Which type of connection is employed for current transformers for the protection of delta- star connected 3-phase transformer?

• A. star-star
• B. delta-star
• C. delta-delta
• D. star-delta

Answer: (D)

The connection of CT on primary and secondary side is always reverse to the connection of transformer.
To protect the power transformer with star-delta against fault, the type of connection of CTs should be in the delta–star connection otherwise the relay will operate when an external fault occurs due to zero sequence currents