## 2 Characterization and Modeling of the Fault Ride-Through Process in Doubly Fed Wind Turbines  
### Characterization of the New Energy Unit Fault Ride-Through  
The new energy grid connection standard serves as the basis for determining the control mechanism of new energy units. To summarize the typical characteristics of fault ride-through control among various new energy units, it is necessary to analyze and compare the new energy grid connection standards of different countries. Grid connection standards and FRT requirements for renewable energy units have been established internationally, with key regulations from China [39], the United States [40], the United Kingdom [41], Australia [42], Germany [43], Denmark [44], and South Africa [45], as comprehensively reviewed in [39; 40; 41; 42; 43; 44; 45]. The literature indicates that although the new energy grid connection standards of different countries vary in specific technical parameters, test conditions, and implementation details, the core principles and control objectives of FRT requirements exhibit significant commonalities. Common features:  
1 All standards require new energy units to maintain grid-connected operation for a certain length of time when the grid voltage drops to a certain level instead of going off-grid immediately and generally stipulate a deep drop tolerance of 0% to 20%. 2 Most standards emphasize the need for the unit to provide a certain amount of dynamic reactive power support during a fault by outputting reactive current proportional to the voltage deviation in order to help the grid voltage recovery. 3 In order to avoid power impact after fault removal, countries generally limit the active recovery rate, requiring the unit active power to gradually restore output at a rate of 10-20% of rated power/second. In addition, some new energy grids with high penetration rates also impose high voltage ride-through (HVRT) requirements to ensure that the units can continue to operate stably during voltage surges. These common features indicate that global new energy grid connection standards adhere to the core principle of \"fault ride-through tolerance, dynamic support, and orderly recovery\" in fault ride-through control. The differences are mainly reflected in the parameter setting and regional grid adaptability. In contrast, the convergence of control objectives lays the foundation for coordinating international standards and the generalized design of units. Figures 1 and 2 show the comparison of new energy low voltage ride-through (LVRT) and high voltage ride-through (HVRT) standards in different countries, which demonstrates the similarities and differences in voltage tolerance range and duration in different countries; Tables 1 and 2 show the comparison of specific requirements of low and high voltage ride-through technical parameters in different countries, including reactive current support ratio, fault ride-through recovery time, and other key indicators. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Country/Standard** & \begin{tabular}{c} **Voltage Spell** \\ **Range** \\ \end{tabular} & \
\begin{tabular}{c} **Reactive Current (Iq)** \\ **Support Formula** \\ \end{tabular} &
\begin{tabular}{c} **FRT Duration** \\ **Requirements** \\ \end{tabular} \\ \hline CHN (GB) & 0.2\(\sim\)0.9 UN & K \(\times\) (0.9-Ut) & 625 ms & Restore to 0.9 UN within 2 s \\ USA (FERC) & 0.15\(-\)0.9 UN & K \(\times\) (0.9-Ut) & 625 ms & Restore to 0.9 UN within 3 s \\ GBR (NGC) & 0.15\(-\)0.85 UN & K \(\times\) (0.85-Ut) & 140 ms & Restore to 0.85 UN within 2.5 s \\ DEU (EON) & 0.0\(-\)0.9 UN & K \(\times\) (0.9-Ut) & 150 ms & Restore to 0.9 UN within 1.5 s \\ DNK (UCTE) & 0.0\(-\)0.9 UN & K\(\times\) (0.9-Ut) & 150 ms & Restore to 0.9 UN within 1.5 s \\ ZAF (RSA) & 0.0\(-\)0.85 UN & K \(\times\) (0.85-Ut) & 150 ms & Restore to 0.85 UN within 2 s, \\  & & & & then to 0.9 UN within 20 s \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of LVRT technical parameters of new energy units in different countries.