Differential Protection Tranformer

First of all, the GE C60 is a Breaker protection relay - it doesn't have differential functions. Are you sure this is what you have, or a T60 Transformer protection relay?

Next, what differential function tripped it, instantaneous or percent differential? Did someone qualified develop the relay settings, including harmonic restraint to ride through the inrush, etc.? Did you enter the correct transformer winding phase compensation so that it accounts for the phase angle difference between primary and secondary windings? Are the CTs wired with the correct polarity, per the manufacturer's instructions? Are the correct CT ratios programmed? This are a few of the items that can cause a false trip on a differential relay. You have some investigation to do!