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#### banzai

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Hi everyone

Why is phase transposition done on high voltage transmission lines, does it have to do any thing with capacitance between earth and the transmission lines ?

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Paging big john.

dspiffy

#### MattHelm

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I am sure Big John will be along shortly to elaborate or correct me, but my understanding is it is to help compensate for both varying capacitance and inductance.

If the phase conductors could occupy the same physical space, their capacitance and inductance to ground would be identical. Since this is not possible, changing them up helps average it out. Or something. I am not a line guy and anything I say about it should be regarded with the same suspicion as phase power calculations from a talking dog.

#### cdnelectrician

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Hi everyone

Why is phase transposition done on high voltage transmission lines, does it have to do any thing with capacitance between earth and the transmission lines ?
Good question! I think it has something to do with line loss. Interested to hear the answer myself!

#### MDShunk

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To some degree, it's done to keep the conductors equal length before, or just after, hard turns.

cdnelectrician

#### Big John

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When you're running multiple circuit conductors a very long distance it means that some conductors end up with a different impedance than others, for example: If one phase were constantly on the outside of a curve, it could be longer and see a higher conductor resistance than the phase on the inside of a curve. Or if another phase is always in the middle, it's gonna see a higher impedance because of the mutual inductance of both the wires on either side of it, etc.

A transposition just swaps around the conductor positions so that each wire experiences the same conditions as the others.

It's no different than how we're supposed to keep all wires the same length when doing parallel runs, only because you can't just coil up a bunch of slack on a transmission line when it gets to the substation, it means you gotta change how the circuit is run so that the conductor properties are still the same at the end.

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Big John beat me to it.

Keeping impedance the same between the phases. Also keeping phase to ground capaciticitance the same.

#### Mshea

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In addition thom the above I also understood that they also do this to deal with capacitance to the earth?

#### Semi-Ret Electrician

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A transposing scheme is a pattern by which the conductors of overhead power lines are transposed at transposing structures. In order to ensure balanced capacitance of a three-phase line, each of the three conductors must hang once at each position of the overhead line.

http://en.wikipedia.org/wiki/Transposition_(telecommunications)

Transposition of High-Voltage Overhead Lines and Elimination of Electrostatic Unbalance to Ground

It's kind of like twisting CAT 5

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In addition thom the above I also understood that they also do this to deal with capacitance to the earth?

It does, sometimes that alone can drive transposition. Short transmission lines can be omitted but longer ones benefit. With horizontal lines its rarely an issue but with vertical ones (phases above one another) the bottom phase has a slightly higher capacitive reactance to ground over the top phase. A few towers in the run will rotate the bottom phase to the middle top and back.

#### magmash

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Phase Transposition is mainly done to balance the capacitance between earth and the transmission lines when using vertical cable towers that hang the wires one over the other with respect to the space between them. when using such towers one of the phases will always be closer to earth and have a greater capacitance, so phase transposition is done to balance that effect of all three phases. its recommended to do a phase transposition every 100 Miles or so, to my experience such transpositions is always tried to be avoided due to the cost of special towers that have to be used for the transposition.

Its also done to equal the transmission lines length when there is much hard turns because the three phases will not have the same impedance, but that is very rarely done.

#### CraigV

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It's kind of like twisting CAT 5
But not for the same reasons. Twisted pair cables for data are twisted to reduce RF interference, via common mode rejection.

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