Differential Protection Of Transformer

Could you show me the star-star CT connection scheme for this transformer protection?

What phase shift are you talking about??

Ynyn0 transformer connection does NOT have a phase shift.

As far as I know, the CT configuration depends on the transformer configuration.

Numerical Relays can compensate for the phase angle difference by the setting and does not require CT secondary circuit arrangement such as delta-connection on the star-connected side of the power transformer which was common for the former transformer protection.

Setting for phase angle matching

The phase angle differences between line currents on each side of the power transformer are corrected by setting according to the hands of a clock as follows:
Rule 1:
If all the windings are star-connected, then take one of the windings as a reference winding and set 1 (= one o'clock) for it. For other winding(s), set the phase angle difference from the reference winding by the expression of the leading angle. One hour corresponds to leading by thirty degrees.

For YNyn0 transformer,i.e.,an autotransformer there will always be a Tertiary winding in delta formation to suppress the flow of zero sequence current due earth fault on secondary side( down - stream) from flowing to the up-stream side.[p]

Therefore to eliminate the differential protection from maloperation during through fault condition ( earth fault ), it becomes necessary to eliminate the zero sequence current from comparison.This is achieved by connecting the CT secondaries by delta connection[p]

As commented by experts previously, this has to be done manually for Electro mechanical and static relays, whereas the same can be carried out by suitably selecting the menu in numerical relays.[p]

Any way the selection of Delta is mandatory requirement for transformers of YNyn0 vector group with tertiary winding with delta connection.