## 2 Characterization and Modeling of the Fault Ride-Through Process in Doubly Fed Wind Turbines
### Structured Model of Electromechanical Transient of Doubly Fed Wind Turbine
#### 2.2.3 Fault Ride-Through Operation Active and Reactive Power Control Module  
In this paper, the fault ride-through active and reactive power control module adopts a three-segment control simulation method, as shown in Figure 8. The active and reactive power control of the model during the whole fault process is decomposed into three-segment control phases during the fault ride-through period (1), the starting point of the fault recovery (2), and the fault recovery process (3), and each of these control phases contains a variety of different control strategies. As shown in Figure 9, the control of the unit before and after the fault can be achieved by modifying the control strategy and key control parameters of each control phase for active or reactive power after determining the power priority. Figure 7: Structured model converter inner loop current control module. 1. Control modeling during fault ride-through;  
The active control methods commonly used during fault ride-through of structured models include specified active power, specified active current, and low-voltage-limited active current control. The reactive control methods are specified reactive power and  
Figure 8: Structured model failure three-segment control area map. Figure 9: Faulty three-segment multi-strategy control. specified reactive current. The mathematical expression for the active control methods during fault ride-through is as follows:  
\[\left\{ \begin{array}{l}P_{\text{ref}}=K_{p}\times P_{0}+P_{\text{set}}\\ I^{\prime}_{\text{dref}}=K_{\text{vp}}\times U_{I}+K_{\text{Id}}\times I_{\text{ d0}}+I_{\text{dset}}\\ I^{\prime}_{\text{dref}}=I_{\text{d0}}\\ I^{\prime}_{\text{dref}}=\frac{I_{\text{LvRT,UP}}}{\text{U}_{\text{UP}}-\text{U }_{\text{down}}}\times(U_{I}-U_{\text{down}})\end{array} \right. \tag{3}\]  
where \(K_{p}\) is the active power coefficient, \(P_{0}\) is the initial active power; \(P_{\text{set}}\) is the active power control value during fault ride-through; \(K_{VP}\), \(K_{\text{Id}}\) are the active current coefficients; \(I_{\text{d0}}\) is the initial active current; \(I_{\text{dset}}\) is the active current control value during the ride-through period; \(I_{\text{LVRT,UP}}\) is the upper current threshold value; \(U_{\text{UP}}\) is the upper voltage threshold value; and \(U_{\text{down}}\) is the lower voltage threshold value. The mathematical expression for the reactive power control mode during fault ride-through is as follows:  
\[\left\{ \begin{array}{l}Q_{\text{ref}}=K_{q}\cdot Q_{0}+Q_{\text{set}}\\ I^{\prime}_{\text{dref}}=K_{\text{vq}}\cdot(U_{\text{set}}-U_{\text{t}})+K_{ \text{Id}}\cdot I_{\text{q0}}+I_{\text{qset}}\end{array} \right. \tag{4}\]  
where \(K_{q}\) is the reactive power coefficient; \(Q_{0}\) is the initial reactive power; \(Q_{\text{set}}\) is the reactive power control value during the ride-through; \(K_{\text{vq}}\) and \(K_{\text{Id}}\) are the reactive current coefficients; \(U_{\text{set}}\) is the fault ride-through voltage threshold; \(I_{\text{q0}}\) is the initial reactive current; \(I_{\text{qset}}\) is the reactive current control value during the ride-through. 2. Fault Recovery Starting Control Model;  
The active control modes for fault recovery starting point control are specified as active current, active current during a fault, and active power during a fault; the reactive control modes are specified as reactive current, initial reactive current, and reactive power during a fault. The mathematical expression for the active control mode of fault recovery starting point control is as follows:  
\[\left\{ \begin{array}{l}I^{\prime}_{\text{dref}}=\min(K_{\text{Id}\_\_\_\_\_} \text{start}\cdot I_{\text{d0}}+I_{\text{dset}\_\_\_\_}\text{start},I_{\text{ d0}})\\ I^{\prime}_{\text{dref}}=I_{\text{d}FRT}\\ P_{\text{ref}}=P_{FRT}\end{array} \right.