How Power Grids Work

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, 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.