What is Closed Delta?

[kwired]

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.

Assuming B is high leg and A to B is the "ghost winding" you are asking why there is 240 volts from A to B, I think.

I believe - you don't have a direct path between A and B like you do with a full delta, but you still have a path from A to C and then C to B - with a 60 degree angle. A to B connected loads will have current passing through both windings at a different rate then if it were a full delta.

 

[dkidd]

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.

Can you back that up with diagrams?

 

[kwired]

Can you back that up with diagrams?

The primary side connections?

If building the bank from standard 7200 line to neutral one bushing transformers you have no choice - you must use two phase conductors and a common neutral for the supply side.

If building the bank from two bushing 12,470 volt primary units you must use all three ungrounded primary conductors, one phase will be common to both units.

Either way you must have three conductors on the primary side and one of them is common to both units.

 

[dkidd]

The primary side connections?

If building the bank from standard 7200 line to neutral one bushing transformers you have no choice - you must use two phase conductors and a common neutral for the supply side.

If building the bank from two bushing 12,470 volt primary units you must use all three ungrounded primary conductors, one phase will be common to both units.

Either way you must have three conductors on the primary side and one of them is common to both units.

So where did

all three primary phases plus neutral.

come from?

 

[Strathead]

Assuming B is high leg and A to B is the "ghost winding" you are asking why there is 240 volts from A to B, I think.

I believe - you don't have a direct path between A and B like you do with a full delta, but you still have a path from A to C and then C to B - with a 60 degree angle. A to B connected loads will have current passing through both windings at a different rate then if it were a full delta.

No, this isn't what i was asking. Thank you anyway. I made the assumption of the 60 degree angle it is the math on the primary side that I am curious about.

 

[jaggedben]

Since this the the PV forum and the OP is a PV guy, let's talk about what would happen if you tried to connect a 240V single phase inverter to an open delta on the phase that doesn't have a transformer coil. (Let's assume the inverter doesn't need a neutral). I have a feeling it's a bad idea, especially if done with a larger inverter. It's related to Strathead's question of what really goes on in that phase.

 

[ggunn]

Since this the the PV forum and the OP is a PV guy, let's talk about what would happen if you tried to connect a 240V single phase inverter to an open delta on the phase that doesn't have a transformer coil. (Let's assume the inverter doesn't need a neutral). I have a feeling it's a bad idea, especially if done with a larger inverter. It's related to Strathead's question of what really goes on in that phase.

FWIW, the times I have had to interconnect to a 240/208/120V high leg delta system the AHJ has wanted us to interconnect on the two 120V to neutral phases and leave the high leg alone.

 

[Besoeker]

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.

I do know something about your systems there. My dear wife is from GA. Her uncle was an electrician. And I have been there (USA) on business quite a few times.

I'm not altogether keen on the idea of 120-0-120 for homes. I see quite a few lost neutral problems aired here. And then there are the different voltage appliances depending on function/purpose.
Here (UK) the vast majority of homes have just 230V line to neutral. Everything runs from that. So a lost neutral, whilst it is still a danger, won't blow things up.

Local distribution transformers in residential and light commercial areas are usually 11kV, 50Hz (closed*) delta primary and 400/233V star (wye) secondary and is a single ONAN unit. Typically, they are around 1MVA to 1.5MVA.

But, in responding to your points, I digress. (Sorry Bob the Badger.)
It's possible that we do have open delta units here. They are covered in my J&P transformer book. But the only time I encountered one was in the Far East. For cost savings?

*To the OP. Draw a triangle. An equilateral meaning all three sides are the same length. Those represent how the three windings are connected on a closed delta. In an open arrangement, ones ide of the triangel representing one winding isn't there.

 

[kwired]

So where did

Originally Posted by kwired
all three primary phases plus neutral.



come from?

I was trying to state that with an open delta you don't need all three primary phases plus neutral, maybe I could have worded it better?

Most of the time they are using wye configuration on the primary side so they use two phases and neutral. But if using line to line voltage rated primary coils you can still connect open delta to all three phases - if neutral is connected it is just to ground the case and not as a circuit conductor.

 

[pv_n00b]

Think of it as a black box experiment. Suppose I have two 3 phase, 480/240V, 3W, 100kW transformers, one open and one closed delta. I connect 3 480V conductors to the primary and three 240V single phase loads on the secondary, one on each phase. Is there any voltage or current measurement that I can take that will allow me to tell which is which? The answer is no, they both act exactly the same outside the box. If I unbalance the loads and take measurements can I tell which is which? Nope, outside the box it looks the same.

Internally they are very different, different number of windings, different winding rating. If you want to look at the internal current flow you apply vector math and it all works out. It's actually a fun exercise in excel.

 

[GoldDigger]

Think of it as a black box experiment. Suppose I have two 3 phase, 480/240V, 3W, 100kW transformers, one open and one closed delta. I connect 3 480V conductors to the primary and three 240V single phase loads on the secondary, one on each phase. Is there any voltage or current measurement that I can take that will allow me to tell which is which? The answer is no, they both act exactly the same outside the box. If I unbalance the loads and take measurements can I tell which is which? Nope, outside the box it looks the same.

Internally they are very different, different number of windings, different winding rating. If you want to look at the internal current flow you apply vector math and it all works out. It's actually a fun exercise in excel.

Actually, if you measure the internal resistance by measuring the line to line voltage drop under load you will see a definite difference between the "open" leg of the triangle and the two closed legs.
If you apply too large a nominally balanced motor load the difference in voltage drop can cause excess current in two of the three phases.

