Closed Delta 120/240V 3-phase service

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In a strictly technical viewpoint one can say that for an N phase star the phase voltages are 360/N degrees apart so that the 3 wire "Edison system" satisfies this criterion just as 3 phase, 4 phase,.... do. All with N hots and a neutral -i.e. star connection. With balanced loads the neutral current is 0 in all these cases. That is the justification for calling it 2 phase.
However, in Europe the 2-phase designation is used rather than 3-wire single phase because of the above viewpoint. (Of course they don't use it there). In North America 2 phase implies two windings in quadrature (and for balanced loads there is a neutral current). The system was started by Edison for DC circuits and somehow, along with the "Edison" name, the alternative single phase center tapped nomenclature came into use.
So, while not normally called that in North America, it is quite justifiably called (without error) a 2 phase system -at least on the secondary of a single phase transformer.
I remember the Scott T back in the 50's but never saw one that I can remember. I do remember the more common open delta transformers but usually the transformers were equal in size.
Reply to
Don Kelly
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The utility cannot insist on anything if the service you order is tariffed by the state public utilities commission they must provide it on demand. The tariff may allow them to surcharge you if the service does not draw enough current to justify the additional expense. If the type of service you want is not tariffed then the utility must apply for a tariff for that new class of service and obtain it prior to connecting the service. If they don't want to provide it then you would be faced with the insurmountable cost of applying for the tariff against their opposition.
Dairy farmers in several states have obtained tariffs for ungrounded delta service to supply customer owned transformers in order to keep utility neutral currents off of the farm and away from their livestock. The utilities initially fought the tariffs very hard for reasons that were not clear to me. Once the problem with dairy cows and stray Multi Grounded Neutral (MGN) currents became well known some utilities began offering special transformers that do not permit the MGN currents to flow on the premise service wiring. I've yet to learn how they accomplish that but if it were my livelihood I'd want the ungrounded Delta supply that was tariffed in spite of the utilities opposition.
Reply to
Tom Horne
Then the two phase system is actually four phase. There are two equal voltages 360/4 degrees apart, each of which could be split with a center tap. It has been known as two phase wherever it was used.
Calling the 240/120V common system "two phase" may not be technically in error but it confuses common usage in the United States.
The Scott-T requires a special transformer, tapped to provide equal quadrature voltages from the equal three phase inputs. I've never seen such a transformer used for any other purpose. They were used to supply motors with two equal windings in quadrature, which was a balanced load, usually on a four wire feed.
Reply to
VWWall
|> So that's the way you would go? What if the utility says they don't want |> to have the loss of an extra set of transformers with MV -> 480 -> 120/240 |> and insist on no more than one transformer per MV -> meter path? |> | | The utility cannot insist on anything if the service you order is | tariffed by the state public utilities commission they must provide it | on demand. The tariff may allow them to surcharge you if the service | does not draw enough current to justify the additional expense. If the | type of service you want is not tariffed then the utility must apply for | a tariff for that new class of service and obtain it prior to connecting | the service. If they don't want to provide it then you would be faced | with the insurmountable cost of applying for the tariff against their | opposition.
I've read many tariffs from various areas. Most have limits on the maximum capacity a utility is required to provide as single phase, and allows the utility to require three phase for higher capacity services. They all seem to indicate that 240 delta services are no longer required to be provided.
In some cases there was some specification of rate compensation when the utility required providing the service at a higher voltage, to account for the loss in the customer transformer. I presume that is when the metering is at the higher voltage. In the example case I described with a large apartment building, presumably metering would be done at the 120/240 end, since there would not be individual transformers for each unit.
| Dairy farmers in several states have obtained tariffs for ungrounded | delta service to supply customer owned transformers in order to keep | utility neutral currents off of the farm and away from their livestock. | The utilities initially fought the tariffs very hard for reasons that | were not clear to me. Once the problem with dairy cows and stray Multi | Grounded Neutral (MGN) currents became well known some utilities began | offering special transformers that do not permit the MGN currents to | flow on the premise service wiring. I've yet to learn how they | accomplish that but if it were my livelihood I'd want the ungrounded | Delta supply that was tariffed in spite of the utilities opposition.
