How Power Grids Work

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.

Electricity generation is the single largest source of greenhouse gas emissions in the United States [source: EPA]. That's why it's so important to develop more renewable sources of energy. In 2014, 67 percent of America's electricity came from fossil fuels like coal and natural gas. Hydroelectric energy was the largest renewable energy source, followed by solar, wind and geothermal power. In 2014, 6 percent of America's electricity was produced by hydropower, while solar, wind and thermal energy together comprised another 5 percent [source: EIA].

No matter what energy source 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.

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:

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.

The power plant produces three different phases of AC power simultaneously, and the three phases are offset 120 degrees from each other. There are four wires coming out of every power plant: the three phases plus a neutral or ground common to all three.

Why three phases? Why not one or two or four? In 1-phase and 2-phase power, there are 120 moments per second when a sine wave is crossing zero volts. In 3-phase power, at any given moment one of the three phases is nearing a peak. High-power 3-phase motors (used in industrial applications) and things like 3-phase welding equipment therefore have even power output. Four phases would not significantly improve things but would add a fourth wire, so 3-phase is the natural settling point.

And what about this "ground," as mentioned above? The power company essentially uses the earth as one of the wires in the power system. The earth is a pretty good conductor and it is huge, so it makes a good return path for electrons. (Car manufacturers do something similar; they use the metal body of the car as one of the wires in the car's electrical system and attach the negative pole of the battery to the car's body.) "Ground" in the power distribution grid is literally "the ground" that's all around you when you are walking outside. It is the dirt, rocks, groundwater and so on of the earth.

The three-phase power leaves the generator and enters a transmission substation at the power plant. This substation uses large transformers to convert or "step up" the generator's voltage to extremely high voltages for long-distance transmission on the transmission grid. Typical voltages for long distance transmission are in the range of 155,000 to 765,000 volts. The higher the voltage, the less energy is lost due to resistance [source: UCSUSA].

A typical maximum transmission distance is about 300 miles (483 kilometers). High-voltage transmission lines are quite obvious when you see them. They are huge steel towers strung out in a line that stretches toward the horizon.

All high-voltage towers have three wires for the three phases. Many towers also have extra wires running along the tops of the towers. These are ground wires and are there primarily in an attempt to attract lightning.

For power to be useful in a home or business, it comes off the transmission grid and is 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 power substation. A power substation typically does two or three things:

The power goes from the transformer to the distribution bus. The bus distributes power to local distribution lines. The bus has its own transformers that can also step down or step up voltage according to local energy needs.

At the bus, there may be two separate sets of distribution lines at two different voltages. Smaller transformers attached to the bus step the power down to standard line voltage (usually 7,200 volts) for one set of lines, while power leaves in the other direction at the higher voltage of the main transformer.

The next time you are driving down the road, you can look at the power lines in a completely different light. On a typical utility pole, the three wires at the top of the poles are the three wires for the 3-phase power. The fourth wire lower on the poles is the ground wire. In some cases there will be additional wires, typically phone, cable TV or Internet lines riding on the same poles.

Lines that carry higher voltage will need to be stepped down further before entering residential buildings and most businesses. This often happens at another substation or in small transformers somewhere down the line. For example, you will often see a large green box (perhaps 6 feet or 1.8 meters on a side) near the entrance to a subdivision. It is performing the step-down function for the subdivision.

You will also find regulator banks located along the line, either underground or in the air. Above-ground regulator banks look like three garbage can-sized transformers held up by two utility poles. They regulate the voltage on the line to prevent undervoltage and overvoltage conditions. The regulator bank works to maintain a steady 7,200 volts running through the neighborhood on three wires (with a fourth ground wire lower on the pole).

Most homes and businesses only need single-phase power, so typically you will see three wires running down a main road, and taps running off on side streets. The taps on utility poles can be configured to deliver single-phase or two-phase power to residences and commercial buildings.

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 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 transformer drum attached to the pole.

In many suburban neighborhoods, the distribution lines are 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 240 volts 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:

The 240 volts enters your house through a watt-hour meter, which measures your electrical consumption so the power company can charge you for putting up all of those wires. In the past, meter readers would periodically check your meter to record your usage. As part of the national upgrade to smart grid technology, millions of residential meters have now been replaced with smart meters that communicate directly with the power company. Not only can the utility read your meter remotely, but it is notified instantly in the case of a power outage, speeding recovery time [source: DOE].

