Dielectric strength of SF6 is

Question

Examoogle · Accepted Answer

Option B: Less than that of oil

Explanation & Concepts:
The dielectric strength of Sulfur Hexafluoride ($SF_6$) gas is approximately 2.5 to 3 times that of air at atmospheric pressure, but it remains significantly lower than that of high-quality transformer insulating oil. Transformer oil possesses a much higher density and molecular complexity, providing superior dielectric breakdown strength compared to gaseous insulators.

Working Principle: The insulation mechanism in $SF_6$ is primarily governed by its high electronegativity; it captures free electrons to form heavy, immobile negative ions, thereby inhibiting the development of electron avalanches. In contrast, transformer oil relies on molecular stability and the suppression of ionic conduction paths under high-field stress. Consequently, while $SF_6$ is an excellent arc-quenching medium, it does not achieve the absolute breakdown voltage levels characteristic of bulk mineral oils.

Key Formulas: $E_b \propto P$ — Dielectric strength $E_b$ of $SF_6$ is directly proportional to its pressure $P$.

Key Points: $SF_6$ is an electronegative gas that effectively quenches arcs.; Dielectric strength of $SF_6$ is highly pressure-dependent.; Transformer oil provides superior insulation for high-voltage liquid-immersed equipment.; At 3 bar pressure, the dielectric strength of $SF_6$ approaches that of oil, but at atmospheric pressure, it is lower.

Advantages: High electronegativity enables rapid arc quenching.; Chemical inertness and thermal stability.

Disadvantages: High global warming potential.; Lower absolute dielectric strength compared to insulating oils at ambient pressures.

Applications: Gas-Insulated Switchgear (GIS).; High-voltage circuit breakers.; Gas-insulated transmission lines (GIL).

Additional Info: Note: The question states 'Less than that of oil', which is factually correct as mineral insulating oil has a breakdown strength ranging from 150-200 kV/cm, whereas $SF_6$ at atmospheric pressure is roughly 89 kV/cm.. Option D is also technically true (as dielectric strength increases with pressure), but in standard comparisons of material properties at nominal states, the comparison with oil is the standard benchmark.

Answer: B is correct — The dielectric strength of $SF_6$ at atmospheric pressure is substantially lower than that of high-quality transformer insulating oil.

Exam Tip: Always verify the pressure state when comparing $SF_6$ dielectric strength; it can outperform air and approach oil characteristics only at elevated pressures (e.g., 3-5 bar).

Examoogle · Answer

Option A: Less than that of air

Examoogle · Answer

Option C: More than that of oil

Examoogle · Answer

Option D: Depend on pressure

Feature	Air	SF6 Gas
Dielectric Strength (1 atm)	1.0 p.u.	3.0 p.u.
Typical Breakdown Strength	~30 kV/cm	150 kV/cm

Correct Answer

Discussion (0)

Correct Answer

Related Questions

Discussion (0)