Relay protection sensitivity is verified by testing whether the relay reliably operates for internal faults while remaining stable under normal or through-fault conditions.

Overview of Sensitivity Verification

The sensitivity of a protective relay ensures it can detect internal faults within the protected zone without being triggered by external disturbances. Verification involves injecting controlled currents into the relay and monitoring its response to confirm it operates at the minimum required fault current and correctly discriminates between internal and external faults .

Step-by-Step Procedure

1. Prepare Test Equipment
   • Primary Injection Test Kit: Injects large currents directly into CT circuits.
   • Secondary Injection Test Kit: Simulates relay inputs with controlled currents and voltages.
   • Ensure all CTs and relay terminals are correctly connected and polarity is verified .
2. Stability Test
   • Inject primary current through one set of CTs with one current directed inward and the other outward.
   • At the relay terminal, no operational current should be observed, confirming stability under through-fault conditions .
3. Sensitivity Test
   • Reverse the polarity of one CT secondary or inject current into only one CT.
   • Gradually increase the primary current until the relay operates.
   • Record the minimum primary current required to trigger the relay, which indicates the relay's sensitivity .
4. Differential Relay Checks
   • Measure CT secondary currents and differential current at the relay.
   • A zero differential current under normal conditions confirms proper CT wiring and stability.
   • When testing sensitivity, a small differential current should trigger the relay, confirming it can detect internal faults .
5. Repeat for All Phases and Tap Settings
   • Perform tests for minimum and maximum tap settings of transformers.
   • Ensure the relay responds correctly across all operational conditions .
6. Documentation
   • Record all measurements on the test form.
   • Verify that all readings fall within manufacturer-specified tolerance limits .

Additional Considerations

• High-impedance differential relays require careful attention to CT connections and bias settings.
• Pilot wire protection requires end-to-end testing to ensure proper relay operation and communication between both ends of the feeder .
• Follow manufacturer guidelines and relevant standards such as IEC 60255 and IEEE C37 series for accurate testing . By following these procedures, engineers can confirm that the relay is sufficiently sensitive to internal faults, stable under external disturbances, and correctly wired, ensuring reliable protection of the electrical system .