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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?
 
D

Don Kelly

Jan 1, 1970
0
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?

---------------
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.
 
| 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
 
D

Don Kelly

Jan 1, 1970
0
----------------------------
| 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

--
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 =60 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.
 
| 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 =60 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.
 
On 3 Dec 2006 12:17:01 -0800 [email protected] wrote:

| [email protected] 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).
 
D

Don Kelly

Jan 1, 1970
0
On 3 Dec 2006 12:17:01 -0800 [email protected] wrote:

| [email protected] 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).

---------

The only advantage I see to 6 phase (or 12 phase) is compactness and this is
actually better for 6 phase-partly for the reason you gave before-
clearances. Where 6 phase has its main advantage is for urban distribution .
This is low voltage stuff=- say 7200V L-N

In terms of power transmission, 2- 3 phase lines will carry as much as a 6
phase line at the same current and voltage to ground (or a 12 or 96 phase
line). Same conductor net cross section and losses and the two 3 phase lines
are electrically independent- giving more flexibility. There is no need to
convert back to 3 phase in either case (simply use alternate lines of the 6
phase system for 3 phase)but there would be problems if both 3 phase pairs
feed into an interconnected system (even in a roundabout way- not a problem
for independent feeders but a definite problem at transmission levels- which
can be handled with judicious transformer connections). Generally the
insulation needs of the typical double circuit tower is due to line to
ground considerations at an expected maximum swing of the insulation
strings, rather than line to line considerations so there is little gain
going to 6 phase unless the ring construction is used in order to gain some
compactness and reduction of right of way (which can be costly in built up
areas) There are added costs in tower construction. Any electrical
advantages? -possibly for EHV in terms of ground level fields but adding a
meter to the tower height would probably be a cheaper way to deal with that
problem.

As for your 96 phase proposal- I suggest that you compare total conductor
size, losses and overall diameter to a 3 phase system. In terms of conductor
cross sections, weight and DC resistivity the 96 #12 wires compare with 3-
4/0 conductors. In practice something equivalent to 6/0 would be needed as
it is AC. In that case the 6/0 will come out ahead in terms of compactness
at the 480V level (conductor centers on a 12mm radius circle rather than a
150mm radius) but will be heavier. Considering bundling (such as 2- 4/0 in
parallel per phase) does better than 96 phase #12. In other words, at too
low a voltage level, the advantage of compactness is lost and there is no
gain electrically but a good deal more complexity.

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.

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.

As for flashover tests on 1.2MV systems, both EPRI in the US and Hydro
Quebec in Canada have facilities for this. I have seen the Hydro Quebec
facility (impressive) and putting the tower top of a 765KV or 1200KV line
inside their main test building is not a problem.
 
| 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.
 
D

Don Kelly

Jan 1, 1970
0
| 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.

-----------
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.
 
| 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?
 
D

Don Kelly

Jan 1, 1970
0
| 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)?
-----------
The decisions on that would be up to the people doing the mechanical design
but I think that you have it right. There is a minimum allowable clearance
to ground-and this varies according to location- open field, road crossings
etc so the towers are already large and expensive. With the clearances
required, the window tower also allows use of two overhead ground wires-
giving better line shielding.
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?
------
Not as stable as the normal four-square base tower (which can also be pulled
down by ide loads if a conductor or two break unbalancing forces). These
have a single base and depend on guy wires for stability. If one tower goes
down, it can pull down others. For changes in direction a normal tower is
used because of the unbalanced pull. The V tower is cheaper, can be dropped
into place by helicopter and has a much simpler foundation. Great in the
mountains.
| 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?
--------
Unless very short, it won't be limited by conductor current carrying
capacity. Rather than use large diameter conductors, a group or bundle of
conductors is used. You can see these bundles with 2 to 4 wires per phase at
voltages from 240KV and above. Bundling gives the L, , C and field
characteristics of a much larger conductor. and is a lot easier to handle
than a much heavier larger diameter conductor.
4- 1 inch diameter conductors spaced 18 inches apart have an equivalent
radius of about 7.5 inches.
Current capacity with ACSR is probably about 2KA (data not on hand).

At that voltage a nominal figure is about 2100MW for a 200 mile line but
this is a crude ballpark figure with a lot of assumptions. The limitations
are mainly allowable phase shift and line charging.
 
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