Technology Trends
A wind turbine generator typically consists of two or three blades 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.
A wind turbine generator has no output at all until the wind speed exceeds its low wind cut-in speed, which is typically between 14 and 22 km/h. Power output then rises until rated output is reached at a wind speed of about 54 km/h, after which power output remains constant until the high wind cut-out speed is reached at about 90 km/h. When the average wind speed exceeds the cut-out speed the wind turbine drops its output to zero to avoid structural damage.
In the early 1980's wind turbine generators were typically around 50 kW capacity with rotor diameters of 20 m. In contrast, wind turbine generators of up to 5 MW and 100 m rotor diameters are currently being tested for operation in off-shore wind farms. The rapid growth in the sector has provided manufacturers with incentives for research and development leading to lower costs per megawatt of installed wind turbine generator capacity. This has also led to improvements in the efficiency of conversion of wind energy to electricity and the addition of safety and grid support features which are helping to raise the limits on integration of wind energy around the world.
Specifically, the latest trends in wind turbine generator technology include the addition of technology which allows them to contribute to grid security in many of the same ways as conventional generators:
- controlling voltage at the output terminals of the wind turbine generator;
- maintaining a consistent output during faults on the grid where voltage drops suddenly;
- maintaining output over a range of grid frequencies around the nominal 50 Hz frequency at which the New Zealand grid operates.
The latest generation of wind turbine generators have the ability to connect together to form large wind farms which can be controlled to exhibit most of the attributes of conventional generators, within the limits of how much the wind blows at any particular time.
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