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Thread: Wild Leg
01-14-2008, 06:48 PM #1
Can someone explain the source and the value of a "wild leg". that is, one of the three phases has a higher voltage than the other two, in 3 Ph service?
I recently read about the use of "open Delta" setup, which uses only 2 transformers (for reason of economy to the utility) to supply the 3 legs, but according to the article, the voltages in that system are balanced.
Where does the higher voltage come from? A higher tap on one transformer? Why would one want one leg to be "hotter?
01-14-2008, 07:07 PM #2
The "wild leg", also known as the "high leg" exists in any 120/240 three-phase service, where there is 120 volts from phase A to neutral, and from phase C to neutral, but there is 208 volts from phase B to the neutral.
It is a necessary characteristic of a phase A to phase C transformer which has a grounded center tap, but which the two other (one other, for "open delta") are not so tapped.
As normal three-phase loads depend only upon the three ungrounded leads, this system provides 240 three-phase, without respect to any ground.
This system also provides for 120 volts single-phase across two of the phases, and 240 volts single-phase also across two of the phases.
Such a system is 66.67 percent efficient as to single-phase distribution, and 100 percent efficient as to three-phase distribution, whereas 120/208 is 100 percent efficient as to single-phase AND three-phase distribution.
A "high leg" can only exist in a grounded delta system, not in a wye system which is inherently grounded through the neutral.
01-14-2008, 07:20 PM #3
Notice the geometric relationship between legs A and C with respect to true neutral N and compare that with leg B and neutral. If the per unit sides of the equilateral triangle ( equilateral do to 120 degree displacement (time) of the 3 phases) are assumed to be 240V., then the trig representations become 240V. * cos 60 = 120 V., the observed voltage of either 1-phase leg to neutral. The 'wild leg" can be seen as 240V. * sin 60 = 207.8V. trigonometric relatinship of the B leg to true neutral.
01-14-2008, 08:02 PM #4
Thank you both very much!
I think I correctly understand that it is the presence of the GROUNDED center tap in one of the three (or two, for open delta) otherwise ungrounded transformers that causes the leg "opposite" the center tap to be at a higher voltage above ground than the other two.
Is high-voltage 3 ph distribution wye with center ground, so the primaries of transformers would be so connected, or is it ungrounded delta? I suppose transformer primaries could be connected delta and secondaries wye, or vice-versa?
It seems to me that if one ran properly grounded wye-connected equipment on a delta system
with a grounded center-tap in one transformer secondary, the two grounds would be at different potentials and a problem would ensue --- but I have apparently been doing wiring in such a system, and have seen no problem.
Does the utility have to be careful to always ground the center-tap between the same two legs to avoid ground currents?
Thank you again for sharing your knowledge.
01-14-2008, 11:00 PM #5
I think you are confusing the grounding conductor with neutral. I believe the whole reason for the grounded delta system is simply to obtain 240V, vice 208 from wye.
Here's two pictures I raided:
First we have grounded Delta.
simply, what you have is 3 secondaries, each of which is 240V. the center tap, grounded, of one winding can be considered separately, as a "single phase" Obviously the center tap gives you 120V on either side.
Now, what you have to do is plot the 3 phases' sine waves on a graph, and add them algebraiclyy. This is where the voltage discrepancy lies. If you start from the center tap, and go to one side, either direction, you measure 120V at the first point. Now, you go to the top winding, the reason you don't get 240--of the second winding +120V--is because the phases are offset. But you ARE adding this winding to your original 120V, hence the top point is "wild" to neutral.
In the case of wye,
You simply have 3 separate 120V windings, so from the center to any one of the three is 120V. Now, because of the phase offset, you can't just add any two and get 240. Because of the angular offset of the 3 phases, the algebraic sum becomes 208