--- title: "Choice of generator circuit breaker" subject: "Power Plant Design" chapter: 4 key: "semester_8/power_plant_design:generator-circuit-breaker-choice" source: https://www.bctengineer.com/q/semester_8/power_plant_design/generator-circuit-breaker-choice site: BCT Engineering --- # Choice of generator circuit breaker Power Plant Design · Chapter 4: Electric System Design of a Power Plant - Asked in: 2082 Shrawan, 6 marks; 2073 Magh, 3 marks (asked 2 times) - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/generator-circuit-breaker-choice - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4 ## Question Which circuit breaker would you think suitable as generator circuit breaker (GCB) in hydropower station, and why? ## Short answer An SF6 generator circuit breaker tested to IEEE C37.013/IEC 62271-37-013 is suitable for hydropower because it interrupts high asymmetry with delayed current zeros, withstands fast TRV, handles high continuous current, out-of-phase switching, and frequent operation. Vacuum GCB suits small/medium units but needs surge protection against chopping. ## Answer A **generator circuit breaker (GCB)** is placed between the generator and the step-up transformer. For a hydropower station, an **SF6 generator circuit breaker** (or a **vacuum** GCB for small and medium units), designed and tested to **IEEE C37.013 / IEC 62271-37-013**, is suitable. A normal distribution breaker built to IEC 62271-100 is not adequate. #### Why a special GCB is needed 1. **High DC component:** the generator circuit has a high X/R ratio, so the fault current DC offset decays slowly; currents can have **delayed current zeros**. The GCB must interrupt such asymmetric currents. 2. **High rate of rise of TRV:** the transformer and generator have small capacitance, so the transient recovery voltage rises very fast; SF6 and vacuum interrupters can withstand it. 3. **High continuous current:** e.g. a 30 MVA unit at 11 kV carries about 1.6 kA; large units carry tens of kA. 4. **Out-of-phase switching:** the GCB must interrupt during unsuccessful synchronisation. 5. **Frequent operation:** hydro units start and stop daily (peaking), so a long mechanical and electrical life is needed. #### Why SF6 (or vacuum) - SF6 has excellent arc-quenching and dielectric strength, handles high currents and asymmetry, and is compact; it is the usual choice for medium and large hydro units. - Vacuum GCBs are compact and maintenance-free for small/medium units; precautions: **current chopping** may cause overvoltage, so surge arresters and RC surge suppressors are fitted near the generator and transformer. - Air-blast and oil breakers are obsolete for this duty. #### Benefits of using a GCB - Generator faults are cleared fast and selectively; the unit is synchronised on the LV side. - Station auxiliaries can be fed from the grid through the GSU when the generator is off (back-feeding), so a separate station transformer may not be needed. - Better protection of the GSU and generator, higher plant availability. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which breaker is suitable as a GCB in a hydropower station? A. Oil circuit breaker B. SF6 (or vacuum for small/medium) breaker tested to IEEE C37.013 / IEC 62271-37-013 C. Air-blast breaker D. Ordinary IEC 62271-100 distribution breaker Answer: B. Generator-duty standards cover the special stresses; ordinary breakers are not adequate. (Key point: Suitable GCB) 2. Why is the DC component a concern for a GCB? A. High X/R makes DC offset decay slowly, causing delayed current zeros B. Low X/R makes current zero early C. DC lowers the continuous current D. It reduces TRV Answer: A. The generator circuit has high X/R, so asymmetric currents may not cross zero for a long time. (Key point: DC component) 3. The TRV in a generator circuit rises very fast because: A. The circuit has large capacitance B. The generator is far from the transformer C. The transformer and generator have small capacitance D. Frequency is high Answer: C. Low capacitance gives a high rate of rise of TRV, which SF6 and vacuum interrupters withstand. (Key point: TRV) 4. A GCB also allows: A. Back-feeding of station auxiliaries from the grid through the GSU when the generator is off B. Removal of the GSU C. Skipping generator protection D. Synchronising on the HV side only Answer: A. The unit can be synchronised on the LV side and auxiliaries fed through the GSU. (Key point: Benefit of GCB) ## Related in Power Plant Design - [Design: 300 m head plant, GCB rating](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2081-300m-gcb) — Power Plant Design, chapter 4, asked 11× - [Precautions for VCB as generator breaker](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Avcb-as-gcb-precaution) — Power Plant Design, chapter 4 - [Generator choice and SCR versus stability](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Agenerator-choice-scr-stability) — Power Plant Design, chapter 4 - [Generator choice and significance of SCR](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Agenerator-choice-scr-significance) — Power Plant Design, chapter 4 - [Vacuum versus SF6 circuit breaker](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Avcb-vs-sf6) — Power Plant Design, chapter 4 - [VCB as GCB and limits versus SF6](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Avcb-gcb-limitations-sf6) — Power Plant Design, chapter 4 ## Related in other subjects - [Reactor to protect circuit breaker of small station](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Areactor-protect-circuit-breaker) — Power Plant Equipment, chapter 4 - [Vacuum circuit breaker](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Avacuum-cb-construction) — Switchgear and Protection, chapter 6, asked 2× - [SF6 circuit breaker](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Asf6-cb-construction) — Switchgear and Protection, chapter 6, asked 2× - [Short circuit MVA of feeder circuit breaker](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Afeeder-cb-short-circuit-mva) — Power System Analysis II, chapter 3, asked 2×