Doubly fed induction generator (DFIG) still shares a large part in today's wind power market. It provides the benefits of variable speed operation cost-effectively, and can control its active and reactive power independently. Crowbar protection is often adopted to protect the rotor-side voltage source converter (VSC) from transient overcurrent during grid voltage dip. But under unbalanced grid voltage condition, the severe problems are not the transient overcurrent, but the electric torque pulsation and dc voltage ripple in the back-to-back VSCs. This paper develops dynamic models in MATLAB/Simulink, validates it through experiments, investigates the behavior of DFIG during unbalanced grid voltage condition, and proposes new controllers in separated positive and negative sequence. Methods to separate positive and negative sequence components in real time are also developed, and their responses to unsymmetrical voltage dip are compared. Simulation results prove that the separated positive and negative sequence controllers limit the torque pulsation and dc voltage ripple effectively.

A hybrid wind park concept which contains both constant speed wind turbine and variable speed wind turbine is proposed in this paper, as it can provide similar power quality and similar low voltage ride through capability with relatively low investment. The objective is to optimize the configuration of a hybrid wind park which has low levelised production cost, high power quality, high reliability and high low voltage ride through capability. This multi-criteria multi-objective optimization problem is solved by a design approach ldquoProgressive Design Methodologyrdquo, which is divided into three stages. The final solution has the lowest levelised production cost with acceptable flicker level and LVRT capability.

Today, doubly fed induction generator (DFIG) gains the dominant market power in the wind energy industry. It provides the benefits of variable speed operation cost-effectively, and can control its active and reactive power independently. Crowbar protection is often adopted to protect the rotor side voltage source converter (VSC) from transient over current during grid voltage dip. But under unsymmetrical voltage dip, the severe problems are not the transient over current, but the electric torque pulsation and DC voltage ripple in the back to back VSCs. This paper develops dynamic models in Matlab/Simulink, investigates the behavior of DFIG during an unsymmetrical voltage dip, and proposes new controllers in separated positive and negative sequence. Methods to separate positive and negative sequence components in real time are also developed, and their responses to unsymmetrical voltage dip are compared. Simulation results prove that the separated positive and negative sequence controller limit the torque pulsation and DC voltage ripple effectively.

The large offshore wind farms are planed to be connected with power grid at transmission level due to their large power capacity and possible long transmission distances. HVDC thyristor bridge as well as other integration methods requires systematical integration studies. In this paper, thyristor bridge models developed in Matlab/Simulink are fast and accurate enough for electromechanical and electromagnetic simulations.