240V Open Delta

[mivey]

Mivey
I am pretty sure that diagram from Widers book has an error. I have never seen a ground connection on the high side. Your going to get circulating currents in the grounds. The grounding of both sides make it into some kind of autotransformer.

We use open deltas in fewer and fewer applications. The most common know is for sewage lift pumps.
Most industrial uses want 4 wire 480 volt and the rest want 208/120 systems.
Someof the older 480 volt users like 3 wire open deltas. They have to use single phase 480 -120/240 or three phase 480-120/208 transformers to get lighting, receptacles etc.

No diagram error. For the 3-phase primary, the neutral is floated. For the 2-phase primary, the primary neutral must be grounded or you only have a single-phase primary input.

 

[rattus]

There is no neutral point provided, but the 3rd conductor is commonly called "the neutral".

It was a neutral until someone took away the third phase!

 

[broadgage]

It would appear to me that some posters are confusing two different types of supply/service.

"Open delta" is normally understood to mean a way of providing 3 phase service, from a 3 phase source, but useing only 2 transformers, and not 3.
The input would be 3 phase, 3 wire.
The advantage of this system is that only 2 transformers need be purchased instead of 3.
It is a cheap way of providing small 3 phase services in rural areas.
The permisable load is only 57% of that could be carried by 3 transformers, and not 67% as might be expected.
If the load grows, then a third transformer may be added later.

The output may be either ungrounded, or one corner may grounded.
Alternativey, the mid point of one transformer may be grounded.
This was popular if the existing service was single phase, 3 wire, 120/240 volt to a home or small business.
The addition of just one extra transformer, probably quite small, would give a 3 phase 240 volt service for central A/C or other large motors.
Note that for such an arrangement, the MV utility supply would have to be 3 phase. The original 120/240 transformer would be connected between say A/B and the additional transformer between either B/C or A/C.

However some posts may not be reffering to open delta at all, but to 3 phase closed delta, derived by means of 3 transformers, from a 3 phase MV supply.
As with open delta, the 3 phase output may be either ungrounded, corner grounded, or have a grounded center tap on one transformer only.
Ungrounded or corner grounded systems can be any desired voltage, but 240 or 480 are common.
Systems with a grounded center tap, could in theory be any desired voltage, but in practice are almost allways 120/240.
Such a system would power 120 volt, 2 wire loads such as lamps, or 240 volt 2 wire loads such as water heaters, or 240 volt 3 wire 3 phase motors.
Such a system will have 208 volts between the grounded center tap and the middle phase, often called the high phase, and coloured orange.
The use of 208 volt loads is not recomended from such a system.

It will be seen that the two systems are somwhat similar, indeed a closed delta system will continue to operate with one missing transformer, becoming an open delta system.

 

[mcclary's electrical]

If you do all of this you no longer have a neutral. Don't confuse grounded conductors with neutrals.

you are assuming that the transformer is a reconnectable 120X240. This is not true of the typical of pole top oil-filled units used by utilities.

Other than that it is a good description of how a set of transformer coils can be reconnected. However it still has nothing to due with all three-phase primary connection require at least three current carrying conductors instead of just two.

It all started when my original post came accross wrong. When I said they are being phased out as 3 phase is more readily available, what I actually meant was, around here, you usually see that type of service that has been added later to a single phase service. It always seems to be an "afterthouight". As the 3 phase distribution system got more and more improvements, you see theses services less and less

 

[mivey]

It all started when my original post came accross wrong. When I said they are being phased out as 3 phase is more readily available, what I actually meant was, around here, you usually see that type of service that has been added later to a single phase service. It always seems to be an "afterthouight". As the 3 phase distribution system got more and more improvements, you see theses services less and less

Some rural areas have such a small load they will only get a 1-phase distribution line and the only option they have is to get a phase converter or a written-pole motor. It does not make economic sense to put in a 3-phase line as it may cost about 50%-60% more.

If the area has enough load growth, that line might get converted to a 2-phase line. It still might not be big enough to justify the cost of a 3-phase conversion as it might cost 30-40% more than the 2-phase conversion.

Even for new construction, it is quite common to have a 2-phase primary because the system load is just not big enough to justify the cost of a 3-phase line. If a new 2-phase line is built in an area that might see growth within the practical life of the structure, we configure the pole tops so they will accommodate the 3rd conductor when the area is ready.

