Center tapped delta 240V 3-phase transformer connections

[mivey]

Cost,cost,cost! With the wye you have to use three transformers, with the open delta, only two transformers are needed. Most POCO's stock only certain size transformers, because they are required to have spares on the lot for emergencys. The cost between hanging three 10 kva transformers versus hanging a 15 kva and a 10 kva is considerable when the payback due to low usage is a lot longer. Especially if only two phases are run on the poles from the main lines. If you have to add another line for a couple of miles, it gets even more expensive.

Very good points.

However, I might point out that you can make a wye with only two transformers as well.

 

[mivey]

We install brand new open delta's with new transformers, and actually use a bracket made for three transformers. http://www.alumaform.com/ClusterMts.pdf

Occasionally you will have a business that is mostly single phase that has one or two motors that needs 240 three phase. Sometimes this motor will not be used everyday, but the bank has to be there, just in case. Sort of like demand charges, the capacity has to be available just in case you want it.

We have one built way out in the country on a single phase line at a two way feed pole. One phase comes in from the south, one phase comes in from the north. The nearest V or 3 phase is 6 miles away. It would cost "a bunch" to reconductor the line just for that small of a load. In today's economy, least cost planning is or should be at the top of the engineering dept's priorities. Linework gets expensive fast. $50-200k per mile depending on construction chosen.


There is also the problem of no load losses to consider. There is one extra pot up there consuming energy that isn't being paid for. That pot humming along for 8759 hrs. a year is money down the drain for a poco. (we can't keep the power on for the whole year...)

Also very good points.

Even with the bad power factor of an open delta, it may still be worth it. It is a balancing act of whether the no load losses or power factor losses are greater as to whether to leave the delta open or to close it.

Please clarify. While it does increase the no-load losses, I don't think closing the delta improves the power factor.

 

[Hv&Lv]

Also very good points.

Please clarify. While it does increase the no-load losses, I don't think closing the delta improves the power factor.

The three pots together in a closed delta will operate at 100% (I know, nothing is 100%)
With the open delta having only two pots and the 30 degree phase shift, the effeciency is only 87%. The other 13% is wasted energy.

 

[gar]

121007-2345 EDT

Hv&Lv:

Those transformers would be toast at full load after a while if the energy losses were 13%.

.

 

[jim dungar]

With the open delta having only two pots and the 30 degree phase shift, the effeciency is only 87%.

Are you confusing the formulas for effective, or equivalent, 3-phase kVA output with efficiency?

When using two equally sized units, you can determine the equivalent 3-phase rating by:
(2 x single kVA x 86.6%) or (3 x single kVA x 57.7%)

While the actual efficiency of each individual transformer remains the same regardless of its connection, the fact they may need to be oversized in order to deliver equal loading could mean an open delta bank could have higher losses than a closed delta. The only way to know is to compare the core and winding losses for the units supplied.

 

[iceworm]

Cost,cost,cost! With the wye you have to use three transformers, with the open delta, only two transformers are needed. Most POCO's stock only certain size transformers, because they are required to have spares on the lot for emergencys. The cost between hanging three 10 kva transformers versus hanging a 15 kva and a 10 kva is considerable when the payback due to low usage is a lot longer. Especially if only two phases are run on the poles from the main lines. If you have to add another line for a couple of miles, it gets even more expensive.

I can't comment on the cost differential - I'm not knowledgable.

One case has 25KVA of copper and steel, capable of 22kva of output. The other has 30kva of copper and steel, capable of 30kva output. The installation crew and the equipment is the same. Number of connections is the same.

It doesn't look like a lot of difference to me. But then again, I'm not knowledgable on the subject.

ice

 

[kwired]

We have one built way out in the country on a single phase line at a two way feed pole. One phase comes in from the south, one phase comes in from the north. The nearest V or 3 phase is 6 miles away. It would cost "a bunch" to reconductor the line just for that small of a load. In today's economy, least cost planning is or should be at the top of the engineering dept's priorities. Linework gets expensive fast. $50-200k per mile depending on construction chosen.

There are many of those kind of builds around here.

There are also many small services that are right on the three phase primary line and they still only supply them with open delta - because they are small loads and it takes less investment to supply with open delta. Most of them are 480 volt open delta supplying irrigation machines that only need 10-15 kVA max.

