technical limitations on three phase 240 volt transformer

Would there be any real _technical_ limitations (as opposed to just fear because such a thing is not common) for the design of a three phase dry
type transformer around a standard "E" core used in typical three phase designs, with each of the 3 secondary windings wound for 240 volts with a center tap on ALL THREE so each winding is really 120/240 volts? Are there any issues with using such a transformer to supply strictly single phase loads, assuming reasonable balance?
One issue I do see is that such a transformer would require more secondary terminal connections. There would be 7 such connections if the neutral were wired in common internally. Assume any requirement for "number of disconnects" would be met by a single three phase primary disconnect.
I've never seen such a transformer in any of the marketing data I've seen from many transformer manufacturers. But I don't see any real reason why such a thing could not be designed and built, if there was a market for it.
I do know this could very simply be done with three single phase 120/240 volt transformers. But what about using TWO separate "E" core dry-type transformers where either the primary or secondary windings are reversed in one of them such that one of them is 180 degrees rotated from the other. Could this split combination be used to supply 120/240 volt single phase loads where one 120 volt leg comes from one transformer, and the other 120 volt leg comes from the other that is 180 degrees offset? What if one of the transformers loses power such as its primary breaker tripping? What about triplen harmonics issues from 2-wire 240 volt loads?
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Been done. The overall economics for it would depend on the use. If the 3 center taps are made common, then you have what is called a 6 phase system. This has been done for some distribution systems (at higher than 240V) where there are some advantages with respect to compactness and insulation (line to line voltage = line to neutral voltage). There are also advantages for rectifier supplies.
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| Been done. The overall economics for it would depend on the use. | If the 3 center taps are made common, then you have what is called a 6 phase | system. This has been done for some distribution systems (at higher than | 240V) where there are some advantages with respect to compactness and | insulation (line to line voltage = line to neutral voltage). There are also | advantages for rectifier supplies.
I have in fact seen (online, available for purchase) one such transformer at a LOWER voltage. It's output was 60 volts L-N on all 6 phases. That would give it 120 volts L-L on the 180 degree phases. It was intended for "technical power" as described in NEC article 647.
I also note you call this 6 phase, yet some of the phases are 180 degrees from each other. FYI, I'd call it 6 phase, too. But if you take 2 of those phases, would you call that branch 2 phase? Normally we don't do that ... that's someting I wish would change. Calling the Edison style split phase system as "2 phases" really does make sense in a certain way of thinking about phases (e.g. how many different vectors from neutral that are involved). Of course saying "2 phases" doesn't completely say what is involved ... the phases could be 180 degrees apart (single phase with Edison split), 120 degrees apart (2 legs of three phase, often seen as the nasty 120/208 type service), 90 degrees apart (the traditional concept of "2 phase"), and even 60 degrees apart (corner grounded open delta). But it would still be natural to refer to them as "2 phases" just as I would refer to "6 phases" on the transformer described in the previous post and "12 phases" for the right side of:
http://phil.ipal.org/usenet/aee/2006-11-30/powerlines.jpg
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wrote:

The 3 wire Edison "single phase" system 240/120 center tapped is referred to as 2 phase in countries which don't use it. That is technically correct in that for an n phase balanced system the phases are 360/n degrees apart. In countries where it is used, we are stuck with the terminology "single phase -center tapped" or "Edison". The argument comes in whether it is 2 phase or single phase center tapped. Note that the Edison system works just as well for DC.
It is common usage to refer to a system with 2 voltages 90 degrees apart as 2 phase. This is not techically correct but does present one of the advantages of a polyphase system- a rotating field in a motor. (two phases 180 degrees apart will give a pulsating field which can be resolved into forward and a backward rotating fields).
The 6 phase system has (adjacent) line to line voltages equal to the line to neutral voltages. 360/6 ` degrees fitting the criterion above. Only with a 3 phase system are the line to line voltages the same between all phases (two phase has only 1 L-L voltage which is equal to itself).
As for two legs of a 3 phase system only part of the system exists - you can treat it as an unbalanced 3 phase system, through symmetrical components, or as an unbalanced 2 phase system using forward /backward components, or simply ignore all this and tackle simple cases directly. Whatever is easiest for the person with the problem.