 

[KWH]

Free Electric

Free Electric

I was told by a local POCO engineer one reason they are doing away with the high leg service is some creative people were tapping the high leg for illegal use since it is not metered.

 

[mivey]

It would appear to me that some posters are confusing two different types of supply/service.

I did not get that impression.

"Open delta" is normally understood to mean a way of providing 3 phase service, from a 3 phase source, but useing only 2 transformers, and not 3.

Hardly, unless you are calling the wye primary with two ungrounded conductors a 3-phase source.

It is a cheap way of providing small 3 phase services in rural areas.

Many times it is the only way as there is only a 2-phase primary. But as you say, sometimes it is used as a cheap way to provide for some small 3-phase loads because the POCO can save a transformer. Any time the 3-phase load is of appreciable size and we have a 3-phase line, we put in a 3 pot bank.

It will be seen that the two systems are somwhat similar, indeed a closed delta system will continue to operate with one missing transformer, becoming an open delta system.

Unless the primary is a floating wye (which is usually the case for a wye-delta bank).

 

[mivey]

I was told by a local POCO engineer one reason they are doing away with the high leg service is some creative people were tapping the high leg for illegal use since it is not metered.

Not true. It is metered.

add: The center-tapped portion is metered just like a single transformer would be. The high-leg has a separate stator. I can post a diagram if you want.

 

[mivey]

I can post a diagram if you want.

As you can see from this modified ABB diagram, the stator for the single-phase portion (winding 1-2) has a 240 volt voltage coil and two current coils (for legs 1 & 2). The high-leg (leg 3) has its own stator with a 208 volt voltage coil and one current coil (for the high-leg).

 

[LarryFine]

"Open delta" is normally understood to mean a way of providing 3 phase service, from a 3 phase source, but useing only 2 transformers, and not 3.

The input would be 3 phase, 3 wire.

Note that for such an arrangement, the MV utility supply would have to be 3 phase. The original 120/240 transformer would be connected between say A/B and the additional transformer between either B/C or A/C.

Two phases and the system neutral can also comprise the "3-wire" primary supply necessary, being what we're calling an "open-Y primary/open-Delta secondary."

So, not only can the third transformer be omitted, so can the third phase. Most of the open-Delta services around here (and there are plenty) receive only two phases.

 

[mivey]

It was a neutral until someone took away the third phase!

More wire theft. Call 911. :grin:

Funny how we can remove a phase conductor and the neutral conductor now becomes a common conductor, which indicates the system can be seen from a different perspective. On the other hand, when we want to start by classifying the neutral as a common conductor, some would say that perspective doesn't make sense.

 

[Sierrasparky]

Out in my neck of the woods we only have 2 phase primary. I have seen this single wire primary in oregon and Washington. I always wonderd if someone played around with the ground wire comming from the pole , or if it became un attached and that person got between it and ground what would happen.

 

[jim dungar]

mivey likes to obfuscate a simple discussion.
He cannot accept the fact that the industry does not 'universally' call 2 hot conductors and a neutral, or common, conductor 2-phase. He has written many posts promoting the practice of calling 120/240 3-wire 2-phase. Now in this thread, he is calling an open-wye configuration 2-phase. It appears mivey recognizes 2-phase systems of 90?, 180?, and 120? separation. I wonder does he call a corner grounded open-delta, 2-phase at 60?? After all it is two phase conductors with one common conductor.

I stand behind my original post

An open delta configuration requires 3-phase.

My reference materials, well at least the ones on my desk right now, (UGLY's, ANSI/IEEE C57.105, and Cooper Power Systems R201-90-2) all call the feed, to open-delta and open-wye transformer connections, as 3-phase.

And, except through the slight of hand of introducing a 3rd wire, mivey has not proven me wrong when I said:

If the utility only has 2-wires it cannot power an open delta.

 

[rattus]

Don't Know:

Don't Know:

Don't know about the industry practices and parlance, but it is clear that only two generators and at least three wires are required to provide open delta service.

IMHO in most cases three phases are present on the pole, but only two are connected to primaries to provide open delta service.

