Wind power has outgrown its teething troubles. Wind turbines that convert the kinetic energy of the wind into electricity with generators of 500 kilowatts and more use proven technology and have significantly reduced the cost of wind power. Driven by a market demand for larger turbines, a technology has been developed for machines with rated outputs of three to five megawatts.
Denmark has set a goal of sourcing 50 percent of its electricity production from wind power. As a result, a development for offshore wind farms has begun. In addition to Denmark, the Netherlands, the United Kingdom and Sweden have gained experience with wind farms just offshore (so-called near-shore wind farms), based on existing technology from the wind turbine industry and from civil engineering and offshore contractors.
A wind turbine is a complicated device that brings together the latest knowledge in aerodynamics, mechanical and electrical engineering. The rotor extracts kinetic energy from the wind and converts it into rotational motion, which is then converted into electrical energy by a generator. The amount of energy in the airflow depends on the wind speed (to the third power). The wind speed at which the turbine begins to deliver its rated power is called the rated wind speed. At higher wind speeds, the rotor blades should be turned more into the wind to avoid mechanical or electrical overload. When the blades are turned even further into the wind, stall occurs, which greatly reduces the torque on the rotor and increases the braking resistance. With a good design, the blades automatically enter stall at high wind speed without the intervention of a control.
Three different types of generators are common in today’s wind turbines. The Danish type, so named because most Danish turbines operate that way, uses a gearbox to work up the rotor speed to drive the (induction) generator. In the induction generator (known in the industry as “squirrel cage” or squirrel cage generator), the rotor (the rotating part) is always slightly behind the rotating magnetic field offered by the stator (the enveloping solid part of the generator). That lag or “slip” varies a bit with the power delivered. Nevertheless, this type of generator is assumed to run at constant speed.
Such a constant-speed turbine is relatively simple in design, robust in construction and therefore reliable in operation. Disadvantages include the lack of regulation for active and reactive (reactive) power and the large fluctuations in power output when wind strength varies because there is no energy buffer available in the form of a large rotating mass. Another design weakness is the gearbox, which suffers greatly from varying wind force and varying torsion on the drive shaft.
Another common concept is the dual-fed induction generator. In this type of generator, both the stator and rotor have a three-phase winding, with which a rotating field can be generated by connecting a three-phase voltage. The rotor windings are fed from the mains by an electronic power converter. This compensates for the difference between the rotor speed and the mains frequency (the “slip” in the Danish type) by offering a rotor current with a variable frequency. This effectively decouples the speed and AC frequency. This makes it possible, within certain limits, to vary the rotor speed in order to reduce noise or to maximize electrical power. In practice, power electronics are mainly used to control reactive power. The position of the blades controls the incoming power of the wind.
The most recent concept is that of the direct-drive wind turbine without a gearbox. The conversion of mechanical to electrical energy here is done with a multi-pole synchronous generator suitable for low speeds. The rotor may contain windings or permanent magnets. The stator’s windings are not connected directly to the power grid, but via power electronics. This allows the turbine to be operated at variable speed while the wind power is controlled through the position of the blades.
As power electronics have become cheaper and more reliable, variable-speed wind turbines can be expected to be the future. In fact, variable speed offers the ability to set the optimal rotor speed for each wind speed, allowing more energy to be extracted from the wind than with a turbine running at a constant speed. That difference is greater than the losses that occur in the power electronics.
The trade-off between a double-fed or a direct-drive generator is complicated. With the dual-fed generator, the power electronics need only handle one-third of the rated power (which saves on cost), but a vulnerable gearbox is still needed. The direct drive has no gearbox, but the generator itself is larger and more expensive, and the power electronics must be heavier.
