For Transformer Differential Protection Devices the empirical dimensioning factor Kd = Kemp for the CTs considering external faults (assuming maximum through-flowing currents) can be taken from the following diagram:

This CT dimensioning assures through fault stability of the differential element. Due to the inbuilt saturation discriminator the CT requirement is independent of the current sensitivity given by the set basic threshold of the tripping characteristic.

The empirical dimensioning factor Kemp (shown in the diagram above) has been determined by investigations using 3-shot auto-reclosing sequences with 450 ms of fault current feed (starting at worst case point on wave) for each shot and 300 ms dead time between shots. In most practical cases faults would be cleared in 100 to 200 ms for external protection operation and the dead time between auto-reclose shots would be longer than 300 ms. This would reduce the flux build-up in the core. Therefore the above shown empirical dimensioning factor Kemp can be considered as being based on a conservative approach.

For internal fault steady-state saturation is permissible with maximum fault currents up to 4 times the steady-state accuracy limit current of the CT. This corresponds to a dimensioning factor of Kd = 0.25 for internal faults.

It is recommended to use CTs of accuracy class 5P (or equivalent).