How Power Grids Work by Marshall Brain
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 sopublic, in fact, that you probably don"t even notice it anymore. Your brainlikely ignores all of the power lines because it has seen them sooften. In this article, we will look at all of the equipment thatbrings electrical power to your home. The next time you look at thepower grid, you will be able to really see it and understand what isgoing on!
The Power PlantElectrical 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 EnergyA breakdown of the major power plants in the United States, by type
The Power Plant: Alternating CurrentSingle-phase power is what you have in your house. You generallytalk about household electrical service as single-phase, 120-volt ACservice. If you use an oscilloscopeand look at the power found at a normal wall-plate outlet in yourhouse, what you will find is that the power at the wall plate lookslike a sine wave, and that wave oscillates between -170 voltsand 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 sinewave is 60 cycles per second. Oscillating power like this is generallyreferred to as AC, or alternating current. The alternative to AC is DC, or direct current. Batteriesproduce 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 toAC, so if you were going to pick one or the other AC would be thebetter choice.The power plant, therefore, produces AC. On the next page,you"ll learn about the AC power produced at the power plant. Mostnotably, it is produced in three phases.
The Power Plant: Three-phase PowerThe 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. If you were to look at the three phases on a graph, they would look like this relative to ground:
There is nothing magical about three-phase power. It is simply three single phases synchronized and offset by 120 degrees.
Why three phases? Why not one or two or four? In 1-phase and2-phase power, there are 120 moments per second when a sine wave iscrossing zero volts. In 3-phase power, at any given moment one of thethree phases is nearing a peak. High-power 3-phase motors (used inindustrial applications) and things like 3-phase welding equipmenttherefore have even power output. Four phases would not significantlyimprove things but would add a fourth wire, so 3-phase is the naturalsettling point.
And what about this "ground," as mentioned above? The powercompany essentially uses the earth as one of the wires in the powersystem. The earth is a pretty good conductor and it is huge, so itmakes a good return path for electrons. (Car manufacturers do somethingsimilar; they use the metal body of the car as one of the wires in thecar"s electrical system and attach the negative pole of the battery tothe car"s body.) "Ground" in the power distribution grid is literally"the ground" that"s all around you when you are walking outside. It isthe dirt, rocks, groundwater, etc., of the earth.
Transmission SubstationThe three-phase power leaves the generator and enters a transmission substation at the power plant. This substation uses large transformers to convert the generator"s voltage(which is at the thousands of volts level) up to extremely highvoltages for long-distance transmission on the transmission grid.
A typical substation at a power plant
You can see at the back several three-wire towers leaving thesubstation. Typical voltages for long distance transmission are in therange of 155,000 to 765,000 volts in order to reduce line losses. Atypical maximum transmission distance is about 300 miles (483 km).High-voltage transmission lines are quite obvious when you see them.They are normally made of huge steel towers like this:
All power towers like this have three wires for the threephases. Many towers, like the ones shown above, have extra wiresrunning along the tops of the towers. These are ground wires and arethere primarily in an attempt to attract lightning.
The Distribution GridFor power to be useful in a home or business, it comes off the transmission grid and is stepped-downto the distribution grid. This may happen in several phases. The placewhere the conversion from "transmission" to "distribution" occurs is ina power substation. A power substation typically does two or three things:It has transformers that step transmission voltages (in the tens orhundreds of thousands of volts range) down to distribution voltages(typically less than 10,000 volts).It has a "bus" that can split the distribution power off in multiple directions.It often has circuit breakers and switches so that thesubstation can be disconnected from the transmission grid or separatedistribution lines can be disconnected from the substation whennecessary.
A typical small substation
The box in the foreground is a large transformer. To its left(and out of the frame but shown in the next shot) are the incomingpower from the transmission grid and a set of switches for the incomingpower. Toward the right is a distribution bus plus three voltageregulators.
The transmission lines entering the substation and passing through the switch tower
The switch tower and the main transformer
Now the distribution bus comes into the picture.
