Circuit breakers
Circuit breakers directly actuated by a protective relay system are usually provided where it is desirable to isolate a faulted transformer with minimum effect on other segments of the power system. They offer the fastest fault-clearing time and highest interrupting capability.
Many of the diagrams in this guide show only relay connections and not circuit breaker location. Wherever possible, the circuit breaker should be included in the relay zone of protection so that a fault in the breaker or leads to the transformer and its bushings is detected.
In some situations, it may be difficult to justify the cost of local circuit breakers. Tripping of remote-source circuit breakers by use of local relays and a communications channel, or by use of a fault-initiating switch (high-speed ground switch) are alternatives.
Transfer trip schemes
Five types of communication channel are in general use for transferring a trip signal to remote circuit breakers: pilot wire, power-line carrier, fiber optic, and microwave or radio. In direct-transfer trip schemes, the receipt of a signal will trip local circuit breakers independently of local relays.
The signal may be a simple application of voltage or audio tones on a pair of wires or may utilize frequency-shift-type audio tones, or frequency-shift carrier.
Frequency-shift equipment employs a guard frequency for channel monitoring and added security against trips by spurious signals. Transformer protective relays will actuate the shift to trip frequency. These schemes have the advantage of speed and the ability to block reclosing of the remote circuit breakers until the faulted transformer is isolated from the system.
Fault-initiating switch (high-speed ground switch)
Remote tripping of circuit breakers can be accomplished by applying a fault (usually solid single phase-to-ground) to the source line so that the remote line relays will detect it and trip the remote circuit breakers. A disadvantage of this scheme is the additional time involved while the ground switch is closing and remote relays in turn detect the fault. Another consideration is that the ground switch phase and the faulted phase on the transformer may be different, thus imposing a multiphase fault on the system.
Disconnecting switch
When remote tripping is used, a power-operated disconnecting switch is usually connected on the source side of the transformer to isolate it from the system. The switch is arranged to open automatically and cancels the remote transfer trip signal or isolates the ground switch from the system. In both cases, this permits the remote breakers to reclose.
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