A distance relay at one end of a transmission line sometimes needs to tell the relay at the other end to trip — in milliseconds. Laying a separate communication cable along hundreds of kilometres of line would be expensive, so the signal is sent over the power line itself. That is Power Line Carrier Communication (PLCC): a high-frequency radio carrier, piggy-backed on the high-voltage conductor.

Figure 2.3: Power Line Carrier Communication (PLCC) — terminal equipment. The wave trap confines the carrier to the protected line while the coupling capacitor blocks 50 Hz power. (Schematic, not to scale.)

How it works

PLCC transmits high-frequency radio carrier signals at 30 kHz to 500 kHz over the high-voltage line, alongside the 50 Hz power. It carries three kinds of traffic: teleprotection (trip and blocking signals between relays), SCADA data, and voice communication between substations.

The terminal equipment

• Wave trap (line trap) — a parallel L–C circuit tuned to resonate at the carrier frequency. It offers negligible impedance (under 0.1 Ω) to the 50 Hz power current, but high impedance (over 400 Ω) to the RF carrier, confining the carrier energy to the protected line section so it does not leak into the rest of the network.
• Coupling capacitor — connects the carrier equipment to the HV line while blocking the 50 Hz high voltage and offering low impedance to the 30–500 kHz RF signal. A power-frequency drain coil at its base safely diverts any 50 Hz current to earth.
• Line Matching Unit (LMU) — matches the transmission-line impedance (300–450 Ω) to the coaxial cable impedance (50–75 Ω) of the RF transmitter/receiver, for maximum power transfer and minimum reflection.

At each end, the transmitter/receiver and the protective relay sit behind this equipment, safely isolated from the high voltage yet able to exchange signals over the very conductor the relay is protecting.

Key points to remember

• PLCC sends a 30–500 kHz carrier over the HV line for teleprotection, SCADA and voice.
• Wave trap (parallel L–C): under 0.1 Ω at 50 Hz, over 400 Ω at RF — it traps the carrier on the protected line.
• Coupling capacitor blocks 50 Hz HV and passes RF; the drain coil diverts 50 Hz current to earth.
• LMU matches line impedance 300–450 Ω to coaxial cable 50–75 Ω for maximum power transfer.

Based on Power System Protection & Switchgear, Complete Examination Solution, Exam 07 May 2026, Q2(c).