6. Wind Turbine Generator Technology
This section briefly describes the main types of WTG currently available. A WTG typically consists of two or three blades39 connected to a hub to form the rotor assembly. The rotor hub connects to a shaft which turns a generator, usually through a gearbox. The electrical output of the generator is then fed to the grid either directly or through a system of power electronics that converts it to the correct grid frequency and voltage.
Selection of generator type for any particular turbine depends on the operating characteristics of the turbine itself, i.e. fixed speed or variable speed. There are currently four common technologies being used for WTGs, as shown in Figure 2.
6.1 Asynchronous - Induction Generator
This is the oldest employed technology and uses a standard "squirrel cage" induction motor directly connected to the grid. The generator is connected to the turbine through a gearbox. The machines cannot supply their own excitation current, which needs to be supplied from the grid, or supplied separately through capacitors. Induction generators are usually only used with fixed speed, stall regulated turbines. The induction generator cannot provide fault ride-through, reactive power or voltage control, frequency regulation or power control, but it does contribute to the power system inertia in the same manner as a standard industrial induction motor.
6.2 Asynchronous - Double Fed Induction Generator (DFIG)
With the double fed induction generator a wound rotor circuit is used, with the windings being externally accessible via slip rings. The generator is connected to the turbine through a gearbox. The rotor current is regulated using power electronics, allowing the generator to operate over a relatively large speed range. The DFIG can potentially contribute reactive power, voltage and frequency regulation, and to a limited extent fault ride-through. It also contributes to the overall power system inertia as the machine stator windings are still grid-connected. An example of this type of generator is the Vestas V90 3 MWWTG proposed by Trustpower for Stage III of the Tararua wind farm.
Figure 2a: Common WTG Technologies
Figure 2b: Common WTG Technologies
Figure 2c: Common WTG Technologies
Figure 2d: Common WTG Technologies
6.3 Synchronous - Unsynchronised
With this option the generator uses either permanent magnets or a standard excitation system. The generator is direct connected to the turbine and allows variable speed operation over a wide range, thereby optimising energy extraction. As the generator frequency will not necessarily be the same as the grid frequency, a four quadrant power converter is used to interface the generator to the grid. This system can provide reactive power, voltage and frequency regulation and, with recent developments, fault ride-through. Power ramp regulation is also being provided with machines of this type. However, owing to the presence of power electronics in the stator connections to the grid, this type of generator cannot contribute to system inertia.
6.4 Synchronous - Synchronised
There is a fourth generator option currently available, although very few in-service units exist. The Windflow Technology WTG located at Gebbies Pass, near Christchurch, uses this technology. With this option a standard synchronous generator is used. The generator is connected to the turbine by means of a variable speed gearbox. In theory this arrangement will overcome the reactive power, voltage, frequency regulation, fault ride-through and inertia concerns that accompany the three main system types. However at this time the ability of the variable gearbox to provide robust speed regulation under all operating and grid conditions has not been totally proven.
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