--- title: "Power line carrier communication" subject: "Power Plant Equipment" chapter: 4 key: "semester_7/power_plant_equipment:plcc-communication" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/plcc-communication site: BCT Engineering --- # Power line carrier communication Power Plant Equipment · Chapter 4: Substation Equipments - Asked in: 2071 Chaitra, 3 marks; 2073 Chaitra, 4 marks (asked 2 times) - Video: [power line carrier communication / PLCC](https://www.youtube.com/watch?v=2ieGFgfDOxA) — Sourabh Sharma Hindi Channel - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/plcc-communication - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-4 ## Question Describe how communication between two electric sub-stations can be made with Power Line Carrier Communication system. ## Short answer PLCC superimposes a 30–500 kHz HF carrier on the power line using coupling capacitors (block 50 Hz but pass HF) and wave traps (confine carrier to the line). The PLC terminal modulates/demodulates speech, data, and protection signals; line matching unit matches impedances. This provides telephony, SCADA data, and teleprotection without separate communication lines. ## Answer **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. ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does the wave trap do in PLCC? A. Offers high impedance to the carrier so it stays on the line, but passes 50 Hz easily B. Blocks 50 Hz power from entering the line C. Matches the coaxial cable to the line D. Modulates speech onto the carrier Answer: A. A parallel LC tuned to the carrier frequency, in series with the line, keeps the signal from leaking into the bus. (Key point: Wave trap) 2. What is the function of the coupling capacitor? A. Passes 50 Hz and blocks the carrier B. Passes the HF carrier to the line and blocks the 50 Hz power frequency C. Stores energy to supply the PLC terminal D. Tunes the line to the carrier frequency Answer: B. A capacitor has high reactance at 50 Hz and low reactance at carrier frequency. (Key point: Coupling capacitor) 3. What does the PLC terminal do at the sending end, and what is the carrier range? A. Rectifies 50 Hz to DC, carrier about 50 Hz B. Amplifies the 50 Hz power, carrier about 30 to 500 MHz C. Modulates speech/data on a carrier of about 30 to 500 kHz D. Filters the surge, carrier about 1 to 5 kHz Answer: C. The terminal modulates the signal onto an HF carrier (about 30 to 500 kHz) and demodulates it at the receiver. (Key point: PLC terminal and carrier) 4. What does the line matching unit do? A. Traps the carrier in the substation bus B. Converts HV to LV for the terminal C. Acts as the power source of the carrier D. Matches cable to line impedance, forms a high-pass filter with the CC and gives surge protection Answer: D. The LMU with the CC matches impedance and filters, and protects the equipment. 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