GRID POWER QUALITY WITH VARIABLE SPPED TURBINES (HYDRO TURBINES AND / OR WIND TURBINES)

Abstract

As the title of this dissertation indicates, variable-speed wind turbines are equipped with special synchronous generator drives which can run at any speed apart from their rated synchronous speed. This can be achieved with the help of the field rotor excitation control system of the generator which adapts the rotational speed of the wind turbine rotor (or the generator rotor, since there is no gearbox) to the wind speed over a relatively wide speed range, so as to produce a variable frequency voltage waveforms. But the electrical system should have a fixed frequency which should be equal to the grid frequency for the system to run in synchronism with the grid. A generator drive connecting a variable-speed mechanical system with a fixed frequency electrical system must therefore contain some kind of a slip or decoupling mechanism between the two systems for the variable speed operation to be possible. This mechanism is provided by the rectifier and the inverter circuit block in the present work. The main principle of this block is to convert the variable frequency (due to the variable speed) into a fixed frequency by rectifying and then inverting to a desired frequency: In wind turbine technology, the doubly-fed induction generator drive (Type C) and the full load converter connected generator drive (Type D) are the two most frequently applied variable speed generator drive concepts. There are also other variable-speed generator drive types, but they are currently not generally applied in wind turbines. It would probably also be possible to use written pole synchronous generators as a way of obtaining variable-speed capability in a generator drive [23]. The Type D variable-speed generator drive which is selected in this dissertation, consist of a traditional synchronous generator combined with power electronics to provide the slip or decoupling mechanism as said before. This configuration is relatively simple to implement and has many advantages over the only other configuration, i.e., Type C where a doubly fed induction generator is used.