Transient stability improvement for DFIG wind turbines using ultracapacitor

Wind energy source is characterized as variable and unpredictable. A random wind speed and blade rotational turbulence can produce fluctuations on the voltage and power supplied into the system. This fluctuated power makes the wind power undispatchable, causing frequency deviations and power outage when wind power penetration is significant. The fluctuating power will impact on power balance and voltage at the point of common coupling. Output power of wind turbine is cubic function of the wind speed. Since wind speed is a nearly random parameter, the output power of the wind turbine is also random process. Even a small variation of wind speed could cause a large variation in the output power. As a result, a large voltage fluctuation may cause voltage variations outside the regulation limit at connection point.;Energy storage devices such as batteries, ultra capacitors, super inductors, and flywheels can be utilized in a hybrid system to solve this problem. A selective control method to mitigate the power fluctuations using the rotor inertia is introduced in the literature. This method is also modified to obtain better energy capturing efficiency. The energy extracting capability using this method is comparable with other methods such as Maximum Power Extraction (MPE) algorithm. In this thesis, a new integrated topology of DFIG wind turbine and Ultracapcitor is introduced. The Ultracapcitor is directly placed on the DC bus of the power conversion device on the rotor. A control technique is developed to adjust the active and reactive power of the turbine, apply power smoothing, and keep the DC bus voltage within an acceptable range.;Matlab Simulink simulation results for various cases are performed on a doubly fed induction generator that verifies the theoretical analysis.