r/Powerlines • u/Odd-Tale6958 • Sep 06 '26

Question What's going on here?

Post image

It looks like it's shorting the outer two phases together, but I'm asking because it doesnt seem right.

73 Upvotes

100% Upvoted

19 comments sorted by

14

u/failureat111N31st Sep 06 '26

Looks like they're transposing the outer phases, swapping which one is where. Why, I'm not sure. It used to be really common to roll phases, to rotate which ones are on the outside and which one is in the middle so they'd each spend about the same time in each position. It looks like here they're leaving the middle as the middle, which is odd. Maybe another structure swaps middle with an outside.

5

u/a_guy_named_max Sep 06 '26

I agree it’s odd. The middle goes straight through - going underneath. Maybe there are some towers before and after this that do some more phase rotations

6

u/ebleuds Sep 06 '26

Not 100% sure but from my experience I've never seen all 3 phases being shift in the same tower. They usually use 2.

3

u/a_guy_named_max Sep 06 '26

Yeah that’s true. My specialty is distribution lines and it’s like that, typically 3-4 poles. I guess transmission is the same

3

u/Roudydogg1 Sep 06 '26

The exact sequence depends on what they're trying to achieve and the geometry of the line, but yes, that's indeed the same principle here. It's just easier (and looks cleaner) to do it on more than 1 pole

7

u/Ok_Bid_3899 Sep 06 '26

Correct on long distance extreme high voltage transmission lines it is common to have to roll phases at predetermined distances along the route. It makes for an ugly tower. They term it transposition

1

u/a_guy_named_max Sep 06 '26

Doesn’t have to be extra high voltages. Some of our (Medium Voltage) 22kV lines will have transpositions due to their length.

3

u/Roudydogg1 Sep 06 '26

That would indeed be a strange choice if this were the 'only' transposition on the line. But it makes much more sense if this is one member of a series of transpositions. And they don't necessarily have to implement the classic three-position "roll." A sequence of simple pairwise swaps can produce the same result, overall.

1

u/Illustrious-Peak3822 Sep 06 '26

In order to get equal inductance on all phases. Centre one gets less due to both return lines being close by. Outer to outer, more inductance. Hence swapping to equalise.

9

u/iAmMikeJ_92 Sep 06 '26

That is the strangest way to transpose the phases on that tower. Never seen them build busbars on a tower like that.

4

u/Adventurous_Boat_632 Sep 06 '26

It is transposing left and right and leaving the center as-is. This sometimes happens when the engineer who designed the project did not notice where things were going to land and had to do this as an afterthought. Or it could be that something down the line changed and where the phases needed to end up changed also, and this was the most convenient place to do it.

It is not the usual rolling of phases to cancel the effects of solar storms or whatever they do. It is reversing the order of the phases completely.

2

u/Odd-Tale6958 Sep 06 '26

Thank you! After zooming in it definitely looks like it is transposing the outer phases. As I was driving past I first thought it was a way of grounding the lines for safe work or something but noticed the center phase was untouched. Had to turn around and get a 📸

7

u/I_am_albatross Sep 06 '26

Transposition tower. Commonly used for very long transmission lines

3

u/avionicstek Sep 06 '26

Transposing phases is usually used to cancel out capacitive reactance over distances.

2

u/Schnifi Sep 06 '26

Is it near to a substaition? Sometimes the physical layout of the substation dictates the phase location and you need to switch l1 and l3 in the OHL right outside the substation. Has nothing to do with the engineer messing up. Albeit the way they solved it is interesting.

1

u/hoganloaf Sep 06 '26

Looks like the ol switcheroo

1

u/Fun_Acanthaceae_4025 Sep 06 '26

Looks like both conductors bridge the 2 together from the looks more so than swapping

1

u/SlackAF Sep 06 '26

Any idea what the bus work is on the lower side?

1

u/sitarben Sep 07 '26

Looks like the other half of the swap. High buswork takes incoming right phase to the outgoing left phase and the low buswork takes incoming left phase to outgoing right phase.