As for the jpg - Left side 2 - independent 3 phase lines or possibly 6 phase Right side could be 4 three phase lines which I doubt, 2- 6 phase lines or a 12 phase line (unlikely). Advantage to 6 phase- none in terms of power transfer but possible compactness in terms of insulation and some decrease in right of way needs which is important in high land cost areas or where reduction of visual impact carries a high price tag. 12 phase has been used -for rectifiers. use a star star with center tapped secondaries and a delta star with center tapped secondaries and tie the secondary neutrals together. feed the rectifiers and get a smoother DC without filtering.
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| The 3 wire Edison "single phase" system 240/120 center tapped is referred to | as 2 phase in countries which don't use it. That is technically correct in | that for an n phase balanced system the phases are 360/n degrees apart. In | countries where it is used, we are stuck with the terminology "single | phase -center tapped" or "Edison". | The argument comes in whether it is 2 phase or single phase center tapped. | Note that the Edison system works just as well for DC.
I'd hope so, as that's how _he_ used it :-)
| It is common usage to refer to a system with 2 voltages 90 degrees apart as | 2 phase. This is not techically correct but does present one of the | advantages of a polyphase system- a rotating field in a motor. (two phases | 180 degrees apart will give a pulsating field which can be resolved into | forward and a backward rotating fields). | | The 6 phase system has (adjacent) line to line voltages equal to the line to | neutral voltages. 360/6 ` degrees fitting the criterion above. Only with a | 3 phase system are the line to line voltages the same between all phases | (two phase has only 1 L-L voltage which is equal to itself). | | As for two legs of a 3 phase system only part of the system exists - you can | treat it as an unbalanced 3 phase system, through symmetrical components, | or as an unbalanced 2 phase system using forward /backward components, or | simply ignore all this and tackle simple cases directly. Whatever is easiest | for the person with the problem. | | As for the jpg - | Left side 2 - independent 3 phase lines or possibly 6 phase | Right side could be 4 three phase lines which I doubt, 2- 6 phase lines or | a 12 phase line (unlikely). Advantage to 6 phase- none in terms of power
It actually was an experiental 12 phase transmission line.
| transfer but possible compactness in terms of insulation and some decrease | in right of way needs which is important in high land cost areas or where | reduction of visual impact carries a high price tag. | 12 phase has been used -for rectifiers. use a star star with center tapped | secondaries and a delta star with center tapped secondaries and tie the | secondary neutrals together. feed the rectifiers and get a smoother DC | without filtering.
Or just split it up into 4 different 3 phase load zones at the load end of the transmission line. It would also be possible with some transformers to put it all back into 3 phases.
Line to adjancent line voltages do go down as the number of phases go up for a constant line to neutral voltage. At 12 phases you have 51.7638%, or for those who like the less precision, 52% ratio between L-N and L-aL. 24 phases would have 26.1052% ... note that is NOT half of the previous figure, though if rounded, it would look like it was.
Of course there is a limit to this. Those lines are going to swing in the wind a certain amount, and as they keep getting closer with more phases, at some point the swinging will bring them in contact. It looks to me like 12 phases would be a limit, and might even be pushing it. 6 phases actually "feels" right to me. But the 12 phase line does have a cool look to it, probably just because it's different. And 6 phases would fit nicely on a traditional transmission tower.
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snipped-for-privacy@ipal.net wrote:

Very strange. It looks difficult to work with, as you would have to thread the cables through the outer ring of steelwork, rather than just hoisting them up, and attaching them to the ends of the crossarms on a conventional pylon.
Just out of interest, where and when was this? I am going to guess either Russia or South Africa, and will probably be totally wrong!
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On 3 Dec 2006 12:17:01 -0800 snipped-for-privacy@mail.croydon.ac.uk wrote:
| snipped-for-privacy@ipal.net wrote: | |> It actually was an experiental 12 phase transmission line. | | Very strange. It looks difficult to work with, as you would have to | thread the cables through the outer ring of steelwork, rather than just | hoisting them up, and attaching them to the ends of the crossarms on a | conventional pylon.
That sure looks like it would be a problem. I'd be more inclined to suggest a single center tower with the support structure on either side of it. That's common for the 6 conductor transmission lines that could be used for 6 phase transmission, with the center crossbar being longer than the top and bottom. The 12 phase line would be harder, but might be doable with a ring structure attached to the tower and the conductors attached to support arms extending outward from the ring.
