SF6 Replacement: Technical Reconstruction and Solutions for Dry Air/N2 Insulation

In the wave of "defluorination" in power equipment, replacing sulfur hexafluoride (SF₆) with dry air or nitrogen (N₂) is not a simple "gas swap," but a huge game of physical properties.

The most core difference lies in arc-quenching capability. SF₆ possesses extremely strong electronegativity and can efficiently capture electrons; its arc-quenching ability is about 100 times that of air. In contrast, dry air and nitrogen have stable molecular structures and basically lack effective arc-quenching performance. If traditional switchgear designs were used, the arc would fail to extinguish, leading to equipment burnout or even explosion.

Facing this physical chasm, the industry has explored a set of effective solutions through the reconstruction of technical routes.

1. Core Contradiction: From "Active Suppression" to "Passive Endurance"

In SF₆ circuit breakers, the gas wears two hats: it acts as both the insulation medium and the arc-quenching medium. When contacts separate and an arc is generated, the SF₆ gas flow forces the arc to extinguish.

However, dry air and nitrogen face physical limitations:

• Lack of Electronegativity: They cannot weaken the arc conductive channel like SF₆, causing the arc to burn longer and at higher temperatures.
• Heat Dissipation Disadvantages: Relying solely on natural convection makes it difficult to handle the instantaneous high heat generated during high-voltage and large-current breaking.

Simply put, SF₆ actively "strangles" the arc, while dry air/nitrogen can only passively "endure" it. Therefore, the strategy must change: let the gas be responsible only for insulation, and introduce other media to handle arc quenching.

2. Breaking the Deadlock: Three Major Engineering Strategies

To address these challenges, mainstream solutions adopt a "hybrid technical route"—a combination of vacuum arc quenching + gas insulation—supplemented by precision structural design.

Functional Decoupling: The Core Intervention of Vacuum Interrupters
This is the fundamental solution to the weak arc-quenching capability of dry air/nitrogen. Since air cannot extinguish arcs effectively, we introduce a perfect "outsider" in this field—the vacuum.

• Principle: Utilize the extremely high dielectric recovery speed of the vacuum interrupter (VI) to cut off the current. The arc in a vacuum environment extinguishes at the first current zero crossing without relying on external gas blowing.
• Division of Labor:
  • Vacuum Interrupter: Solely responsible for "breaking current," undertaking the arc-quenching task.
  • Dry Air/Nitrogen: Solely responsible for "phase-to-ground and phase-to-phase insulation," filling the switchgear body to isolate high-voltage components.
• Advantage: This completely bypasses the shortcoming of poor arc-quenching performance in eco-friendly gases, achieving true "zero-carbon" emissions (since Nitrogen/Air GWP=0). Products like Schneider's RM AirSeT and Siemens' Blue GIS currently use this route.

Mechanical "Precision Braking": Puffer-Type Stagnation Point Design
Although vacuum is mainly used for arc quenching, arcs may still occur when disconnectors break small currents (e.g., capacitive currents) or when acting as load switches. At this point, how to utilize the weak air flow field to assist in arc extinction becomes key. Manufacturers like ABB have innovatively applied "puffer-type" technology.

• Stagnation Point Effect: Through precisely designed piston and nozzle structures, the dry air inside the chamber is compressed during the movement of the moving contact. When the airflow sprays at high speed towards the arc area, gas dynamics principles are used to form a "stagnation point" where velocity is zero.
• Mechanism: This "stagnation point" generates local high pressure. On one hand, it compresses the arc diameter through the "thermal pinch effect," increasing arc resistance; on the other hand, the high-density gas boosts local insulation strength, preventing arc reignition.
• Effect: This design acts like "precision braking" for the airflow, transforming originally disordered air movement into directional high-pressure gas blowing, compensating for the inherent lack of arc-quenching ability in dry air.

Coordination Optimization of Grounding Safety and Operating Mechanisms
Since dry air/nitrogen lacks the strong insulation and arc-quenching capabilities of SF₆, extreme caution is required during grounding operations.

• Busbar-Side Grounding: To avoid accidents caused by difficult arc quenching when line-side grounding switches close short-circuit currents, new design schemes tend to adopt busbar-side combined functional grounding switches.
• Interlocking Mechanism: Through dual mechanical and electrical interlocks, it is ensured that the upstream circuit breaker absolutely does not open when the grounding switch is closed. This avoids forcibly cutting load current in air which lacks arc-quenching capability.
• High-Speed Closing: Addressing the weaker insulation capability of eco-friendly gases, some designs increase the closing speed of switches to reduce pre-strike time and lower the risk of contact ablation.

3. Conclusion

Replacing SF₆ with dry air or nitrogen is essentially an engineering art of "maximizing strengths and avoiding weaknesses." We acknowledge and accept the reality that they are "weak" in arc-quenching capability (less than 1% of SF₆), so we no longer force them to perform the high-difficulty task of "cutting current." Instead, we position them as pure insulation barriers. By introducing vacuum arc-quenching technology, coupled with puffer-type structural optimization and intelligent control strategies, we have successfully bypassed the shortcomings of physical properties, providing a feasible path for building a green, safe, and future power grid.