--- title: "Carrier current and bus-bar protection" subject: "Switchgear and Protection" chapter: 7 key: "semester_6/switchgear_and_protection:carrier-current-busbar-protection" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/carrier-current-busbar-protection site: BCT Engineering --- # Carrier current and bus-bar protection Switchgear and Protection · Chapter 7: Protective Relays - Asked in: 2071 Magh, 8 marks; 2075 Bhadra, 8 marks (asked 2 times) - Video: [SGP501 Protection of Busbars](https://www.youtube.com/watch?v=dEus3dQ5v50) — Lectures in Electrical Engineering - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/carrier-current-busbar-protection - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-7 ## Question Explain the following types of protection with suitable diagrams: i) Carrier current protection ii) Bus-bar protection. ## Short answer Carrier current protection sends a high-frequency signal over the power line itself via coupling capacitors and line traps to provide fast unit protection. A blocking scheme trips only for internal faults. Bus-bar protection commonly uses differential relays: CT secondaries of all connected circuits are paralleled; the relay trips all breakers on the bus for any non-zero sum current. ## Answer #### i) Carrier current protection **Carrier current protection** is a unit protection for long transmission lines in which information about the currents at both ends is sent over the **power line itself** using a high-frequency carrier signal (50–500 kHz). It allows high-speed, simultaneous tripping at both ends for faults anywhere on the line, where pilot wires would be too long. **Equipment at each end** - **Transmitter and receiver** (carrier set) producing and detecting the HF signal. - **Coupling capacitor**: connects the carrier set to the HV line and blocks power-frequency voltage. - **Line trap (wave trap)**: a parallel LC circuit tuned to the carrier frequency, in series with the line; it confines the carrier to the protected section and passes 50 Hz. - **Relays**: directional or distance relays and a fault detector. ```text Bus A Bus B │ CB Line trap ═══════ Line ═══════ Line trap CB │ ├──□───[LC]──┬────────────────────────┬──[LC]───□──┤ │ │ Coupling cap. Coupling cap. │ │ Transmitter/Receiver Transmitter/Receiver │ │ Relays ───── trip ───── Relays ``` **Directional comparison (blocking) scheme** 1. Directional relays at both ends see the fault direction. 2. For an **internal fault**, power flows into the line at both ends; both relays see "forward" and no blocking carrier is sent; both breakers trip. 3. For an **external fault**, the relay at the end nearest the fault sees power flowing out of the line; it starts its transmitter and sends a **blocking signal** to the remote end, which prevents tripping. **Phase comparison scheme:** the phase of currents at the two ends is compared via the carrier; in-phase (internal fault) → trip; 180° out of phase (through fault) → block. **Advantages:** fast (1–3 cycles), simultaneous tripping at both ends, permits high-speed auto reclosing, no separate communication channel needed; the carrier can also be used for telephony and telemetry. #### ii) Bus-bar protection **Bus-bar protection** detects faults on the busbars of a substation and trips all breakers connected to the faulty bus section. Busbar faults are rare but severe, so the scheme must be fast and very stable for external faults. **Methods** 1. **Back-up protection by remote relays:** the bus is cleared by the time-delayed relays of the connected circuits; simple but slow. 2. **Fault bus (frame leakage) protection:** used for metal-clad switchgear; the frame is insulated from earth and earthed through a single CT and relay; an earth fault to the frame trips all breakers. 3. **Differential protection (most common):** based on Kirchhoff's current law: the sum of currents entering a healthy bus is zero. ```text Feeder1 Feeder2 Feeder3 Source │CT │CT │CT │CT ════╪════════════╪══════════╪═════════╪════ Bus │ │ │ │ └─────┬──────┴────┬─────┴────┬────┘ │ CT secondaries paralleled └────────[ R ]─────────┘ differential relay ``` - CTs of the same ratio are placed in every circuit connected to the bus, and their secondaries are connected in parallel with the relay. - **Normal load or external fault:** ΣI = 0, so no current flows in the relay. - **Bus fault:** all circuits feed the fault; ΣI = total fault current, and the relay trips all breakers on that bus. - **High impedance differential scheme** is widely used: a high-resistance relay with a stabilising resistor prevents false tripping when a CT saturates during a heavy external fault. **Biased (percentage) differential** relays are also used. - Large stations divide the bus into **zones** with separate protection, plus a **check zone** to avoid false trips. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the function of the line trap (wave trap) in carrier current protection? A. To block 50 Hz current B. To couple the carrier set to the line C. To confine the carrier signal to the protected section while passing power-frequency current D. To amplify the carrier signal Answer: C. The parallel LC circuit is tuned to the carrier frequency, blocking it from leaving the section but passing 50 Hz. (Key point: Role of line trap) 2. In the directional comparison (blocking) scheme, what happens for an external fault? A. Both ends trip at once B. The end nearest the fault sees power flowing out of the line and sends a blocking signal to the remote end C. No carrier is ever sent D. The remote end sends a trip signal Answer: B. For an external fault the near-end relay sees reversed power, starts its transmitter, and the blocking carrier prevents the remote end from tripping. (Key point: Directional comparison blocking) 3. On what principle does bus-bar differential protection work? A. Kirchhoff's current law: the sum of currents entering a healthy bus is zero B. Voltage drop across the bus C. Gas formation in the bus chamber D. Comparing carrier frequency at both ends Answer: A. For normal load or external faults the CT secondary currents sum to zero; a bus fault makes the sum equal to the fault current and the relay trips. (Key point: Busbar differential principle) 4. Why is a high-impedance differential scheme (with stabilising resistor) and a check zone used in busbar protection? A. To make tripping slower B. To avoid false tripping when a CT saturates on a heavy external fault C. To increase the carrier frequency D. To measure load current Answer: B. CT saturation produces spill current; the high-resistance relay with stabilising resistor and the check zone prevent false trips. (Key point: CT saturation and check zone) ## Related in Switchgear and Protection - [Ring bus rearrangement and zones](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aring-bus-comparison) — Switchgear and Protection, chapter 1 - [Earth fault protection: 20 MVA alternator](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aearth-fault-20mva-5-ohm) — Switchgear and Protection, chapter 7 - [Differential protection: 11 kV, 100 MVA generator](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Agenerator-differential-11kv-6-ohm) — Switchgear and Protection, chapter 7 - [Prospective current, cut-off current, time delay fuse](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aprospective-cutoff-time-delay-fuse) — Switchgear and Protection, chapter 3 - [Balanced circulating current: 10 kV alternator](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acirculating-current-10kv-10-ohm) — Switchgear and Protection, chapter 7 - [Cut-off and time-current characteristics of fuse](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Afuse-cutoff-time-current) — Switchgear and Protection, chapter 3 ## Related in other subjects - [Bus-bar reactor scheme: 30 MVA and 50 MVA](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Abus-bar-reactor-30-50-mva) — Power Plant Equipment, chapter 4, asked 2× - [Bus-bar reactor scheme with four generators](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Abus-bar-reactor-four-generators) — Power Plant Equipment, chapter 4, asked 2× - [High voltage bus bar types](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Ahv-busbar-types) — Power Plant Design, chapter 4 - [Reactor function and ring bus-bar scheme](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aring-bus-reactor-scheme) — Power Plant Equipment, chapter 4