Answer:

To protect the power transformer against faults, current transformers (CTs) are used. CTs are used to measure the current flowing through the transformer and provide this information for protection and monitoring purposes. The correct type of connection for CTs in a Y-Y power transformer with a neutral grounded is the D-D connection.

Explanation:

The D-D connection refers to the connection of CTs in the delta winding of the power transformer. In this connection, the secondary winding of the CT is connected in delta configuration.

Reasons for using D-D connection:

1. Primary Fault Current: The D-D connection provides accurate measurement of the primary fault current. This is important for protection purposes as it allows for quick and accurate fault detection and isolation.

2. Zero-Sequence Current: The D-D connection allows for the measurement of zero-sequence currents. Zero-sequence currents are the unbalanced currents in the system that flow in the neutral conductor. By measuring these currents, faults such as earth faults can be detected and isolated.

3. Ground Fault Detection: The D-D connection is especially important for detecting ground faults. In a Y-Y power transformer with a neutral grounded, ground faults can occur in the primary or secondary windings. By using CTs in the D-D connection, these ground faults can be accurately detected and isolated.

4. Vector Group: The D-D connection is in line with the vector group of the transformer. The vector group specifies the phase relationship between the primary and secondary windings. By using the D-D connection, the CTs are correctly aligned with the vector group, ensuring accurate measurement and protection.

In summary, the correct type of connection for CTs in a Y-Y power transformer with a neutral grounded is the D-D connection. This connection allows for accurate measurement of primary fault currents, detection of zero-sequence currents, detection of ground faults, and alignment with the vector group of the transformer.