--- title: "VCB as GCB and limits versus SF6" subject: "Power Plant Design" chapter: 4 key: "semester_8/power_plant_design:vcb-gcb-limitations-sf6" source: https://www.bctengineer.com/q/semester_8/power_plant_design/vcb-gcb-limitations-sf6 site: BCT Engineering --- # VCB as GCB and limits versus SF6 Power Plant Design · Chapter 4: Electric System Design of a Power Plant - Asked in: 2070 Bhadra, 3+5 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/vcb-gcb-limitations-sf6 - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4 ## Question What particular precautions should be taken while using vacuum circuit breakers as a generator circuit breaker? State the limitations of vacuum circuit breaker in comparison to SF6 circuit breaker in HV application. ## Short answer Precautions: install surge arresters and RC snubbers against chopping overvoltages, ensure type-testing for generator duty. Limitations: VCB voltage per break limited to ~40 kV, requiring series bottles; chopping current higher than SF6; SF6 provides restrike-free capacitive switching and higher continuous current, dominating above 72.5 kV. ## Answer #### Precautions when a VCB is used as a generator CB A generator breaker sees a very high fault current with a large **DC component** (because of the high X/R ratio), **delayed current zeros**, and a very fast **TRV** rate of rise. A VCB interrupts so well that it can **chop** the current before natural zero. On the inductive generator and transformer windings this gives $V = L\,di/dt$ overvoltages and multiple re-ignitions. So: 1. Install **metal-oxide surge arresters** close to the breaker on both sides. 2. Install **surge capacitors or RC snubbers** at the generator terminals to slow the voltage rise and damp chopping overvoltages. 3. Choose a breaker **type-tested for generator duty** (IEEE C37.013 / IEC 62271-37-013), including asymmetrical breaking with high DC content and out-of-phase switching. 4. Check the **TRV/RRRV** capability; add TRV capacitors if needed. 5. Make sure the continuous current rating and temperature rise are adequate (forced cooling if needed). 6. Use **low-chopping contact material** (e.g. Cu–Cr) and monitor vacuum integrity. #### Limitations of VCB compared with SF6 in HV application | Point | VCB | SF6 CB | |---|---|---| | Voltage per break | Up to about 36–40 kV; series interrupters needed above | One break handles 145–245 kV, more with series | | HV use (>72.5 kV) | Limited, few products | Standard up to 800 kV | | Current chopping | Higher; switching overvoltages | Very low chopping | | Capacitive switching | Risk of restrike | Restrike-free | | Continuous current | Limited by contact size | Very high (several kA) | | Insulation outside bottle | Needs separate insulation | Gas gives insulation too (GIS) | | Size at HV | Bulky with many breaks | Compact | **Summary of VCB limitations at HV:** the dielectric strength of a vacuum gap does not rise linearly with gap length, so a single bottle cannot be built economically above about 40 kV. Several bottles in series need voltage grading. Chopping overvoltages are higher and the continuous current rating is lower. For these reasons SF6 dominates above 72.5 kV, although SF6 is a greenhouse gas. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why are surge capacitors used with a VCB on a generator? A. Cool contacts B. Cut DC offset C. Slow voltage rise and damp chopping overvoltages D. Raise current Answer: C. They limit the overvoltages caused by current chopping. (Key point: Chopping) 2. Why does a generator breaker face difficult duty? A. Slow TRV B. High X/R gives large DC component and delayed current zeros C. Low fault current D. Low voltage Answer: B. Asymmetry can delay current zero, demanding type-tested capability. (Key point: DC component) 3. Why is a single vacuum bottle limited to about 40 kV? A. Bottles are too big B. Contacts melt C. Gas pressure D. Dielectric strength does not rise linearly with gap length Answer: D. At higher voltage many bottles in series are needed, requiring grading. (Key point: Voltage limit) 4. Which is standard above 72.5 kV? A. VCB B. Air-blast only C. SF6 CB D. Oil CB Answer: C. SF6 handles high voltage per break, with low chopping and compact size. (Key point: HV choice) ## Related in Power Plant Design - [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 - [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 - [Choice of generator circuit breaker](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Agenerator-circuit-breaker-choice) — Power Plant Design, chapter 4, asked 2× - [Design: 310 m head plant, HV breaker](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2080-310m-hvcb) — Power Plant Design, chapter 4, asked 3× - [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× ## Related in other subjects - [Arc extinction in vacuum circuit breakers](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Avacuum-cb-arc-extinction) — Switchgear and Protection, chapter 6 - [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× - [Puffer type SF6 circuit breaker](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Apuffer-sf6-cb) — Switchgear and Protection, chapter 6