If a power transformer has a star connected primary and a delta connected secondary then the CT connections on its primary and secondary sides should be:

If a power transformer has a star connected primary and a delta connected secondary then the CT connections on its primary and secondary sides should be: Delta and Star respectively.

Correct Option: Option 4

Detailed Explanation:

In power transformers, the configuration of the primary and secondary windings impacts the Current Transformer (CT) connections. When the primary side of a transformer is star-connected and the secondary side is delta-connected, it is crucial to ensure that the CT connections are made appropriately to maintain accuracy in current measurement and protection schemes.

Reasoning for Delta and Star CT Connections:

1. Star-Connected Primary Winding:

• The star connection on the primary winding results in line currents being equal to the phase currents. This simplifies the CT connections on the primary side.
• To accurately measure the line currents in the star connection, the CTs should be connected in a delta configuration. This is done to account for phase displacement and to ensure correct phase relationships for protection and metering purposes.
• In a star connection, the voltage across each phase winding is lower (line voltage/√3), which reduces insulation requirements and makes the system efficient for high-voltage transmission.

2. Delta-Connected Secondary Winding:

• The delta connection on the secondary winding causes the line currents to differ from the phase currents. The relationship between line and phase currents in a delta connection is that the line current is √3 times the phase current.
• To accurately measure the line currents in the delta connection, the CTs should be connected in a star configuration. This is necessary to ensure proper current transformation and phase angle relationships for protection devices like differential relays.
• Delta connections on the secondary side also help in providing a path for the third harmonic currents, which eliminates distortion and ensures stable operation.

3. Phase Displacement Between Star and Delta Connections:

• In a star-delta transformer, there is a 30° phase displacement between the primary and secondary windings. This displacement must be accounted for in CT connections to ensure proper operation of protection systems.
• By connecting CTs in delta on the primary side and star on the secondary side, the phase displacement is effectively compensated, ensuring that the protection systems, such as differential relays, function correctly.

4. Protection Scheme Accuracy:

• One of the primary reasons for choosing specific CT connections is to ensure the accuracy of protection schemes like differential protection.
• If the CTs are not connected in the appropriate configuration, the phase displacement and current magnitude differences between the primary and secondary windings may lead to incorrect operation of protection devices.
• By using delta CT connections on the star side and star CT connections on the delta side, the current magnitudes and phase relationships are matched, ensuring the correct operation of the protection system.

Important Information:

To further analyze the question, let’s evaluate the other options:

Option 1: Delta and Delta respectively

This option is incorrect because using delta CT connections on both the primary (star-connected winding) and the secondary (delta-connected winding) would not correctly account for the phase displacement between the windings. This could lead to inaccuracies in current measurement and protection schemes.

Option 2: Star and Delta respectively

This option is incorrect because using star CT connections on the primary (star-connected winding) would not account for the phase displacement and would result in incorrect current measurements. Additionally, delta CT connections on the secondary (delta-connected winding) would not align with the required phase relationships for protection schemes.

Option 3: Star and Star respectively

This option is incorrect because using star CT connections on both the primary and secondary sides would fail to account for the phase displacement between the star and delta windings. This misalignment would cause protection systems, such as differential relays, to malfunction.

Option 4: Delta and Star respectively

This is the correct option because:

• Delta CT connections on the primary (star-connected winding) ensure proper phase alignment and current transformation.
• Star CT connections on the secondary (delta-connected winding) account for the phase displacement and ensure accurate current measurement and protection system operation.

Option 5: Not provided

This option is invalid as it does not offer a relevant solution to the given problem.

Conclusion:

The correct CT connections for a power transformer with a star-connected primary and a delta-connected secondary are delta on the primary side and star on the secondary side. This configuration ensures proper current transformation, accurate phase alignment, and reliable operation of protection systems. Other options fail to meet these requirements and would result in inaccuracies or protection system malfunctions.