Transformer Fault Current and C57.12

[mbk2k3]

Good afternoon folks,

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

[ol 1]
[li]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?

[/li]

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

 

[jghrist]

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

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.