Closed Delta 120/240V 3-phase service

In the case of a commercial building served by 3 phase - closed delta
240/120 Vservice, what is the best way to detect a ground fault on a
three phase motor circuit on phase C were it to come into contact with
the ground?
Whereas the service transformer on phase AB has the only centered
tapped (neutral) windings 120V phase-to-neutral and 240V
phase-to-phase and phase C (not connected to the phase AB transformer)
is the wild phase.
It seems to me that (without additional protective equipment), if
phase C accidently touches the ground there would be a voltage
imbalance, but the overcurrent protection would not necessarily trip
off.
Am I wrong by assuming this?
Also, what, in general, are the pros and cons of a 120/240V 3 phase
closed delta connection?
Is this a good choice for a small commercial building with a 3 story
hydraulic elevator and mostly 120V. appliance and lighting loads?
Beachcomber
Reply to
Beachcomber
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I don't know what you mean by the first question. In answer to your second; In general terms you want the 240 system when your predominate load is small to medium hp 3-phase motors. If you are going to supply primarily lighting and receptacle loads, then a 208 4-wire system is an obvious advantage.
Reply to
Long Ranger
The 3d leg is 208v above ground and it would trip the O/C device if it faults.
Reply to
gfretwell
| In the case of a commercial building served by 3 phase - closed delta | 240/120 Vservice, what is the best way to detect a ground fault on a | three phase motor circuit on phase C were it to come into contact with | the ground?
A GFI device? All 4 wires (A,B,C,N) would run through the CT together.
| Whereas the service transformer on phase AB has the only centered | tapped (neutral) windings 120V phase-to-neutral and 240V | phase-to-phase and phase C (not connected to the phase AB transformer) | is the wild phase.
It is 208 volts relative to ground.
| It seems to me that (without additional protective equipment), if | phase C accidently touches the ground there would be a voltage | imbalance, but the overcurrent protection would not necessarily trip | off. | | Am I wrong by assuming this?
That depends on how solid the ground path is. If the impedance is high to get back to the source, maybe not enough current will flow. That could easily be the case if it contacts earth. It could even be the case if it contacts a metal part of the building. But if the impedance is low enough, you get high current.
Still, if there is any risk of a ground fault, and especially one that could be contacted by a person as part of the ground path, then ground fault protection would be a good thing to have. A GFI device?
| Also, what, in general, are the pros and cons of a 120/240V 3 phase | closed delta connection?
If there is a loss of one phase in this system it can lead to trouble. Delta can have back-energizing issues as well as lopsided loading if a phase is lost. And 240D/120 would be quickly out of balance if the 120 volt loads predominate. Alternatively, a Scott-T version of that could help in limited 3-phase loading environments.
| Is this a good choice for a small commercial building with a 3 story | hydraulic elevator and mostly 120V. appliance and lighting loads?
208Y/120. Configure the elevator system for 208 volts. It is the more common electrical system in the USA, now. An elevator manufacturer not being able to do 208 volts is limit their market severely.
Reply to
phil-news-nospam
The protective device will trip if phase C is grounded, just as for the other phases. The delta system will not allow full utilization of the available kVA, since one phase can not be used for the single-phase loads. With mostly 120V lighting & appliances, I would go 208/120 4-wire wye, so you get the benefit of all three phases for the 120V loads. Run the elevators & A/C on 208 3-phase.
Benjamin D Miller, PE
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Reply to
Ben Miller
I have only seen center tapped "closed delta" a few times, notably in the alley next to the Hog's Breath Salloon in Key West (look at the pole behind the stage) and I bet from the looks of it they added the 3d transformer as the load increased. Usually it is "open delta" with 2 transformers as a cheap way to get some 3 phase to a customer who has mostly single phase loads. The open delta was actually pretty popular in the light industrial areas of East Naples (Florida).