One possible reason they fought those tariffs was because they believed that the ungrounded transformers would be subject to damage caused by a lightning strike involving an arc between primary and secondary when the secondary is not grounded. This could damage the winding insulation.
The special transformers are probably ones that do have an ungrounded secondary, but have a special arc gap device to allow a lightning strike or surge to jump across between primary and secondary, reducing the risk of damage to the winding insulation.
Reply to
phil-news-nospam
| |> |> | |> |> |> |> >> B C |> |> >> \ / |> |> >> \ / |> |> >>A----N----D |> |> >> / \ |> |> >> / \ |> |> >> F E |> |> > |> |> >>A-N and N-D are both wound on the first core. B-N and N-E are both wound |> |> >>on the second core. C-N and N-F are both wound on the third core. A third |> |> >>of the 120/240 loads would be served from A-N-D. A third of the 120/240 |> |> >>loads would be served from B-N-E. A third of the 120/240 loads would be |> |> >>served from C-N-F. Half of the 208Y/120 loads would be served from A,C,E |> |> >>and N. Half of the 208Y/120 loads would be served from B,D,F and N. |> |> > |> |> >I always wondered if either any electric company offered such a service, |> |> >or if there was any such thing as a breaker panel that would support it. |> |> |> |> Are we not forgetting about the primary windings for such a scheme? |> | |> | ...and how does this "six phase" differ from a center-tapped three- |> | phase? |> |> When most or all of the loads are single phase, the "6 star" can keep the |> loading balanced over the 3 incoming phases, while also providing genuine |> 240 volts to those single phase loads. 240DCT or 240DVCT puts most of |> the loading on one phase angle (which might be 2 incoming phase lines). |> 208Y/120 balances the loads, but there's no 240. 240Y/139 is a bit rich |> on the L-N circuits. 220Y/127 is sometimes a compromise. | | Maybe I'm dense, but I see *no* difference between the two schemes. | ELectrically, I don't see how you could tell one from the other.
A saw a bunch of different schemes in this thread. Maybe you can list the specific schemes you think are alike but treated as different.
|> |> Also, doesn't six phase distribution mean six phase transmission? Six |> |> phase transmission for any distance is going to be horrorendously |> |> expensive just to keep a few apartment buildings at 120/240 V. |> | |> | No need for six-phase transmission to get six phases at the |> | "apartment building". Like you, I don't see the point though. |> |> Which would you be willing to give up? Having 240 volts (and instead you |> settle for a wimpy 208 volts) or having the phases balanced (this is more |> of a utility concern)? | | Before I can tell you my preference in donuts, I need to see the | difference between six and half a dozen. ;-)
Can you describe an electrical system configuration which is capable of:
1. Supplying 120 and 240 volts (not 208 volts) in single phase to all single phase loads.
2. Divides up the single phase load in three equal groups in order to maintain a balance on each of the three phases of the utility supply.
... that is not the "6 star" I described?
Reply to
phil-news-nospam
I don't know if it is official, but a lot of utilities, engineers, and electricians call it "split-phase" in the USA. The term seems to eliminate some of the ambiguity of "two-phase".
Someone could claim that the 208V coming into an apartment is "two phase" since by one definition, you have two of the three phases available at the panel, but electrically it just resolves into single phase.
Then there is this interesting circuit.
H N H
| | | | |- switch | | | | |____120V lamp____|______120V lamp_____|
If the two hots are connected to a single phase 120/240 split phase circuit, the behavior of the lamps is different when the switch is closed, then when the two hots are connected to a 120/208 V. supply from a 3 phase service (such as for a large condo or apartment building).
Beachcomber
Reply to
Beachcomber
CORRECTION - That should say, the behavior of the lamps is different on the two different types of service when the switch is "OPEN".
Sorry, my bad...
Beachcomber
Reply to
Beachcomber
And what do do if the peaks and dips didn't coincide properly?
Actually neutralising was more an exercise with older HF transmitters before they made more use of grounded-grid stages.
Reply to
Stuart
| |>Calling the 240/120V common system "two phase" may not be technically in |>error but it confuses common usage in the United States. |>> | | I don't know if it is official, but a lot of utilities, engineers, and | electricians call it "split-phase" in the USA. The term seems to | eliminate some of the ambiguity of "two-phase".
