Turbines and Power Quality Issues
The buyer of a wind turbine does not need to concern himself
with local technical regulations for wind turbines and other
equipment connected to the electrical grid. This responsibility
is generally left to the turbine manufacturer and the local power
For the people who are technically minded,
we go into some of the electrotechnical issues involved in connecting
a turbine to the grid on this page.
The term "power quality" refers to the voltage stability,
frequency stability, and the absence of various forms of electrical
noise (e.g. flicker or harmonic distortion) on the electrical
grid. More broadly speaking, power companies (and their customers)
prefer an alternating current with a nice sinusoidal shape, such
as the one in the image above. (If you are not familiar with
the basics of alternating current (AC) it may be useful to consult
the Reference Manual about
this subject before continuing).
(and Stopping) a Turbine
Most electronic wind turbine controllers are programmed to let
the turbine run idle without grid connection at low wind speeds.
(If it were grid connected at low wind speeds, it would in fact
run as a motor, as you can read about on the generator
page). Once the wind becomes powerful enough to turn the
rotor and generator at their rated speed, it is important that
the turbine generator becomes connected to the electrical grid
at the right moment.
Otherwise there will be only the mechanical
resistance in the gearbox and generator to prevent the rotor
from accelerating, and eventually overspeeding. (There are several
safety devices, including fail-safe brakes, in case the correct
start procedure fails, which you may have read in the section
on Wind Turbine Safety).
Starting with Thyristors
If you switched a large wind turbine on to the grid with a normal
switch, the neighbours would see a brownout (because of the current
required to magnetise the generator) followed by a power peak
due to the generator current surging into the grid. You may see
the situation in the drawing in the accompanying browser window,
where you see the flickering of the lamp when you operate the
switch to start the wind turbine. The same effect can possibly
be seen when you switch on your computer, and the transformer
in its power supply all of a sudden becomes magnetised.
Another unpleasant side effect of using a
"hard" switch would be to put a lot of extra wear on
the gearbox, since the cut-in of the generator would work as
if you all of a sudden slammed on the mechanical brake of the
power thyristors in wind turbines get very hot when they are
activated. They have to be equipped with aluminium heat sinks
and fans as you see in the picture to the right.
© 1998 Soren Krohn
prevent this situation, modern wind turbines are soft starting,
i.e. they connect and disconnect gradually to the grid using
thyristors, a type of semiconductor continuous switches
which may be controlled electronically. (You may in fact have
a thyristor in your own home, if you own a modern light dimmer,
where you can adjust the voltage on your lamps continuously).
Thyristors waste about 1 to 2 per cent of
the energy running through them. Modern wind turbines are therefore
normally equipped with a so called bypass switch, i.e.
a mechanical switch which is activated after the turbine has
been soft started. In this way the amount of energy wasted will
Grids, Grid Reinforcement
If a turbine is connected to a weak electrical grid, (i.e. it
is vary far away in a remote corner of the electrical grid with
a low power-carrying ability), there may be some brownout / power
surge problems of the sort mentioned above. In such cases it
may be necessary to reinforce the grid, in order to carry the
fluctuating current from the wind turbine.
Your local power company has experience in
dealing with these potential problems, because they are the exact
mirror-image of connecting a large electricity user, (e.g. a
factory with large electrical motors) to the grid.
Flicker is an engineering expression for short lived
voltage variations in the electrical grid which may cause light
bulbs to flicker. This phenomenon may be relevant if a wind turbine
is connected to a weak grid, since short-lived wind variations
will cause variations in power output. There are various ways
of dealing with this issue in the design of the turbine, mechanically,
electrically, and using power electronics.
Islanding is a situation which may occur if a section
of the electrical grid becomes disconnected from the main electrical
grid, e.g. because of accidental or intended tripping of a large
circuit breaker in the grid (e.g. due to lightning strikes or
short circuits in the grid). If wind turbines keep on running
in the isolated part of the grid, then it is very likely that
the two separate grids will not be in
phase after a short while.
Once the connection to the main grid is re-established
it may cause huge current surges in the grid and the wind turbine
generator. It would also cause a large release of energy in the
mechanical drive train (i.e. the shafts, the gear box and the
rotor of the wind turbine) much like "hard switching"
the turbine generator onto the grid would do.
The electronic controller of the wind turbine
will therefore constantly have to monitor the voltage and frequency
of the alternating current in the grid. In case the voltage or
frequency of the local grid drift outside certain limits within
a fraction of a second, the turbine will automatically disconnect
from the grid, and stop itself immediately afterwards. (Normally
by activating the aerodynamic brakes as explained in the section
on wind turbine safety).