Center tapped delta 240V 3-phase transformer connections

[jcbabb]

I have a building that currently is fed from the utility with a 240V 3-phase delta service. There are two transformers, a 50kVA and 100kVA.



I have often used the following diagram (that I found on this site some time ago)to describe this type of service, but now I am second guessing my assumptions.



I just don't see how the two transformers can make up each part of the diagram. Is the connection in reality an open delta? See below:

 

[infinity]

I would say open Delta.

 

[augie47]

Samw. I'd say you have an open delta. Fairly common at one time for small commercial locations with a small 3 phase load as compared to their single phase load, such as a grocery with a 3 phase compressor.

 

[Hv&Lv]

We still build at least one open delta a month.

 

[Open Neutral]

Here's a PG&E diagram

 

[jcbabb]

Can any of you speak to the significance of the open delta? What are its limitations from the customer's side versus a closed delta? I would guess that it limits current on the A and C phases.

We are adding on to this building and removing the 240V 3P service entrance. I will be bringing in a new 480Y277 or 208Y120 service. From the new MDP, I will need a transformer to service the existing 240V 3P MDP. I do not have an exhaustive list of the existing loads, but I do have peak demand from the utility in kW. What is the best way to go about sizing the 240 delta transformer?

Thanks for the responses guys.

 

[kwired]

Really depends on what the loads are.

Do the loads have to be supplied with 240 volt or is 208 acceptable?

Is there going to be any remaining 3 phase loads connected or will any existing three phase loads be changed to the 480 volt system?

If it is supplying mostly 120 volt loads then there is interest in trying to use 120/208 wye system if possible.

If you do have significant load and do need 240 volts instead of 208 volts you will likely not want to use open delta supplied by your new service as this will unbalance phase current on which ever two phases you connect it to. If all that is supplied is single phase loads and you used a single phase transformer you have the same issue. But you are not too likely to find a single unit transformer that is open delta design - that is typically done by building an open delta system out of single phase transformers

 

[jcbabb]

kwired
Really depends on what the loads are.

Do the loads have to be supplied with 240 volt or is 208 acceptable?

Is there going to be any remaining 3 phase loads connected or will any existing three phase loads be changed to the 480 volt system?

If it is supplying mostly 120 volt loads then there is interest in trying to use 120/208 wye system if possible.

If you do have significant load and do need 240 volts instead of 208 volts you will likely not want to use open delta supplied by your new service as this will unbalance phase current on which ever two phases you connect it to. If all that is supplied is single phase loads and you used a single phase transformer you have the same issue. But you are not too likely to find a single unit transformer that is open delta design - that is typically done by building an open delta system out of single phase transformers

kwired,

There are several 3-phase condensers that will remain. They are pretty old units and I'm not sure what going to 208 would do to them. Also as I said, I am not positive exactly what all of the existing loads are which is why I was thinking I would maintain the 240V 3P connection. My intent was to provide an off-the-shelf 3 phase 480V primary to 240V delta secondary w 120V center tapped tx, and I guess I am assuming that this would be a closed delta.

Hope that made sense.

 

[kwired]

kwired,

There are several 3-phase condensers that will remain. They are pretty old units and I'm not sure what going to 208 would do to them. Also as I said, I am not positive exactly what all of the existing loads are which is why I was thinking I would maintain the 240V 3P connection. My intent was to provide an off-the-shelf 3 phase 480V primary to 240V delta secondary w 120V center tapped tx, and I guess I am assuming that this would be a closed delta.

Hope that made sense.

Makes sense, and I find it hard to believe you would find an open delta transformer without making it a special order, that configuration is usually field made out of two single phase units. You would also need larger units than you would need if building a closed delta system - or at least a larger unit for the A-C phase if there is a high amount of 120/240 load. Even a full delta transformer may need oversized if there is a lot of load from A-C

 

[jim dungar]

kwired,

There are several 3-phase condensers that will remain. They are pretty old units and I'm not sure what going to 208 would do to them. Also as I said, I am not positive exactly what all of the existing loads are which is why I was thinking I would maintain the 240V 3P connection. My intent was to provide an off-the-shelf 3 phase 480V primary to 240V delta secondary w 120V center tapped tx, and I guess I am assuming that this would be a closed delta.

