 Generating 3-phase alternating
current
How
it happens: Passing a magnet over a coil (winding) of wire causes
electrons to move along the coil, creating electricity. This is
called magnetic induction.
Generators use magnetic induction to
produce alternating current AC which is a type of electricity
where electrons move back and forth along a wire.
Small
household generators
use magnets to produce 120-240 volt single-phase AC, but the grid uses
a large electromagnetic generator to produce 30,000 volt 3-phase AC. See image Read more
The
grid delivers alternating current (AC) to homes and
businesses.
Solar and wind generation must be converted into same voltage and
frequency AC power before being dispatched to the grid. All appliances,
computers etc are rated for AC power as shown on product label.
''At
the center of conventional (coal, oil, gas, atomic, hydro)
power stations is a generator, a rotating machine that converts
mechanical energy into electrical energy by creating relative motion
between a magnetic field (north and south pole magnet) and a conductor
(coils A B C).''
In general, with coal, oil, gases, atomic,
hydro powered generators, the electromagnet inside generator spins past
3 coils (or windings) of wire that are spaced exactly 120° apart inside
the generator. Or more exactly, a central rotor spins inside a stator,
like a large electric motor. Hydroelectric generator Video Resource pdf Resource .pdf
Alternating
current:
As the electromagnet passes each coil, the coil becomes energized with
a pulse of electric power that reverses each time the north and south
poles pass. Back and forth, the electrons race down the coil and out
onto a wire, momentarily stop and then race back the other direction,
creating what is called alternating current. While voltage rises and
falls with the momentary halt and reversal of electrons, the action
happens so fast that the average voltage is always above zero and
functions as a level voltage.
Frequency
The electrons make a complete cycle, there and back, 60 times per
second in the Americas and 50 times per second in rest of world. This
is called Hz, hertz, frequency, or cycles. Frequency is monitored
closely so multiple power plants (solar, coal, gas, hydro, wind etc)
can safely dispatch along the same transmission line without damaging
generators, equipment etc, and so end users receive stable, predictable
power.
3-phase
transmission:
3 Hot wires and a Neutral leave the generator: A wire is connected to
one end of each of the 3 coils, and those 3 wires leave the power plant
and become the 3 Hot wires found in 3-phase power that is transmitted
across the grid.
Each
of the 3 Hot wires is called a phase ... thus 3-phase
The
other ends of the 3 coils are joined together to form the Neutral wire
that travels with the Hot wires (see image), and is bonded to the ground rod array
located under the power plant and switchyard, and is bonded to a ground
at each tower, pole, substation, and installation along the way.The 3
Hot phase conductors and Neutral are electromagnetically coupled at the
generator, thus helping the Neutral act as a shield
against lightning, reducing magnitude of surge, as long as Neutral is
grounded at each pole.
Three-phase with 3 hot wires and a neutral is most efficient method for
transmitting electric power.
Out of phase:
Since the generator magnet passes each coil at a different moment, each
Hot wire carries electrons that are 'out of phase' with electrons on
the other Hot wires, and also out of phase with the Neutral.
Out-of-phase means the electrons on one Hot wire are accelerating
different directions at any given moment from the electrons on any
other Hot wire or the Neutral.
Energizing
the Load:
When out of phase wires are connected to a load, the electrons
oscillate back n forth between each wire, going back and forth through
the load and energizing the load. A load is a light, fan, motor, oven, clothes dryer etc.
With
single-phase, the electrons travel back and forth between
two wires, either Hot-to-Neutral or Hot-Hot.
With
three-phase
the electrons travel back and forth between three Hot wires, each leg
interacting with the other as the generator spins, and where the
best explanation becomes mathematical and exceeds the scope of this
page. |
Using
the formula, the primary volts or E1
is 7200 volts, and secondary volts E2
is 240 volt residential single-phase service. And let's say the main breaker
for the residence is 250 amps, then I2
is 250 amps. 
Switching
3 phase loads 
High
voltage conductors are suspended high above the ground and spaced far
apart, utilizing air as an insulator. This is to prevent electricity
from jumping (arcing) to other wires, or to the transmission tower, or
to the earth. The higher the voltage, the more space is
needed to
keep the voltage under control.
