Kil-Hun Kim, “이중여자 유도형 풍력발전시스템의 계통 불평형 전압으로 인한 직류단 전압의 리플 저감,” 아주대학교 공학석사 학위 논문, 2010. > Paper

Wind Energy is one of the fastest growing renewable energy systems. The demand for connecting large-scale wind farms to the

Power grid is still on the rise. Many large scale wind farms employ DFIG (Doubly Fed Induction Generator) with variable speed wind

turbines. The DFIG is a wound rotor induction motor with its stator windings which is directly connected to grid and which rotor

winding is connected to the grid through a Back-to-Back converter. The Rotor Side Converter controls the active and reactive power

of the DFIG independently. The Grid Side Converter is used to regulate the dc-link voltage between the two converters and its power

factor is usually set to unity. To achieve it, various control strategies may be used. The distribution systems in these areas can be

usually unbalanced, because many wind turbines are connected in weak areas where heavy unsymmetrical loads, transitory faults,

voltage dip, and unsymmetrical transformer winding or transmission impedance. The unbalanced voltage on the grid side converter

may produce large voltage ripple in the dc link voltage, unequal non-sinusoidal output currents, unequal heating, and power loss.

And they may shorten the lifetime of the dc capacitor in the grid-connected wind turbines with DFIG. Even a very small unbalanced

voltage can create a large unbalanced current. These effects cause oscillation of torque and power at the generator that can lead to

speed pulsations, acoustic noise, increased losses and shorter insulation life. Based on the symmetrical components analysis,

oscillating torques are the results of the