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How Power Grids Work from How Stuff Works
Electrical power is a little bit like the air you breathe: You don't really think about it until it is missing. (9) Power is just
"there," meeting your every need, constantly.
It's only during a (10) power failure when you walk into a dark room and instinctively hit the useless light switch, that
you realize how important power is in your daily life.
Power travels from the power plant to your house through an amazing system called the (11) power distribution grid.
The grid is quite public -- if you live in a suburban or rural area, chances are it is right out in the open for all to see. It is so
public, in fact, that you probably don't even notice it anymore. Your brain likely ignores all of the (12) power lines
because it has seen them so often.
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The Power Plant
Electrical power starts at the power plant. In almost all cases, the power plant consists of a (13) spinning electrical
generator. Something has to spin that generator -- it might be a water wheel in a (14) hydroelectric dam a large diesel
engine or a (15) gas turbine. But in most cases, the thing spinning the generator is a (16) steam turbine. The steam might
be created by burning (17) coal, oil or natural gas. Or the steam may come from a (18) nuclear reactor.
A breakdown of the major power plants in the United States, by type Photo courtesy U.S. Department of Energy
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The Power Plant: Alternating Current
(19) Single-phase power is what you have in your house. You generally talk about household electrical service as singlephase, 120-volt AC service. Power power like this is generally referred to as AC, or (23) alternating current. The
alternative to AC is DC, or (24) direct current. (25) Batteries produce DC: A steady stream of electrons flows in one
direction only, from the negative to the positive terminal of the battery.
AC has at least three advantages over DC in a (26) power distribution grid:
1. Large (27) electrical generators happen to generate AC naturally, so (28) conversion to DC would involve an
extra step.
2. (29) Transformers must have (30) alternating current to operate and we will see that the power distribution grid
depends on transformers.
3. It is easy to convert AC to DC but (31) expensive to convert DC to AC, so if you were going to pick one or the
other AC would be the better choice.
The power plant, therefore, produces AC. On the next page, you'll learn about the AC power produced at the power
plant. Most notably, it is produced in three phases.
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A typical substation at a power plant
Transmission Substation
The power leaves the generator and enters a (43) transmission substation at the power plant. This substation uses large
transformers to convert the generator's (44) voltage (which is at the thousands of volts level) up to extremely high
voltages for long-distance transmission on the transmission grid.
You can see at the back several three-wire towers leaving the substation. Typical voltages for long distance transmission
are in the range of 155,000 to 765,000 volts in order to (45) reduce line losses. A typical maximum transmission distance
is about (46) 300 miles (483 km). High-voltage transmission lines are quite obvious when you see them. They are
normally made of huge (47) steel towers like this:
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A typical small substation
The Distribution Grid
For power to be useful in a home or business, it comes off the transmission grid and is (50) stepped down to the
distribution grid. This may happen in several phases. The place where the conversion from "transmission" to
"distribution" occurs is in a (51) power substation A power substation typically does two or three things:
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
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It has (52) transformers that step transmission voltages (in the tens or hundreds of thousands of volts range)
down to distribution voltages (typically less than 10,000 volts).
It has a (53) “a bus” that can split the distribution power off in multiple directions.
It often has (54) circuit breakers and switches so that the substation can be (55) disconnected from the
transmission grid or separate distribution lines can be disconnected from the substation when necessary.
The box in the foreground is a large transformer. To its left are the incoming power from the transmission grid and a set
of switches for the incoming power. Toward the right is a distribution bus plus three voltage regulators.
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Distribution Bus
The power goes from the transformer to the distribution bus:
In this case, the bus distributes power to two separate sets of (56) distribution lines at two different voltages. The
smaller transformers attached to the bus are stepping the power (57) down to standard line voltage (usually 7,200 volts)
for one set of lines, while power leaves in the other direction at the (58) higher voltage of the main transformer. The
power leaves this substation in two sets of three wires, each headed down the road in a different direction:
The next time you are driving down the road, you can look at the power lines in a completely different light. In the
typical scene pictured above, the three wires at the top of the poles are the three wires for the (59) 3-Phase power. The
fourth wire lower on the poles is the ground wire. In some cases there will be additional wires, typically (60) phone or
cable TV lines riding on the same (61) poles.
As mentioned above, this particular substation produces two different voltages. The wires at the higher voltage need to
be (62) stepped-down again, which will often happen at another substation or in small transformers somewhere down
the line. For example, you will often see a large green box (perhaps 6 feet/1.8 meters on a side) near the entrance to a
subdivision. It is performing the step-down function for the (63)_sub-division.
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At the House
And finally we are down to the wire that brings power to your house! Past a typical house runs a set of poles with one
phase of power (at 7,200 volts) and a (66)_ground wire (although sometimes there will be two or three phases on the
pole, depending on where the house is located in the distribution grid). At each house, there is a (67)_transformer drum
attached to the pole, like this[picture above on right]:
In many suburban neighborhoods, the distribution lines are (68)_underground and there are green transformer boxes at
every house or two.
The transformer's job is to reduce the 7,200 volts down to the (69)_240 that makes up normal household electrical
service. Let's look at this pole one more time, from the bottom, to see what is going on:
There are two things to notice in this picture:

There is a bare wire running down the pole. This is a (70)_grounding wire. Every utility pole on the planet has
one. If you ever watch the power company install a new pole, you will see that the end of that bare wire is
stapled in a coil to the base of the pole and therefore is in direct contact with the earth, running 6 to 10 feet (1.8
to 3 m) underground. It is a good, solid ground connection. If you examine a pole carefully, you will see that the
ground wire running between poles (and often the guy wires) are attached to this direct connection to ground.
The 240 volts enters your house through a
typical watt-hour meter like this one:
The meter lets the power company charge you for putting up all of these wires.
Answers:
Section 1 Electrical Grid from Wikipedia, the free encyclopedia
1)Electrical Grid; 2) Generating plants; 3)Transmission lines; 4) Transformers; 5) power industry; 6) Electricity
generation; 7) Electric power transmission; 8) Electricity distribution
Section 2 How Power Grids Work from How Stuff Works
9) Power; 10) power failure; 11) power distribution grid; 12) power lines; 13) spinning electrical generator; 14)
hydroelectric dam; 15) gas turbine; 16) steam turbine; 17) coal; 18) nuclear reactor; 19) Single-phase power; 20)
oscilloscope; 21) sine wave; 22) cycles per second; 23) alternating current; 24) direct current; 25) Batteries; 26) power
distribution; 27) electrical generators; 28) conversion; 29) Transformers; 30) alternating; 31) expensive; 32) 120 degrees;
33) three phases; 34) ground; 35) peak; 36) improve; 37) earth; 38) conductor; 39) electrons; 40) metal body; 41)
negative; 42) the ground; 43) transmission substation; 44) voltage; 45) reduce; 46) 300; 47) steel towers; 48) wires; 49)
lightning; 50) stepped-down; 51) power substation; 52) transformers; 53) bus; 54) circuit breakers; 55) disconnected; 56)
distribution lines; 57) down; 58) higher voltage; 59) 3-phase; 60) phone; 61) poles; 62) stepped down; 63) subdivision;
64) undervoltage; 65) regulator bank; 66) ground wire; 67) transformer drum; 68) underground; 69) 240 volts; 70)
grounding wire