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# Primary Fault Current Question

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## Primary Fault Current Question

Question:
1000kva 34.5kv to 480v secondary transformer 5% imp. Apply 480v to primary side and ground secondary. What is primary fault current?

I don't know what I am doing wrong, I am using the full load amps and fault current equations and I am not arriving at this answer.

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Originally Posted by anderssok09
Question:
1000kva 34.5kv to 480v secondary transformer 5% imp. Apply 480v to primary side and ground secondary. What is primary fault current?

I don't know what I am doing wrong, I am using the full load amps and fault current equations and I am not arriving at this answer.
One way to do it is to convert the impedance percentage back to the actual impedance value to solve with the new voltage.

So from the nameplate SBase = 1000kVA and VBase = 34.5kV, then ZBase = VBase2/SBase
ZBase = 34.5kV2/1000kVA = 1190
Since the impedance is 5%, the actual impedance seen by the high side is
ZBase*5% or 1190*.05 = 59.5 Ohms

Now we just have an impedance and a voltage we are putting across it to find the current V=IR.

So VLN/R or I = (480/1.732)/59.5 Ohms = 4.65A

Another way to do this would be to find the primary side fault current at given levels, and multiply by the ratio of voltage you are applying.

IPrimary Fault @ 34.5 = 1000kVA/(34.5kV*1.732*.05) = 334.71A

334.71A * (480/34500) = 4.65A

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Originally Posted by jrm5116
One way to do it is to convert the impedance percentage back to the actual impedance value to solve with the new voltage.

So from the nameplate SBase = 1000kVA and VBase = 34.5kV, then ZBase = VBase2/SBase
ZBase = 34.5kV2/1000kVA = 1190
Since the impedance is 5%, the actual impedance seen by the high side is
ZBase*5% or 1190*.05 = 59.5 Ohms

Now we just have an impedance and a voltage we are putting across it to find the current V=IR.

So VLN/R or I = (480/1.732)/59.5 Ohms = 4.65A

Another way to do this would be to find the primary side fault current at given levels, and multiply by the ratio of voltage you are applying.

IPrimary Fault @ 34.5 = 1000kVA/(34.5kV*1.732*.05) = 334.71A

334.71A * (480/34500) = 4.65A
Thanks!

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