Hope that made sense.

You can go closed delta, but why not not just recreate the open delta arrangement you have?
Many times it is easier to get off-the shelf single phase units that to get a single three phase 240/120V one (this is one of the advantages of the open delta).

 

[jcbabb]

You can go closed delta, but why not not just recreate the open delta arrangement you have?
Many times it is easier to get off-the shelf single phase units that to get a single three phase 240/120V one (this is one of the advantages of the open delta).

Jim,

That's good info to know. As an engineer, often I am out of touch with actual parts availability. I just flip through the catalogs and use what's listed to help me design. It is also easier (jn my opinion) to spec a 240 delta tx rather than outline how to connect two singles to create the open delta arrangement. By "easier", I also mean "less chance for change orders".

What about the possibility of simply feeding it with a 208Y120 transformer? What are the chances that there is existing equipment that won't work properly? This is a church, if that helps.

 

[Open Neutral]

As told to me:

Open Delta is used to save money; it needs 2 xfmrs, not 3. It may save on not need the third leg. [Gather that's not an issue here, where the pigiron is on the pole, vice 1000 feet away via underground duct.]

OD is big where there is lots of 120/240 single-phase load, and less {but more than zero} 3∅ need.

An alternative is 2 services: one solely for the 3∅, the other for the 120/240 single phase needs.

The downsides:

1. You need to derate the transformers since the windings do double duty. (57%)
2. It can be hard to get all three legs balanced; and motors don't like imbalanced current.
3. It's not exactly 120 degrees apart.

Closed delta would keep your 120/240v, and solves 2 & 3. It can be done with 3 single-phase xfmrs, or one 3∅ one.

 

[jim dungar]

OD is big where there is lots of 120/240 single-phase load, and less {but more than zero} 3∅ need.

In the early days of rural electrification, the Open Delta was used for facilities that required more 3-phase than 1-phase.
If the loads are predominantly 1-phase then 240/120V systems can usually be converted to 208Y/120 fairly easily.
If the loads a 3-phase, then switching them to 208V instead of 240V may be an issue.

The downsides:

1. You need to derate the transformers since the windings do double duty. (57%)
2. It can be hard to get all three legs balanced; and motors don't like imbalanced current.
3. It's not exactly 120 degrees apart.

You do not 'derate' the transformers. You simply use a different formula for determining the equivalent 3-phase output. And yes, an open delta bank has only 57.7% of the output of a closed delta (assuming all equal sized units and no 120V loading).

Balancing and phase angles are not much of an issue if the loads are predominantly 3-phase (e.g. single phase loading is limited to 5%).

In Wisconsin, maybe 75% of the 240/120 services are open delta.

 

[kwired]

To change to 120/208 you need to examine your loads and see how much of an issue it will be.

Motors will often be just fine - but may need to have overload settings adjusted if they are operating near full load capacity as they will draw more current @ 208 volts.

Anything with resistance heating elements will have reduced capacity - it will still work but will take longer to heat the objects being heated, but could be a problem if the amount of heat @ 240 was marginal to begin with.

120 volt loads will not change.

Most hermatic compressors are dual 208/230 rated - and will likely require little additional attention - they will draw more current than they did before but nameplate ratings likely require same conductor size for either voltage, could need larger fuses or breakers if originals were marginal at holding starting current.

In most churches HVAC is biggest load - unless there is a kitchen with a big demand involved - still HVAC is more of a continual load than the kitchen normally is. If you are going to increase the service and feed the old equipment from a separately derived system you likely still want to consider full delta or wye just to balance what is likely the largest loads in the place across all three phases.

 

[Hv&Lv]

In the early days of rural electrification, the Open Delta was used for facilities that required more 3-phase than 1-phase.
If the loads are predominantly 1-phase then 240/120V systems can usually be converted to 208Y/120 fairly easily.
If the loads a 3-phase, then switching them to 208V instead of 240V may be an issue.