I think 6 phase transmission would be adequate. There is the option to take 3 phases to one load and the other 3 phase to another load at the far end, or use transformers to recombine 6 phases to 3 phases for a single big load.
But I did envision trying out my own little 96 phase transmission "line" by attaching a bunch of THWN wires around the outside of a big PVC tube. But that would be a lot of weird little transformers to get all those phase angles. But with a L-N voltage of 120 volts, L-L would only be 7.852579877 volts in this case (carried to my arithmetic precision extremes). Or at a L-N voltage of 480 volts and AWG #12, it could carry over 1 MW. Even cheap magnet wire at AWG 22 at 120 volts could still carry 1/4 MW. Of course that's 96 parallel conductors.
| Just out of interest, where and when was this? I am going to guess | either Russia or South Africa, and will probably be totally wrong!
I recall it was a test facility in Tennessee, USA. I saw another picture that showed these 2 test lines didn't run very far (from one end to the other of the test facility).
I have seen other pictures showing big arcs from Russian tests of 1.2 Mv which may even have been DC (they did a lot of DC research, too).
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| Increasing the number of phases to 6 will allow a more compact line using a | ring structure and going to 12 phases at the same L-N voltage will not | increase the diameter so that it is actually more compact -probably the | reason for a test line. The governing factors in going to more than 3 phase | are a)potential cost savings which depend very much on distance, voltage | level and other factors. | b)special applications such as rectifiers as has been done for at least 80 | years.
With a support structure that wraps around the conductors, some of the gains in compactness are then lost. But it looks to be rather compact between the towers. Still, if you had conductors with 4 times the size in cross section, arranged in a triangle, wouldn't it fit in just about the same space for the same L-G voltage?
| The shortness of the test lines indicate that what was of concern was the | actual fields associated with these configurations and also an attempt to | get an estimate of costs and problems of construction per tower as well as | assessment of problems with conductor swing etc.
I think the conductor swing could be an issue. Sure, the L-L voltage is lower, but the distance the wires move in the wind can still be just as much while having to keep clear a smaller distance.
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wrote:

----------- The problem of stringing conductors going through a tower "window" exists at present for many lines - mainly in the 300KV and up range. This line didn't appear to be at that level. For a given line to line voltage- one could consider the conductors places around the circumference of a circle. For 3 phase, 6 phase, 3n phases, this circle's diameter will be the same if not affected by conductor radius. This would be the case for the lines pictured. In that case, there is really no advantage to a more compact structure as the diameter of the "window" would be determined by line to neutral voltage. The structure for 6 or higher phases will involve more complex support and more insulator strings. There are also mechanical limits on the minimum conductor size. If the voltage is low- then one can get a more compact structure with 6 phases but the benefit is not likely to extend to use of more phases. Practical limit of any advantage - possibly 240KV if right of way costs are high. At higher voltages, considerations of surface fields would require larger conductors (or bundles of 2 to 4 conductors per phase, spaced 30 to 50cm apart) than what current considerations would require so that is a disadvantage. Line capacitance and inductance will be affected adversely in comparison to 3 phase. On the other hand, for distribution, spans are short and voltages are low so a 6 phase line may have 7 conductors (neutral in the centre) and insulation requirements low enough that an insulating yoke holding the conductors is practical and additional yokes can be placed as spacers at different points along the span so that swing effects are eliminated (you have correctly hit upon a serious consideration). Possibly such a yoke could be used on longer spans at higher voltages just as spacers are used with bundled conductors but there are cons as well as pros for that at transmission voltage levels. Another problem with swing is that, for conductor held fairly rigidly at the towers, there are side stresses on the conductors. This may not be a problem as many lines now use V strings to support conductors and these limit movement at the tower (that is why they are used- reduces the need to allow for insulator swing) and aren't falling down.
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| The problem of stringing conductors going through a tower "window" exists at | present for many lines - mainly in the 300KV and up range. This line didn't | appear to be at that level.