Reply to
gfretwell
| | |>Beachcomber wrote: |>> In the case of a commercial building served by 3 phase - closed delta |>> 240/120 Vservice, what is the best way to detect a ground fault on a |>> three phase motor circuit on phase C were it to come into contact with |>> the ground? |>> |>> Whereas the service transformer on phase AB has the only centered |>> tapped (neutral) windings 120V phase-to-neutral and 240V |>> phase-to-phase and phase C (not connected to the phase AB transformer) |>> is the wild phase. |>> |>> It seems to me that (without additional protective equipment), if |>> phase C accidently touches the ground there would be a voltage |>> imbalance, but the overcurrent protection would not necessarily trip |>> off. |>> |>> Am I wrong by assuming this? |>> |>> Also, what, in general, are the pros and cons of a 120/240V 3 phase |>> closed delta connection? |>> |>> Is this a good choice for a small commercial building with a 3 story |>> hydraulic elevator and mostly 120V. appliance and lighting loads? |>> |>> Beachcomber |> |> |>The protective device will trip if phase C is grounded, just as for the |>other phases. The delta system will not allow full utilization of the |>available kVA, since one phase can not be used for the single-phase loads. |>With mostly 120V lighting & appliances, I would go 208/120 4-wire wye, so |>you get the benefit of all three phases for the 120V loads. Run the |>elevators & A/C on 208 3-phase. |> |> |>Benjamin D Miller, PE |>
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|> | | I have only seen center tapped "closed delta" a few times, notably in | the alley next to the Hog's Breath Salloon in Key West (look at the | pole behind the stage) and I bet from the looks of it they added the | 3d transformer as the load increased. Usually it is "open delta" with | 2 transformers as a cheap way to get some 3 phase to a customer who | has mostly single phase loads. | The open delta was actually pretty popular in the light industrial | areas of East Naples (Florida).
I've seen a few setups with one big fat transformer and one small one. Usually, the big one has 2 bushings and the small one has 1 bushing. On the secondary, the big one has 3 wired lugs and the small one has only 2 wired lugs. I'm guessing these are arranged as Scott-T. Where I have seen these is in business (not industrial) areas of small towns in West Virginia and Ohio, usually servicing stores or restaurants. Some of those smaller pole pigs were the smallest pole pigs I've seen. My guess is the smallest ones were no more than 3 or 5 kVA.
Reply to
phil-news-nospam
The two transfomer delta around here is just 2 of the expected 3 in true delta. They connect A to C and leave B floating off of the AB transformer that is center tapped. As long as you have a well balanced 3 phase load it works fine. We did get in trouble in the computer biz since we had some single phase loads derived from the incoming 3 phase inside the machine. I have had to juggle machines around or "roll the phases" to balance the phase loads.
Reply to
gfretwell
| |>| I have only seen center tapped "closed delta" a few times, notably in |>| the alley next to the Hog's Breath Salloon in Key West (look at the |>| pole behind the stage) and I bet from the looks of it they added the |>| 3d transformer as the load increased. Usually it is "open delta" with |>| 2 transformers as a cheap way to get some 3 phase to a customer who |>| has mostly single phase loads. |>| The open delta was actually pretty popular in the light industrial |>| areas of East Naples (Florida). |> |>I've seen a few setups with one big fat transformer and one small one. |>Usually, the big one has 2 bushings and the small one has 1 bushing. |>On the secondary, the big one has 3 wired lugs and the small one has |>only 2 wired lugs. I'm guessing these are arranged as Scott-T. Where |>I have seen these is in business (not industrial) areas of small towns |>in West Virginia and Ohio, usually servicing stores or restaurants. |>Some of those smaller pole pigs were the smallest pole pigs I've seen. |>My guess is the smallest ones were no more than 3 or 5 kVA. | | The two transfomer delta around here is just 2 of the expected 3 in | true delta. They connect A to C and leave B floating off of the AB | transformer that is center tapped. As long as you have a well balanced | 3 phase load it works fine. We did get in trouble in the computer biz | since we had some single phase loads derived from the incoming 3 phase | inside the machine. I have had to juggle machines around or "roll the | phases" to balance the phase loads.
