高压线为什么恰好是三根线？三相输电一次讲透（原声文稿）
Transcript of 高压线为什么恰好是三根线？三相输电一次讲透 （中文完整文稿）。
Look at this giant steel tower power line with its three wires, so where's the return wire? Here's something strange.
Your lamp gets by on 120 volts and even it needs two wires, one out and one back. In your house, that wire back is called the neutral.
But this line can run at 500 ,000 volts and it has no wire set aside for the trip home. and yet the current still gets home.
So why do power lines have three wires?
Let's take it one step at a time, starting with the one rule that never breaks.
Current only flows in a loop, out from its source and back home.
Say you want to send three loads of power across a state the obvious way. That's three separate loops, three wires out and three wires back.
6 wires, and every one of them costs money and wastes a little power as heat.
But look inside the generator at the far end.
It has three separate sets of windings, coils of wire, and a magnet sweeps past them one after another.
That gives you three identical waves of push, each a third of a cycle behind the last. Engineers call each of those waves a phase.
Here comes the odd move.
Take the three return wires and join them into one. It's still three loops and they simply share the road home. That's four wires. How much current is on that shared wire?
Freeze the waves at one instant and say the load on all three is even. Wire one is carrying a thousand amps out.
At that same instant, wires two and three are each bringing 500 back.
A thousand out and a thousand back. back, so the shared wire has nothing left to carry.
Those numbers are only an example, so let's move the clock.
A moment later, wire 1 carries nothing, wire 2 carries 866 out, and wire 3 brings 866 back.
A moment after that, wires 1 and 3 each send 500 out, and wire 2 brings back a thousand. Stop the clock anywhere you like. Add the three and you get zero.
What do you do with a wire that carries nothing? You stop buying it. That's it.
Those giant lines don't have a wire set aside for the trip home because each of the three is the way back for the other two.
If that's hard to picture, think of a plumbing tee. Three pipes meeting at one fitting.
Water can't pile up inside the fitting, so whatever comes in has to leave through the others.
In the machines at the ends of the line, that fitting is a center point where the three sets of windings join. The rule still stands.
Current goes home. It just goes home on the other two wires.
That explains the missing wire.
But I slipped a word past you back there. Even. Real lines are never perfectly even.
The wires don't all sit the same distance from each other and from the ground, and big loads can pull harder on one wire than the others. So the three don't quite balance. What happens then?
It depends on whether that fitting has a fourth way out.
If the center point connects to nothing else, there's no way out.
Water can't pile up, so the three currents stay uneven, but they still add to zero.
The price shows up in the voltage, which goes lopsided, high on one wire and low on another.
If the center points are tied to the earth at both ends, and on the giant lines they usually are, that's a drain in the fitting.
What doesn't cancel goes home through the ground, with the earth acting as a wire. It's there on a normal day, usually small.
We looked for a dependable number and couldn't find one, so you won't get one from us.
So the giant lines have no wire set aside for the trip home.
That's a different thing from saying current doesn't come back. It always comes back.
Taking three wires off the line doesn't come free though.
Compare them fairly with the same wire, the same current, the same voltage to ground, and an even load.
Three wires carry what six carried with half the wire and half the heat lost along the way. The bill comes in voltage.
The push between any two of the wires is about 1 .73 times the push from one wire to the ground.
On a 500 ,000 volt line, each wire sits about 290 ,000 volts above the ground, with the full 500 ,000 between neighbors. That's why they hang so far apart.
And the long insulator strings hold each wire off the grounded steel.
Nobody could be sure the idea would hold up over distance until somebody built it.
In August of 1891, a line in Germany carried three -phase power about 110 miles at 15 ,000 volts, from a river to an exhibition in Frankfurt.
More than three -quarters of the power got through.
Two years later, America's first commercial three -phase plant opened in the hills near Redlands, California. You'll hear it said that Tesla invented all this.
His work mattered, but several men reached the same idea independently in the late 1880s.
