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Transformer Fault Current and C57.12

Transformer Fault Current and C57.12

Transformer Fault Current and C57.12

(OP)
Good afternoon folks,

I have some questions regarding fault currents and calculating these fault currents for oil-filled transformers:

  1. When calculating the fault current [full load current / impedance of transformer], do we use full load current at base rating or the full rating with cooling fans running?

  2. Are limitations of fault current that a transformer can withstand according to C57.12.00 (Section 7) based on external faults only?

  3. i.e. full load current / impedance = fault current transformer can withstand for external faults only?
    i.e. if a transformer winding failed, its probably going to generate fault currents higher than this FLA/impedance value depending on utility contribution and where the fault is in the windings?

RE: Transformer Fault Current and C57.12

In North America, where IEEE/ANSI standards are used, use the base rating (ONAN). I think that IEC uses the maximum rating.

Quote (IEEE Std 57.12.00 7.1.1)

Liquid-filled transformers shall be designed and constructed to withstand the mechanical and thermal stresses produced by external short circuits under the conditions specified in 7.1.3, 7.1.4, and 7.1.5.

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