Understanding an Open-Delta Bank

[fio1022]

Hi all,
I don't know where else to ask this question and I have been trying to find the answer for a while now(no one seems to know for sure).
I am an apprentice lineman for my local utility company and I'm trying to understand how an open delta bank provides TRUE three phase power.
A regular wye or closed delta bank has three transformers with three phases feeding the primary side, 3 phases in 3 phases out(3 different phase angles).An open delta only has two transformers with two primary feeds.2 phases in 3 out?I don't get it.
Is an open delta bank secondary supplying true three phase power(120 degree phase shift)or is it two phases compensating for the lack of a third.
Just wondering if I am far off track on my thinking...
Any information is appreciated.

[dbrown20]

Very good question. It is a little bit out of the expertise of the average lineman. Probably more in the realm of a good Electrical engineer. Usually with something of this sort I try to find a good reference book that I can hopefully understand. Usually the math required in things of this nature are a little bit beyond my ken.

The following are some excerps from a publication from GE that I picked up one place where I worked. It is a manual that is more geared toward Power Transformers or as some refer to them Sub Station Transformers. However the principles still hold true I'm sure. With this and what little I know maybe you can get a little better understanding. Probably someone else will chime in with some more info also.

Here's the GE stuff. " Although this connection delivers three-phase currents which are approximately symmetrical to a three-phase symetrical load, the currents flowing in the high voltage circuit are not equal nor are they 120 degrees apart.
The maximum safe output of the bank operating in this manner is 58% of a 3 pot Wye/Delta bank. The system is grossly unbalanced, both electrostatically and electromagnetically."

Here's what little I know about them. They will parallel with a 3 pot Wye/Delta bank. Done that many times. You can add an additional wing or power pot with out interuption. Done that a few times. Most companies limit their load to about 20 HP total 3 phase.

Here's something else I noticed in this book and I just thought I'd throw it in also as it's interesting. "Units of widely different impedances may be used to form a Y-delta bank without appreciably affecting the current division. Furthermore, there is no danger of excessive circulating currents in the delta, though the ratios are not the same, for these currents cannot be reproduced in the Y and therefore they are magnetizing currents and necessarily very small."

If you wish perhaps you might contact GE about such a book as this. It's not to be confused with the little GE Distribution handbook that is so common everywhere. This small book was compiled by the Power Transformer Department, Pittsfield,Massachusetts. dbrown20

[topgroove]

Here is a pretty good picture of an open delta. clink on the link below. After the picture loads if you click on it again it will enlarge it.

http://www.dropshots.com/day.php?use...7&ctime=184203

[Trampbag]

Open Delta is a 3 phase transformer bank using 2 transformers. In essence one side of the secondary Delta is physically missing, however an electrical measurement is still possible across the missing side and so it is 3 phase. It will phase in with a closed delta bank. It is not as efficient as a 3 transformer bank, but saves one transformer. It also can save one conductor on the primary, whether using Wye or Delta primary, which can be a great savings as well. A lighting connection can be used in an open Delta bank. An open Delta connection can be used in an emergency when one transformer of three in a Delta bank is inoperable or burnt out.

No such hook up in a Y secondary is possible, always use three transformers.

[dbrown20]

Open Delta is a 3 phase transformer bank using 2 transformers. In essence one side of the secondary Delta is physically missing, however an electrical measurement is still possible across the missing side and so it is 3 phase. It will phase in with a closed delta bank. It is not as efficient as a 3 transformer bank, but saves one transformer. It also can save one conductor on the primary, whether using Wye or Delta primary, which can be a great savings as well. A lighting connection can be used in an open Delta bank. An open Delta connection can be used in an emergency when one transformer of three in a Delta bank is inoperable or burnt out.

No such hook up in a Y secondary is possible, always use three transformers.

Yes a hookup with a Wye secondary is possible. It is possible to use 2 transformers for example in a 3 phase 208 or to use 3 transformers for a 3 phase 208. Either hookup can be made in a Wye with only 2 primary phases and the system neutral. Very uncommon but actually have the 3 pot example in the area where I now work. The 2 pot hookup must have one pot with external secondary bushings. We talked about this once before several years ago. dbrown20

[hifihaxor]

wow dbrown, i never thought about it but you may be right. with a 100kva or larger, with the 4 sec bushings, connect it like parallel except leave one leg unconnected... the common leg is your neutral and ties to the other pots neutral, and theres your 3 wye phases. ive never tried or seen it but it looks like it would work.

anyone have 3 phase txes in one overhead can? we dont use t hem here, but ive seen them in cali.. 3 pri bushings, 4 sec... really weird wheen i first saw one.

swamp is absolutly correct. ive seen banks with a 75 lighting pot and a 10 or 15 wing pot to feed a gas station and a half block of houses... since we have many areas that are mixed use with houses and businesses open deltas are useful. some old neighborhoods (the gables) have open delta residential services... many houses had three phase well pumps, airconditioners and.. elevators!! in practice, the lighting pot is usually larger in a closed delta bank as well, figuring if you have max load on the wind pots, then youll still need some additional capacity to run single phase loads.

cwye/cdelta banks arent used much in miami because of ferroresonance issues, especially on the 23kv areas.. many older industrial areas have them though. for those in the area, my favorite bank is at merrill stevens drydock on nw7ave and north river drive, its a feeder dead end platform bank. closed delta, very old equpiment, alllllll original and still in excellent condition.

i asked my boss once why on cwye/cdeltas the primary neutral is left floating, he said its in an attempt to balance the currents on the primary side.

[44kv]

in nc here on the duke system. we call them stack banks, 3 prim,4 sec. ive seen them in 30 kva, 75,150. im sure there are more but when they go bad we trade them out for 3 pot banks.

[topgroove]

i asked my boss once why on cwye/cdeltas the primary neutral is left floating, he said its in an attempt to balance the currents on the primary side.

If the primary neutral is grounded on a bank of wye delta distribution transformers, the transformers may be overloaded to the point of burnout by circulating currents caused by unbalanced primary voltages.
In the event of a phase loss the transformer bank will try to replace the missing phase. Its success in replacing the missing phase will depend on the relative size of the transformer bank compared to the load connected to the circuit past the point of phase loss.
Been there, Done that.
Note. It is not the connection to ground but the connection to the system neutral which is the cause of the issues. This is a fine point as the system neutral is well grounded.
When a primary phase is lost, transformer bank primary fuses frequently blow . Transformers burn out often. When two primary phases are lost, there is a backfeed of current from both unpowered transformers. These voltages will be determined by the connected load past the point of phase loss, but the maximum sum of the two voltages will be the normal voltage. That's when the freezers and refrigerators start burning up.

[dbrown20]

I have never worked where it was legit to ground the primary neutral on a 3 pot Wye/Delta bank. I have seen it done but it is considered to be the wrong way. dbrown20

[dbrown20]

I have never worked where it was legit to ground the primary neutral on a 3 pot Wye/Delta bank. I have seen it done but it is considered to be the wrong way.

Grounding the high side of a 3 pot bank, hooked Wye/Delta can cause this bank to act as a grounding bank as all imbalance in the vicinity will travel through the bank and all faults and lightening strikes in the area will go through it also subjecting it to a greater likilhood of damage. Ever transf. book I have read mentions this always. dbrown20