A Robust Power Control Strategy to Enhance LVRT Capability of Grid-Connected DFIG-Based Wind Energy Systems

This paper presents a new robust and effective control strategy to mitigate symmetrical voltage dips in a grid-connected doubly-fed induction generator (DFIG) wind energy conversion system without any additional hardware in the system. The aim is to control the power transmitted to the grid so as to keep the electrical and mechanical quantities above their threshold protection values during a voltage dip transient. To achieve this, the references of the powers are readjusted to adapt the wind energy conversion system to the fault conditions. Robust control strategies, combining the merits of sliding mode theory and fuzzy logic are then proposed in this paper. These controllers are derived from the dynamic model of the DFIG considering the variations in the stator flux generated by the voltage drop. This approach is found to yield better performance than other control design methods which assume the flux in the stator to remain constant in amplitude. This control scheme is compliant with the fault-ride-through grid codes which require the wind turbine generator to remain connected during voltage dips. A series of simulations scenarios are carried out on a 3 MW wind turbine system to demonstrate the effectiveness of the proposed control schemes under voltage dips and parameters uncertainties conditions.