Analysis And Control Of Doubly Fed Induction Generator Based Wind Turbine Under Grid Disturbances

The main objective of this work is to study the behavior of the doubly fed induction generator (DFIG) in wind energy conversion system (WECS) under grid disturbances. In this dissertation, vector control or field oriented control (FOC) is presented to analyze the behavior of the DFIG under grid disturbances. The flux linkage is used as a basic variable to present the DFIG model. The dq-axes transformation from the three-phase stationary reference frame to two-phase stationary reference frame and from two-phase stationary reference frame to two-phase rotating reference frame is achieved to create the whole DFIG model. The transformation between these axes is achieved in two steps. The first step is to transform the three-phase stationary reference frame of the machine to two-phase stationary reference frame, and in the second step transform the two-phase stationary to two-phases dq rotating reference frame before applying them to the machine model. The model of DFIG is achieved by using the dq parameters to representing the machine in a rotating reference frame. The grid voltage oriented control strategy is applied for the grid side converter (GSC) in order to maintain the DC link voltage constant and control the reactive power exchanged between the rotor and the grid. The vector of the grid voltage is aligned in phase with the d-axis, so the q-axis components becomes null. By assuming the supply voltage to be constant, the dq components of the grid currents can be used to control the active and reactive powers respectively. The phase locked loop (PLL) is used to detect the angular position of the grid voltage, and provides the phase synchronous angle for the dq transformation blocks. The DC link voltage is kept constant through the outer PI voltage controller by using the difference between the measured and the reference value of the DC bus voltage, and the output of this process is the reference d--axis grid current. The difference of the actual and the reference of the dq--axes of the grid converters currents is used to generate the suitable signals for the GSC which processed by the inner PI current controller. The stator voltage oriented vector control strategy is applied for the rotor side converter (RSC) by aligning the stator voltage with the d--axis to implement control action. The dq--axes rotor currents are used to control the DFIG in order to maximize the turbine output power. To simplify the control action, and to calculate the d--axis rotor reference currents, the magnetization current im is assumed to be constant, the stator flux component is assumed to be zero, and the leakage inductance is negligible. The dq--axes rotor reference currents are carried from the speed error through the PI controller and from the errors of the reactive power respectively, and then the appreciated signals is applied to the RSC. The models of GSC and the RSC are used to simulate the machine, and the results are carried out using MATLAB/SIMULINK.The results show:in normal condition, the GSC control strategy made a clear decoupled of active and reactive power, and the flux in the rotor as well as the stator maintained constant. When the grid voltage suddenly rises, the stator and the rotor currents are decreased, and therefore, the reactive power decreased. During the sudden reduction in the grid voltage, the stator and rotor currents are increased, and due to some amount of power absorb by the converters, the value of the DC link voltage increased and then settled to its rated value. The change in wind speed level increases the rotor speed, and the pitch angle will be in its optimal value to capture the maximum power from the wind, but when the wind speed is higher than the rated, the pitch angle will increase, and no more power will be captured. The small variation in the system frequency will not affect in the system reliability, but when the change in large value is happened, a clear disturbance in the system will be shown. When the reactive power demand increases, the rotor and stator currents are also increased, while the rotor speed is decreased.