So I could call the setup I described earlier as "three split-phase"?
| Someone could claim that the 208V coming into an apartment is "two | phase" since by one definition, you have two of the three phases | available at the panel, but electrically it just resolves into single | phase.
You could derive three phase from it with 2 transformers that have a 120 volt primary and an isolated 120 volt secondary. Wire up one transformer primary to one 120 volt supply L-N. Wire up the other to the other 120 volt supply L-N. Now wire the secondaries in series with one of them having a reversed phase. Connect the end of the other one to neutral, and you end up with 120 volts at a phase angle 120 degrees away from the two supply phases. The resultant diagram would look like:
A * \ / \ \ / \ N C / / B
A-N and B-N are original supply connections, so wiring things this way is ultimately an autotransformer. C-N is the newly derived phase that completes the three phase system. The power factor on the two supply phases will be awful, depending on how much that derived phase is used.
It could also be done NOT as an autotransformer with transformers that have 2 120 volt secondaries. Many step-up/down transformers might have dual 120 volt primaries and secondaries allowing them to be wired as 120 and/or 240 volt isolation transformers. Wire primaries in parallel for 120 volts, and wire secondaries arranged as above to do the same trick as an isolation transformer.
| Then there is this interesting circuit. | | H N H | | | | | | | |- switch | | | | | | |____120V lamp____|______120V lamp_____| | | | If the two hots are connected to a single phase 120/240 split phase | circuit, the behavior of the lamps is different when the switch is | closed, then when the two hots are connected to a 120/208 V. supply | from a 3 phase service (such as for a large condo or apartment | building).
Yes, that is true. This is also why the NEC has rules like 310.15(B)(4)(b).
Reply to
phil-news-nospam
| |>Then there is this interesting circuit. |> |>H N H |> |>| | | |>| |- switch | |>| | | |>|____120V lamp____|______120V lamp_____| |> |> |>If the two hots are connected to a single phase 120/240 split phase |>circuit, the behavior of the lamps is different when the switch is |>closed, then when the two hots are connected to a 120/208 V. supply |>from a 3 phase service (such as for a large condo or apartment |>building). |> |>Beachcomber |> | CORRECTION - That should say, the behavior of the lamps is different | on the two different types of service when the switch is "OPEN". | | Sorry, my bad...
Actually, I misread your diagram. I have a normal fixed space font and you apparently have a variable space font. You can never reliably draw a diagram in text with such a font because there is no standard for the spacing of such fonts. Only someone using exactly the same font can see exactly what you drew.
If you have any way to switch to a fixed space font like "Courier" when editing a posting, that would be best because it will display as you create it with any fixed space font (all characters are the same width and the spacing you create is maintained the same).
So it seems what you drew is a "switched open neutral". Well, what can I say but: you're not supposed to allow having an open neutral. Clearly the exact behaviour of an open neutral varies by system configuration.
Try a 120 volt delta-vee system :-)
Reply to
phil-news-nospam
I just found a good reference:
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There seems to be a lot of misinformation about this now almost extinct circuit. The need for an 87%, (sqrt3/2), tap is unique. One reference said it was still in use in Philadelphia.
Reply to
VWWall
I looked at my photo again, and it is understandable what is going on after all. See for yourself:
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(full size)
It's not clear the large transformer has 3 MV bushings, but it does.
One connection from the large transformer goes only to the small one, 3 others (1 connected to a bare neutral) go below it and two additional leads from the small can go downward, for a total of 5 leads that run down the pole.
There's another (probably) 120/240V service going left to right (with a drop to the upper right) that's appears not to be involved or connected, except for the neutral.
It appears to me, if the large can is delta with the lead to the small can the high leg, that this is a 5 wire 2 phase circuit, or what many would call 4 phase, 4 poles spaced at 90 degrees and a neutral. Can anyone suggest an alternate possibility?
Reply to
Michael Moroney
Specifically, the six-phase "star" and three-phase center-tapped wye. With any two phases any number of others is a few transformers away.
Look harder at a center-tapped wye. There is only 60degrees between the "negative" of phase-A and Phase-B.
