Transformer Damage curves

[luis7006]

Hello,

I am in doubt about the correct ANSI factor to determine
the damage curve of a transformer.
According to the well know procedure, it is said that if the transformer windings are delta-delta a factor of 0.87 has to be considered for all current values.
In the same way, if delta- wye grounded transformer, then, the factor is 0.58.

My question is:
What about delta -high resistance wye grounded and delta- low resistance grounded wye??
The same factor of 0.58 has to be used?

I have been thinking about it and if it is a high resistance grounding then the fault current is almost zero and the performance would be the same as delta-ungrounded wye, then the factor would have to be 1, isnt it??
I will thank your opinions

 

[dpc]

There's no 0.87 factor for delta-delta transformer. For a through fault, both primary and secondary windings see the same per unit current.

The 0.87 factor is used to determine relay coordination between primary and secondary protection for phase-to-phase through faults on the secondary side.

For the delta-wye transformers, for resistance grounded transformers, you don't really need to shift the damage curve to check primary protection. It is a moot point because the ground fault current can never reach the transformer damage curve.