Sensitivity in relay protection refers to the ability of a protective relay to detect and respond to the smallest fault current within its designated protection zone.

Definition and Importance

Sensitivity is a critical parameter in relay protection that ensures a relay can detect internal faults, even when the fault current is low due to high impedance or distance from the relay . A highly sensitive relay can operate correctly for minimal fault currents, preventing equipment damage and maintaining system stability . Insufficient sensitivity may result in undetected faults, leading to prolonged outages or cascading failures in the power system .

Factors Affecting Sensitivity

1. Relay Type and Settings: Differential relays, overcurrent relays, and distance relays have different sensitivity requirements. Proper calculation of current and voltage thresholds ensures the relay responds to all internal faults without false tripping .
2. Current Transformer (CT) Accuracy: CTs must accurately reproduce primary currents at the relay terminals. Incorrect CT connections or polarity can reduce sensitivity .
3. Fault Location and Impedance: Faults far from the relay or with high resistance may produce low currents. Sensitivity must be sufficient to detect these conditions .
4. System Coordination: Sensitivity must be balanced with selectivity and dependability to avoid unnecessary tripping of upstream or downstream relays .

Testing Sensitivity

Relay sensitivity is verified through primary and secondary injection tests:

• Primary Injection Test: Large currents are injected into CT circuits to simulate real fault conditions. The minimal current required to operate the relay is determined .
• Secondary Injection Test: Controlled currents and voltages are applied directly to the relay inputs to confirm proper operation and sensitivity .
• Differential Relay Testing: Currents are injected with reversed CT polarity to ensure the relay responds only to internal faults and not to through-faults, confirming both sensitivity and stability .

Practical Considerations

• High-Voltage Systems: In HV substations, sensitivity must be high enough to detect low-level faults without compromising selectivity or security .
• Trade-offs: Increasing sensitivity may reduce security or selectivity. Modern microprocessor-based relays allow simultaneous high sensitivity, speed, and reliability, but careful design and coordination are still required .
• Documentation and Calibration: All sensitivity tests and settings should be recorded and periodically verified to maintain protection reliability .

Summary

Sensitivity ensures that protective relays detect all internal faults within their zone, even under low-current conditions, while maintaining system stability and minimizing unnecessary outages. Proper calculation, CT accuracy, testing, and coordination are essential to achieve optimal sensitivity in HV and MV substations.