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Sensitivity coefficients of various relay protection systems

Fiber and communications infrastructure knowledge brief Published 05 Dec 2020

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The sensitivity coefficient of a relay protection should be greater than 1, typically defined as the ratio of the minimum fault current to the relay's operating current.

Definition and Purpose

The sensitivity coefficient (Ks) of a protective relay is defined as the ratio of the minimum short-circuit current that the relay must detect to the minimum current required for the relay to operate. This coefficient ensures that the relay can reliably detect faults even under low-current conditions, such as remote faults or lightly loaded lines.

Recommended Values

For overcurrent relays, the relay should operate when the fault current exceeds 1. 1 to 1. 3 times the set start current, depending on the relay characteristic (long-time inverse or very/inverse time) and network conditions.

A Ks value greater than 1 ensures that the relay is sensitive enough to detect the minimum fault current while avoiding false tripping during normal load conditions. In transformer protection, sensitivity is adjusted to detect through-faults and inrush currents without unnecessary tripping, often using differential or current-restrained schemes.

Practical Considerations

Selectivity and coordination: The sensitivity must be balanced with selectivity to ensure that only the relay closest to the fault operates first, while backup relays act if the primary relay fails. System conditions: Calculations should consider maximum load current, minimum fault current, and voltage variations to ensure reliable operation under all conditions. Relay type: Different relay types (overcurrent, distance, differential) have specific sensitivity requirements based on their operating principles and the network configuration.

In summary, the sensitivity coefficient should be set above 1, carefully calculated based on the minimum fault current and relay operating current, while considering coordination, selectivity, and system-specific conditions to ensure reliable and safe protection of the power network.

Sensitivity coefficients of various relay protection systems - CJC FIBRE CONSULTANTS [PDF]

Sensitivity coefficients of various relay protection systems

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This reference is intended for preliminary fiber and communications infrastructure research. Optical interface, fiber type, port or core capacity, electrical input and output, enclosure protection, cooling, installation limits and applicable standards must be verified for the specific project.

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