# A PMSG Wind Energy System Featuring Low-Voltage Ride-through via Mode-Shift Control
###### Abstract  
Low-voltage ride-through (LVRT) and grid support capability are becoming a necessity for grid-tied renewable energy sources to guarantee utility availability, quality and reliability. In this paper, a swap control scheme is proposed for grid-tied permanent magnet synchronous generator (PMSG) MW-level wind turbines. This scheme shifts system operation from maximum power point tracking (MPPT) mode to LVRT mode, during utility voltage sags. In this mode, the rectifier-boost machine-side converter overtakes DC-link voltage regulation independently of the grid-side converter. The latter attains grid synchronization by controlling active power injection into the grid to agree with grid current limits while supporting reactive power injection according to the sag depth. Thus grid code requirements are met and power converters safety is guaranteed. Moreover, the proposed approach uses the turbine-generator rotor inertia to store surplus energy during grid voltage dips; thus, there is no need for extra hardware storage devices. This proposed solution is applied on a converter topology featuring a minimal number of active switches, compared to the popular back-to-back converter topology. This adds to system compatibility, reducing its size, cost and switching losses. Simulation and experimental results are presented to validate the proposed approach during normal and LVRT operation. wind energy conversion systems (WECS); PMSG; LVRT; symmetrical voltage sag/dip; machine-side converter (MSC); grid-side converter (GSC); swap control; grid interface; reactive power control +
Footnote †: journal: Applied Sciences  
able to perform reactive power compensation and smooth grid connection. Moreover, the generator is fully decoupled from the grid, which adds to its robustness against utility faults. This makes the WECS better in terms of fault ride through compliance that can be achieved without any additional hardware devices. In addition, wind energy conversion efficiency is the highest compared to other types of turbines [6]. Finally, these systems can achieve higher power density with multiple generators and by using the distributed drive train concept [3]. To summarize, PMSG-based wind turbines have gained great attention on wind farms due to their gearless operation, smaller maintenance cost, high efficiency, as well as their full controllability range, which makes them more favorable than their counterpart. Normally, WECS feature two power conversion stages that employ an AC/DC machine-side converter (MSC) at the generation end and a DC/AC grid-side converter (GSC) at the grid side. PMSG MSCs are full-scale power converters that must be rated the same as the generator capacity; thus, the system converter size, cost and complexity increase. Commonly, there are two types of MSCs: active front-end converters or passive front-end ones (diode rectifier) that can be assisted with a boost DC/DC converter. In an attempt to reduce the converter size and cost, passive front-end converter topology can be an attractive solution. It features less active switches and in turn less gate-drive circuits, reducing converter implementation and control complexity. Moreover, less switching losses are experienced, especially at low and average turbine speed conditions [7,8]. That is why, for low and medium power ranges, passive front-end converters are considered a predominant solution [9]. However, a trade-off for using this topology is met with higher torque ripples due to the uncontrolled nature of the generator stator currents [7,9,10]. The dramatic scale-up of wind energy penetration and integration to the electrical network result in several challenges in terms of utility, stability, and reliability. Consequently, revised grid codes urge wind power plants to maintain system continuity even under abnormal operation and transients. Among these challenges, LVRT capability has lately gained much attention as one of the main integration issues that have been standardized in many countries.