Disturbance Observer Based Control for Torque Ripple Mitigation in PMSG-Based Wind Turbine During Unbalanced Grid Voltages

This paper presents a composite current controller for Back-to-Back (BTB) converters in wind energy conversion systems (WECSs) to cope with balanced as well as unbalanced grid voltages. The composite controller consists of a combination of a state feedback controller with a disturbance observer (DO). The idea is to use an observer to estimate the offset, representing the mismatch between the actual system and the nominal model including unknown disturbances, which is then canceled by the state-feedback controller. The proposed controller is designed for both machine-side converter (MSC) and grid-side converter (GSC) to control the power flow injected into the grid under both balanced and unbalanced grid voltages. The proposed controllers for MSC and GSC are coordinated to control the power exchange between the grid and the permanent magnet synchronous generator (PMSG) with good power quality. More precisely, such a control scheme is used to mitigate the effect of the second harmonic oscillation in dc bus voltage that arises during unbalanced grid voltages. Simulation tests were performed to test the efficacy of the proposed controller considering both balanced and unbalanced grid voltages. The obtained results provide evidence to the effectiveness of the proposed controller to maintain a good quality current for both PMSG and the host grid even in the presence of voltage unbalance.