PUBLICATIONS

Wind energy is one of the fastest growing renewable energy systems. The demand for the connection of large-scale wind parks to

the power grid is still on the rise. Many large-scale wind parks employ the DFIG with variable speed wind turbines. The main

advantage of DFIG wind turbine systems is in the four-quadrant active and reactive power control, and the small converter size The

unbalanced voltages on the grid side converter may produce a large voltage ripple in the dc link voltage, unequal to the non-

sinusoidal output currents, unequal heating, and power losses. They may also shorten the lifetime of the dc capacitors in the grid-

connected wind turbines with DFIG. A very small unbalanced voltage can create a large unbalanced current. These effects can cause

an oscillation of the torque and power in the generator that can lead to speed pulsations, acoustic noise, increased losses and shorter

insulation life. The instantaneous oscillation of the three phase active power produces the oscillation in the dc link voltage. The

oscillation component has a frequency band which is twice of that of the grid frequency. Based on a symmetrical components

analysis, the oscillating torques are the result of the interaction of the different sequence components of the balanced and

unbalanced grid voltages and currents. This paper proposes a control method for the DFIG under unbalanced grid voltages. The ain

purpose is to reduce the torque oscillation and the dc link voltage for the control of both the RSC and the GSC.