Relay protection must be sufficiently sensitive to reliably detect all internal faults while avoiding operation for external or normal load conditions.

Key Principles of Sensitivity

Sensitivity in relay protection refers to the ability of a relay to detect the smallest fault current within its designated protection zone. A relay must operate reliably under actual conditions that produce the least operating tendency, ensuring that even low-magnitude faults are detected without delay or failure . Sensitivity is critical for:

• Detecting faults at the far end of feeders or lines where fault currents are reduced due to line impedance .
• Ensuring backup protection operates if the primary relay fails, which requires multi-stage protection with graded sensitivity .
• Maintaining system stability by isolating only the faulted section and preventing unnecessary outages .

Sensitivity Requirements in Practice

1. Minimum Fault Current Detection: Relays must be set to detect fault currents above the lowest expected fault level in the protected zone. For example, a feeder relay may need to detect currents as low as 150 A to ensure coverage along the entire line .
2. Internal Fault Verification: Differential or unit protection relays are tested to confirm they respond to internal faults while remaining stable for external faults. This involves injecting primary currents and observing relay operation .
3. Coordination with Other Relays: Sensitivity must be balanced with selectivity to ensure that only the closest relay to the fault operates first, while upstream relays act as backup .
4. Testing and Commissioning: Sensitivity tests involve gradually increasing injected currents until the relay operates, recording the minimum current required for reliable operation. This ensures the relay meets its design sensitivity under real conditions .

Additional Considerations

• Speed and Reliability: High sensitivity must not compromise the relay's speed or reliability. Relays should operate promptly for faults but avoid false trips due to load currents or transient conditions .
• Multi-Stage Protection: Using multiple relays with different sensitivity settings allows for both primary and backup protection, ensuring comprehensive coverage and system stability .
• Device Type and Characteristics: Sensitivity requirements vary depending on relay type (current, voltage, impedance, differential) and operating characteristics (definite time, inverse time, or multifunctional numerical relays), .

In summary, relay protection sensitivity requirements ensure that all internal faults are detected reliably, even at low fault currents, while maintaining selectivity, speed, and system stability. Proper testing, coordination, and multi-stage protection are essential to meet these requirements.