- 2075 Bhadra · 8 marks
Compare MOCB/BOCB and ABCB/puffer SF6
Short answer
i) Minimum oil CB uses only quenching oil with solid insulation, is smaller and lower fire risk than bulk oil CB which uses large oil for quenching and insulation; MOCB oil deteriorates faster. ii) Air blast CB uses external compressed air, noisy, high chopping; puffer SF6 CB generates its own SF6 blast, silent, very fast, low maintenance, no exhaust.
Compare the performance and characteristics and working principle of: i) Minimum oil circuit breaker vs Bulk oil circuit breaker ii) Air blast circuit breaker vs puffer type SF6 circuit breaker.
The comparison covers working principle, performance and characteristics of each pair.
i) Minimum oil CB vs Bulk oil CB
Working principle (common): the arc between separating contacts decomposes oil into a gas bubble (mainly hydrogen); high pressure and cooling by hydrogen deionise the gap so that the arc goes out at current zero.
| Point | Minimum oil CB | Bulk oil CB |
|---|---|---|
| Role of oil | Arc quenching only | Arc quenching and insulation to earth |
| Tank | Small porcelain chamber, kept live | Large earthed steel tank |
| Oil quantity | About 10% of BOCB | Very large |
| Arc control | Turbulator (axial/side vent) always used | Plain break or explosion pot |
| Size, weight | Small, light | Heavy, bulky, large foundations |
| Fire risk | Low | High |
| Oil deterioration | Fast (small volume) | Slow |
| Maintenance | Frequent oil change | Oil filtering, tank cleaning |
| Range | 3.3–132 kV | Up to 33 kV |
ii) Air blast CB vs Puffer type SF6 CB
Working principle:
- ABCB: a blast of compressed air from a separate reservoir is blown on the arc; it cools the arc and sweeps away ions, and air is exhausted to atmosphere.
- Puffer SF6: the moving contact drives a piston that compresses SF6 in a cylinder and blows it through a nozzle over the arc. SF6 is electronegative and absorbs free electrons, so the gap recovers very fast. Gas stays in a sealed enclosure.
| Point | Air blast CB | Puffer SF6 CB |
|---|---|---|
| Medium | Air at 20–30 bar | SF6 at about 5–7 bar |
| Gas supply | External compressor and receiver | Self-generated by piston; no compressor |
| Dielectric strength | Low (needs high pressure) | 2.5–3 times air |
| Arc time constant | Larger | Very small |
| Current chopping | High; resistors needed | Negligible |
| Noise | Very loud exhaust | Silent (closed system) |
| Breaks per pole at EHV | More | Fewer |
| Maintenance | Compressor, valves, leaks | Very low; gas leak monitoring |
| Environment | Clean (air) | SF6 is a strong greenhouse gas |
| Cost | High (with air plant) | Moderate |
| Present use | Mostly replaced | Standard for 33–765 kV |
Remarks
SF6 puffer breakers combine the speed of air blast breakers with silent, compact and low-maintenance operation, which is why they have replaced both oil and air blast types at transmission voltages.
✓ Answer checked by Claude Opus 5.5 (high effort)
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