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Topic Title: 11 kV spur lines in Scotland
Topic Summary: Questions about these power lines
Created On: 24 January 2011 01:16 PM
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 24 January 2011 01:16 PM
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ColMacF

Posts: 2
Joined: 24 January 2011

This is for my information/understanding only.

There are many long, two wire, 11 kV spurs in Scotland. My understanding is that these wires are separate phases at 11 kV phase to phase and with the phases 180 degrees apart. Is that correct?

If correct, does that mean that they are 11 divided by 2 kV phase to ground?

Also if correct, what is the nature of the connection from the main 11 kV 3 phase line where the phases are 120 degrees apart? How is it done?
 24 January 2011 04:21 PM
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fshillit

Posts: 18
Joined: 14 August 2002

The two wires are indeed seperate phases but are 120 degrees apart (as they are taken from 2 of 3 phases of a 3 phase line). The phase to ground voltage is 11/sqrt(3)=6.35kV.
 24 January 2011 07:36 PM
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slittle

Posts: 3463
Joined: 22 November 2007

You could well find that the LV fed from these provides you with two LV phases at 180 degrees apart (460v between phases)

Quite common down here in rural Essex


Stu
 24 January 2011 08:03 PM
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ColMacF

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Joined: 24 January 2011

Thanks for that quick response. So it's easy enough to take the two phases from the 3 phase lines. Does that phase separation not give unbalanced load and losses from currents to neutral/earth at the user end?

Do you only get 180 degree phase difference in 'split-phase' systems where the secondary side of a transformer is tapped in the middle and both ends?
 24 January 2011 09:30 PM
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slittle

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Yes, the 180 degree phase difference is done in the transformer although it seems to be a fairly common arrangement across lots of DNO's.

I've never seen anything else on my patch. If it's two phase HV, then it's either single phase (230) or two phase (460)

Stu
 25 January 2011 02:44 PM
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Paul1966

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Joined: 21 December 2004

One thing you have to keep in mind is that in order to discern any sort of phase difference you have to be comparing at least three points in the system.

The two phases of the spur tapped from a three-phase line will be 120 degrees out of phase with each other when referenced to a common third point, e.g. earth potential. But as far as just measuring across those two lines is concerned, all you'll see is a simple sinusoidal waveform of 11kV. The primary of the transformer connected at the end of that spur line is just a single winding with a two-wire connection, so with no third reference point it makes no difference whether the two lines are 120 degrees out of phase when referenced to earth, because the primary winding does not that third reference point.

Single-phase spurs like this are very common in this area of rural Norfolk as well, sometimes feeding a small transformer with just a single 240V secondary, but often to a single-phase 3-wire 240/480V system for small collections of houses, farms, and so on.

Load balancing can still be carried out as far as possible, simply by connecting some single-phase spurs across A & B, some across A & C, and some across B & C from the three-phase 11kV line.
 27 January 2011 12:09 PM
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Angram

Posts: 551
Joined: 23 March 2009

Yes these two phase lines are everywhere in UK where there is low density demand.
Annoying if you are re-locating your business with 3-phase motors !
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