Open delta load calc

[jim dungar]

And that term is specifically identified for its use in that section. To continue with this use even after jumper, smart$, & bob started discussing the different line-line windings seems strange to me.

So, do you feel it is inappropriate to call a 3-phase bank of multiple 1-phase core and coil assemblies a 'transformer', even though they may be contained in a single enclosure or 'tank'?

 

[mivey]

So, do you feel it is inappropriate to call a 3-phase bank of multiple 1-phase core and coil assemblies a 'transformer', even though they may be contained in a single enclosure or 'tank'?

If I want to call a bank of transformers a transformer, I see no problem as long as we are on the same page as to what we are discussing. Separate cores is a minor difference. If someone comes up to me and starts discussing the individual transformers in the bank, and I say "those are not individual transformers", I find that strange. Don't you?

Since the OP apparently said he did not have separate transformers (he did not agree with the discussion of separate transformers making up the bank), I concluded he was taking about a single enclosure.

Do you feel it is appropriate to call three separate windings one winding once they are connected together?

 

[jim dungar]

The OP said he had an open-delta transformer, he never came back into the discussion.

While I have never seen one, I did not find it strange enough to question the existence of a single enclosure containing a bank of windings connected in an open-delta configuration.

 

[mivey]

The OP said he had an open-delta transformer, he never came back into the discussion.

While I have never seen one, I did not find it strange enough to question the existence of a single enclosure containing a bank of windings connected in an open-delta configuration.

Good grief. I never said they did not make an open delta in a single enclosure, only that I have not seen one. It seems like the OP has one. Just follow the posts:

OP #1: "Speaking with another designer I work with, he has a difference of opinion on load calcs for an open delta service." {initial post about open delta}

jumper #2: "...The AC xformer would need to be bigger than the AB xformer" {discussion about individual transformers in the bank}

smart $ #3: "...the AC xfmr would need likely be sized larger than the AB." {discussion about individual transformers in the bank}

bob #4: "The larger transformer will carry 100% of the single phase load and each transformer will carry 58% of the three phase load" {discussion about individual transformers in the bank}

OP #5: "Maybe im missing something here. I only have one transformer... "{The OP comes back to the discussion to say he does not know why they are discussing individual transformers as he only has one}

The strange part would be to have two pots hanging on a pole, and have someone discussing the individual pots, and then for you to say: "There is only one transformer"

 

[jim dungar]

The strange part would be to have two pots hanging on a pole, and have someone discussing the individual pots, and then for you to say: "There is only one transformer"

Just like the the informal NEC definition I quoted, I often (but definitely not always) refer to a bank of single-phase units by the singular 'transformer'.

 

[mivey]

Just like the the informal NEC definition I quoted, I often (but definitely not always) refer to a bank of single-phase units by the singular 'transformer'.

If I don't know what's outside, I call it "the transformer" as well. If it is a bank, I call it a bank for clarity when talking with utility personnel (and a pad-mount a pad, and pots in an enclosure an enclosure). If talking to the customer, I would just call it the transformer as they usually don't know the difference.

 

[Smart $]

If I don't know what's outside, I call it "the transformer" as well. If it is a bank, I call it a bank for clarity when talking with utility personnel (and a pad-mount a pad, and pots in an enclosure an enclosure). If talking to the customer, I would just call it the transformer as they usually don't know the difference.

Looking back at the OP and follow-up post, I believe he is referring to "transformer" in the general sense.

Getting back to the part about how to calculate an open-delta service, it is generally assumed all single phase loads will be connected A to C (the center-tapped winding). If it is known the POCO is going to supply power via an open delta "transformer", all that is necessary is to separate the calculated single phase loads from the calculated three phase loads and supply the POCO with these kVA values. They will determine the "transformer" size(s).

If one has to determine sizes, first let's explain the why the cores are oversized. In the following example we have a typical 3? load connected to a [closed] delta secondary operating at 100Vrms (just to make calc's easier).

So here our calculated load is: 17.32A ? 100V ? √3 = 3kVA.
And as seen by the "transformer": 10A ? 100V ? 3 = 3kVA

Now let's compare to the same load powered by an open-delta secondary.

Here our calculated load is the same: 17.32A ? 100V ? √3 = 3kVA.
But as seen by the "transformer": 17.32A ?100V ? 2 = 3.464kVA

As such, each winding must be sized for 1.732kVA. That gives us a ratio of 1.732 to 3, which is 57.7% of the load.

From there, the single phase load connected A to C can be arithmetically added to the "lighter" core, while the "kicker" can be as determined.

PS: I wish they would call the "lighter" core the "lighting" core, or "major" core, or something of the sort, so as not to confuse it with a core being lighter rated.

