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Old 02-20-2015, 01:50 AM  
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Default Corner grounded open delta

Ok so this has had me confused for quite sometime now. Or POCO has a lot of these around, works like a typical wild leg as far as panel/distribution is concerned. Call it poor-mans three phase, or farmers three phase. Due to the lack of a third primary wire needed.

The primaries is where the confusion is, they only use TWO phases on the pole, hence the corner ground, open delta configuration. How in the world do you put TWO phases in and get THREE out?

I'm completely fine with the secondary side of the cans as they are the same as in a closed delta 240 3phase typically used in a majority motor load facility with minimal need for the 120 side. Just one less can, it's this corner ground primary 2phase into 3 that has me all

TWN
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Old 02-20-2015, 02:04 AM  
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Ground is bonded to the 3rd phase.

The other 2 are 120 degrees out of phase.

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Old 02-20-2015, 02:21 AM  
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Ground is bonded to the 3rd phase.

The other 2 are 120 degrees out of phase.

Correct me if I am wrong, is that picture not a corner grounded closed delta?

Borgi

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Old 02-20-2015, 02:33 AM  
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The messenger acts as the 3rd phase. Basically one of the phases becomes a grounded conductor and in turn POCOs can leave it uninsulated. By grounded I do not mean a true neutral, but rather at ground potential or technically a grounded phase.

Once inside the building the grounded conductor splits into a grounded and grounding conductor at the first disconnect.

In this case the grounded conductor is the 3rd phase.

The same potential exits between it and the other phases, just except its near ground potential.
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Old 02-20-2015, 07:44 AM  
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The messenger acts as the 3rd phase. Basically one of the phases becomes a grounded conductor and in turn POCOs can leave it uninsulated. By grounded I do not mean a true neutral, but rather at ground potential or technically a grounded phase.
I'm of the understanding that PoCos use a Y distribution system. I always thought an open Delta primary consisted of two phase conductors and a neutral. Are you saying the "neutral" in this case is actually a grounded 3rd phase?
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Old 02-20-2015, 08:38 AM  
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I'm of the understanding that PoCos use a Y distribution system. I always thought an open Delta primary consisted of two phase conductors and a neutral. Are you saying the "neutral" in this case is actually a grounded 3rd phase?
Yes.

POCOs used to supply just about anything for a secondary, some still do.
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Old 02-20-2015, 08:48 AM  
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Here is an old discussion that might be useful.
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Old 02-20-2015, 08:52 AM  
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Yes.

POCOs used to supply just about anything for a secondary, some still do.
So if PoCo has a grounded phase conductor on the distribution is it correct to assume it's leaving the generation plant as a corner grounded system?

If not, then where (as where in the distribution system) to they ground one of the phases and how do they keep it from being a close to short circuit to the grounded neutral of the Wye distribution?

Thanks.
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Old 02-20-2015, 09:07 AM  
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So if PoCo has a grounded phase conductor on the distribution is it correct to assume it's leaving the generation plant as a corner grounded system?
You mean LV (low voltage) or MV (medium voltage)? Almost all MV is either ungrounded delta, multi grounded wye or uni grounded wye. HV is always grounded wye. At the MV level corner grounded delta is exceptionally rare.

For services they used to hook the 600 volt and under seondaries anyway you wanted.


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If not, then where (as where in the distribution system) to they ground one of the phases and how do they keep it from being a close to short circuit to the grounded neutral of the Wye distribution?

Thanks.
Pole pigs are 2 winding transformers with each winding isolated from one another. Its a separately derived system. The primary can be grounded wye, ungrounded wye or delta while the secondary can be anyone one of those not having to match. Some connections like ungrounded wye grounded wye are not feasible, but something like delta grounded wye or ungrounded wye corner grounded delta is doable and operational.


An open delta bank can either be open wye grounded, open delta or just open delta open delta. Secondary can either be center grounded, corner grounded or ungrounded. Secondary grounding is irrelevant from primary grounding.

A classic example is an 12kv ungrounded delta line. Only 3 wires and no ground or neutral wire. primary is hooked in delta, secondary in wye and grounded. You now have 4 wires and a grounded neutral even though the primary has none.
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Old 02-20-2015, 09:40 AM  
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Originally Posted by Borgi View Post
Correct me if I am wrong, is that picture not a corner grounded closed delta?

Borgi
Same theory but adding a third transformer bumps ups the capacity around 33%.
We have plenty of open delta.
Closed delta is quite the exception.
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Old 02-20-2015, 12:44 PM  
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Ok so this has had me confused for quite sometime now. Or POCO has a lot of these around, works like a typical wild leg as far as panel/distribution is concerned. Call it poor-mans three phase, or farmers three phase. Due to the lack of a third primary wire needed.

The primaries is where the confusion is, they only use TWO phases on the pole, hence the corner ground, open delta configuration. How in the world do you put TWO phases in and get THREE out?

I'm completely fine with the secondary side of the cans as they are the same as in a closed delta 240 3phase typically used in a majority motor load facility with minimal need for the 120 side. Just one less can, it's this corner ground primary 2phase into 3 that has me all

TWN


Maybe this picture is more like what you're asking about. This is an open delta, ( two transformers, one per phase ).

It's not corner grounded as you can see, ( well not in the traditional sense maybe ), but this may be what your utility is using.

