# A PMSG Wind Energy System Featuring Low-Voltage Ride-through via Mode-Shift Control
## 5 Simulation Results
### Case 1: 70% Symmetrical Grid Voltage Dip
During the sag, the MSC and GSC will switch actions as previously explained, where the MPPT is seized and the MSC takes charge of DC-link voltage regulation, which is achieved decoupled from the grid current control achieved by the GSC. This decoupled action prevents DC-link voltage ripple propagation to the grid, reducing grid current THD during the dip from 6% (when applying non swap control) to 3% (with the proposed swap control). The GSC takes control of the grid interface and supports by injecting reactive power into the grid while limiting active power injection. Thus, DC-link voltage rise and grid currents' increases are eliminated. According to the voltage dip depth, the GSC will operate to provide full reactive power support with no active power exported to the grid. In order to achieve the latter, the grid \(q\)-axis current reference \(I_{q}^{*}\) is adjusted to \(-I_{N}\) in accordance with Equation (36), while the \(d\)-axis current command \(I_{d}^{*}\) is set to zero; thus, grid current increase is eliminated. The DC-link voltage \(V_{dc}\) is now being regulated solely by the MSC to its rated value. Meanwhile, MPPT is halted, so the boost converter current \(I_{b}\) will drop to zero, allowing the energy surplus to be stored in the generator's rotating mass. Accordingly, PMSG line current \(i_{a-gen}\) and PMSG active power \(P_{gen}\) are reduced to zero in response to the MPPT termination. Consequently, there is a torque mismatch in the mechanical system turbine-generator, which causes the PMSG rotor speed \(n\) to increase due to the reduction \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Results during** & \multicolumn{2}{c}{**During 70\% Voltage Sag**} & \multicolumn{2}{c}{**During 30\% Voltage Sag**} \\ \hline \multicolumn{4}{c}{**No Mode-Shift Control**} & **With Mode-Shift Control** & **No Mode-Shift Control** & **With Mode-Shift Control** \\ \hline \(I_{b}\) (A) & 1 & 0 & 1 & 0.6 \\ \hline \(I_{d}\) (A) & 1.2 & 0 & 1.1 & 0.6 \\ \hline \(I_{q}\) (A) & 0 & \(-1\) & 0 & \(-0.8\) \\ \hline \(I_{grid}\) peak (A) & 1.2 & 1 & 1.1 & 1 \\ \%THD & 6\% & 3\% & 5.5\% & 4\% \\ \hline \(V_{dc}\) (V) & 1.2 & 1.05 & 1.1 & 1.02 \\ \hline \(P_{grid}\) (W) & 0.336 & 0 & 0.77 & 0.325 \\ \hline \(Q_{grid}\) (W) & 0 & 0.28 & 0 & 0.45 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Simulation performance results. in the electromagnetic torque \(T_{e}\)[27]. However, due to the large inertia of the mechanical wind turbine system, speed increase is maintained within the safety operating margins of the generator and does not exceed 130%. After the dip is cleared, both converters switch back to their normal control actions prior to the fault. The MSC achieves MPPT, controls the PMSG active power and maintains the generator speed. Speed \(n\) is recovered as well as the electromagnetic torque \(T_{e}\), and the energy stored in the inertia is transferred smoothly to the grid. DC-link voltage regulation, as well as grid synchronization at unity power factor, is restored by GSC, and the system recovers at t = 1.1 s.