 

[mivey]

...first let's explain the why the cores are oversized. In the following example ...

Nice illustrations & coverage.

 

[Smart $]

Nice illustrations & coverage.

Thank you!

Also regarding...

...

As such, each winding must be sized for 1.732kVA. That gives us a ratio of 1.732 to 3, which is 57.7% of the load.

...

It could also be looked at as oversizing the "transformer" by [at least] 15.47%

3.464 ? 3 ? 100% = 115.47%​

 

[jumper]

Looking back at the OP and follow-up post, I believe he is referring to "transformer" in the general sense.

Getting back to the part about how to calculate an open-delta service, it is generally assumed all single phase loads will be connected A to C (the center-tapped winding). If it is known the POCO is going to supply power via an open delta "transformer", all that is necessary is to separate the calculated single phase loads from the calculated three phase loads and supply the POCO with these kVA values. They will determine the "transformer" size(s).

If one has to determine sizes, first let's explain the why the cores are oversized. In the following example we have a typical 3? load connected to a [closed] delta secondary operating at 100Vrms (just to make calc's easier).

So here our calculated load is: 17.32A ? 100V ? √3 = 3kVA.
And as seen by the "transformer": 10A ? 100V ? 3 = 3kVA

Now let's compare to the same load powered by an open-delta secondary.

Here our calculated load is the same: 17.32A ? 100V ? √3 = 3kVA.
But as seen by the "transformer": 17.32A ?100V ? 2 = 3.464kVA

As such, each winding must be sized for 1.732kVA. That gives us a ratio of 1.732 to 3, which is 57.7% of the load.

From there, the single phase load connected A to C can be arithmetically added to the "lighter" core, while the "kicker" can be as determined.

PS: I wish they would call the "lighter" core the "lighting" core, or "major" core, or something of the sort, so as not to confuse it with a core being lighter rated.

Cool. Bookmarked for future study. Thanks.

 

[Hameedulla-Ekhlas]

Looking back at the OP and follow-up post, I believe he is referring to "transformer" in the general sense.

Getting back to the part about how to calculate an open-delta service, it is generally assumed all single phase loads will be connected A to C (the center-tapped winding). If it is known the POCO is going to supply power via an open delta "transformer", all that is necessary is to separate the calculated single phase loads from the calculated three phase loads and supply the POCO with these kVA values. They will determine the "transformer" size(s).

If one has to determine sizes, first let's explain the why the cores are oversized. In the following example we have a typical 3? load connected to a [closed] delta secondary operating at 100Vrms (just to make calc's easier).

So here our calculated load is: 17.32A ? 100V ? √3 = 3kVA.
And as seen by the "transformer": 10A ? 100V ? 3 = 3kVA

Now let's compare to the same load powered by an open-delta secondary.

Here our calculated load is the same: 17.32A ? 100V ? √3 = 3kVA.
But as seen by the "transformer": 17.32A ?100V ? 2 = 3.464kVA

As such, each winding must be sized for 1.732kVA. That gives us a ratio of 1.732 to 3, which is 57.7% of the load.

From there, the single phase load connected A to C can be arithmetically added to the "lighter" core, while the "kicker" can be as determined.

PS: I wish they would call the "lighter" core the "lighting" core, or "major" core, or something of the sort, so as not to confuse it with a core being lighter rated.

Nice information

 

[Hameedulla-Ekhlas]

See this example also

Open Delta Connection:

The open delta transformer connection can be made with only two transformers instead of three (figure 1-8). This connection is often used when the amount of three phase power needed is not excessive, such as a small business. It should be noted that the output power of an open delta connection is only 87% of the rated power of the two transformers. For example, assume two transformers, each having a capacity of 25 kVA, are connected in an open delta connection. The total output power of this connection is 43.5 kVA (50 kVA x 0.87 = 43.5 kVA).

Another figure given for this calculation is 58%. This percentage assumes a closed delta bank containing 3 transformers. If three 25 kVA transformers were connected to form a closed delta connection, the total output would be 75 kVA (3 x 25 = 75 kVA). If one of these transformers were removed and the transformer bank operated as an open delta connection, the output power would be reduced to 58% of its original capacity of 75 kVA. The output capacity of the open delta bank is 43.5 kVA (75 kVA x .58% = 43.5 kVA).

The voltage and current values of an open delta connection are computed in the same manner as a standard delta-delta connection when three transformers are employed. The voltage and current rules for a delta connection must be used when determining line and phase values of voltage current.

 

[KP2]

how to calculate an open-delta service

Thanks for your illustrstions and examples. They are so helpful.