 

[pv_n00b]

Actually, if you measure the internal resistance by measuring the line to line voltage drop under load you will see a definite difference between the "open" leg of the triangle and the two closed legs.
If you apply too large a nominally balanced motor load the difference in voltage drop can cause excess current in two of the three phases.

You are correct in your two points but I was looking at how the transformer looks to a load under normal working conditions. That's why I said by making voltage and current measurements only, not measurements that show the internal design of the transformer. Obviously if you start probing the internal makeup of the transformer you will see differences. Overloading the transformer to get measurements outside of normal working conditions also would not apply.

I was pointing out that under normal usage measuring what the load sees you can't tell the difference between a closed and open delta and this is one of the things that makes them useful.

 

[GoldDigger]

You are correct in your two points but I was looking at how the transformer looks to a load under normal working conditions. That's why I said by making voltage and current measurements only, not measurements that show the internal design of the transformer. Obviously if you start probing the internal makeup of the transformer you will see differences. Overloading the transformer to get measurements outside of normal working conditions also would not apply.

I was pointing out that under normal usage measuring what the load sees you can't tell the difference between a closed and open delta and this is one of the things that makes them useful.

But it is exactly under normal usage that the difference in internal impedance (reflected in differing terminal voltages) causes problems with voltage regulation. This is, among other things, the reason that single phase loads across the open side of the delta are discouraged, and even fully balanced three phase loads should not be too large relative to the transformers' nominal capacity.

I do not consider looking at the voltage drop at the black box terminals under load "looking at the internal design of the transformer". I consider it looking at the transformer characteristics, whatever their source, that affects the performance of the loads.
A three transformer closed delta with one of the pots having a higher %Z relative to the others or a smaller size can have the same performance problems as an open delta.

 

[don_resqcapt19]

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.

That must be a regional thing....wye connected pole transformers are common around here.

 

[jaggedben]

But it is exactly under normal usage that the difference in internal impedance (reflected in differing terminal voltages) causes problems with voltage regulation. This is, among other things, the reason that single phase loads across the open side of the delta are discouraged, and even fully balanced three phase loads should not be too large relative to the transformers' nominal capacity.

....

It seems to me that open deltas therefore ought to be high-leg so that I (the electrician) know which leg is only supposed to be used for three phase loads. So to speak to the OP's question, open deltas are (hopefully) a subset of high-leg deltas. (Closed deltas are another story).

I'd also note that, without taking any electrical measurements, one way to tell if a service is open delta (albeit maybe not 100% reliably) is to look outside and count the cans hanging from the pole.

 

[ActionDave]

That must be a regional thing....wye connected pole transformers are common around here.

Wye connected pole transformers are common here too, pad mounted deltas are not.

 

[kwired]

It seems to me that open deltas therefore ought to be high-leg so that I (the electrician) know which leg is only supposed to be used for three phase loads. So to speak to the OP's question, open deltas are (hopefully) a subset of high-leg deltas. (Closed deltas are another story).

I'd also note that, without taking any electrical measurements, one way to tell if a service is open delta (albeit maybe not 100% reliably) is to look outside and count the cans hanging from the pole.

Open or closed delta doesn't matter, if the midpoint of one phase is grounded, you will have a high leg on open or closed delta. If midpoint is not grounded - then you need to either corner ground the system or set it up as an ungrounded system and provide a ground fault monitoring system, again can be open or closed delta.

 

[ActionDave]

It seems to me that open deltas therefore ought to be high-leg so that I (the electrician) know which leg is only supposed to be used for three phase loads....

If it is an existing installation it's the leg with all the breaker blanks.

 

[pv_n00b]

But it is exactly under normal usage that the difference in internal impedance (reflected in differing terminal voltages) causes problems with voltage regulation. This is, among other things, the reason that single phase loads across the open side of the delta are discouraged, and even fully balanced three phase loads should not be too large relative to the transformers' nominal capacity.

I do not consider looking at the voltage drop at the black box terminals under load "looking at the internal design of the transformer". I consider it looking at the transformer characteristics, whatever their source, that affects the performance of the loads.
A three transformer closed delta with one of the pots having a higher %Z relative to the others or a smaller size can have the same performance problems as an open delta.

I've never read anything saying that connecting a load across the open phase is discouraged. I'd like to see a good source for that.

There will be a small variation in the terminal voltage under load because the 2 transformers are operating under different power factors. But that's not due to a variation in the internal impedance, even though it might look like it. It's due to the transformers having different apparent currents with corresponding different voltage drops. But it's not a large difference, the difference is only 30° and the internal impedance of a good transformer is pretty low. In a lab with a good test setup, no noise, balanced load and sensitive measurement equipment you can tell it's an open delta. Someone with a DMM making measurements in the service entrance panel in a commercial building with unbalanced loading trying to tell if it is an open delta or closed delta? I would say they are not going to be able to tease out that difference in voltage regulation. It's going to look pretty much the same as a closed delta in the field.

This is a good analysis of the math behind open deltas:

http://www.aast.edu/pheed/staffadmi...formers connection.pdf&stafftype=staffcourses

 

[kwired]

I've never read anything saying that connecting a load across the open phase is discouraged. I'd like to see a good source for that.

Even a three phase load will be connected across the open phase.

One must consider the fact that the source doesn't have the same capacity as it would if it were a full delta with all three coils the same capacity.

Such applications of this kind of system are usually where there is limited three phase load, a rather fixed load, or in remote locations where it is deemed more practical then bringing the third phase to the location, over sizing the open leg transformer or even both transformers from what they would be if a closed delta may be necessary if there is going to be significant load on it, but other then the remote location application one is probably better off just building a closed delta in the significant load applications.