Application of audio-tone systems for protective relaying can be divided into two categories:

Transformer and Circuit Breaker Failure Protection or Other Direct Trip Applications
These applications are termed “direct transfer tripping.” The audio-tone system functions as the communication link to extend relay tripping circuits to remote circuit breaker locations.

These type of direct transfer tripping applications have the greatest difficulty in meeting the relay demands. These systems, with few exceptions, cannot have fault detector supervision of the receivers and the security against undesired tripping rests solely with the audiotone equipment.

Transmission Line Protection
Audio-tone systems function as the communication link for pilot relaying schemes employed for transmission line protection. Transfer trip schemes, including direct underreaching, permissive underreaching, and permissive overreaching protection are used extensively with telephone channels.

Directional comparison blocking and phase comparison schemes are used primarily with power line carrier but they are also employed with audio-tone systems over telephone channels and microwave channels. Tripping with line protective schemes can be made dependent on line relay and fault detector relay operation.

Transformer and Circuit Breaker Failure Protection
Transfer tripping schemes using audio-tone systems over telephone channels have been used extensively for transformer protection where high voltage breakers have been omitted or for breaker backup protection where system arrangement places a backup breaker(s) at a remote location. The operation of a frequency shift audio-tone system for remote clearing is described in the following paragraph.

During normal conditions, a guard signal(s) is transmitted continuously. Receipt of the guard signal by the tone receiver produces blocking of the breaker trip circuit. At the same time, the guard signal provides continuous monitoring of the tone system.

When the protective relays detect abnormal operation, they initiate removal of the guard signal(s) and transmission of the trip signal(s). The absence of guard and the reception of the trip constitutes a valid trip condition to effect remote clearing.

Transmission Line Protection
Pilot relaying systems applicable for the protection of power-transmission lines and for which audio tone channels may be used are briefly described below. The transmission lines may have two or more terminals each with circuit breakers for disconnecting the line from the rest of the power system.

All of the relaying systems described can be used on two-terminal or multiterminal lines. These relaying systems program the automatic operation of the circuit breakers during power system faults.

Where possible, fault detector relays should supervise the receiver relay trip circuits to improve security.

However, their use should not be a substitute for an audio-tone system of highest reliability, since the greatest noise levels are likely to occur at the very instant when the fault detectors have operated, that is during a power system fault.

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