Naked Science Forum
General Science => General Science => Topic started by: theThinker on 23/07/2019 17:33:25
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from internet:
"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..."
And then it says the neutral wire is connected to the ground.
My question now is how many conductor wires are there in overhead lines if the power station carries power through overhead lines.
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4
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For transmission of 3-phase, yes you need 4 wires. The ground wire (on the big towers at least) is usually the middle one connected directly to the towers without insulators.
It's also the one that birds are more likely to sit on. For lower voltage lines, the birds are not so picky and might choose otherwise, especially during cold weather when the live lines might be a little warm.
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4
Is your "4" definite without exception?
I have searched the internet and get confusing answers. Some articles seems to say 3 overhead lines are sufficient. The power station's fourth neutral is just grounded with 3 overhead conductors. Any return current to the power generator is transmitted with the literal earth ground as a wire over thousands of miles.
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In principle, if you have 3 conductors exactly 120 degrees out of phase and carrying the same current, there is no current flow in the 4th conductor.
The 4th wires is there to carry the "unmatched" current.
It's connected to earth for convenience and safety.
In practice power pylons often have 7 wires
A pair for each phase and a neutral at the top (which also doubles up as a lightning conductor (and bird perch))
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In the words of a lawyer "it all depends".
For longrange transmission you would probably use a high voltage (400 kV or more) 3-wire "delta" system with a protective earth, feeding into a stepdown transformer providing 14 kV 3 + N + E outputs for underground connection, thence 440V 3 + N + E. For domestic use you would take any one phase, neutral and earth to give you 240V rms, but for industrial use you might use the 440V with no neutral as a delta or with the neutral as a star configuration, depending on the difference between starting and running torque of the motor you are driving.
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"Star" configuration is used for residential AC supply, as it is referenced to Earth for safety reasons, and the short distance means the extra wire doesn't cost too much. This has 4 wires in the street: 3 phases + Neutral.
"Delta" configuration is used for long distance transmission, as there is no fourth conductor that costs money. (Note that I said there is no fourth conductor from the transformer, so you can't connect it to Earth.) This is typically visible as 3 or 6 thick conductors, and 1 or 2 lightning arrester wires. The lightning arrester wires are much thinner, since they don't need to carry current on a regular basis.
In practise, transformers are often built with windings for Delta on one side, and Star on the other, as any current or voltage imbalance on one side becomes more balanced on the other side.
And if a transmission line is Delta at one end (3 wires only) and Star at the other (4 wires), it is possible to Earth the center of the Star end, preventing the transmission line from rising above Earth potential, which would bring insulators closer to breakdown voltage.
Having a Star-Star transmission line, with both centers Earthed brings the possibility of low-frequency Earth currents caused by Coronal Mass Ejections to overload the transformers and cause a power outage, such as happened in Quebec in 1989.
https://en.wikipedia.org/wiki/March_1989_geomagnetic_storm#Quebec_power_blackout
Also note that some high-capacity transmission lines use DC, and there is an even number of conductors, with center-tapped Earth (plus lightning arrester wires).
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Many thanks for the clarifications given.
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It's interesting!