Page "Power line communication" Paragraph 7

Utility companies use special coupling capacitors to connect 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 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 protective 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.