 

[LarryFine]

Funny how we can remove a phase conductor and the neutral conductor now becomes a common conductor, which indicates the system can be seen from a different perspective. On the other hand, when we want to start by classifying the neutral as a common conductor, some would say that perspective doesn't make sense.

We have the same argument during our "is a center-tapped secondary two phases?" discussions. :roll:

 

[LarryFine]

I have seen this single wire primary in oregon and Washington.

Are you talking about SWER?

 

[LarryFine]

I stand behind my original post

An open delta configuration requires 3-phase. An open delta primary requires three 'lines'. An open wye primary requires two 'lines' and a neutral. If the utility only has 2-wires it cannot power an open delta.

I concur with all of that. To supply an open Delta, the source must be three-phase, even if only two lines and the neutral are supplied. There must be three primary conductors, one of which may be the neutral.

 

[LarryFine]

Now in this thread, he is calling an open-wye configuration 2-phase.

That's the exact same scenario as two lines and the neutral from a 208Y/120v source.

 

[mivey]

mivey likes to obfuscate a simple discussion.
He cannot accept the fact that the industry does not 'universally' call 2 hot conductors and a neutral, or common, conductor 2-phase.

Jim refuses to acknowledge that 2 hot conductors and a neutral, or common, conductor is also called 2-phase by a large section of the industry.

Jim refuses to acknowledge that the naming convention he likes to promote is not a universal fit and he tries to bend the systems to fit his restrictive naming convention. I never said that the industry should universally call it by one name or another.

I do promote the idea that if people in our industry would seek to understand where the names came from by starting with a more general definition, then they would not get confused when the different naming conventions and system configurations do not match. They would also be able to see how some of the naming conventions are derived sub-sets of the general case.

Understanding where the names come from and why they apply for specific cases is not a move to drop the names we have become accustomed to.

He has written many posts promoting the practice of calling 120/240 3-wire 2-phase.

Jim is mis-representing what I have said.

The point I have tried to make in other posts is that the 3-wire 120/240 volt single-phase system maps to the same space as that occupied by a using more generalized naming conventions of poly-phase systems. Understanding generalized case helps understand why there is no conflict between the two. But that does not mean we should go re-naming everything, only that a broader understanding will show why the traditional name works for the specific case.

Just because the traditional name works for a specific case does not mean we can use the specific case to create a general naming convention. I have shown numerous times why this does not work, and actually falls apart for other specific cases.

Now in this thread, he is calling an open-wye configuration 2-phase.

Because it is common in the utility industry (my current field).

It appears mivey recognizes 2-phase systems of 90?, 180?, and 120? separation. I wonder does he call a corner grounded open-delta, 2-phase at 60?? After all it is two phase conductors with one common conductor.

If you would start listening to what I am saying, maybe you would understand and you wouldn't have to wonder because you could figure it out for yourself. The source has three, equal-magnitude, phase-displaced voltages so it would normally be labeled as a 3-phase source.

Now for how it is used:
Let the corners of the delta be a, b, and c. Let's ground a.

If the load uses the voltage Vba OR Vca, then it is a single-phase load and I am using the source as a single-phase supply.

If the load uses the voltage Vba AND Vac AND Vcb, then it is a three-phase load and I am using the source as a three-phase supply.

If the load uses the voltage Vba AND Vac (or any other combination of two equal magnitude, phase-displaced voltages), then it is a two-phase load and I am using the source as a two-phase supply.

That's right: The 3-phase source can supply all three systems.

What happens if we ground at midpoint instead? You have a combination of a three-phase configuration and a single-phase configuration that can supply 1, 2, and 3 phase loads.

I stand behind my original post
My reference materials, well at least the ones on my desk right now, (UGLY's, ANSI/IEEE C57.105, and Cooper Power Systems R201-90-2) all call the feed, to open-delta and open-wye transformer connections, as 3-phase.

By common utility language, the feed to open-delta would be 3-phase and the feed to the open-wye would be 2-phase.

And, except through the slight of hand of introducing a 3rd wire, mivey has not proven me wrong when I said:

That statement by itself is not wrong. But that was not what you asked. You wanted to know why I posted what I did. I told you why. If you don't want to listen, so be it.

 

[rattus]

To be more precise, we can say that the open delta uses two phases of a three phase distribution system. It is unlikely that the power plant generates only two phases.