How does it divide anything? An imbalance can still be placed on any one (or two).
I still don't see the difference between that and a center-tapped wye. Again, I'm not a power jock, so may be missing something subtile.
Reply to
krw
Sure. I meant as the primary transmitter operator. A first-class wasn't necessary 24/7. The guy that maintained the indian didn't need a first. ;-)
Reply to
krw
I remember one, to this day;
As the licensed transmitter operator, if the antenna lights fail, do you:
A) Climb the tower and replace the light bulb B) Cut down the tower C) Dismantle the transmitter D) Call the FAA and advise them of the danger
My choice at the time was E) laugh my ass off.
Reply to
krw
| |> |> | |> |> |> |> | |> |> |> |> |> |> >> B C |> |> |> >> \ / |> |> |> >> \ / |> |> |> >>A----N----D |> |> |> >> / \ |> |> |> >> / \ |> |> |> >> F E |> |> |> > |> |> |> >>A-N and N-D are both wound on the first core. B-N and N-E are both wound |> |> |> >>on the second core. C-N and N-F are both wound on the third core. A third |> |> |> >>of the 120/240 loads would be served from A-N-D. A third of the 120/240 |> |> |> >>loads would be served from B-N-E. A third of the 120/240 loads would be |> |> |> >>served from C-N-F. Half of the 208Y/120 loads would be served from A,C,E |> |> |> >>and N. Half of the 208Y/120 loads would be served from B,D,F and N. |> |> |> > |> |> |> >I always wondered if either any electric company offered such a service, |> |> |> >or if there was any such thing as a breaker panel that would support it. |> |> |> |> |> |> Are we not forgetting about the primary windings for such a scheme? |> |> | |> |> | ...and how does this "six phase" differ from a center-tapped three- |> |> | phase? |> |> |> |> When most or all of the loads are single phase, the "6 star" can keep the |> |> loading balanced over the 3 incoming phases, while also providing genuine |> |> 240 volts to those single phase loads. 240DCT or 240DVCT puts most of |> |> the loading on one phase angle (which might be 2 incoming phase lines). |> |> 208Y/120 balances the loads, but there's no 240. 240Y/139 is a bit rich |> |> on the L-N circuits. 220Y/127 is sometimes a compromise. |> | |> | Maybe I'm dense, but I see *no* difference between the two schemes. |> | ELectrically, I don't see how you could tell one from the other. |> |> A saw a bunch of different schemes in this thread. Maybe you can list |> the specific schemes you think are alike but treated as different. | | Specifically, the six-phase "star" and three-phase center-tapped | wye. With any two phases any number of others is a few transformers | away.
I don't know which you mean by "three-phase center-tapped wye". Maybe you can pick it out from this police lineup:
* * * * * * \ \ / * * * | \ / \ \ \ / / \ / | | \ / \ N---* *---N---* / \ / | *---N---* N * / / \ / \ / | | / / / \ *---N---* *---N---* *---N---* | / * * * * *
1 2 3 4 5 6 7
If you don't see the guilty party, maybe you can draw a picture :-)
Note that drawings are NOT to scale (relative to voltage).
|> |> |> Also, doesn't six phase distribution mean six phase transmission? Six |> |> |> phase transmission for any distance is going to be horrorendously |> |> |> expensive just to keep a few apartment buildings at 120/240 V. |> |> | |> |> | No need for six-phase transmission to get six phases at the |> |> | "apartment building". Like you, I don't see the point though. |> |> |> |> Which would you be willing to give up? Having 240 volts (and instead you |> |> settle for a wimpy 208 volts) or having the phases balanced (this is more |> |> of a utility concern)? |> | |> | Before I can tell you my preference in donuts, I need to see the |> | difference between six and half a dozen. ;-) |> |> Can you describe an electrical system configuration which is capable of: |> |> 1. Supplying 120 and 240 volts (not 208 volts) in single phase to all |> single phase loads. | | Look harder at a center-tapped wye. There is only 60degrees between | the "negative" of phase-A and Phase-B.
I don't know which I need to look at.
|> 2. Divides up the single phase load in three equal groups in order to |> maintain a balance on each of the three phases of the utility supply. | | How does it divide anything? An imbalance can still be placed on | any one (or two).
