The selected current ratio of CTs in each side of power transformer is proportional to the rated current of the same side; Also CTs secondary interconnections are in opposite of power transformer interconnections. (E.g. star in primary and delta in secondary side of power transformer). The differential protective relay is static base and low impedance type.
According to the technical recommendations, all secondary protection circuits (containing: CTs, star point of relay current elements,…) shall be grounded in one point.
But here there is a problem. Studies and observations has shown that protection system sometimes is unstable in the sever earth faults which are out of zone.
A young and clever electrical engineer who works in the consultant technical office has studied this problem and believes that the way of secondary circuits grounding (grounding of star point of relay current elements) is the reason of that mentioned instability. What do you think?
How can you explain the reason of that mentioned differential protection instability?