## 2 Characterization and Modeling of the Fault Ride-Through Process in Doubly Fed Wind Turbines
### Characterization of the New Energy Unit Fault Ride-Through
\begin{table}
\begin{tabular}{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} **Continuous Operation Time During Fault** \\ \end{tabular} \\ \hline CHN (GB) & 1.1\(\sim\)1.3 Un & K \(\times\) (Ut-1.1) & (1.1\(\sim\)1.2) Un: Remain connected for 1 s \\  & & (1.2\(\sim\)1.25) Un: Remain connected for 1 s \\  & & (1.25\(\sim\)1.3) Un: Remain connected for 0.5 s \\ \hline USA (WECC) & 1.1\(\sim\)1.2 Un & K \(\times\) (Ut-1.1) & (1.1\(\sim\)1.15) Un: Remain connected for 3 s \\  & & (1.15\(\sim\)1.175) Un: Remain connected for 2 s \\  & & & (1.175\(\sim\)1.2) Un: Remain connected for 1 s \\  & & & \(>\)1.2 pu: Tripping allowed \\ \hline AUS (NER) & 1.1\(\sim\)1.3 Un & K \(\times\) (Ut-1.1) & (1.1\(\sim\)1.3) Un: Remain connected for 0.6 s \\  & & & Recover to 1.1 Un within 0.9 s \\ \hline DEU (EON) & 1.2\(\sim\)1.4 Un & K \(\times\) (Ut-1.2) & (1.1\(\sim\)1.15) Un: Remain connected for 60 s \\  & & & (1.15\(\sim\)1.2) Un: Remain connected for 5 s \\  & & & \(>\)1.2 pu: Remain connected for 0.1 s \\ \hline DNK (UCTE) & 1.2\(\sim\)1.3 Un & K \(\times\) (Ut-1.2) & (1.2\(\sim\)1.3) Un: Remain connected for 0.1 s \\  & & & (1.1\(\sim\)1.15) pu: Remain connected for 2 s \\  & & & (1.15\(\sim\)1.2) pu: Remain connected for 0.16 s \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of HVRT technical parameters of new energy units in different countries. Figure 2: High-voltage ride-through standards for new energy units in various countries. As can be seen from the comparison of the waveforms of the new energy unit fault ride-through criteria in Figures 1 and 2, the fault ride-through process of new energy units can be divided into three segments, which are the fault initial phase, the fault ride-through period, and the fault recovery process, in which the fault initial phase starts from the instant of grid fault occurrence and lasts for a short period (usually from a few milliseconds to tens of milliseconds); the fault ride-through period is the stage from the fault stabilization to the fault clearance, and the duration depends on the grid standard (e.g., the low voltage needs to be maintained from 150 ms to 1.5 s); and the fault recovery process is the transition stage from the fault clearing (voltage restoration) to the return of the unit to the normal operation state. It lasts from hundreds of milliseconds to several seconds. Although there is a high transient risk in the initial stage of the fault, the crowbar or chopper protection circuits of the converter can respond quickly in milliseconds, making the risk in this stage completely controllable and having a limited impact on the system. The latter two phases rely on active control strategies, which must match the dynamic characteristics of the grid and are directly related to the unit's adaptability to the grid; therefore, their control performance is the most critical in the fault ride-through process. During the fault ride-through period, the short transient shock of the initial stage has been attenuated. The new energy unit switches to the fault ride-through control strategy, under which the new energy unit needs to maintain a specific non-off-grid continuous operation duration according to the depth of the fault voltage drop and outputs the corresponding reactive currents to support the recovery of the transient voltage; and in the stage of the fault recovery process, the unit has to be switched to the normal control mode again after the fault is cleared, and the external active current is output to the grid. During the fault recovery process, after the fault is cleared, the unit should immediately switch back to normal control mode. The external active and reactive power output should then be gradually restored to the expected value at a specific rate or according to a preset curve.