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#128365 - 01/25/03 08:22 PM Short Circuit 1000kva 3 ph 1.6% Impedance 277/480
HouseOfPower Offline
Member

Registered: 01/25/03
Posts: 11
Loc: Houston Texas
I have another question.

On a short circuit current of a 1000 kva, 3 phase transformerbank with a 1.6% impedance operating at 277/480

First I did: 1000 x 1000= 1,000,000 / 277

=3610
3610/ 1.732= 2084
2084/ .016= 130,272.38?

Did I goof up with the impedance? Shoul it be divided by the .016 or 1.6 ?

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#128366 - 01/25/03 10:50 PM Re: Short Circuit 1000kva 3 ph 1.6% Impedance 277/480
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
I came up with an SCA of 83,619.64 Amps at the Transformer (3Ø Bolted Fault, Infinite Primary, 0.9 of the %Z for tolerances).

Here's the formula I used to acheive the resulting 83619.64 amps:

1: Determine the Full-Load Amperes of the Transformer:

I(FLA) = KVA × 1000 ÷ E(L-L) × 1.73
[ "I(FLA)" = Full-Load Amperes, "E(L-L)" = Line-To-Line Voltage ]

2: Find Transformer's Multiplier:

Mpl. = 100 ÷ 0.9 × %Z
[ "Mpl." = Multiplier, "0.9" figures worst-case scenario tolerance of Impedance percentage ]

3: Determine Transformer's Let-Through Current:

I(SCA) = I(FLA) × Mpl.

So using your Transformer's ratings, here's what it looks like:

1: I(FLA) = 1204.2 Amps
( 1000 × 1000 ÷ 480 × 1.73 )

2: Mpl. = 69.44
( 100 ÷ 0.9 × 1.6 )

3: I(SCA) = 83619.644444- Amperes
( 1204.2 × 69.44 )

Check my math and let me know if there are any errors.

Scott35 S.E.T.
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#128367 - 01/26/03 09:06 AM Re: Short Circuit 1000kva 3 ph 1.6% Impedance 277/480
Nick Offline
Member

Registered: 08/13/01
Posts: 603
Loc: Riverside, CA
Scott,
Your math checks out.
Just out of curiousity I ran this through some software programs. Here are the results:
Bussmann-83,531.135
EDR-83,532
This is using your .9 of %Z value.
Pretty close huh?

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#128368 - 01/26/03 09:15 AM Re: Short Circuit 1000kva 3 ph 1.6% Impedance 277/480
HouseOfPower Offline
Member

Registered: 01/25/03
Posts: 11
Loc: Houston Texas
Ok, I wasn't working the problem right. Looks like it is back to the drawing board. thanks

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#128369 - 01/26/03 11:22 AM Re: Short Circuit 1000kva 3 ph 1.6% Impedance 277/480
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Nick,

Closer than I even expected!!!

I ran the numbers by hand! (feel free to make jokes or laugh!).

Scott35 S.E.T.
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#128370 - 01/26/03 06:07 PM Re: Short Circuit 1000kva 3 ph 1.6% Impedance 277/480
Gwz Offline
Member

Registered: 04/29/02
Posts: 199
Given:
KVA = 1000
E = 480V
3Ø = 1.732
% Z = 1.6
.9 = factor if KVA great than 25

using continuous calculator input the formula would be:

KVA * 1000 / 480V / 1.732 = * 100 / 1.6%Z / .9 = SCA

1000*1000/480/1.732=*100/1.6/.9= 83531.135061158155846377555384484 short circuit amperes.

That is by using the calculator in Windows Acessories on the computer programs file.

Just think - - - Could have used 1.732050807568877293527446341115059 for the square root of 3.

Then it would be 83528.684778591690467180089771695 Short Circuit Amperes.

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#128371 - 01/26/03 06:58 PM Re: Short Circuit 1000kva 3 ph 1.6% Impedance 277/480
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
1.6%Z? That is one stiff transformer.

Padmount? Delta-Wye?

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#128372 - 01/26/03 07:09 PM Re: Short Circuit 1000kva 3 ph 1.6% Impedance 277/480
HouseOfPower Offline
Member

Registered: 01/25/03
Posts: 11
Loc: Houston Texas
After the 1.732 you multiply by 100

Where and how do you get that figure?

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#128373 - 01/26/03 07:14 PM Re: Short Circuit 1000kva 3 ph 1.6% Impedance 277/480
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
It's likely that is needed to convert 1.6% into 0.016 per-unit impedance.

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#128374 - 01/27/03 04:33 AM Re: Short Circuit 1000kva 3 ph 1.6% Impedance 277/480
Gwz Offline
Member

Registered: 04/29/02
Posts: 199
That is not the formula.

It is a way to keep using the ( hand ) calculator with-out stopping to enter a previous calculation into the final answer.

Of course the 1.6%Z sounds low for that KVA.

Maybe it's one of the super efficient transformer (super cooled coils) or that ( amorrous steel) [ I know that is not the correct spelling, but it is a special conductor/or/core used for some experimental transformers. It was mentioned in the EC&M magazine about 10 years ago.].

The local PoCo has 50KVA pad mounts with 0.9 %Z.

These 0.9 %Z units are 7.2KV/120/240V 1Ø 3W pad mounts.

[This message has been edited by Gwz (edited 01-27-2003).]

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