Objective Questions on SF6 CB – A Complete MCQ Series

Hint: Higher dielectric strength and arc-quenching ability of SF6 gas

Hint: SF6 has approximately 2.5 times the dielectric strength of air at the same pressure and temperature conditions.

Hint: Puffer breakers use mechanical energy to compress SF6 gas which is then released as a high-pressure blast to extinguish the arc.

Hint: SF6 becomes supercritical at 37.6 bar and 45.6°C (3.76 MPa and 45.6°C). Above this point, it exhibits both liquid and gas properties.

Hint: Most HV SF6 breakers operate at 4-6 bar gauge pressure at 20°C to ensure adequate dielectric strength and arc quenching capability.

Hint: SF6 has a GWP of 23,500 over 100 years, making it one of the most potent greenhouse gases, though it is chemically stable and non-toxic.

Hint: HF forms when SF6 decomposes in the presence of moisture and is highly corrosive to metals and insulators.

Hint: High purity (≥99.9%) is essential to prevent contamination that could reduce dielectric strength and promote internal corrosion.

Hint: Self-blast breakers utilize the thermal energy from the arc to heat and expand the gas, creating pressure for arc interruption.

Hint: Modern SF6 breakers typically interrupt within 2-3 cycles (25-35 ms at 50/60 Hz) including relay time and mechanical operation.

Hint: SF6 is highly electronegative, rapidly capturing free electrons and preventing arc reformation after current zero crossing.

Hint: Low gas pressure compromises dielectric integrity and interrupting capability; the breaker should be isolated immediately to prevent failure.

Hint: Double-pressure systems typically maintain a 15:1 to 25:1 ratio, though they are now largely obsolete in favor of single-pressure designs.

Hint: SF6 sublimes (solid to gas) at -63.8°C at 1 atm. Below this temperature, it solidifies, which is critical for cold climate installations.

Hint: Copper-tungsten alloys (typically 70-80% Cu, 20-30% W) provide excellent arc erosion resistance and electrical conductivity.

Hint: Molecular sieves absorb moisture and acidic decomposition products, maintaining gas purity and preventing corrosion.

Hint: SF6 dielectric strength increases approximately linearly with pressure in the practical operating range of 1-10 bar.

Hint: High-voltage SF6 breakers require substantial contact travel (150-200 mm for 245 kV) to develop sufficient nozzle pressure and arc length.

Hint: IEC 62271-100 (formerly IEC 60056) defines ratings, testing procedures, and performance requirements for AC circuit breakers.

Hint: In self-blast breakers, low fault currents may not generate sufficient thermal energy to create the pressure needed for successful interruption.

Hint: Modern sealed SF6 breakers should maintain leak rates below 0.5% per year, with many achieving less than 0.1% per year.

Hint: Rotating arc breakers use magnetic coils to create an axial magnetic field that forces the arc to rotate, distributing thermal stress.

Hint: GIS can reduce substation footprint by 90% or more compared to AIS, making it ideal for urban or space-constrained locations.

Hint: Moisture reduces dielectric strength significantly and promotes formation of corrosive decomposition products leading to insulation failure.

Hint: Moisture should be maintained below 50 ppmv (parts per million by volume) to prevent condensation and ensure dielectric integrity.

Hint: Thermal cycling tests verify that seals and gaskets maintain integrity across the rated temperature range (-40°C to +60°C typically).

Hint: TRV is the voltage that appears across breaker contacts after current zero; the breaker must withstand this without reignition.

Hint: The nozzle design and gas pressure directly determine the arc-quenching blast intensity, which is critical for interruption.

Hint: Capacitor switching involves interrupting low current with high rate-of-rise restriking voltage, potentially causing reignition.

Hint: SF6 efficiently extinguishes arcs with minimal energy transfer to contacts, resulting in very low erosion even after thousands of operations.

Hint: Quality SF6 breakers are designed for 10,000 mechanical operations (no-load) and 100-200 full-rated interruptions.

Hint: SO2 formation indicates thermal or electrical stress from arcing; trending SO2 levels helps assess internal condition.

Hint: Grading capacitors distribute the system voltage evenly across multiple series-connected interrupter units.

Hint: Prestrike during closing can cause voltage doubling across open phases, potentially damaging connected equipment like transformers.

