We have 2 - 2MVA, 22/0.433kV Xfrmrs feeding two 415V LV Switchboards and in turn various LV loads. The switchboards have a bus-tie to enable the LV loads to be powered even if one supply/Xfrmr fails. The LV Switchboards incomers have a REF relay. The phase CTs are 3000/5A. I understand that the all the CTs should have the same ratio as the currents need to balance out during normal operation. Mismatched CTs will cause 'Spill Current' to flow through the relay path.

Your comments!

 

You are absolutely correct. The star point of the transformer also should have the Diff CT with the same CT ratio or else any "out of the zone" ground fault can cause unbalance in the summation currents flowing in the CT's, the currents may spill over and activate the relay if above the setting thresholds. However it should be made sure that CLASS PS CT's are used for REF protection and wired with proper polarity checks before commissioning.