Sensitivity verification ensures that a relay protection device reliably detects internal faults within the protected zone while remaining stable for external faults.

Purpose of Sensitivity Verification

Sensitivity verification is performed to confirm that a relay protection device operates correctly for internal defects in the protected zone and does not respond to external faults. This is crucial for maintaining system reliability, safety, and selectivity in power networks .

Testing Principles

Relay protection, especially differential protection, relies on the current balancing principle between current transformers (CTs) at the boundaries of the protected zone. Sensitivity tests check whether the relay can detect minimal fault currents and operate as intended .

Equipment Required

• Primary Injection Test Kit: Injects large currents directly into CT circuits to simulate real fault conditions.
• Secondary Injection Test Kit: Simulates relay inputs with controlled currents and voltages.
• Clamp Meter: Measures current non-intrusively.
• Digital Multimeter: Measures voltage, resistance, and continuity .

Step-by-Step Sensitivity Test Procedure

1. Connect the CTs: Inject primary current via one set of CTs with one current flowing inward and the other outward. A zero differential current at the relay terminals indicates proper CT wiring and stability.
2. Reverse Polarity Test: Reverse the polarity of one CT and inject primary current through only one CT. The relay should operate, confirming sensitivity.
3. Determine Minimal Operating Current: Gradually increase the injected primary current until the relay operates. Record the minimal current required for operation.
4. High-Impedance Differential Relays: Apply test voltage to one transformer winding while short-circuiting the opposite winding, ensuring the relay responds correctly to internal faults .

Verification Cycle and Maintenance

• For high-reliability systems (60 kV and above), relay protection devices should be verified annually.
• For lower voltage systems (10 kV), calibration is typically performed every two years.
• Regular verification ensures correct operation, intact circuits, and accurate settings, reducing maintenance costs and improving system safety .

Benefits

• Confirms proper relay operation for internal faults.
• Ensures stability against external faults.
• Supports safe and economic operation of the power system.
• Provides reliable data for maintenance and commissioning decisions . Sensitivity verification is a critical part of relay protection testing, ensuring that protective devices respond accurately to faults, maintain system stability, and safeguard both equipment and personnel.