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%IMPEDANCE CALCULATION OF TRANSFORMER

%IMPEDANCE CALCULATION OF TRANSFORMER

%IMPEDANCE CALCULATION OF TRANSFORMER

(OP)
Dear Sir

Please let us know what is the relation between %impedance ,BIL & short circuit current of the transformer

RE: %IMPEDANCE CALCULATION OF TRANSFORMER

No relationship involving BIL.  Inverse relationship between impedance and short circuit current.

RE: %IMPEDANCE CALCULATION OF TRANSFORMER

(OP)
Thank you for your response.

i want to know the relation between impedance and short circuit current.

if possible kindly send me the formula

RE: %IMPEDANCE CALCULATION OF TRANSFORMER

Short Circuit Current = (Transformer Full Load Amps)/Zpu.

Where:

Tx FLA = the full amps at the base transformer rating, and

Zpu = per unit impedance of the tranformer= 100 x %Z.

Example: short circuit through a transformer with 10%Z will be 10 times its full load amps, assuming it is fed by an infinite source.

RE: %IMPEDANCE CALCULATION OF TRANSFORMER

Testing and rating is done with the transformer at operating temperature. A cold transformer will test with a slightly lower impedance. With small dry type transformers (less than 10 KVA) the difference is noticeable.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: %IMPEDANCE CALCULATION OF TRANSFORMER

Quote (rcwilson):

assuming it is fed by an infinite source
Very common assumption, very uncommon source.

If you're talking big transformers at high transmission voltages it can be a reasonable assumption.  If you're talking small transformers connected into the distribution system it may not be such a good assumption.

RE: %IMPEDANCE CALCULATION OF TRANSFORMER

also Zpu = %Z/100  

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla
 

RE: %IMPEDANCE CALCULATION OF TRANSFORMER

Quote by DB
"If you're talking big transformers at high transmission voltages it can be a reasonable assumption.  If you're talking small transformers connected into the distribution system it may not be such a good assumption. "

True but many utilities will use this when giving a fault current value to a customer.

RE: %IMPEDANCE CALCULATION OF TRANSFORMER

"also Zpu = %Z/100 "

 Thanks for the correction.

Isc = FLA /Zpu = (FLA x 100%)/%Z

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