transforming 3 phase with two transformers

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electrofelon said:
I recently learned that this could be done. Are there any disadvantages? Why do utilities usually use a bank of three instead of two?
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Yes there are disadvantages. Running three phase off of two transformer windings has a reduced power output by a factor of 1/√3 or 0.577. This also in a round-about way answers the second question.

From a document by Cooper Power Systems

The capacity of an open delta bank is only 57.7% of a
delta-delta bank of the same size units. That is, three 25
kva transformers connected delta-delta would have a
capacity of 75 kva. Two 25 kva transformers in an open
delta bank would have a capacity of only 43.2 kva. It is
convenient to remember that in an open delta bank only
86.6% of the capacity in the transformers is realized.
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One reason is cost.

Why provide 100kVA of capacity when the customer only needs 80kVA. Less kVA supplied may mean less operating costs to the utility. If the customer's power requirements grow, the utility only needs to add one transformer instead of replacing three of them.
 
jim dungar said:
One reason is cost.

Why provide 100kVA of capacity when the customer only needs 80kVA. Less kVA supplied may mean less operating costs to the utility. If the customer's power requirements grow, the utility only needs to add one transformer instead of replacing three of them.
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That's the other side of the coin?advantages (OP inquired about the disadvantages only :smile: ).

Preceding statement in same Cooper Power Systems document:

...this connection can be used in an
emergency in case one of the transformers in a deltadelta
bank fails. This type of bank is also used to supply
power to a three-phase load which is temporarily light
but which is expected to grow. When the load increases
to a point where the two transformers in the bank are
overloaded, an increase in capacity of 1.732 times can
be obtained by adding another unit of the same size and
using the delta-delta connection.
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zbang said:
FWIW, some equipment manufacturers won't warrent/support their products when connected to an open-delta supply. IIRC this includes most television and radio transmitters.
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I'm curious why?

Even with phase-loss control?
 
The leg that doesn't have a transformer across it has much worse voltage regulation than the other two. Imbalances become much worse
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What about a single balanced load such as a kiln?

The motivation for this is I am getting a 1 kva wind turbine and the output needs to be stepped down. Thus theoretically I would either need 3 .33kva tranformers or two .57 kva. For the delta delta, this would likely mean 3 .5 kva due to availability, however I could get by with two .5 kva for the open delta, because my output after voltage drop would be lower than 1kva so the .866 kva total should be fine (going with three .25 kva in a delta delta for .75 kva capacity would be pushing it).
Is anyone aware of any differences in electrical efficiency of delta-delta vs open delta?
 
Smart $ said:
That's the other side of the coin—advantages (OP inquired about the disadvantages only :smile: ).
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I thought it was a two part question. You answered the disadvantage part, I answered the why utilities do it part.
 
jim dungar said:
I thought it was a two part question. You answered the disadvantage part, I answered the why utilities do it part.
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My interpretation of the second part was, "Why do utiliies not do it?"

...either way, they do it (or don't do it) in whichever fashion is most beneficial and least costly to them!
 
electrofelon

The reason one leg of the three phase has worse regulation is that two of the legs are supplied directly by a transformer, the third leg is the open affected by voltage drop thru the other two transformers, instead of just one.

It seriously affects three phase motors started under load, such as submersible pumps. It would have less effect on resistive loads, without inrush.

Jim T
 
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