Three had proved itself, but in 1895 the builders at Niagara Falls looked at the same choice and built their generators on two phases with four wires.
Those were serious engineers. Why isn't your grid built that way?
To see why, give every number the same three tests. Do the wires cancel so you can leave the return wire off? Is the power steady?
And will it spin a motor by itself? Start with steady.
On a single wave, the push falls to zero and climbs back 120 times a second, and the power falls to nothing right along with it.
Now picture three lamps, one on each phase.
When one is at the bottom, the other two are up.
Add the power of all three and the total doesn't move.
It's a flat line at every instant as long as the load is even.
A big motor does, because steady power means a steady twist on the shaft instead of a shake. Next, the motor.
Think of the wave at a football stadium.
Nobody runs around the stadium.
Each section just It stands up at its moment and sits back down, and the wave goes around anyway.
A motor's three sets of windings are bolted in place, spaced around the inside, and fed a third of a cycle apart.
That makes the magnetism sweep around the inside of the motor, and it drags the spinning part along.
Trade any two of the wires, and the wave runs backward, and so does the motor.
One phase can't do that.
It's a single section standing and sitting, with no direction at all.
That's why a single phase motor needs a helper winding to get going, usually with a capacitor. Now run the tests.
Start with two wires, plane out and back.
They cancel perfectly, and on wire alone, that pair can match three phases.
It loses on the other two tests.
The power still falls to nothing 120 times a second, and a motor can't tell which way to turn.
Next, two waves a quarter of a cycle apart.
You get smooth power and they'll spin a motor, which is what Niagara's generators made.
But they don't cancel evenly.
You need four wires, or three with one carrying extra, and either way that's twice the heat for the same power.
Even at Niagara, the line to Buffalo the next year left on three wires.
Now three waves, a third of a cycle apart.
They cancel, the power stays steady, and the motor spins by itself.
Three is the smallest number that passes all three tests.
Then why would anyone ever try more?
Four phases is really two opposite pairs, with one more of everything at each end and no new trick to show for it.
6 is the interesting one, because 6 got built.
Run six waves and neighboring wires sit closer together in the cycle, so the voltage between neighbors drops.
That lets you raise the voltage on the same line, if the towers and insulators can take it.
Engineers estimated more than 70 % more power on an existing double line.
A utility in New York actually tried it.
Starting in July of 1992, it ran a line a mile and a half long as six phases for three years.
The bill was at the ends of the line. It needed special transformers.
And a six -phase line means six breakers and six sets of bus bars, where a three -phase line has three. Go up to 12 phases and each step gains you less.
One of the engineers who built such lines said the substation work sort of defeats the economics.
The same engineer also said high phase counts could compete if a new wave of line building ever came. So 6 is still alive as an idea.
It's a bigger bill for more power.
And 3 remains the smallest number that does the whole job.
Hit subscribe. Kilowatt Nation takes apart the grid you live next to, one piece at a time.
Now go look at a real tower, because you can often count more than 3.
- Those thin wires on the very top look a lot like a return wire. They aren't.
Those are shield wires.
They're usually tied to the steel tower, they sit up there to catch lightning, and many of them carry fiber optic lines.
They aren't carrying your power.
When something shorts out, they help carry that surge back, along with the earth.
You may also see a few cables held apart by little spacers. That bundle is one wire doing one job.
and six groups on one tower is simply two lines sharing the same steel.
The rule of thumb is to count in threes.
If the number won't divide by three, you're looking at a shield wire, or, on a smaller line, a neutral. You will find a few big lines with two wires.
Most of those run on direct current, which has no waves to cancel, so it needs its own way back.
All of that was the giant lines. lines. Your street is different.
The big line ends at a substation, and the voltage often steps down more than once on its way to you.
At the last step, the transformer's low side usually starts something the towers never had. A fourth wire.
That's the neutral from your house, tied to the earth again and again along the way. It runs back to the substation and stops there.
That transformer's high side usually has no center point at all.
