On Mon, 22 Oct 2007 05:38:07 -0700 email@example.com wrote:
| Iam doing an english technical report and have a question to ask to
| help finish the report. I was wondering what the electrical power
| would be on the electrical lines in a residential area feeding the
| houses before it goes to the transformers for the houses, and what the
| power would be to a industrial building. I know that the power to an
| industrial would vary from plant to plant depending on what they do,
| but if I can get an idea it would help. Thanks
This is probably a little more than you are asking for, but maybe there
will be some piece of info you can add to the paper to explain why some
of the voltages are used.
Since your email address is @charter.net I'll presume you are in the USA.
The electrical power industry divides power lines into three voltage
categories labeled LV for low voltage, MV for medium voltage, and HV for
high voltage. In general, MV is used for what is called distribution,
and HV is used for what is called transmission. Transmission usually
goes directly from one substation (often at a power generating plant)
to another. Distribution goes from a substation to multiple transformers
that serve typical customers.
The common house/office voltage is officially 120 volts, although many
people still refer to it as 110 volts and say "one ten". 110 volts was
the standard Thomas Edison used for his first power service in New York
City. He also used the idea (although I don't know if he invented it)
of splitting the voltage between two polarities (his system was DC so it
would be plus and minus), so it was actually 220 volts between the two
far wires, and only 110 volts between either of those wires and the wire
in the middle. This allowed him to connect light bulbs to 110 volts and
get some of the advantages of higher voltage distribution (to run wires
at greater distances without so much loss of voltage due to resistance
in the wires).
Most homes in the US are supplied with true single phase power, using
the split phase system that Edison uses, but with AC. So the voltages
are 120 volts between either of the 2 "hot" wires, and the center wire
which is grounded and called "neutral". The voltage between the 2 hot
wires is 240 volts. With some variation in voltages, this system is
also used in some other parts of the world, notably North and Central
America, parts of northern South America, most of the Caribbean, as well
as Japan (only 100 volts) and Taiwan. Some places get as much as 127
In other parts of the world, homes and offices typically get voltages
that range from 220 to 240 volts between the hot wire and ground. Many
also get three phase power at voltages from 380 to 416 volts measured
between any two (out of two or three connected) hot wires.
The AC frequency in the USA is 60 Hz (cycles per second). Most of the
rest of the world uses 50 Hz. Some places have the lower voltage with
50 Hz and a few places have the higher voltage with 60 Hz. Japan and
Brazil have both power frequencies within the same country.
In some cases, notably large buildings, three phase power is cheaper to
provide than single phase power, and even though homes still typically
only get 2 hot wires fed, (three phase has 3 hot wires with the cycle
timing shifted so each wire is 1/3 of a cycle different from the others)
the voltage between the 2 hot wires is only 208 volts instead of 240.
Industrial buildings making heavy use of electrical power will get that
power typically at a voltage of 277 volts, with 480 volts between any
pair of hot wires. It's almost always three phase because large motors
need it. If single phase loads are the main usage, it still needs to be
well balanced usage over the three phases used in the distribution. In
a few places these voltages will be 347 volts and 600 volts, mostly in
Canada. Heavy industry will often use much higher voltage and have power
bills in the millions of dollars. Many coal mines use a voltage that is
a combination 577 volts and 1000 volts. Many industrial facilities and
electrical railways have special power frequencies. A few even have DC
power. In other places in the world, industrial power can be available
in many other voltages, such as the combination 690/400 volts in Europe.
The distribution of power in a typical residential neighborhood in the
USA will have a mix of LV to feed 2 to 6 homes from one larger transformer
and MV to feed those transformers. The LV will be the 120/240 single
phase power described above. The MV will typically be between 6900 volts
and nearly 8000 volts, with 7200 or 7970 volts being common in many areas.
That is the voltage between a hot wire and the grounded wire. The voltage
between two hot wires in a three phase system (the distribution is almost
always three phase, or just one hot wire) is about 1.732 times as much, a
figure derived from the mathmatics of sine waves (trigonemtry) and is the
square root of three.
In possibly rare cases, the distribution can be an even higher voltage,
as much as 34,500 volts. This would typically be used only if there are
special reasons such as a greater distance from the last substation, or
heavy power users in the area.
When you observe overhead power lines, notice how many separate wires are
on insulator stand-offs, and how big those insulators are. Big insulators
suggest a higher voltage (but do not assume always so). One or two wires
suggests single phase while three or four wires suggests three phase. In
some cases you will see even more wires, such as six or more. These may
simply be separated circuits for special cases, such as backup power to
a hospital, or a separate circuit that will split apart further away from
Transmission lines supply power from one area to another. You can usually
see several transmission lines from a power generating plant substation
going off in a few directions to supply power to many locations or connect
to the power grid (many transmission lines interconnected with substations).
The voltage on these can range from 69,000 volts to 765,000 volts or more in
a few rare cases (above this level is much more of a technical challenge).
The power industry refers to the voltage levels of 120 to 600 volts as LV
(low voltage). But inside a home, the designations change and we use "high
voltage" to refer to the 120/240 volts you could easily be electrocuted with
and "low voltage" for a reduced voltage that is 30 volts or less, typically
only 12 volts, commonly used for special lighting systems. Incandescent
lights work better when the voltage is lower because it uses a shorter and
thicker filament that runs at a higher temperature (this actually produces
more light and less heat for a given amount of power, and that light is a
whiter quality of light better suited for task lighting). The lower voltage
also allows for the use of uninsulated track lighting systems since contact
with this voltage is much less likely to hurt anyone, and requires touching
two wires since the systems are required to be isolated from ground.
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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