Transformer Impedance

We received information about newly planned transformer parameters. It states: “Transformer… with a nominal 5.75% impedance. The impedance per ANSI C57.12.51 is to be within 7.5% of nominal”. I haven’t seen this standard, but I assume that 7.5% is not another value of impedance but rather manufacturing tolerance on the value of nominal (or design) impedance. So, I might have a transformer with real impedance of (5.75 +/-0.075)%. Is that true?


Since it is virtually impossible to determine exactly the impedance theoretically, the measured impedance is commonly a guaranty by the transformer manufacturer within +/- 7.5% of the specify rated impedance.

Short circuit calc. is commonly done before the transformer build. Therefore, it is good practice to reduce the specify impedance in 7.5% to obtain the worst case with the larger SC current.

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