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Utility applications
Utility companies use special coupling capacitors to connect low-frequency radio
transmitters to the power-frequency AC conductors. Frequencies used are in the
range of 24 to 500 kHz, with transmitter power levels up to hundreds of watts.
These signals may be impressed on one conductor, on two conductors or on all
three conductors of a high-voltage AC transmission line. Several different PLC
channels may be coupled onto one HV line. Filtering devices are applied at
substations to prevent the carrier frequency current from being bypassed through
the station apparatus and to ensure that distant faults do not affect the
isolated segments of the PLC system. These circuits are used for control of
switchgear, and for protection of transmission lines. For example, a protection
relay can use a PLC channel to trip a line if a fault is detected between its
two terminals, but to leave the line in operation if the fault is elsewhere on
the system.
While utility companies use microwave and now, increasingly, fiber optic cables
for their primary system communication needs, the power-line carrier apparatus
may still be useful as a backup channel or for very simple low-cost
installations that do not warrant a fibre drop.
There are also some very low-bit rate power line communication systems used for
automatic meter reading.
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