IEEE C57.12; Transformer Fault Testing

[fluxcap]

IEEE C57.12 includes single phase to ground, two phase to ground and three phase faults as withstand tests/design criteria for liquid filled transformers.

Is it logical to assume that a transformer should withstand a three phase TO GROUND thru fault on the secondary as well. This is sometimes (and perhaps too often) applied when a system is energized into a set of safety grounds or a grounded three phase switch.

This is assuming that the transformer protection is set properly to clear within the damage curve timing(durations of 2 sec for cat II, III, IV of the C57 spec).

I have reviewed the positive and zero sequence models and can see no (electrical) theoretical difference between the three phase to ground and a straight three phase fault (on the wye secondary system). Anything going on electro-mechanically that I am neglecting? or should we expect a transformer to hold up to this type of thru fault?

Thanks in advance.

 

[stevenal]

If all poles close simultaneously there is no difference. In reality, pole closures are somewhat staggered, so the fault evolves from SLG to DLG to 3phase.