* Question

What is the allowed signal?

* Answer

1. Basic Concept

In power systems and protective relaying, an allowed signal generally refers to a signal that the system recognizes as a valid trigger for protective action. In the original context, the high-frequency signal is the typical allowed signal:

Receiving a high-frequency signal is a necessary condition for a protection trip. Without it, the protection will not operate.

At the same time, receiving a high-frequency signal is also a sufficient condition to activate protection tripping, meaning that once the signal is present, the protection system is permitted to act.

In other words, the allowed signal defines the logical condition under which the protection relay is authorized to trip.

2. Why a High-Frequency Signal

Traditionally, power line carrier systems used high-frequency signals transmitted over power lines to coordinate protection between substations.

High-frequency communication is fast, reliable, and resistant to noise.

It provides a clear, dependable basis for determining whether a fault exists.

By combining the high-frequency signal with relay logic, the system ensures that only valid conditions trigger a trip, reducing the chance of false operations.

3. Evolution with Optical Fiber

With the widespread adoption of optical fiber in modern power systems, signal transmission methods have expanded:

Optical fiber supports higher bandwidth and lower latency compared to traditional carrier channels.

It is less vulnerable to electromagnetic interference.

Fiber links allow not only high-frequency “allowed signals” but also digital messages (for example, IEC 61850 GOOSE messages) to be exchanged directly between protective relays.

This transition has greatly improved the speed, reliability, and flexibility of protection communication systems.

Summary

An allowed signal in protective relaying is the system-recognized condition that permits tripping. Historically, a high-frequency signal transmitted over power lines served this role—both as a necessary and sufficient condition for protective action. With the rise of optical fiber, the transmission of allowed signals has become faster, more robust, and more versatile, supporting both analog high-frequency and modern digital communications.