Electrical power is a little bit like the air you breathe: You don't really think about it until it is missing. Power is just "there," meeting your every need, constantly. It is only during a 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. You use it for heating, cooling, cooking, refrigeration, light, sound, computation, entertainment... Without it, life can get somewhat cumbersome.
Power travels from the power plant to your house through an amazing system called the power distribution grid.
Nuclear Power Image Gallery
Power grid distribution lines can be above or under ground. See more nuclear power pictures.
The Power Plant
Electrical power starts at the power plant. In almost all cases, the power plant consists of a spinning electrical generator. Something has to spin that generator -- it might be a water wheel in a hydroelectric dam, a large diesel engine or a gas turbine. But in most cases, the thing spinning the generator is a steam turbine. The steam might be created by burning coal, oil or natural gas. Or the steam may come from a nuclear reactor like this one at the Shearon Harris nuclear power plant near Raleigh, North Carolina.
No matter what it is that spins the generator, commercial electrical generators of any size generate what is called 3-phase AC power. To understand 3-phase AC power, it is helpful to understand single-phase power first.
Photo courtesy U.S. Department of Energy
A breakdown of the major power plants in
the United States, by type
The Power Plant: Alternating Current
Single-phase power is what you have in your house. You generally talk about household electrical service as single-phase, 120-volt AC service. If you use an oscilloscope and look at the power found at a normal wall-plate outlet in your house, what you will find is that the power at the wall plate looks like a sine wave, and that wave oscillates between -170 volts and 170 volts (the peaks are indeed at 170 volts; it is the effective (rms) voltage that is 120 volts). The rate of oscillation for the sine wave is 60 cycles per second. Oscillating power like this is generally referred to as AC, or alternating current. The alternative to AC is DC, or direct current. 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 power distribution grid:
- Large electrical generators happen to generate AC naturally, so conversion to DC would involve an extra step.
- Transformers must have alternating current to operate, and we will see that the power distribution grid depends on transformers.
- It is easy to convert AC to DC but 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. Continue here
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