How Power Grids Work
Inside this Article
1. Introduction to How Power Grids Work
2. The Power Plant
3. The Power Plant: Alternating Current
4. The Power Plant: Three-phase Power
5. Transmission Substation
6. The Distribution Grid
7. See more ?
Nuclear Power Image Gallery
Power grid distribution lines can be above or under ground. See more nuclear power pictures.
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. 冗长的
Power travels from the power plant发电厂 to your house through an amazing
system called the power distribution grid. 配电网
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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 power lines because it has seen them so often. In this article, we will look at all of the equipment that brings electrical power to your home. The next time you look at the power grid, you will be able to really see it and understand what is going on!
The Power Plant
Electrical power starts at the power plant. In almost all cases, the power plant consists of a spinning electrical generator旋转的发电机. Something
has to spin that generator -- it might be a water wheel水轮机 in a
hydroelectric dam,水电大坝 a large diesel engine 柴油发电机or a gas
turbine.燃气涡轮机 But in most cases, the thing spinning the generator is a steam turbine蒸汽轮机. The steam might be created by burning coal,
oil or natural gas. Or the steam may come from a nuclear reactor 核反
应堆like this one at the Shearon Harris nuclear power plant 核电站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.三相交
流电源 To understand 3-phase AC power, it is helpful to understand single-phase power单相电源 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 示波器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正弦波, 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.交流电 The
alternative to AC is DC, or direct current.直流电 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:
1. Large electrical generators happen to generate AC naturally, so
conversion to DC would involve an extra step.
2. Transformers 变压器must have 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 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.
The Power Plant: Three-phase Power 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三根相线 plus
a neutral or ground 地线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 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大功率三相电机 (used in industrial
applications) and things like 3-phase welding equipment 三相焊接设备
therefore have even power output平稳的输出功率. 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.电
力系统 The earth is a pretty good conductor and it is huge, so it makes a good return path for electrons. 电子回路(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 负极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地下水, etc., of the earth.
Transmission Substation输电变电站
The three-phase power leaves the generator发电机 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 high voltages for long-distance transmission on the transmission grid.输电电网
A typical substation at a power plant
You can see at the back several three-wire towers三相输电塔 leaving the
substation. Typical voltages for long distance transmission are in the range of 155,000 to 765,000 volts in order to reduce line losses.线路
损耗 A typical maximum transmission distance is about 300 miles (483 km). High-voltage transmission lines高压输电线 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 three phases.三相
电 Many towers, like the ones shown above, have extra wires running along the tops of the towers. These are ground wires 接地线and are there
primarily in an attempt to attract lightning. 防雷
The Distribution Grid配电网
For power to be useful in a home or business, it comes off the transmission grid and is stepped-down逐级降压 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.配电
站 A power substation typically does two or three things:
; It has transformers that step transmission voltages输电电压 (in
the tens or hundreds of thousands of volts range) down to
distribution voltages 配电电压(typically less than 10,000 volts).
; It has a "bus" 总线that can split the distribution power off in
multiple directions.
; It often has circuit breakers 断路器and switches 开关so that the
substation can be disconnected from the transmission grid or
separate distribution lines can be disconnected from the substation
when necessary.
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 incoming power from the transmission grid and a set of switches for the incoming power. Toward the right is a distribution bus配电总线 plus three voltage regulators.
稳压器
The transmission lines entering the substation and passing
through the switch tower 控制塔
The switch tower and the main transformer
Now the distribution bus comes into the picture. Distribution Bus
The power goes from the transformer to the distribution bus:
In this case, the bus distributes power to two separate sets of distribution lines at two different voltages. The smaller transformers attached to the bus are stepping 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 power leaves this substation in two sets of three wires, each headed down the road in a different direction:
The wires between these two poles are "guy wires张索" for
support. They carry no current.
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 on the right, 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
In some cases there will be additional wires, typically phone or wire.
cable TV lines riding on the same poles.
As mentioned above, this particular substation produces two different voltages. The wires at the higher voltage need to be 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 subdivision. 分支线路
Regulator Bank稳压器
You will also find regulator banks located along the line, either underground or in the air. They regulate the voltage on the line to prevent undervoltage电压过低 and overvoltage电压过高 conditions.
A typical regulator bank
Up toward the top are three switches that allow this regulator bank to
be disconnected for maintenance when necessary: