Open Delta System: Finding kVA Rating of V-V Bank

[ranju]

While studying a numerical on open delta system .., we have to find total kVA rating of V-V bank so obviously it should be equal to = root 3 *(kVA rating per transformer in V-V bank)..
But in the solution , its is taken as =root 3 *(kVA rating per transformer in delta-delta )..!
So , isn't it a mistake in the textbook..??

 

[jim hardy]

a picture is worth a thousand words
and several turned up from a searchhttp://forums.mikeholt.com/showthread.php?t=124662&page=4

If one has to determine sizes, first let's explain the why the cores are oversized. In the following example we have a typical 3Ø 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.

and this link has a paragraph that makes things pretty clear
http://www.purduecal.edu/cpmi/NSF%20Courses/ECET-212/CLASSPRESENTATION/Three-phase-Transformers.pdf
page 12 of 17

Open Delta Connection
•
The open delta transformer connection can be made with only two instead of three
transformers as shown in figure- 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.

that should help you figure out what your textbook's author did, and whether his presentation is wrong or just somehow mis-leading.

old jim

 

[ranju]

that should help you figure out what your textbook's author did,

but the kVA ratings of the two differs so how can we use kVA rating of delta-delta for V-V connection??

 

[jim hardy]

but the kVA ratings of the two differs so how can we use kVA rating of delta-delta for V-V connection??

Without knowing what your author wrote, that's impossible to answer.

The above examples make it clear how to figure the kva each transformer has to handle in each case,
so i don't see what more could one need to completely describe the requirements for a particular connection?

Are you asking about transformers, or are you asking about what somebody (who i don't know) wrote in some book that I've never seen ?

 

[ranju]

Without knowing what your author wrote, that's impossible to answer.

the author simply wrote that to find total kVA rating of V-V bank its is taken as =root 3 *(kVA rating per transformer in delta-delta )

 

[jim hardy]

the author simply wrote that to find total kVA rating of V-V bank its is taken as =root 3 *(kVA rating per transformer in delta-delta )

hmmmmObserve in that Mike Holt link referenced up there in post #2
where he starts with three 1 KVA transformers serving a 3 KVA load

that with Δ-Δ you can use the total of all 3 transformers, 3KVA

If one has to determine sizes, first let's explain the why the cores are oversized. In the following example we have a typical 3Ø 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.

But with open delta, to supply a 3 kva load required two 1.732 KVA transformers...as he showed.

What if instead of upsizing the transformers to supply the full 3 KVA , he just downsized the load to match what the two remaining transformers can deliver?
How many KVA could he get out of two 1 KVA transformers connected open Δ ?

Hmm.
Now his two transformers can only supply ten amps to the lines
so all three line currents would have to be reduced to 10 amps..
KVA three phase still equals Vline X I line X √3, and that = 100 volts X 10 amps X √3 = 1.732 KVA
which looks to me like √3 X transformer rating.

So i can see nothing wrong with the statement you cited.
With three transformers in Δ-Δ you get 3X transformer rating
but with two transformers open Δ you don't get 2X transformer rating you get √3 X transformer rating.

1/√3 is 0.577, which rounds to 58% , the number in that Purdue link.

Looks consistent to me.