Distribution BusThe power goes from the transformer to the distribution bus:
In this case, the bus distributes power to two separate sets ofdistribution lines at two different voltages. The smaller transformersattached to the bus are stepping the power down to standard linevoltage (usually 7,200 volts) for one set of lines, while power leavesin the other direction at the higher voltage of the main transformer.The power leaves this substation in two sets of three wires, eachheaded down the road in a different direction:
The wires between these two poles are "guy wires" for support. They carry no current.
As mentioned above, this particular substation produces two differentvoltages. The wires at the higher voltage need to be stepped downagain, which will often happen at another substation or in smalltransformers somewhere down the line. For example, you will often see alarge green box (perhaps 6 feet/1.8 meters on a side) near the entranceto a subdivision. It is performing the step-down function for thesubdivision.
Regulator BankYou will alsofind regulator banks located along the line, either underground or inthe air. They regulate the voltage on the line to prevent undervoltageand overvoltage conditions.
A typical regulator bank
Up toward the top are three switches that allow this regulator bank to be disconnected for maintenance when necessary:
Atthis point, we have typical line voltage at something like 7,200 voltsrunning through the neighborhood on three wires (with a fourth groundwire lower on the pole):
TapsA house needs only one of the three phases, so typically you will see three wires running down a main road, and tapsfor one or two of the phases running off on side streets. Picturedbelow is a 3-phase to 2-phase tap, with the two phases running off tothe right:
Here is a 2-phase to 1-phase tap, with the single phase running out to the right:
At the HouseAnd finally weare down to the wire that brings power to your house! Past a typicalhouse 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 phaseson the pole, depending on where the house is located in thedistribution grid). At each house, there is a transformer drum attached to the pole, like this:
In many suburban neighborhoods, the distribution lines are underground and there are green transformer boxes at every house or two.Here is some detail on what is going on at the pole:
The transformer"s job is to reduce the 7,200 volts down to the 240 voltsthat makes up normal household electrical service. Let"s look at thispole 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 groundingwire. Every utility pole on the planet has one. If you ever watch thepower company install a new pole, you will see that the end of thatbare wire is stapled in a coil to the base of the pole and therefore isin 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 apole carefully, you will see that the ground wire running between poles(and often the guy wires) are attached to this direct connection toground. There are two wires running out of the transformer and three wires running to the house.Thetwo from the transformer are insulated, and the third one is bare. Thebare wire is the ground wire. The two insulated wires each carry 120volts, but they are 180 degrees out of phase so the difference betweenthem is 240 volts. This arrangement allows a homeowner to use both120-volt and 240-volt appliances. The transformer is wired in this sortof configuration:
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.
Safety Devices: FusesFuses and circuit breakers are safety devices.Let"s say that you did not have fuses or circuit breakers in your houseand something "went wrong." What could possibly go wrong? Here are someexamples:A wire comes loose in a lamp and directly connects power to ground.A mouse chews through the insulation in a wire and directly connects power to ground.A person is hanging a picture in the living room and the nailused for said picture happens to puncture a power line in the wall,directly connecting power to ground.
The power then enters the home through a typical circuit breaker panel like the one above.
Safety Devices: Circuit Breakers
Inside the circuit breaker panel (right) you can see the two primary wires from the transformer entering the main circuit breakerat the top. The main breaker lets you cut power to the entire panelwhen necessary. Within this overall setup, all of the wires for thedifferent outlets and lights in the house each have a separate circuitbreaker or fuse:
If the circuit breaker is on, then power flows through the wirein the wall and makes its way eventually to its final destination, the outlet.
What an unbelievable story! It took all of that equipment to get power from the power plant to the light in your bedroom.
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The next time you drive down the road and look at the power lines, orthe next time you flip on a light, you"ll hopefully have a much betterunderstanding of what is going on. The power distribution grid is trulyan incredible system.