into DFIG-WTs by controlling the state of electronic power switches during grid faults. In this way, the wind turbine configurations can be modified to restrain stator overcurrent and rotor overcurrent, provide power support, and restrict overvoltage, such that the LVRT capability of DFIG-WTs can be enhanced. External retrofit techniques are widely used in pre-installed wind turbines for simplicity. However, economic concerns still act as a major barrier for external retrofit techniques, and the difficulties in modifying the original control structure also restrict the application of external installations. Although external modifications are not the future direction of LVRT solutions, the installation of external LVRT devices with specific function is necessary. Internal control techniques apply advanced control theories to strengthen the LVRT capability of DFIG-WTs. By making some compensations on voltage and current or improving the control performance in nonlinear uncertain systems, internal control modifications can provide better control effects during grid voltage dips. Because the LVRT capability of DFIG-WTs can be enhanced without any external auxiliary circuits, internal control techniques have economic advantages over external retrofit techniques. Thus, internal control techniques are preferred in newly installed wind turbines, and they have broad prospects for future development. Inspired by the summary and comparative results in References [1] and [4], the above-discussed LVRT techniques and their effects are presented in Tables 1 and 2 to indicate the functions of various LVRT solutions. In this table, \(\surd\) indicates that the chosen LVRT technique provides effective control to eliminate the grid fault effect, - indicates that the chosen LVRT solution has no effect on the fault, and * indicates that the chosen LVRT technique may deteriorate the grid fault effect. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline
**Active** & **Reactive** & **DC-Link** & **High** & **Rotor** & **Stator** & **Rotor** & **Stator** & **High** & **High** & **High** & **High** & **LVRT Capacity** \\
**Power** & **Power** & **Voltage** & **DC-Link** & **Voltage** & **Voltage** & **Voltage** & **Current** & **Current** & **Rotor** & **Rotor** & **Stator** & **Technique** \\
**Support** & **Support** & **Oscillations** & **Voltage** & **Oscillations** & **Oscillations** & **Oscillations** & **Oscillations** & **Oscillations** & **Voltage** & **Current** & **Current** & **Current** \\ \hline – & * & – & \(\surd\) & – & * & \(\surd\) & – & – & – & \(\surd\) & – & Rotor crowbar \\ – & – & \(\surd\) & \(\surd\) & – & – & – & – & – & \(\stard\) & – & DC-chopper \\ – & \(\surd\) & – & \(\surd\) & – & \(\surd\) & – & – & \(\surd\) & – & Series dynamic resistor \\ \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & – & \(\surd\) & \(\surd\) & – & \(\surd\) & – & Energy storage system \\ – & – & \(\surd\) & \(\surd\) & – & \(\surd\) & – & – & \(\surd\) & – & – & Series braking \\ – & \(\surd\) & \(\surd\) & \(\surd\) & – & \(\surd\) & – & \(\stard\) & – & – & STATCOM \\ – & – & – & – & – & \(\surd\) & – & – & – & \(\surd\) & Fault current limiter \\ \(\surd\) & – & \(\surd\) & \(\surd\) & – & \(\surd\) & – & – & – & – & Series grid side \\  & & & & & & & & & & & & converter \\ \end{tabular}
\end{table} 
Table 1: Influences of external LVRT techniques on low-voltage faults.