Voltage-source converter control under unbalanced grid conditions for a variable speed wind energy system

This paper presents a grid-side model and control for small wind turbines connected to the grid using a back-to-back converter. The control is developed to transmit all power generated by an induction machine to the grid and to match grid requirements. The grid synchronization is achieved by a Double Synchronous Reference Frame Phase Locked Loop (DSRF-PLL) even during grid faults or unbalanced conditions. The study of the DC-bus power leads to the compensator design which regulates the DC-bus voltage to a reference signal. Active and reactive power control is implemented so the system is able to act on each power independently. Thus, a vector oriented field control on currents is built to transfer all the power from the wind turbine to the grid and to keep the power factor to 1. Simulation results for the system performances under unbalanced conditions are presented.