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 Post subject: 3-Phase Transformer ModelPosted: Wed Jul 28, 2010 8:08 pm

Joined: Wed Jul 21, 2010 7:54 am
Posts: 4
Location: OK
If I have a 300 kVA utility transformer bank made up of 3 single phase 100 kVA transformers each with an impedance of 1.9%, is the equivalent impedance of the 300 kVA transformer simply 3 x 1.9, or is it the square root of 3 times 1.9?

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 Post subject: Posted: Thu Jul 29, 2010 9:51 am
 Arc Level

Joined: Wed Jun 04, 2008 9:17 am
Posts: 428
Location: Spartanburg, South Carolina
RJohnson wrote:
If I have a 300 kVA utility transformer bank made up of 3 single phase 100 kVA transformers each with an impedance of 1.9%, is the equivalent impedance of the 300 kVA transformer simply 3 x 1.9, or is it the square root of 3 times 1.9?

1.9%. The impedance is a per phase value.

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 Post subject: Posted: Fri Jul 30, 2010 8:04 am
 Arc Level

Joined: Tue Jan 13, 2009 5:00 pm
Posts: 579
How are the transformers connected? Wye or delta?

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 Post subject: Posted: Sat Jul 31, 2010 10:42 am

Joined: Wed Jul 21, 2010 7:54 am
Posts: 4
Location: OK
Connected delta-wye.

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 Post subject: Posted: Mon Aug 02, 2010 8:52 am
 Arc Level

Joined: Tue Jan 13, 2009 5:00 pm
Posts: 579
Edited: Sorry I misremembered. Connections make no difference. 1.9% using 100 kVA and line to neutral voltage to get ohms secondary equates to 1.9% using 300 kVA and line to line voltage.

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 Post subject: Posted: Tue Aug 03, 2010 6:38 am

Joined: Fri Jan 30, 2009 10:04 am
Posts: 10
Location: Toronto
RJohnson wrote:
Connected delta-wye.

No matter how the 3 single phase transformer connected, the impedance should be the same as the single phase tested value

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 Post subject: Posted: Tue Jun 07, 2011 12:32 pm

Joined: Mon Apr 13, 2009 11:54 am
Posts: 8
Location: Midland, Tx
What is one of the transformers has a different nameplate impedance?

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 Post subject: Posted: Fri Jun 10, 2011 8:35 am
 Arc Level

Joined: Tue Jan 13, 2009 5:00 pm
Posts: 579
Then there is no equivalent three phase impedance.

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 Post subject: Posted: Sun Jun 12, 2011 6:08 pm

Joined: Fri Jan 30, 2009 10:04 am
Posts: 10
Location: Toronto
Can 3 single phase transformers with different IZ% be connected to a transformer bank?

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 Post subject: Posted: Sun Jun 12, 2011 10:33 pm
 Sparks Level

Joined: Mon Mar 29, 2010 5:02 am
Posts: 136
J.T wrote:
Can 3 single phase transformers with different IZ% be connected to a transformer bank?

It is my understanding that they can be, but they still have to be within a very tight range..

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