Interrupting Current and Transformer Impedance with OA and FA rating

[kewo]

What is the interrupting current rating required for a circuit breaker to protect this transformer?

Single phase transformer:
OA, 55C: 8333 kVA
OA, 65C: 9333 kVA
FA, 55C: 10000 kVA
%Impedance: 10.2% (at 75C, OA Base)

Is it:
1. FLA = 8333kV / 5.04kV = 1653A; IC = 1653A/.102 = 16206A,
2. FLA = 9333kV / 5.04kV = 1852A; IC = 1852A/.102 = 18157A, or
3. FLA = 10000kV / 5.04kV = 1984A; IC = 1984A/.102 = 19451A?

The %Impedance does say "75C, OA Base" so would IC be either (1) or (2)?

How is the 75C taken into account? Would the impedance actually be lower, thus a higher IC, since the ratings are listed at a lower temperature (55C and 65C instead of 75C)?

Thanks.

 

[dpc]

What is the interrupting current rating required for a circuit breaker to protect this transformer?

If you are trying to protect this transformer, the circuit breaker would be on the primary side of the transformer, not the secondary. So the short circuit current seen by this breaker would be a function of the upstream available fault current, not the transformer impedance.

What you are calculating is the short circuit current on the secondary side of the transformer.

You would use the OA rating, which is 8333 kVA to compute the fault current on the secondary side, since that is what the % impedance is based on.

The temperature only effects the resistance, so there is very little impact on the short circuit current since the transformer impedance is mainly inductive.

 

[kewo]

dpc,

Thank you for the response. You're correct, what I mean to protect is from a fault on a bus after the transformer and circuit breaker.

The %impedance does say OA, but there are two: 8333kVA and 9333kVA. Why not use the 9333kVA rating?

 

[dpc]

If this transformer was made in the US or made to ANSI standards, the impedance is based on the lowest self-cooled rating. Sometimes the kVA base is more clearly stated on the nameplate, but if not, I would assume the lowest rating.

 

[kewo]

I'll check on that. Thank you!

 

[prc]

If the trf is from IEC world,the rated impedance will be on the maximum rating.Also please check impedance at extreme tappings.Many times it may be much below the impedance at rated tap.In that case you should use the lowest impedance to find out the maximum SC on LV side.