can see the internal cooling fan moving inside this generator.
It is mounted at the end of the rotor, which is hidden
inside the shining magnetic steel cylinder, called the stator.
The radiator-like surface cools the generator.
It is hard to see the details on a real life
generator like the one to the right.
Therefore, we'll take it apart and make some
simplified models on the next pages.
wind turbine generator converts mechanical energy to electrical
Wind turbine generators are a bit unusual,
compared to other generating units you ordinarily find attached
to the electrical grid. One reason is that the generator has
to work with a power source (the wind turbine rotor) which supplies
very fluctuating mechanical power (torque).
These pages assumes that you are familiar
with the basics of electricity, electromagnetism, and in particular
alternating current. If any of the expressions volt (V), phase,
three phase, frequency, or Hertz (Hz) sound strange to you, you
should take a look at the Reference
Manual on Electricity, and read about alternating
current, three phase alternating
and induction, before
you proceed with the following pages.
On large wind turbines (above 100-150 kW) the voltage (tension)
generated by the turbine is usually 690 V three-phase alternating
current (AC). The current is subsequently sent through a transformer
next to the wind turbine (or inside the tower) to raise the voltage
to somewhere between 10,000 and 30,000 volts, depending on the
standard in the local electrical grid.
Large manufacturers will supply both 50 Hz
wind turbine models (for the electrical grids in most of the
world) and 60 Hz models (for the electrical grid in America).
Generators need cooling while they work. On most turbines this
is accomplished by encapsulating the generator in a duct, using
a large fan for air cooling, but a few manufacturers use water
cooled generators. Water cooled generators may be built more
compactly, which also gives some electrical efficiency advantages,
but they require a radiator in the nacelle to get rid of the
heat from the liquid cooling system.
and Stopping the Generator
If you connected (or disconnected) a large wind turbine generator
to the grid by flicking an ordinary switch, you would be quite
likely to damage both the generator, the gearbox and the current
in the grid in the neighbourhood.
You will learn how turbine designers deal
with this challenge in the page on Power
Quality Issues, later.
Choices in Generators and Grid Connection
Wind turbines may be designed with either synchronous or asynchronous
generators, and with various forms of direct or indirect
grid connection of the generator.
Direct grid connection mean that the generator
is connected directly to the (usually 3-phase) alternating current
Indirect grid connection means that the current
from the turbine passes through a series of electric devices
which adjust the current to match that of the grid. With an asynchronous
generator this occurs automatically.