The apparent sizing on that picture suggest to me that the line was merely in the MV range, not the HV range, and thus more suited for distribution.
| For a given line to line voltage- one could consider the conductors places | around the circumference of a circle. For 3 phase, 6 phase, 3n phases, this | circle's diameter will be the same if not affected by conductor radius. This | would be the case for the lines pictured. In that case, there is really no | advantage to a more compact structure as the diameter of the "window" would | be determined by line to neutral voltage. The structure for 6 or higher | phases will involve more complex support and more insulator strings. There | are also mechanical limits on the minimum conductor size. | If the voltage is low- then one can get a more compact structure with 6 | phases but the benefit is not likely to extend to use of more phases. | Practical limit of any advantage - possibly 240KV if right of way costs are | high. | At higher voltages, considerations of surface fields would require larger | conductors (or bundles of 2 to 4 conductors per phase, spaced 30 to 50cm | apart) than what current considerations would require so that is a | disadvantage. Line capacitance and inductance will be affected adversely in | comparison to 3 phase.
Is this why the very HV stuff is constructed with 3 conductors all equal distant from ground, as opposed to a triangle with 1 conductor above the other 2 conductors? Or is it just too impractical to have so much height (e.g. protection wires then have to higher and the whole assembly has to be taller and more subject to tipping over)?
I remember seeing a picture of one of those large transmission lines that have the V-shaped tower going up to a wide cross brace holding up 3-inline conductors, possibly in the 765kV range ... with about 20 such towers in the picture, all tipped over and iced up. The entirety of what was in the picture was affected, suggesting the possibility that much more of that line was affected. Just how stable is that tower design?
| On the other hand, for distribution, spans are short and voltages are low so | a 6 phase line may have 7 conductors (neutral in the centre) and insulation | requirements low enough that an insulating yoke holding the conductors is | practical and additional yokes can be placed as spacers at different points | along the span so that swing effects are eliminated (you have correctly hit | upon a serious consideration). Possibly such a yoke could be used on longer | spans at higher voltages just as spacers are used with bundled conductors | but there are cons as well as pros for that at transmission voltage levels. | Another problem with swing is that, for conductor held fairly rigidly at the | towers, there are side stresses on the conductors. This may not be a problem | as many lines now use V strings to support conductors and these limit | movement at the tower (that is why they are used- reduces the need to allow | for insulator swing) and aren't falling down.
There will still be some swing, maximum at the mid point between 2 towers. Given a specific conductor size and tower spacing, the conductors will have a specific required sag, and that looseness would give a specific amount of mid-air swing. Too many phases around a circle would quickly bring them into contact or arc distance. The N-phase design would have to be more rigid to be practical, I'd think. You could scale up N to such an extreme (disregarding the complexity of creating all the phases) that the wires would just end up bumping into each other or being too thin.
Anyway, I can't see any advantage to more than 6 phase lines and clearly see that in transmission application, even 6 phases is a burden.
What is the largest practical conductor sizing when considering multiple conductors braced together? E.g. what is the highest capacity 765kV three phase transmission line you could envision ever being built?
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Whether the Edison system is 2 phase or single phase is subject to much debate. If you look at the voltage on each leg, it's easy to say it's two phase. However, if you look at the _power_ of each leg (into a resistive load), you'll see the two legs have the same phase, yet 3 phase still has 3 sine waves shifted from each other by 120 degrees, and the same is true for n phase systems (n odd). There is also the other system described as "two phase", that is, two hots shifted from each other by 90 degrees and a neutral. (there are also 4 and 5 wire versions of this, the latter some may want to call "4 phase") It, too, has two power phases shifted from each other by 90 degrees.
It's worth noting that it's possible, using a clever arrangement of tapped transformers, to convert "m phase" to "n phase" (m & n > 1) IF you define the 90 degree version as "2 phase". The simplest of these is the Scott-T arrangement to convert between 3 phase and (90 degree) 2 phase.

Judging from the insulators in that .jpg, it's pretty certain the power is _produced_ with the phase shift between adjacent conductors of 60 degrees (left) and 30 degrees (right), since on the left, all the insulators are the same size since phase phase and phase neutral voltages are all the same (for adjacent phases) and the second has smaller insulators between adjacent phases than to the tower. This makes sense since adjacent phases are about 0.5176 times the line-neutral voltage from each other. How it's _used_ at the load end, I don't know, but for the right tower, I see problems if it's multicircuited, since if you have to switch off one circuit, the insulators between the dead circuit and adjacent ones will be subject to nearly double the voltage they're subjected to when both are on.
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