Power line carrier communication

Power Line Carrier Communication (PLCC) uses the high-voltage transmission line itself as the medium to send speech, data, telemetry and protection signals between two substations, by superimposing a high-frequency carrier (about 30–500 kHz) on the 50 Hz power.

 Substation A                           Substation B
  bus                                         bus
   |                                           |
  [WT]=========== HV line ================[WT]
   |                                           |
  [CC]                                        [CC]
   |                                           |
  [LMU]                                      [LMU]
   |                                           |
 PLC terminal                           PLC terminal
 (tx/rx: speech, data, teleprotection)

Components and working

1. PLC terminal (transmitter/receiver): modulates the speech/data/protection signal on to an HF carrier at the sending end and demodulates it at the receiving end.
2. Coupling capacitor (CC): high-voltage capacitor that passes the HF carrier to the line but blocks the 50 Hz power frequency.
3. Line matching unit (LMU): matches the impedance of the coaxial cable to the line and, with the CC, forms a high-pass filter; it also provides surge protection and earthing switch.
4. Wave trap (line trap): a parallel LC circuit in series with the line, tuned to the carrier frequency. It offers high impedance to the carrier, so the signal does not leak into the substation bus, but negligible impedance to 50 Hz power.

The carrier travels along the line conductors (phase-to-earth or phase-to-phase coupling) to the other substation, where it is picked up through its own CC and LMU.

Uses

• Telephone (speech) between substations and load dispatch centre.
• Teleprotection: carrier-aided distance and inter-tripping.
• SCADA data and telemetering.

Merits

• No separate communication line needed; the line is strong and reliable.
• Economical over long distances.