Borgi

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Old 02-20-2015, 01:34 PM  
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This may help:

http://www.cooperindustries.com/cont...rs/R201902.PDF


The big question is, does this bank only supply 240 volts, or is it 120/240, which makes a difference.
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Old 02-20-2015, 01:41 PM  
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Originally Posted by meadow View Post
You mean LV (low voltage) or MV (medium voltage)? Almost all MV is either ungrounded delta, multi grounded wye or uni grounded wye. HV is always grounded wye. At the MV level corner grounded delta is exceptionally rare.

For services they used to hook the 600 volt and under seondaries anyway you wanted.



Pole pigs are 2 winding transformers with each winding isolated from one another. Its a separately derived system. The primary can be grounded wye, ungrounded wye or delta while the secondary can be anyone one of those not having to match. Some connections like ungrounded wye grounded wye are not feasible, but something like delta grounded wye or ungrounded wye corner grounded delta is doable and operational.


An open delta bank can either be open wye grounded, open delta or just open delta open delta. Secondary can either be center grounded, corner grounded or ungrounded. Secondary grounding is irrelevant from primary grounding.

A classic example is an 12kv ungrounded delta line. Only 3 wires and no ground or neutral wire. primary is hooked in delta, secondary in wye and grounded. You now have 4 wires and a grounded neutral even though the primary has none.
Correct me if I'm wrong, but SCE in California uses a lot of 12 kv ungrounded delta for their distribution, right? I remember seeing a lot of their system connected that way, or so I thought it was.
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Old 02-20-2015, 01:44 PM  
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FWIW, National Grid does not offer delta for any new services, and they do whatever they can to eliminate old delta services off of their system. Delta was once as common as wye is now. They don't like having to stock odd delta transformers in their supply yards.
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Old 02-20-2015, 01:51 PM  
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Originally Posted by meadow View Post
This may help:

http://www.cooperindustries.com/cont...rs/R201902.PDF


The big question is, does this bank only supply 240 volts, or is it 120/240, which makes a difference.
TWN, if you look at figure 21 on page 12, I think that is the drawing that addresses what I think you are asking about.

I think you get the 3-phase output because the 2 "hot" phases are not 180 out when referencing to the neutral. I believe that is what you are asking about.
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Old 02-20-2015, 02:01 PM  
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Correct me if I'm wrong, but SCE in California uses a lot of 12 kv ungrounded delta for their distribution, right? I remember seeing a lot of their system connected that way, or so I thought it was.
Im not sure exactly sure how SCE grounds their X0s, but most of Cali is unigrounded wye, just the neutral is not brought out of the substation while the remaining is indeed ungrounded delta with some reactance/resistance earthing in there as well.

A common misconception is that if a distribution line is 3 wires with no neutral its automatically supplied via ungrounded delta . Not so, in fact most of the time in new construction its a solid ground to earth, just no neutral or ground wire is run out of the substation. This is done because running a line totally ungrounded can cause serve overvoltages from an arcing ground fault, grounding solidly or through a properly sized impedance stops this.


MX hopefully can chime in.
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Old 02-20-2015, 02:13 PM  
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Open deltas always attract sparkys looking up poles here.....~CS~
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Old 02-20-2015, 02:25 PM  
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FWIW, National Grid does not offer delta for any new services, and they do whatever they can to eliminate old delta services off of their system. Delta was once as common as wye is now. They don't like having to stock odd delta transformers in their supply yards.
I doubt its that They can whip up a 240 volt delta bank with the same resi pigs in stock now. Not an issue for them.

What primarily drives utilities getting rid of delta secondary services is the preference to use a wye grounded primary. Grounded wye primary has 2 major advantages (even though it has a dozen disadvantages): only 1 bushing and arrestor is required; 2 Ferroresonance is unlikely if not impossible.

While a wye grounded primary delta secondary is a functioning connection it has one major draw back: the closed delta secondary turns the transformer bank into a grounding bank. In fact grounded wye delta transformers are used anytime an artificial neutral is needed because they do the job so well.

Any phase angle imbalances (like voltage regulators causing neutral shift) will cause current to circulate in the secondary. Often this isn't a major issue, but the real problem comes up when a phase faults to ground. The resulting voltage imbalance from a primary fault leads to the bank trying to balance the primary voltages through the secondary by drawing large amounts of currents which will blow cutout fuses if not damaging the pigs all together over time. For this reason POCOs avoid anything delta secondary because it does not play well with a primary wye.

The solution of course would be to leave the primary wye floating (or go delta primary) and it is definitely doable as the bank will continue to work. However, that will require 2 bushing pigs; in addition to opening up the very Ferroresonance risk POCOs want to avoid.

California on the other hand is different. Utilities are bound to EMF and PUC95 laws which mandate that any current carrying conductor be insulated from earth. This forces the use of delta primaries so POCOs have no choice. Because just about any secondary connection will tolerate a delta primary California POCOs have no problem supplying ungrounded, corner grounded and 120/240 3 phase delta services in addition to the classic wye. This is why every other 3 phase service is delta or open delta.

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Old 02-20-2015, 04:43 PM  
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IIRC, the poco's use open deltas here when they fry a pig (love that Meadow) to buy time...

~CS~
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Old 02-20-2015, 05:08 PM  
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IIRC, the poco's use open deltas here when they fry a pig (love that Meadow) to buy time...

~CS~
Id imagine. Believe it or not, if you pull one cutout on a grounded wye delta bank the customer will continue to have 3 phase power. The 2 remaining pigs might overload but at that point its an open delta.
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