Fuses and circuit breakers are safety devices. Let's say that you did not have fuses or circuit breakers in your house and something "went wrong." What could possibly go wrong? Here are some examples:

When a 120-volt power line connects directly to ground, its goal in life is to pump as much electricity as possible through the connection. Either the device or the wire in the wall will burst into flames in such a situation. (The wire in the wall will get hot like the element in an electric oven gets hot, which is to say very hot!).

A fuse is a simple device designed to overheat and burn out extremely rapidly in such a situation. In a fuse, a thin piece of foil or wire quickly vaporizes when an overload of current runs through it. This kills the power to the wire immediately, protecting it from overheating. Fuses must be replaced each time they burn out, which is why very few homes still use them.

A circuit breaker uses the heat from an overload to trip a switch, and circuit breakers are therefore resettable. Power enters the home through a circuit breaker panel. Inside the circuit breaker panel are two primary wires from the transformer entering the main circuit breaker at the top. The main breaker lets you cut power to the entire panel when necessary. Within this overall setup, all of the wires for the different outlets and lights in the house each have a separate circuit breaker or fuse.

If the circuit breaker is on, then power flows through the wire in the wall and makes its way eventually to its final destination, the outlet.

If you live in states like Arizona, New Mexico and Nevada, it pays to have solar panels. The city of Tucson, Arizona averages more than 3,800 hours of sunshine each year [source: Current Results]. In the past, if you wanted to generate your own power using renewable resources like solar panels or wind turbines, you would have to operate "off the grid" — disconnected from the power grid run by your local electrical utility.

Now, thanks to upgrades in technology and changes in policy and regulations, most states and utility companies allow individuals to generate their own power and remain linked with the larger grid.

How does it work? Armed with a special electrical meter and some current inversion equipment, homeowners can tap renewable resources like sunshine and wind to supplement the electricity they receive from the grid. If it's a cloudy day in Tucson, folks with grid-connected homes don't have to read in the dark. They can use as much or as little electricity from the main grid as they want.

Even better, if homeowners are able to generate more power than they need, the local utility will buy the excess power from them, essentially "turning back the meter" [source: DOE].

Just after 4 p.m. on Aug. 14, 2003, a huge swath of the eastern U.S. and Canada went dark. The massive 2003 blackout affected 50 million people across eight U.S. states and the Canadian province of Ontario. Power in some areas wasn't restored for two days [source: USCPSOTF].

Although blackouts of this magnitude are rare, they draw attention to weaknesses in the power grid system. The U.S.-Canadian power grid is actually composed of three separate grids: the Eastern Interconnection, the Western Interconnection, and the ERCOT Interconnection (also known as the Texas Interconnection). Within each of these three grids, hundreds of power plants must produce electricity to keep up with constantly shifting power demands across the grid.

If one power plant goes down, or one high-voltage transmission line is cut, or one generator fails, then other units must pick up the slack. Usually this isn't a problem. But when demand for electricity is high — as it was on that hot August afternoon in 2003 — isolated failures in the grid can trigger a cascade of breakdowns leading to millions of people going without power.

The cause of the 2003 blackout was a combination of human error, computer malfunctions and overgrown trees near power lines. It started with a few generators going offline in Northern Ohio because of mechanical trouble. The load was shifted to nearby generators, but overgrown trees made contact with overhead power lines, causing those lines to trip. Utility companies in the region didn't have adequate monitoring systems in place, so workers failed recognize the severity of the situation before generators across the region became overloaded and shut down [source: USCPSOTF].

Like a circuit breaker, components of a power grid can "trip" and shut off when a load is dangerously high. The result is that the load is passed on to other parts of the grid network, which may also shut down, creating a domino effect that leads to a blackout.

While human error and computer malfunctions can be remedied, there's not much utility companies can do about bad weather. Severe weather — high winds, lightning strikes, heavy snowfall — is the No. 1 cause of power outages in America [source: DOE].

The next time you drive down the road and look at the power lines, or the next time you flip on a light, you'll hopefully have a much better understanding of what is going on. The power distribution grid is truly an incredible system.