# Open delta system

1. May 9, 2015

### 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..??

2. May 10, 2015

### jim hardy

a picture is worth a thousand words
and several turned up from a search

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 [Broken]
page 12 of 17

old jim

Last edited by a moderator: May 7, 2017
3. May 11, 2015

### ranju

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

4. May 11, 2015

### jim hardy

The above examples make it clear how to figure the kva each transformer has to handle in each case,
so i dont 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 dont know) wrote in some book that i've never seen ?

5. May 11, 2015

### ranju

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 )

6. May 12, 2015

### jim hardy

hmmmm

Observe 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
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.

Last edited: May 12, 2015