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What is the relay protection factor

Fiber and communications infrastructure knowledge brief Published 16 Aug 2024

Prepared for preliminary research by the METRA knowledge desk. Confirm optical interfaces, port or core capacity, electrical input and output, enclosure protection, thermal design and installation limits against current standards and product documentation.

The relay protection factor quantifies how effectively a protective relay detects and isolates faults, often expressed through parameters like the Plug Setting Multiplier (PSM) and Multiplying Factor (MF).

Definition and Purpose

The relay protection factor is a measure of a relay's sensitivity and effectiveness in responding to fault currents. It indicates how much the actual fault current exceeds the relay's pickup setting, helping engineers determine the relay's operating speed and selectivity. This factor ensures that only the faulty section of the system is isolated, minimizing disruption to the rest of the network.

Key Parameters Influencing Relay Protection Factor

Plug Setting Multiplier (PSM): Represents how many times the actual current exceeds the relay's pickup current. A higher PSM results in faster relay operation according to the inverse definite minimum time (IDMT) curve. Time Setting Multiplier (TSM): Scales the base operating time derived from the relay's characteristic curve, allowing coordination between upstream and downstream relays.

Lower TS

M values lead to faster tripping. Multiplying Factor (MF): Adjusts the relay's internal measurements to match actual primary system values, compensating for CT/PT ratios and measurement losses.

Role in Protection Coordination

The relay protection factor is critical for selectivity, ensuring that only the closest relay to a fault trips while upstream relays act as backup. Proper calculation prevents overreaching or underreaching, which could either leave faults uncleared or cause unnecessary outages. It also helps in defining the pickup current and operating time for overcurrent and earth-fault relays, ensuring reliable and safe operation of the power system.

Practical Application

In practice, engineers use the relay protection factor to: Determine pickup currents for overcurrent and ground fault relays. Set time-current characteristics for inverse-time relays. Ensure coordination between multiple relays along a feeder or transmission line.

Compensate for CT/PT ratios and system measurement inaccuracies using the MF. By carefully adjusting these parameters, the relay protection factor ensures fast, reliable, and selective fault isolation, maintaining system stability and minimizing service interruptions.

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What is the relay protection factor
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Technical note

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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