Development of laboratory doubly fed induction generator for wind energy research

Abstract

This thesis studies the basic concept of doubly-fed induction generators (DFIG) and develops a laboratory model to simulate DFIG wind turbine generators (WTG). “Doubly-fed” refers to the three-phase stator and rotor windings, both of which have electric power exchange with the ac power system. Different from synchronous generators installed in coal, oil, gas, hydro, and nuclear power plants, asynchronous DFIG generators are widely used for wind energy conversion because of the diversity of wind power. Through the control of back-to-back PWM converters connected between the DFIG rotor and power system, a DFIG can operate at variable speed but constant stator frequency. Below rated wind speed, the DFIG controls the torque on the turbine shaft to track the best operating point (i.e. at best tip-speed ratio). Above rated wind speed, the pitch angle of the turbine blades is adjusted to limit the power captured from the wind. DFIG can provide power factor regulation by controlling the reactive power exchange with the grid. In this laboratory DFIG experiment, a DC motor is open-loop controlled as a prime mover with variable mechanical power output. A wound-rotor induction motor is mechanically coupled to the DC motor and operated as a DFIG. An IGBT inverter is connected to a variable DC voltage source in order to provide a controllable three-phase voltage applied to the rotor windings. The reactive power output is controlled by setting the magnitude of the rotor excitation voltage. The active power (torque), reactive power output and speed can be controlled by setting the frequency of the rotor excitation voltage. Through manually adjusting the DC input and rotor excitation voltage, the laboratory DFIG is able to operate at a variable simulated wind speed (4 – 25 m/s) with desired power output following the GE 1.5 MW WTG. As an advanced control strategy, decoupled d-q vector control for DFIG using back-to-back converters is studied. Under a stator-flux oriented reference frame, for the rotor-side converter, the rotor d- component (i.e. vrd, ird) controls the stator reactive power (rotor excitation current), while the rotor q- component (i.e. vrq, irq) controls the stator active power (electrical torque); for the supply-side converter, the d- component (i.e. vd, id) controls the DC-link voltage, while the q- component (i.e. vq, iq) controls the reactive power.