When we say that differential protection CTs on star connected transformer winding are connected in delta, does this mean that CT's primary winding is connected in star while its secondary winding is connected in delta?

Also in case of differential protection, is CT's primary winding always connected in star?

 

There is always a phase displacement between the primary and secondary winding currents (aproximately 30 degrees), for differential protection we need to balance these phase displacements. For that, we have to interchange the CT connections.

 

Hi,
Let us assume a power transformer having Delta Star configuration. This means that the primary (HV) winding is Delta connected & the secondary (LV) winding Star connected. As already mentioned by Kannan, the connection of the CT connected to the Differential relay has to be connected in reverse manner (i.e. HV side is star fashion & LV side in delta fashion). This is also the reply to your second question.