Electrical - Wind Energy: Renewable Energy and the Environment - page 130

1.0

0.5

0.0

-0.5

-1.0

Time

FIGURE 7.1 Two sine waves with different frequencies.

The implications are that electric power needs to be transmitted at high voltages. In the summer
time, as air temperature increases, the transmission lines are further limited in the amount of power
they can carry. High current and high temperatures also lead to more sag in the transmission lines.
Wind turbines with generators at 240 or 480 V need to be fairly close to the load or the utility line.
With higher voltages, smaller-diameter wire can be used. Transformers change the voltage, so wind
farms will have a transformer with every large turbine to increase the voltage for transmission. The
transformer may be at the top in the nacelle, which means the power wires down the tower can be
smaller.
Capacitance: Capacitors are devices for storing charge. An example of a capacitor is two metal
plates separated by a small distance. Capacitors are not used for long-term storage because the
charge leaks away.
Inductance: Inductors are devices for storing magnetic fields. An example of an inductor is a
coil of wire.
Electric field: Electric fields, E , originate or terminate on charged particles. If a charged particle
feels a force, it is in an electric field.

F q

E

(7.7)

Magnetic field: Magnetic fields, B , are due to moving charges or intrinsic spin (a property of
particles just like charge is a property of particles). Some materials have a magnet field, and they
are called permanent magnets. Permanent magnet alternators use rare earth atoms, which are more
expensive than iron, nickel, and cobalt. If a moving charge feels a force at right angles to its motion,
it is in a magnetic field. Also, changing electric fields create changing magnetic fields, and changing
magnetic fields create changing electric fields. Maxwell formulated the theory of electromagnetism
in all of its elegance of four equations, appropriately called Maxwell's equations. This is the theo-
retical basis for all of the electric power industry and communication by electromagnetic waves,
which we accept as commonplace today.