Doubly-fed induction generator ride-through fault capability using resonant controllers for asymmetrical voltage sags

Modern grid codes worldwide determine specific requirements for the connection of wind energy conversion systems (WECSs) to the grid. One of the requirements is the ride-through fault capability (RTFC) or low-voltage ride-through (LVRT), which defines that the plants must not be disconnected from the grid at certain levels of voltage sags and also must contribute to network stabilisation. The LVRT of the doubly-fed induction generator (DFIG) technology is a well-known problem, mainly during unbalanced grid voltage conditions. The present study extends the studies presented in the literature, analysing the DFIG behaviour during asymmetrical voltage sags using a frequency-domain modelling. A modified resonant controller is proposed to improve the system response during voltage sags, and the analysis of the RTFC based on the converter limits is carried out. The proposed strategy is validated with simulation results of a 2 MW-WECS and experimental results of a 25 kW scaled test bench.