Its windings are joined in a triangle, so the street's return current can't climb onto the towers.
Why add back the wire we just took off?
Because of how the street is built, the three wires usually split up.
A branch takes just one of them, plus the neutral, down your block, and a branch like that is cheap to build.
But one hot wire has no partners left to cancel with.
All of that current has to come home on the neutral and through the earth.
One engineer described a line with 42 amps going out and only 22 back on the wire. The rest was in the dirt.
Back on the main line, where all three wires still run together, the neutral carries only what's left over.
Parts of California are the exception, with more lines that have no neutral at all than you'll usually find in North America.
up on the towers the way home is the other two wires on your block the way home is the neutral and the ground that's the whole difference you can see that arrangement on your own pole the streets main wires are usually on top anywhere from 2400 to 34 ,500 volts below them sits the neutral then the wires to the houses and lowest of all phone and cable the arrangement varies by utility, so treat that as a rough guide.
The bill from the towers shows up here too, that same 1 .73.
On one common line, each wire is 7 ,200 volts to the neutral and 12 ,470 between wires.
Your house usually lives on just one of the three.
You've also got three wires coming from the pole to your house, two hots and a neutral.
In a typical house, those two hots aren't two of the three phases.
Both come from the same one, through the transformer on your pole.
Since each block hangs on one wire, utilities connect different loads to different wires to keep things about even.
It drifts over time, and a crew may have to move a transformer to another wire.
Ever had a storm leave your house dark while the neighbors' lights stay on, on the same street? Here's one way that happens.
A storm drops a branch on one of the three, and the fuse protecting just that wire opens.
Your block goes dark, and a neighbor fed from another wire still has lights. It's common, too.
One big California utility says an outage on a single phase is more than twice as likely as an outage on all three.
Your neighbor could also be on a different line altogether, or the trouble could be in the wire to your own house, so report it either way.
If you see a wire down, stay far away and call it in. Dark doesn't mean dead.
And if your lights are brightening and dimming, that can be a broken neutral, a different problem, and a dangerous one. Call the utility right away.
For most houses, losing a wire means the lights go out.
A three -phase motor has a worse problem because it may not stop.
Go back to the stadium and take out one section.
A running motor will usually keep turning on the two wires it has left, getting hotter.
The work doesn't shrink, so it all comes through two wires.
As a rule of thumb, the current on each jumps about 1 .73 times what it was.
A motor that's already stopped generally can't restart.
It just sits there drawing heavy current. Ordinary breakers aren't designed to catch this.
The motor's overload protection may, but on a lightly loaded motor, it can miss it.
Some motors take it better than others, but without protection, a motor can overheat and burn out.
One July evening in the late 1990s, an egg farm in Millington, Maryland lived through a version of this.
Lightning struck somewhere off the property, and the power went out.
The farm had seven chicken houses, more than half a million chickens, and ventilation fans that needed three -phase power.
Without those fans, a federal court later wrote, the birds die within an hour or so.
A backup generator started by itself. Then a coolant hose ruptured and the generator quit. While a worker tried to fix it, the power came back on.
That should have been the good news.
But it came back as single -phase, the kind your house runs on, and about 100 fan motors burned out.
A second generator failed too, at least partly because of those motors. About 500 ,000 chickens died.
The court doesn't say exactly what reached those fans, only that it was single phase and that it burned them out.
Not too much power, the court said, the wrong type of power.
And at least one big utility's rules make protecting a three -phase motor from losing a phase the customer's job.
Here's the whole lesson on one card.
Current always goes home. On the giant lines each of the three wires is the way back for the other two.
Three is the smallest number that cancels, stays steady and spins a motor by itself.
Your street adds a neutral because your block hangs on just one wire.
Next time you pass under one of those towers, count them again in threes and leave out the thin ones on top. The return wire was never missing.
You were counting it the whole time.
The wires from your poles transformer to your house are a different three, with their own video on your screen.
Click it and you'll see how 7200 volts becomes your 120.