What is CT and PT test?
Current Transformer (CT) and Potential Transformer (PT) Testing
Current transformers (CTs) and potential transformers (PTs), also known as voltage transformers (VTs), are essential components in electrical power systems. They are used to step down high currents and voltages to safer, measurable levels for metering, protection, and control systems. Testing these transformers is crucial to ensure they function correctly and reliably. Here’s an overview of what CT and PT testing involves:
Current Transformer (CT) Testing
Current transformers are tested to ensure they accurately measure the current in a power system and provide proportional secondary current for metering and protection devices.
1. Ratio Test:
- Purpose: Verify the accuracy of the transformation ratio.
- Method: Apply a known current to the primary winding and measure the output current from the secondary winding. The ratio of these currents is compared to the CT’s rated ratio.
2. Polarity Test:
- Purpose: Confirm the correct polarity of the CT connections.
- Method: Apply a small AC or DC voltage and check the phase relationship between the primary and secondary windings.
3. Excitation or Saturation Test:
- Purpose: Determine the excitation characteristics and the saturation point of the CT.
- Method: Gradually increase the voltage applied to the secondary winding and measure the corresponding current. This test identifies the knee point voltage, beyond which the CT saturates.
4. Burden Test:
- Purpose: Ensure the CT can drive the connected burden (load) without significant accuracy loss.
- Method: Connect the CT to its rated burden and measure the secondary current to ensure it is within acceptable limits.
5. Insulation Resistance Test:
- Purpose: Check the insulation quality between the windings and the core.
- Method: Use a megohmmeter to measure the resistance between the windings and between the windings and the core.
6. Phase Angle Test:
- Purpose: Measure the phase displacement between the primary and secondary currents.
- Method: Use a phase angle meter to verify that the CT maintains the correct phase relationship.
Potential Transformer (PT) Testing
Potential transformers are tested to ensure they accurately step down high voltages to safe levels for metering and protection devices.
1. Ratio Test:
- Purpose: Verify the accuracy of the transformation ratio.
- Method: Apply a known voltage to the primary winding and measure the output voltage from the secondary winding. The ratio of these voltages is compared to the PT’s rated ratio.
2. Polarity Test:
- Purpose: Confirm the correct polarity of the PT connections.
- Method: Apply a small AC voltage and check the phase relationship between the primary and secondary windings.
3. Excitation Test:
- Purpose: Determine the excitation characteristics and ensure the PT operates correctly under different voltage conditions.
- Method: Apply varying voltage levels to the primary winding and measure the corresponding secondary current.
4. Burden Test:
- Purpose: Ensure the PT can drive the connected burden without significant accuracy loss.
- Method: Connect the PT to its rated burden and measure the secondary voltage to ensure it is within acceptable limits.
5. Insulation Resistance Test:
- Purpose: Check the insulation quality between the windings and the core.
- Method: Use a megohmmeter to measure the resistance between the windings and between the windings and the core.
6. Phase Angle Test:
- Purpose: Measure the phase displacement between the primary and secondary voltages.
- Method: Use a phase angle meter to verify that the PT maintains the correct phase relationship.
Conclusion
CT and PT testing ensures the accurate performance of these transformers, which is critical for the safety, reliability, and efficiency of electrical power systems. Proper testing can prevent measurement errors, protection malfunctions, and potential damage to equipment, thereby maintaining the overall integrity of the power system.
Aug 01,2024