You do not 'derate' the transformers. You simply use a different formula for determining the equivalent 3-phase output. And yes, an open delta bank has only 57.7% of the output of a closed delta (assuming all equal sized units and no 120V loading).

Balancing and phase angles are not much of an issue if the loads are predominantly 3-phase (e.g. single phase loading is limited to 5%).

In Wisconsin, maybe 75% of the 240/120 services are open delta.

We still build them where single phase is much larger than the three phase load. If the three phase is too large, or larger than single phase, we close the bank with the third pot (If we have a third phase avaliable).
When we size an open delta, we must "upsize" the bank to account for the poor power factor and the fact that the bank is good for 86.6% of rating.

 

[jim dungar]

We still build them where single phase is much larger than the three phase load. If the three phase is too large, or larger than single phase, we close the bank with the third pot (If we have a third phase avaliable).
When we size an open delta, we must "upsize" the bank to account for the poor power factor and the fact that the bank is good for 86.6% of rating.

For those that may be questioning sizing:

Assuming equal sized 15kVA single phase transformers;
closed delta = 3 times the rating of one single phase unit = 3 x 15 = 45kVA equivalent

open delta method 1 = 57.&% of full capacity 3-phase = .577 x 3 x 15 = 25.96kVA equivalent (great method for figuring out how much capacity is left if a closed delta bank becomes an open delta due to the loss of one unit)
open delta method 2 = 86.6 % of sum of transformers = .866% x (15 + 15) = 25.98kVA (method often used for figuring out capactiy of a new bank)

 

[Open Neutral]

In the early days of rural electrification, the Open Delta was used for facilities that required more 3-phase than 1-phase.
If the loads are predominantly 1-phase then 240/120V systems can usually be converted to 208Y/120 fairly easily.
If the loads a 3-phase, then switching them to 208V instead of 240V may be an issue.

Two issues here: 208 3∅ vs. 240 3∅; and open vs. closed. [2 xfmrs/primaries vs 3.] The voltage will be a bigger nut on an existing rework than a new job, I'd say.

PG&E's guru told me they sell {240V} Open Delta for those customers needing more single phase than 3. Iffen we had lots of 3∅ demand, they'd go closed. They do sell 240 Closed Delta.

You do not 'derate' the transformers. You simply use a different formula for determining the equivalent 3-phase output.

Poor wording by me acknowledged. You put it far better.

Balancing and phase angles are not much of an issue if the loads are predominantly 3-phase (e.g. single phase loading is limited to 5%).

But PG&E uses "duplex" xfrms with ratings such as 25/10, 50/10 - the bigger number is the "lighter" with centertap; the smaller is the "stinger" leg. That alone says that most of the load is single phase. Plus various manufacturers warn about imbalance, and preach how to reduce same. [The Franklin pump I've mentioned here...]

In Wisconsin, maybe 75% of the 240/120 services are open delta.

I assume you mean "240/120 3∅ services." That strikes me as unamazing; I'd think most big users were 208 wye.

 

[Open Neutral]

The PG&E "duplex" transformers are seen in Table 9 on page 7.

 

[jim dungar]

I assume you mean "240/120 3∅ services." That strikes me as unamazing; I'd think most big users were 208 wye.

240/120 is the correct way to describe this voltage. My personal preference is to add the descriptors 3PH and 4W (unless I am joining a conversation), but they are not part of the IEEE/ANSI methodology.

Of course most users are 208Y/120 not 240/120 3PH 4W. Most of the major utilities prefer not to supply 240/120 on new construction. Many of the rural suppliers do offer 240/120 for sites that are mainly single-phase but have some 3-phase loads, and for some locations (i.e. well pumps) where there are 3-phase motors but very little single-phase.

 

[hillbilly1]

Depending on how many 240 volt loads you are having to refeed, you may consider a buck/boost transformer set up, just go with the 120/208 wye for everything else. Full deltas though are not that hard to get, I have done the same conversion for a paper recycling plant.