Hint: Due to environmental concerns, SF6 must be recovered, purified, and recycled using proper equipment; atmospheric release is prohibited in many jurisdictions.

Hint: Puffer mechanisms typically achieve compression ratios of 5:1 to 8:1 to generate sufficient pressure for effective arc extinction.

Hint: The blast valve directs high-pressure SF6 gas from the compression chamber through the nozzle toward the arc during contact separation.

Hint: The arc-quenching nozzle experiences direct exposure to high-temperature arcs and decomposes over time, requiring periodic inspection.

Hint: Standard breaking capacities for 420 kV SF6 breakers range from 31.5 kA to 63 kA, with some special designs reaching higher values.

Hint: Gas analyzers measure moisture content, SF6 purity, and decomposition products to assess gas quality and identify potential issues.

Hint: Density monitors have thermal time constants of 2-5 minutes to prevent false alarms during rapid temperature changes while maintaining accurate pressure compensation.

Hint: IEC 60376 specifies the quality and handling requirements for new SF6 gas, while IEC 60480 covers reused gas.

Hint: Hybrid designs minimize SF6 quantity while maintaining performance, addressing environmental concerns.

Hint: SF6 exhibits rapid dielectric recovery after current zero, crucial for withstanding the steep TRV associated with short-line faults.

Hint: GIS typically operates at 3-4 bar gauge for optimal dielectric performance while minimizing liquefaction risks.

Hint: SO2 formation typically indicates significant thermal decomposition from internal arcing or overheating.

Hint: Rupture discs protect the enclosure from catastrophic failure during internal arcs by venting high pressure.

Hint: Hydraulic mechanisms typically operate at 200-350 bar to provide the high forces needed for quick contact movement.

Hint: SF6 exhibits dielectric recovery rates of 30-50 kV/μs, significantly higher than air or oil, enabling successful interruption.

Hint: Contact erosion is primarily proportional to the square of interrupted current and the number of operations.

Hint: Modern 245 kV SF6 breakers typically close in 40-60 ms, including operating mechanism response time.

Hint: The charging motor compresses the operating springs which store energy for subsequent opening or closing operations.

Hint: At 5 bar absolute, SF6 provides approximately 150 kV breakdown voltage for a 10 mm uniform field gap.

Hint: Mechanical endurance tests verify the breaker can perform the specified number of operations without mechanical failure.

Hint: Arc contacts are designed to withstand arc erosion, protecting the main contacts which carry continuous current.

Hint: Heaters prevent SF6 liquefaction at low temperatures, maintaining proper gas density for dielectric and interrupting performance.

Hint: Operating coils typically have resistances of 100-200 ohms to limit current while providing sufficient magnetic force.

Hint: Current chopping during capacitive current interruption can generate overvoltages due to trapped energy in the capacitive circuit.

Hint: Trip-free design ensures the breaker can open even if a close command is maintained, providing protection against closing onto faults.

Hint: Modern monitoring systems trend multiple mechanical parameters to detect degradation before failure occurs.

Hint: A 145 kV SF6 breaker typically contains 30-50 liters of gas volume per interrupter unit.

Hint: The Basic Insulation Level (BIL) must coordinate with system protection levels to ensure proper surge withstand capability.

Hint: Contact bounce results from kinetic energy conversion during contact impact, potentially causing prestrike and contact erosion.

Hint: Megger testing checks insulation resistance of bushings, operating rods, and control circuits to detect moisture or contamination.

Hint: Internal arcs generate rapid pressure rises of 5-8 times normal operating pressure, which the enclosure must withstand.

Hint: Tie rods mechanically link multiple interrupter units to ensure simultaneous contact movement across all breaks.

Hint: Magnetic actuators offer reduced maintenance, faster operation, and higher reliability compared to spring or hydraulic mechanisms.

Hint: Dew point directly correlates with moisture content in SF6 gas, critical for preventing condensation and maintaining dielectric strength.

Hint: SF6 is chemically stable and can be stored indefinitely in sealed containers without degradation.

Hint: The number of interrupter units in series is primarily determined by the system voltage, with higher voltages requiring more breaks.

Hint: The blast tube channels compressed SF6 gas from the puffer cylinder to the nozzle throat for effective arc quenching.

Hint: Multiple methods including UHF sensors, acoustic detection, and gas analysis can identify partial discharge activity.