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Joined: Mar 2005
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Yes, but it's meaningless, as the connections on the secondary define which phase is which- it's on the other end that you have to worry about rolling or transposition.
[This message has been edited by SteveFehr (edited 10-29-2006).]
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What is a high leg delta?
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Joined: Mar 2005
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High leg delta is where you have a 4-wire 240V Delta, where instead of all 3 phase carriers being 139V to ground like they would be in 240/139V wye, one of the 240V legs is center-tapped at the transformer, and the center-tap grounded and connected to the neutral. (+120 and -120, just like at a residential pole pig). The other delta- the "high leg" is now at a higher potential, 207V to ground.
So, it ends up where instead of having 3 phases, each 240V between them and 139V to ground, you end up with 3 phases- each of which still have 240V between them, but two of these are now 120V to ground, and the third is 207V to ground.
The advantage with this is that you can supply 3-phase loads and 120V 1-phase loads from a single distribution system without any additional transformers.
[This message has been edited by SteveFehr (edited 10-29-2006).]
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How many xfmrs (pole pigs) does it take for the high leg delta? Also, what in the world is 139V to ground? Is that peak, RMS, or Stevo measurement?
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240V RMS / Sqrt(3) = 139V RMS. You could call it 220/127V or 230/133V if you wanted to, all depends where in the distribution system it is, what with voltage drop at all. Pole pigs aren't high leg delta, as least not normal 1-phase residential ones. They're a single center-tap step-down transformer that drops the line voltage to 240V with a grounded center-tap for the neutral. +7.2kV GND
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o-----------------o
o--------o--------o
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+120 GND -120
(Neutral)[This message has been edited by SteveFehr (edited 10-29-2006).]
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Joined: Aug 2006
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I've never seen 139 VAC to ground anywhere or 220/127V, for that matter. I thought utilities don't like to go lower than 0.95 PU (per-unit)? Don't 2 or 3 pole pigs make up your 3-phase Delta? Also, why would you divide 240VAC by the sqrt(3) to get 139V, when from the high leg to ground you would get 208V to ground?
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Joined: Mar 2005
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That's because nobody ever runs a 240V WYE, there's not much point. (Wouldn't surprise me if there are a few out there, though!) If it's (normal) delta or wye, it's going to be 480/277V or 208/120. But if there was a 240V wye, it would be 240/139V.
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The 240Y/139 (actually 230Y/133)construction is the standard voltage in 230V drive isolation transformers.
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What is a "rolled" primary?
Let's say you are going to parallel two transformers, but you get the primary rotation on one different from the other. So, one will give a 30 degree shift "leading" to the secondary and the other will give a 30 degree "lagging" shift. What happens is the phases of one end up being 60 degrees out from the other, and no amount of swapping on the secondary will correct it. How many xfmrs (pole pigs) does it take for the high leg delta? You can do it with two primary phases and two pots (open delta) or 3 primary phases and three pots (closed delta). The 3 pot bank has better voltage regulation than the open one. [This message has been edited by WFO (edited 11-11-2006).]
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