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Principle of Four-Five Relay Protection

Fiber and communications infrastructure knowledge brief Published 09 Oct 2023

Prepared for preliminary research by the ALFA PRO 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.

Four-Five relay protection is a time-coordinated overcurrent protection scheme that isolates faults selectively by using a combination of instantaneous and time-delayed relay settings.

Overview

The Four-

Five relay protection scheme is primarily used in overcurrent protection of power systems, especially in distribution networks. Its main goal is to quickly detect and isolate faults while ensuring that only the affected section of the network is disconnected, maintaining system stability and continuity of supply.

Operating Principle

Time Coordination: The scheme uses a combination of instantaneous (4) and time-delayed (5) elements. The "4" element typically represents a definite-time overcurrent relay, which operates immediately when the current exceeds a preset threshold. The "5" element represents a time-overcurrent relay, which introduces a delay to allow upstream or downstream relays to operate first if necessary.

Selective Fault Isolation: By coordinating the time settings of multiple relays along a feeder or bus, the Four-Five scheme ensures that only the relay closest to the fault trips first, preventing unnecessary disconnection of healthy parts of the network. Current Measurement: Relays measure current through current transformers (CTs) installed at strategic points. The measured current is compared against the relay settings.

If the current exceeds the threshold, the relay initiates a trip signal to the circuit breaker. Backup Protection: The time-delayed element acts as a backup. If the primary relay fails to operate, the delayed relay ensures that the fault is eventually cleared, maintaining system safety.

Applications

Distribution Feeders: Protects feeders from short circuits and overloads while maintaining selectivity. Transformers and Substations: Provides coordinated protection for transformers and bus sections. Industrial Systems: Ensures minimal disruption in complex industrial power networks by isolating only the faulty section.

Key Advantages

Speed and Reliability: Immediate response to severe faults via the instantaneous element. Selectivity: Only the affected section is disconnected, preventing widespread outages. Coordination: Time-delayed elements allow upstream and downstream relays to operate in a coordinated manner.

Backup Safety: Ensures fault clearance even if the primary relay fails. In summary, the Four-Five relay protection scheme combines instantaneous and time-delayed overcurrent relays to achieve fast, selective, and reliable fault isolation, making it a widely used method in power system protection.

Principle of Four-Five Relay Protection - ALFA PRO COMMUNICATION SYSTEMS (Pty) Ltd [PDF]

Principle of Four-Five Relay Protection

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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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