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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