Abstract
The doubly-fed induction generator (DFIG) based wind energy conversion system (WECS) is a mature and promising renewable energy solution amid growing global energy demand. As supply reliability and system resiliency become increasingly important, low voltage ride-through (LVRT) capability has become mandatory for such systems. During a voltage dip at the AC bus, where the WECS delivers power, it must stay connected and aid voltage recovery by supplying reactive power, as per local grid standards. While ON-grid LVRT strategies have been extensively researched, OFF-grid LVRT operations have received little attention. For greater source reliability, WECSs should also operate in OFF-grid mode, where similar voltage dip issues can arise. This paper proposes a hybrid LVRT strategy for DFIG-WECS in OFF-grid mode. A battery energy storage system (BESS) maintains the DC bus voltage, while a grid-forming converter (GFC) holds the AC bus voltage with its limited current capacity. The DFIG rotor disconnects from the rotor side converter (RSC) and connects to a crowbar during the voltage fall and rise period. Between these two periods, the DFIGRSC provides reactive power. The method is validated using a MATLAB/SIMULINK model in an OPAL-RT real-time simulation, demonstrating that under different wind velocities, the OFF-grid DFIG-WECS withstands different levels of voltage dips while supporting voltage recovery.