What is Closed Delta?
- Thread starter SunFish
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Carultch
Senior Member
- Location
- Massachusetts
High leg delta, corner-grounded delta, and standard delta, are all subsets of "closed delta". Meaning that all three transformer coils are connected in a closed loop among the phases.
The opposite is an open delta, which only has two transformer coils, such as AB and BC, but no CA.
Neither high-leg delta nor a corner-grounded delta is a subset of a closed delta configuration as both configurations can be supplied by an open-delta transformer bank.
Sierrasparky
Senior Member
- Location
- USA
- Occupation
- Electrician ,contractor
I am amazed how many in the field don't know this simple part of the Delta High leg setup. I am glad you clarified this.
- Occupation
- Licensed Electrician
A little more follow up on what Smart $ said.
A closed delta uses three transformers an open uses two. Most delta systems will have the centre tap of one of the transformers grounded which gives you your high leg. The only time that you don't have a high leg on a delta is if it's corner grounded or ungrounded.
kwired
Electron manager
- Location
- NE Nebraska
Full or closed delta would be preferred if there is a lot of phase to phase power loads.
An open delta comes in handy when there is limited three phase load but a more significant amount of line to neutral loads, or for limited three phase load applactions where maybe the POCO saves some expense by only using a two transformer system or even only has to run (possibly miles) of two primary phases plus neutral instead of all three primary phases plus neutral.
An open delta comes in handy when there is limited three phase load but a more significant amount of line to neutral loads, or for limited three phase load applactions where maybe the POCO saves some expense by only using a two transformer system or even only has to run (possibly miles) of two primary phases plus neutral instead of all three primary phases plus neutral.
JFletcher
Senior Member
- Location
- Williamsburg, VA
Yes.
If I understand correctly, a closed or full delta would have 3 phases input, 3 xfmrs (or one with 3 windings), and 3ph, 3wire* output, full power capacity, no neutral, e.g., 12470/480V. Line-line on the secondary is 480V between any two phases, and 3ph equipment is also 480V. *Delta-wye would have 4 wires.
A high leg delta uses two primary phases on 2 xfmrs to get 3ph output, using one (larger) center-tapped xfmr. Secondary is 3ph, 4 wire, with the neutral center tapped on the AC leg. Thus, A-->N is 120V, C-->N is 120V, but B-->N is 208V (high/wild/stinger). Phase to phase on any side is 240V. Used for smaller 3ph loads and for single phase 120V loads. Does not have the capacity of a similar sized full delta service.
If your area has overhead service, look at the poles. Those with one small and one large trashcan (xfmr) are high leg systems. 3 identically sized xfmrs could be delta or wye secondary, usually wye.
If I understand correctly, a closed or full delta would have 3 phases input, 3 xfmrs (or one with 3 windings), and 3ph, 3wire* output, full power capacity, no neutral, e.g., 12470/480V. Line-line on the secondary is 480V between any two phases, and 3ph equipment is also 480V. *Delta-wye would have 4 wires.
A high leg delta uses two primary phases on 2 xfmrs to get 3ph output, using one (larger) center-tapped xfmr. Secondary is 3ph, 4 wire, with the neutral center tapped on the AC leg. Thus, A-->N is 120V, C-->N is 120V, but B-->N is 208V (high/wild/stinger). Phase to phase on any side is 240V. Used for smaller 3ph loads and for single phase 120V loads. Does not have the capacity of a similar sized full delta service.
If your area has overhead service, look at the poles. Those with one small and one large trashcan (xfmr) are high leg systems. 3 identically sized xfmrs could be delta or wye secondary, usually wye.
- Occupation
- Licensed Electrician
No. Closed delta or open delta is determined by the number of transformers, not the voltage level of the secondary.
Or one three phase transformer with the windings in a delta configuration perhaps?
- Occupation
- Licensed Electrician
Perhaps, but within the context of here in the States, you are only going to see delta power derived from pole mounted transformers. There may be some exceptions, but they are rare.
JFletcher
Senior Member
- Location
- Williamsburg, VA
:?
My "yes" was to the OP's question:
"Is a "Closed Delta" different than a "Delta High-Leg"?"
The rest of my comment backs up your correction about # of xfmrs to make each.
- Occupation
- Licensed Electrician
I did not realize that.
You can have a high leg on both an open and a closed delta. Do we agree?
Using different voltages in each example threw me off.
JFletcher
Senior Member
- Location
- Williamsburg, VA
I should have quoted the OP first to avoid confusion.
Agreed, tho I dont think I've ever seen a closed high leg delta service. Two pot high leg set-ups are much more common, yes?
- Occupation
- Licensed Electrician
Gotta be a regional thing. There are a bunch of delta systems where I live, most are closed, but I can take you to an open delta quite easily. Most are 240V, most are high leg. Corner grounds are very rare and ungrounded is non-existent.
My background is industrial. Pole mounted isn't something I had to deal with. Here or in the States.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
True for residential other than multi-family. Both industrial and apartment complexes out West may have pad mount transformers and all underground power service.
You are not likely to see an open delta with pad mount. Those would either be complete three phase or single phase with one transformer.
Electric-Light
Senior Member
Closed would be the normal delta.
Open delta looks like this.
http://forums.mikeholt.com/showthread.php?t=155814&p=1507707#post1507707
Closed / open should be self explanatory.
You'll have the same voltages. Open delta often built with two different sized transformers and it is used to provide three phase service from a pole that primarily consists of single phase loads. The second and usually smaller transformer is used exclusively for three phase loads, such as irrigation pump or shop equipment.
Homes usually get 120/240 with a center tap transformer.
Open delta looks like this.
http://forums.mikeholt.com/showthread.php?t=155814&p=1507707#post1507707
Closed / open should be self explanatory.
You'll have the same voltages. Open delta often built with two different sized transformers and it is used to provide three phase service from a pole that primarily consists of single phase loads. The second and usually smaller transformer is used exclusively for three phase loads, such as irrigation pump or shop equipment.
You guys probably arrange transformres different than how we arrange our transformers. You have three 50 hurts 240 tranformres without centre taps to get 415Y/240 which is not far from how we get commercial power using three 60 Hz 277 transformers for 480Y/277.
Homes usually get 120/240 with a center tap transformer.
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kwired
Electron manager
- Location
- NE Nebraska
A lot of full delta systems here - mostly 240 volt systems. If you are supplying a lot of 240 volt loads and limited 120 volt loads why go with a 208 volt wye system?
Seen two or even three single phase padmounts connected to make a three phase bank before. Most of them were older and they would not likely use three anymore these days but rather a single core three phase unit, but I have seen more recently a new two unit open delta, but that has even been 10-15 years ago.
I don't believe it is common at all to see a delta secondary supplying the end user over there.
Same here although I can take you to a good number of corner grounded and one ungrounded around here.
Roger
Strathead
Senior Member
- Location
- Ocala, Florida, USA
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- Electrician/Estimator/Project Manager/Superintendent
On a side note. When you have a two transformers providing 3 phase delta How do you size the transformers and what happens power and amperage wise on the line side of the transformer? I assume that the phase angle of 60 between the transformers must give you a formula that somehow ends up with the 240 say across the "ghost" transformer, but the entire concept is a little hard for me to get my head around.
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