## 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
#### 2.2.1 Mechanical Side Control and FM Inertia Control Modules
In Figure 4, \(\omega_{\text{ref}}\) is the best reference value of generator speed under the actual wind speed; \(P_{\text{gen}}\) is the active power output from the generator; \(P_{\text{ref}}\) is the reference value of active power output from the operation state control module; \(T_{r}\) is the time constant of rotational speed measurement; \(T_{pr}\) is the time constant of electromagnetic power measurement; \(Kc\) is the transfer coefficient; \(K_{p}\), \(K_{\text{i}}\) are the coefficients of the rotational speed control proportionalintegral link; \(K_{pp}\) and \(K_{pi}\) are the electromagnetic power proportional integration link coefficients; \(\beta_{\max}\) and \(\beta_{\min}\) are the maximum and minimum limits of the pitch angle; and \(\mathrm{d}\beta_{\max}\) and \(\mathrm{d}\beta_{\min}\) are the maximum and minimum speeds of change of the pitch angle. 2. FM Inertia Control Module;  
The FM inertia control module is used to provide inertia characteristics similar to those of a conventional generator when simulating frequency changes. For example, when the frequency drops, the rapid control of power electronics and the inertia of the unit rotor are utilized to temporarily increase the unit's active power output and reduce the frequency drop for a short period. Since the FM inertia control module is mostly an optional function in new energy units, and its influence on system identification or steady-state operation is negligible, it usually presents a closed state in the structured model control. The control model of the FM inertia control module is shown in Figure 5.  
In Figure 5, \(f\) is the actual voltage frequency of the system grid point; \(f_{\mathrm{ref}}\) is the reference value of the system frequency; \(\Delta f_{\mathrm{ab\_pos}}\) is the dead zone of the positive frequency deviation; \(\Delta f_{\mathrm{max}}\) is the maximum value of the positive frequency deviation; \(\Delta f_{\mathrm{min}}\) is the maximum value of the negative frequency deviation; \(\mathrm{d}P/\mathrm{d}f_{\mathrm{UP}}\) is the power change rate corresponding to positive frequency deviation; \(\mathrm{d}P/\mathrm{d}f_{\mathrm{DN}}\) is the power change rate corresponding to negative frequency deviation; \(\mathrm{d}f_{\mathrm{max}}\) is the upper limit of the power change; \(\mathrm{d}f_{\mathrm{min}}\) is the lower limit of the power change; dbwi is the inertia control positive frequency deviation dead zone; \(T_{\mathrm{lpwi}}\) is the inertia control first-order inertia time constant; \(K_{\mathrm{wi}}\) is the inertia control scaling coefficient; \(T_{\mathrm{wowi}}\) is the inertia control differential time constant; \(P_{\mathrm{mxwl}}\) is the upper limit of the inertia control  
Figure 4: Wind turbine model pitch angle control module. Figure 5: FM inertia control module. power change; \(P_{\text{mwul}}\) is the lower limit of the inertia control power change; urlwi is the upper limit of the inertia control power change rate; and drlwi is the inertia control power lower limit of the rate of change of inertia control power.