I do know of one 75 HP well supplied from an open delta - but is an older service, they seldom supply that large of a load with open delta, and this one happens to be one phase from north and other from south kind of situation. In fact if they were wanting a new service to same location today the POCO would likely make them pay whatever they are asking to build a three phase line, and would not even consider the open delta for that big of a load.

 

[Smart $]

Are you confusing the formulas for effective, or equivalent, 3-phase kVA output with efficiency?

When using two equally sized units, you can determine the equivalent 3-phase rating by:
(2 x single kVA x 86.6%) or (3 x single kVA x 57.7%)

While the actual efficiency of each individual transformer remains the same regardless of its connection, the fact they may need to be oversized in order to deliver equal loading could mean an open delta bank could have higher losses than a closed delta. The only way to know is to compare the core and winding losses for the units supplied.

...

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)

26kVA maximum output from summed 30kVA rating is only 86.6% efficient (synonymous with effective). As I explained in another thread, the term efficient is not limited to the 'engineering' definition.

 

[iceworm]

... There are also many small services that are right on the three phase primary line and they still only supply them with open delta - because they are small loads and it takes less investment to supply with open delta. Most of them are 480 volt open delta supplying irrigation machines that only need 10-15 kVA max. ....

Does the economics of 480 open D for 15kva loads apply to the OPs 240/120 questions? I suspect not.

ice

 

[iceworm]

... 26kVA maximum output from summed 30kVA rating is only 86.6% efficient (synonymous with effective). As I explained in another thread, the term efficient is not limited to the 'engineering' definition.

S$ -
Perhaps, in a technical discussion, it should be. Otherwise :? :?

ice

 

[jim dungar]

26kVA maximum output from summed 30kVA rating is only 86.6% efficient (synonymous with effective). As I explained in another thread, the term efficient is not limited to the 'engineering' definition.

It looks like you are working backwards from what the potential supply, rather than working forward from what is needed.

If the requirement is only for 25kVA, why would you use 30kVA of transformation when 26kVa would be sufficient?

 

[Smart $]

S$ -
Perhaps, in a technical discussion, it should be. Otherwise :? :?

ice

Not efficient, meaning not effective, inefficient, or ineffective, to some degree, is technically correct usage of the word. Additionally, the 'engineering' definition is only valid when using the word efficiency... AND in the proper context.

If we succumb to your suggestion, we would be limiting our vocabulary for the linguistically impaired. In other words, the technical discussion becomes less technical. :happyyes:

 

[Smart $]

It looks like you are working backwards from what the potential supply, rather than working forward from what is needed.

If the requirement is only for 25kVA, why would you use 30kVA of transformation when 26kVa would be sufficient?

No matter how you look at it, two 15kVA-rated transformers (arithmetically summed at 30kVA) are required for 26kVA of secondary system capacity. It is not very difficult to realize the degree of effectiveness.

 

[jim dungar]

It is not very difficult to realize the degree of effectiveness.

Effectiveness (adj): producing the intended outcome.
(from most sources - effectiveness is interested obtaining the end result, not the cost of getting to it.)

Given the intent is to supply 26kVA of load, then the open delta is as equally effective as the closed delta.

 

[Smart $]

Effectiveness (adj): producing the intended outcome.
(from most sources - effectiveness is interested obtaining the end result, not the cost of getting to it.)

Given the intent is to supply 26kVA of load, then the open delta is as equally effective as the closed delta.

Effectiveness is a noun

Effective can be a noun but is typically used as an adjective.

Otherwise what you say is true... but definitions often do not relate all contextual uses. For example:

Two persons purchase an identical product. One pays twice as much as the other. Both produced the intended outcome. But if I ask which person was more effective, are you going to say neither?

 

[mivey]

The three pots together in a closed delta will operate at 100% (I know, nothing is 100%)
With the open delta having only two pots and the 30 degree phase shift, the effeciency is only 87%. The other 13% is wasted energy.

That is not the way it works. I believe you are talking about % utilization. The cost to step up a size is generally smaller than the cost to add a separate transformer.

There are loss floors as well as we only have so many different sizes. For example:
Consider the case of 15-20 kVA worth of 120/240v single-phase load and 5 kVA of 240v three-phase load. The smallest units we keep for general use are typically 10-15 kVA units. So we might have a 25 kVA lighting pot and a 10 kVA pot to make the open delta. As you know, adding the third 10 kVA pot to close the delta does not make sense for this location.