Autonomous wind turbines with Doubly-Fed Induction Generators

In this paper, a vector control mechanism has been suggested and applied to simulate the operation of a Doubly-Fed Induction Generator (DFIG) attached to an isolated off-the-grid wind turbine, to demonstrate that these generators can be used at lower costs to control the power quality in stand alone wind turbines. In order to achieve this end, the alignment of the d-axis of the synchronously rotating reference frame is imposed along the stator flux vector. A wide range of sub and super synchronous operating speeds are investigated using a 5th order model. Artificially generated wind speeds are used to acquire variable speed operation. Twenty seconds simulations are presented by feeding the stator d and q current components. The input current components have been drawn from the simulation of a rotor short circuited, grid connected DFIG, starting from rest. The synchronous frequency ωs, and stator voltage components Vrefds and Vrefqs are imposed as constant demands in the control equations. Simulations are carried out by independently iteratively solving the differential equations for the magnetizing current and the two rotor current components. Solutions of these equations respectively generate the demands for the d-component of the rotor current, and the two components of the rotor voltage. The demand for the second component of the rotor current is obtained directly from the field alignment condition. The stator current input data also includes a fault induction response to a stator voltage step change. Simulation results faithfully reproduce the initial conditions, indicating the validity of the vector control scheme for stand alone operations of the DFIG.