# A PMSG Wind Energy System Featuring Low-Voltage Ride-through via Mode-Shift Control
## 5 Simulation Results
### Case 2: 30% Symmetrical Grid Voltage Dip  
The system performance has been assessed for a three-phase symmetrical dip of 30% at the PCC occurring at t = 0.6 s for 150 msec. Similar to the 70% case, the system response was analyzed first with the conventional non-swap control methodology as shown in Figure 9, where the MSC continues to extract maximum power during the dip interval. The grid voltage \(v_{grid}\) drops to 0.7 p.u., while the grid current \(i_{grid}\) will increase above the nominal value. The GSC is unable to maintain the DC-link voltage \(V_{dc}\) regulated level due to the energy mismatch between the grid active power \(P_{grid}\), which has been significantly reduced, and the extracted generator power \(P_{gen}\). No reactive power support is provided during the fault; thus, the grid reactive power \(Q_{grid}\) is maintained at zero.  
Figure 10 shows the system responses when the proposed control is applied during the same three-phase 30% symmetrical voltage dip. At t = 0.6 s, grid sag is detected, MPPT is seized, and power converters control duties are swapped. The MSC will take control of the DC-link voltage regulation while the GSC will control reactive and active power injection into the grid. According to the proposed control scheme, since the grid voltage \(v_{grid}\) falls between 0.5 and 0.9 p.u., reactive power support is required, and active power is set, accordingly, to prevent grid over currents.  
This is achieved by setting the \(q\)-axis current \(I_{q}^{*}\) controller command using (36) the \(d\)-axis current command \(I_{d}^{*}\), which is set as to \(\sqrt{I_{N}^{2}-I_{q}^{*2}}\) to keep the grid currents \(i_{grid}\) at nominal value. Grid current THD during the dip decreased to 4% due to the proposed decoupled control. Meanwhile, the DC-link voltage \(V_{dc}\) is maintained by the MSC within safe limits for safe ride-through and limiting the rise compared to the conventional control technique. After the fault is cleared at t = 0.75 s, GSC and MSC actions are restored, MPPT by MSC, while GSC injects the extracted power into the grid and maintains fixed DC-link voltage. Speed and torque are retained, and the system fully returns to normal operation at t = 1.1 s.  
Figure 7: Simulation results during 70% symmetrical grid voltage without the proposed control scheme: (**a**) abc grid voltages \(v_{grid}\) and d– axis grid voltage \(V_{d}\) (p.u.); (**b**) abc grid currents \(i_{grid}\) and d– axis grid current \(I_{d}\) (p.u.); (**c**) d– axis currents \(I_{d}\) and reference \(I_{d}^{*}\) with q– axis currents \(I_{q}\) and reference \(I_{q}^{*}\); (**d**) DC– link voltage \(V_{dc}\) and reference \(V_{dc}^{*}\) (p.u); (**e**) grid active power \(P_{grid}\) and reactive power \(Q_{grid}\) (p.u); (**f**) boost chopper current \(I_{b}\) and reference \(I_{b}^{*}\) (p.u); (**g**) PMSG Line- line voltage \(v_{ab-gen}\) and phase a– current \(i_{a-gen}\); (**h**) PMSG active power \(P_{gen}\) (p.u); (**i**) PMSG speed \(n\) and torque \(T_{e}\) (p.u).  
Figure 8: Simulation results during 70% symmetrical grid voltage dip with the proposed control scheme: (**a**) abc grid voltages \(v_{grid}\) and d– axis grid voltage \(V_{d}\) (p.u.); (**b**) abc grid currents \(i_{grid}\) and d-axis grid current \(I_{d}\) (p.u.); (**c**) d– axis currents \(I_{d}\) and reference \(I_{d}^{s}\) with q– axis currents \(I_{d}\) and reference \(I_{d}^{s}\); (**d**) DC– link voltage \(V_{dc}\) and reference \(V_{dc}^{s}\) (p.u); (**e**) grid active power \(P_{grid}\) and reactive power \(Q_{grid}\) (p.u); (**f**) boost chopper current \(I_{b}\) and reference \(I_{b}^{s}\) (p.u) (**g**) PMSG Line-line voltage \(v_{ab-gen}\) and phase a-current \(i_{a-gen}\); (**h**) PMSG active power \(P_{gm}\) (p.u); (**i**) PMSG speed \(n\) and torque \(T_{c}\) (p.u).