instantenous over current of power transformer

There is no simple answer for this question. For the proper protection of the transformer, you first need to know the transformer damage curve and the inrush current information. Once you have this information, do the protection coordination and find the appropriate multiples of setting. The instantaneous multiplier setting shall be a number so that the transformer is protected for short circuit current but it is not tripped due to the inrush current during the initial energize.

For better understanding about the transformer relay setting, I would suggest you to open the document from the link below and read the Transformer section, page 15 to 22, especially the figures in page 21 and 22. The vertical portion of the curve represents the instantaneous protection. This curve shifts to right as the setting multiples are increased. The protection curve should remain at the right side of the inrush current point of the transformer (shown as X in the figure) and left side to the downward extension of the damage curve.

http://epowerengineering.com/guides-part-5.pdf

This document is a very good one for protection coordination for the electrical devices.

- MS