What I wonder is how 240 volt L-L loads would do with either the closed or open 240D/120 systems, when connected between the high-leg and either pole of the 120/240 side. Consider the simple 240 volt water heater, assuming it treats both wires as hot, which it must in USA single phase systems. Also consider a computer PSU that has two-pole switching and can handle a 230 volt German Schuko that has no polarity to it (e.g. either wire it gets from being plugged in could be the hot). I do remember reading some power utility requirements that nothing but three phase loads is allowed to connect to the high-leg. But what is the effect of doing so?
Reply to
phil-news-nospam
I'm wondering if the simplicity of a single phase 240/120 V service for a small elevator building is in some way superior to the slightly more complex 3 phase- 208/120 V. wye service.
Can a small hydraulic passenger elevator serving a maximum of 3 flloor run OK on a 240 V. single phase circuit? If wired the other way, this would probably be the only 3 phase load.
My concern with the 3 phase is that if the utility loses a phase, a 208V 3 phase motor will stop rotating and burn up (without special protection). I've seen this happen in a condo building. Also, because of the partially powered delta-wye at the transformer and some of the 120 V lines might go to something like 68 V or so.
I know that there are probably protective devices that can be installed to protect this. I wonder though, if it might just be better to stick with a 120/240 V. single phase service.
Any thoughts?
Beachcomber
Reply to
Beachcomber
It alters the voltage between the phases if you have a L/L across the missing leg it will drag down that voltage
Reply to
gfretwell
|>What I wonder is how 240 volt L-L loads would do with either the closed or |>open 240D/120 systems, when connected between the high-leg and either pole |>of the 120/240 side. Consider the simple 240 volt water heater, assuming |>it treats both wires as hot, which it must in USA single phase systems. |>Also consider a computer PSU that has two-pole switching and can handle |>a 230 volt German Schuko that has no polarity to it (e.g. either wire it |>gets from being plugged in could be the hot). I do remember reading some |>power utility requirements that nothing but three phase loads is allowed |>to connect to the high-leg. But what is the effect of doing so? |> | | I'm wondering if the simplicity of a single phase 240/120 V service | for a small elevator building is in some way superior to the slightly | more complex 3 phase- 208/120 V. wye service.
It would be simpler. For a small building it can be OK. For a larger building, the utility would want more balance in the phase loading.
One option is to use a three phase transformer where all three secondary windings are 240 volt center tapped to 120/240 volts. What you get are SIX separate hot phase wires, and a neutral. I'll label the phase wires going clockwise as A,B,C,D,E,F. You can have 2 three phase systems from this, connecting to A,C,E or B,D,F. They would be 208Y/120. And you can have 3 single phase systems from this, connecting to A,D and B,E and C,F. Divide the tenants that get single phase three equal ways and this way they get genuine 120/240. This is actually a bit more complex than the common systems (for example you would have 3 separate distributions for the single phase). You probably can't meter the whole thing at once. But I'd prefer such a system, myself.
If for some reason you really need 240 volts on the threee phase loads, that either forces the 240D/120 system, or just 240D, or a 240Y/139 system just for three phase.
Ultimately, I'd prefer to have 3 120/240 volt systems plus 480Y/277 for the three phase loads.
Reply to
phil-news-nospam
I am obviously missing something. How do you connect the center taps of three windings together, and then connect them in a wye, and not blow up the transformer? And how do you get 240/120 & 208Y/120 from the same windings?
Reply to
Ben Miller
They could also be an open delta, where most of the load is single-phase. The big one is center tapped, and handles all of the 120V loads. The small one only handles three-phase loads, perhaps a roof top A/C unit. I saw this exact setup at a factory that was having problems with one of their VFD's. The voltage imbalance was terrible. We brought in a genset and proved that the power system was to blame. When the system was originally installed, the building was a warehouse with only an A/C unit on the three-phase, so it worked great. Nobody thought to check it when the manufacturing equipment moved in with heavy three-phase loads.
