CT Connection for Differential Relay

Are you idle & just speculating? What is your real problem, not imaginary one?

What will happen? well, without additional circuitry tied on downstream, the insulation on the CT wires runs the risk of breaking down. It's a bit like asking what happens when a train passes a red signal.

The CTs need to be connected per the requirements of the particular differential relay(s) you are using. Consult the instruction manual to determine what they want.

The older style electromechanical and solid-state numerical differential relays required the CTs to be connected in wye configuration on a delta winding of the power transformer and delta-connected on a wye winding. This cancelled out the phase shift inherent in a delta-wye transformer's primary to secondary waveforms. If you connect the CTs as you have described (delta to delta and wye to wye), you will trip the differential relay very quickly as load is applied to the transformer.

Modern microprocessor-based relays do an analog to digital conversion of the currents, so they can compensate for the phase shift mathematically. Those relays typically want you to connect all the CTs in wye configuration, and then you tell the relay what the transformer's winding configuration is (or it may even detect the phase shift and compensate automatically).

Thank you very much

Note also that if your relays require a delta CT connection, the order of the delta (the order of the connection of the delta - which phase polarity end connects to which phase's opposite end) matters. Again, the manufacturer's instruction manual will guide you.