# A PMSG Wind Energy System Featuring Low-Voltage Ride-through via Mode-Shift Control
## 8 Conclusions  
This paper introduces a cost-effective WECS featuring efficient LVRT capability via a proposed modification to the classical control technique without adding any auxiliary hardware devices. The PMSG is addressed rather than DFIG due to its superior fault tolerance capabilities. However, the PMSG-based WECS requires a full-power converter topology, which makes it an expensive alternative. To compensate for this tradeoff, a passive front-end rectifier assisted by a boost chopper is used as the MSC followed by a three-phase current-controlled VSI as the GSC. This topology is employed rather than the popular back-to-back topology since the former topology is assisted with fewer active switches, gate drive circuits and switching losses, especially at low and average wind speeds. Thus, converter cost, size, and implementation and control complexity are reduced, making it a favorable solution for low and medium power ranges and for distant WES generators. However, these merits are achieved at the cost of higher torque pulsations and noise. \begin{table}
\begin{tabular}{c c c} \hline \multicolumn{3}{c}{**Generator Type**} \\ \hline \multicolumn{3}{c}{**DHIG**} & **PMSG** \\ \hline Components’ limitations & \begin{tabular}{c} Gear box and slip rings \\ (require more maintenance) \\ \end{tabular} & \begin{tabular}{c} Multi-pole PMSG \\ (heavy and big) \\ \end{tabular} \\ \hline Reliability & Less & None \\ \hline Failure rate & More & Less \\ \hline VAR and P control & Partial control & Complete control \\ \hline LVRT capability & Less & None \\ \hline Converter capacity & Partial power converter & Full power converter \\ \hline Converter size and cost & less & None \\ \hline \multicolumn{3}{c}{**MSC**} \\ \hline Active front-end & \begin{tabular}{c} Passive front-end+ boost chopper \\ \hline Active switches and gate drives number & More & Less \\ \hline Size and cost & More & Less \\ \hline Implementation complexity & More & Less \\ \hline Control complexity & More & Less \\ \hline Switching losses at low and medium speeds & More & Less \\ \hline Switching losses at high speeds & Less & More \\ \hline Generator-side control & Full & Limited \\ \hline Generator torque ripples and noise & Less & More \\ \hline Generator current harmonics & Less & More \\ \hline \multicolumn{3}{c}{**Action during LVRT**} \\ \hline \multicolumn{3}{c}{Non-swap} & \
\begin{tabular}{c} Swap \\ \end{tabular} \\ \hline MSC & MPPT &
\begin{tabular}{c} Serre MPPT \\ DC-link voltage regulation \\ \end{tabular} \\ \hline GSC & DC-link voltage regulation and grid current \(d\)-component control & Grid current \(d\) and \(q\) components’ control \\ \hline Decouple between \(V_{de}\) and \(t_{pd}\) & No & Yes \\ dynamics & & \\ \hline VAR injection & No & Yes \\ \hline Surplus energy & In DC-link causing \(V_{de}\) and \(t_{gal}\) increases, otherwise extra & In rotor inertia with no need of extra hardware mitigating \\ hardware device is required & \(V_{de}\) and \(t_{gal}\) rises \\ \hline \end{tabular}
\end{table} 
Table 6: Comparison between the considered WECS components and applied control versus those of classical WECS systems [3; 4; 5; 6; 7; 8; 9; 10]. During voltage dips, the mode-shift (swap) LVRT feature, proposed in this paper, maintains the DC-link voltage and grid currents at safe boundaries without any need for additional hardware requirement. Moreover, it decouples the DC-link voltage control from grid current control during faults, which adds to system robustness. This modification serves the main objective of achieving an economic fault-tolerant flexible fault-ride-through solution applicable for existing or future PMSG-WECS. Besides, any energy surplus is stored in the rotor inertia; thus, the proposed approach is most favorable for MW direct-driven PMSG wind turbines, which possess a large moment of inertia  
Simulation results of the considered 1.5 MW PMSG-based system, under 70% voltage dip, verified the superiority of the proposed swap approach over the classical non-swap one.