Reply to
Ben Miller
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He is essentially describing a 6 phase secondary. Each 120 to neutral and separated by 60 degrees but you can get 240 between the legs that are 180 degrees apart. as well as 208 between the legs that are 120 degrees apart. So you can have 2 -120/208V Ysystems with common neutral as well as 3 120/240V single phase circuits as well as some 120V D with one terminal tied to neutral.
There is no real advantage but the chance of problems with mixed up wiring will be increased.
The question then arises- why bother?
Reply to
Don Kelly
|> One option is to use a three phase transformer where all three |> secondary windings are 240 volt center tapped to 120/240 volts. What |> you get are SIX separate hot phase wires, and a neutral. I'll label |> the phase wires going clockwise as A,B,C,D,E,F. You can have 2 three |> phase systems from this, connecting to A,C,E or B,D,F. They would be |> 208Y/120. And you can have 3 single phase systems from this, |> connecting to A,D and B,E and C,F. Divide the tenants that get single |> phase three equal ways and this way they get genuine 120/240. This |> is actually a bit more complex than the common systems (for example |> you would have 3 separate distributions for the single phase). You |> probably can't meter the whole thing at once. |> But I'd prefer such a system, myself. |> | | I am obviously missing something. How do you connect the center taps of | three windings together, and then connect them in a wye, and not blow up the | transformer? And how do you get 240/120 & 208Y/120 from the same windings?
Maybe an ASCII art picture will help:
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.
You could also get 120 volts from A-B or B-C or C-D or D-E or E-F or F-A, but you would not want to.
You could also ignore the center tap and rewire it for 416Y/240.
If the windings can be split and wired in parallel, you could rewire it for 208Y/120 with double the amperage.
A transformer with dual secondary 120 volt windings on each of the three cores would be quite flexible, being able to be configured for any of these three systems (but you would have 12 wires coming off the cores on the secondary side).
Reply to
phil-news-nospam
I agree with Mr Kelly
They use delta vee to save money, why would they do this 6 pole thing?
Reply to
gfretwell
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. I strongly suspect the answer to both is an absolute No Way, esp. if you think about how the heck you'd design the panel. You'd have to invent nonstandard breakers that would do lots of skipping to be useful. Of course such a service would really be useful only in an apartment building where there would be multiple standard panels, mostly residential split phase (1/3 A-D 1/3 B-E 1/3 C-F) and a 3 phase panel for elevator service, so no need for an actual panel for this 7 wire service.
Reply to
Michael Moroney
Do the elevator companies even offer a single phase elevator system in the size you need? (how many HP is such an elevator motor). Since the system has to control the motor, possibly reversing it, you may not be able to simply replace the motor with a single phase motor.
Do you have a need for 240V (not 208V) residential devices? If not, there's no real advantage of 240V delta or open delta over standard 208Y/120 service. The only real place I see where 240V delta has a place is residential service with a small 3 phase load, such as a workshop or large A/C unit. That's simply because homes have 240V and 120/240V single phase appliances.
I have seen open delta 3 phase in residential areas of Baton Rouge LA for whole house A/C. 1 large and one tiny transformer.
Reply to
Michael Moroney
I would assume no effect, as long as the insulation was rated for 208VAC to ground vs. 120VAC to ground, and you don't overload the possibly small high leg transformer. The insulation won't be an issue for anything designed for European use where either leg may be hot, since either leg must handle 240V. One overload problem is when something is connected across the missing leg of an open delta system. The transformers have to deal with a higher VA per delivered watt, plus more copper losses. Combine this with a small high leg transformer and you may see voltage sag.
Reply to
Michael Moroney

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