Interposing current-transformer (CT) is used in differential protection of a transformer to

• A. match the standard ratio of CTs used on both sides of the transformer winding
• B. minimize the effect of saturation of the main CTs
• C. increase the sensitivity of the relay
• D. limit the main CT secondary current

Answer: (A)

The main function of an interposing CT is to balance the currents supplied to the relay where there would be an imbalance due to the ratios of the main CTs.
Interposing CTs are equipment with a ratio that can be selected by the user to achieve the balance required.
An interposing CT is installed between the secondary winding of the main CT and relay.