Of course an imbalance can always happen, and likely will. But it is a statistical thing. If a building has 54 apartments, connecting 18 of them to phase A, 18 to phase B, and 18 to phase C, would be "balanced" enough for utility purposes. If the supply were coming in as 120/240DCT, then all of the apartments would be on just one of the phases and the phase loading would be as lopsided as if the entire building were supplied with single phase power. If the utility insists on balancing the phases and rejects single phase service for this reason, they will reject 120/240DCT. If the supply is 208Y/120, that would satisfy the utility (18 apartments supplied with phases A and B, 18 suppleid with B and C, and 18 supplied with C and A). But it would not satisfied the need to have 240 volts.
|> ... that is not the "6 star" I described? | | I still don't see the difference between that and a center-tapped | wye. Again, I'm not a power jock, so may be missing something | subtile.
Maybe you saw something I didn't see. Check the above police lineup.
Reply to
phil-news-nospam
| snipped-for-privacy@world.std.spaamtrap.com (Michael Mor| |> snipped-for-privacy@ipal.net writes: | |>>| |>>| One weird setup I've seen, and I'm not totally sure exactly what it is: |>>| 1 large 3 phase transformer (single can) with a small transformer can |>>| below it on a pole. The small transformer has NO MV connection nor even a |>>| MV bushing! It sits below the drops to the homes/businesses. The only |>>| thing I can think it could be is the big can is 240V delta, and the high |>>| leg powers the small can, with is a 208V/240VCT. The big can powers half |>>| the residential services plus any 3 phase services, the small can powers |>>| the other residential services. It's sort of Scott-T relative to the big |>>| can. But this is a guess, I actually don't know how it's wired. I even |>>| took a photo to try and figure it out. There are quite a few of these |>>| setups in Philadelphia. | |>>If the big can is already 3 phase, why would anyone need to also have a |>>Scott-T? That doesn't make sense. It must be something else. How many |>>MV bushings on each can? | |>3 on the big can, 0 (yes, zero) on the small can. | |>I called it Scott-T like because I'm guessing the small can is fed from |>the 240 delta high leg to neutral, which would be 90 degrees shifted from |>the 240VCT winding. Assuming my guess at what it is is correct at all. | |>> How many secondary lugs? | |>4 on the big can, I believe 4 on the small can. | | I looked at my photo again, and it is understandable what is going on | after all. See for yourself: | |
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|
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(full size) | | It's not clear the large transformer has 3 MV bushings, but it does. | | One connection from the large transformer goes only to the small one, 3 | others (1 connected to a bare neutral) go below it and two additional | leads from the small can go downward, for a total of 5 leads that run down | the pole. | | There's another (probably) 120/240V service going left to right (with a | drop to the upper right) that's appears not to be involved or connected, | except for the neutral. | | It appears to me, if the large can is delta with the lead to the small can | the high leg, that this is a 5 wire 2 phase circuit, or what many would | call 4 phase, 4 poles spaced at 90 degrees and a neutral. Can anyone | suggest an alternate possibility?
It looks to me like what is going on is that the first can provides common 208Y/120 like:
B / / A-----N \ \ C
And then the second can turns one of the phases into 120/240 like:
B / / A-----N-----A' \ \ C
Most of the loading is 3 phase where 208Y/120 serves the need, but some load needs genuine 240 or split phase 120/240 and they didn't want to hang a 2nd can up high to do it directly from MV (and can't bring the MV down to where the 2nd can is now).
Just a guess.
Reply to
phil-news-nospam
#2 be the culprit. Your "split-phase" three-phase from another post.
Close enough for government work.
#2
Sure, but your "six-phase" is no different than the normal three- phase "wye". At least I don't see it.
Nope. You saw it, just one of us isn't "getting it" (I could easily be missing something - 65h work weeks do that after a while).
Reply to
krw
Except, there is no connection to one lug of the big can except to the small can, so you get either:
B / / N-----A' \ \ C
or maybe
B / / A-N \ \ C
(trying to show a different voltage on A)
or some combination. Not too useful IMO.
There are quite a few of those things in inner city Philly, as well as some other odd practices.
Reply to
Michael Moroney

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