No, long-term loaded operation is not permitted. A slight SF6 pressure drop that has not yet reached the lockout value only indicates the equipment temporarily retains the capability for loaded operation. However, enhanced monitoring is mandatory and an outage for corrective action must be requested promptly. Continued long-term operation poses severe safety hazards.
Core Principle: SF6 Dual Functions & Graded Protection
SF6 gas in high-voltage switchgear (circuit breakers, GIS, etc.) performs two critical functions:
- Arc Quenching: During breaker opening/closing, arc temperatures reach tens of thousands of degrees. SF6 rapidly cools and extinguishes the arc via its high electronegativity and thermal conductivity.
- Insulation: SF6 dielectric strength is approximately 2.5–3 times that of air, serving as the key insulating medium between live parts and between live parts and the enclosure.
Both functions are strongly correlated with gas density (pressure). Crucially, the SF6 compartment is a fixed volume — pressure varies significantly with temperature (rises when hot, drops when cold). Industry standard practice uses 20°C as the reference temperature for pressure conversion and comparison to avoid misjudgment from temperature fluctuations.
Equipment uses density relays (with temperature compensation) to set graded protection thresholds:
| Threshold |
Definition |
| Rated Pressure |
Standard operating pressure; arc-quenching and insulation performance at optimum. |
| Alarm Value (Warning) |
Slightly below rated; arc-quenching and insulation still adequate but deviated from normal range — alerts O&M personnel for prompt action. |
| Lockout Value (Protection) |
Further reduced pressure; arc-quenching and insulation insufficient — forces prohibition of open/close operations to prevent flashover or explosion. |
Relationship: Lockout Value < Alarm Value < Rated Value. As pressure drops, alarm triggers first; continued decline triggers lockout.
Why Not Long-Term Operation?
Although pressure has only dropped slightly and has not reached lockout — meaning the equipment can still operate safely temporarily — the real danger is not the low pressure itself, but the ongoing leakage trend.
Leakage is progressive: seal aging, flange seepage, welding defects — none self-heal. Drop by drop, month by month, pressure will eventually reach lockout, at which point the breaker is forcibly locked out and cannot operate.
Fault trip risk is extremely high: if a system fault occurs during pressure decline requiring a trip, insufficient arc-quenching may fail to extinguish the arc, leading to phase-to-phase short circuit or equipment explosion.
Insulation flashover risk: reduced pressure shrinks insulation margin; under overvoltage (lightning, switching) internal flashover may occur.
Leakage Judgment Method
Core method: compare adjacent pressure readings under identical temperature conditions.
At the same temperature, if adjacent recorded pressure values differ by (0.1–0.3) × 10⁵ Pa, suspect leakage exists. Pressure readings are temperature-sensitive; comparison must be at same temperature, or use the density relay reading (already temperature-compensated). Specific leakage rate grading and handling strategies per equipment manufacturer procedures.
Correct Handling Procedure
| Step |
Action |
| 1. Confirm Alarm Level |
Check density relay indication; verify current pressure is in alarm zone (above lockout, below rated). |
| 2. Enhance Monitoring |
Record current pressure; increase patrol frequency; focus on pressure trend. |
| 3. Report to Dispatch & Request Outage |
Standard practice. Equipment may remain loaded but must schedule outage for repair ASAP to prevent further decline to lockout. |
| 4. Implement Safety Pre-Control (Before Outage) |
Execute in sequence: ① Remove operation fuse (prevents mis-trip if lockout circuit unreliable); ② Apply dedicated lockout tool to mechanically block mechanism (ensures no mis-operation under low pressure); ③ Re-install operation fuse (ensures fail-safe protection circuit remains active). |
| 5. Identify Root Cause |
Use leak detector to locate leak points. Common locations: weld seams, flange sealing surfaces, arc chamber top covers, pipe joints. |
| 6. Replenish or Repair |
If no leak found (normal long-term pressure drop): qualified technician performs live gas replenishment using SF6 purity ≥99.995%. Recharge slowly near rated pressure to avoid overpressure shock to seals. If leak found: immediately report to dispatch, transfer load, then outage for repair. |
SF6 Decomposition Product Safety Alert
Pure SF6 is non-toxic, odorless, and non-flammable. After arc decomposition, it produces toxic, corrosive byproducts:
Toxic Gases: Thionyl fluoride (SOF₂), sulfuryl fluoride (SO₂F₂), sulfur tetrafluoride (SF₄), sulfur dioxide (SO₂).
Toxic Solids: Metal fluoride powders (copper fluoride, aluminum fluoride, etc.) — white or gray-white powder.
Safety Requirements:
- PPE mandatory for leak handling or maintenance: protective gloves, goggles, respirator.
- Before entering sealed gas compartment: ventilate thoroughly, verify gas concentration safe.
- Decomposition products are strongly corrosive — skin contact → wash immediately with soap and water.
Environmental Compliance Requirements
SF6 is among the most potent greenhouse gases known. Per IPCC assessment reports:
- Global Warming Potential (GWP): ~23,500× CO₂
- Atmospheric lifetime: ~3,200 years
Environmental Requirements:
- SF6 must be recovered using dedicated recovery equipment during leak handling and maintenance — venting to atmosphere is strictly prohibited.
- Recovered SF6 must be purified; re-use only after passing quality testing.
- Maintain gas usage log at maintenance site for full traceability.
Moisture Content Monitoring
Leakage means not only SF6 loss but potential moisture ingress. Moisture is a key driver of SF6 equipment insulation degradation:
Mechanism 1: Moisture + SF6 decomposition products → HF (hydrofluoric acid) & H₂SO₃ (sulfurous acid) → corrodes internal metal parts & insulation.
Mechanism 2: Moisture condenses at low temperature → reduces surface insulation strength → flashover risk.
Monitoring Requirements:
- When pressure drop detected, simultaneously test SF6 moisture content.
- Operational limits (per standards):
- Gas compartments communicating with arc chamber: ≤ 300 μL/L (ppm)
- Gas compartments not communicating with arc chamber: ≤ 500 μL/L (ppm)
- Exceedance → schedule gas drying treatment or replacement.
Summary
SF6 pressure slightly below rated but above lockout = temporary loaded operation permissible, but long-term operation PROHIBITED.
Guiding Principle: Alarm doesn't always mean immediate trip; lockout means no operation. Check alarm first, then pressure, assess trend, confirm leak point.
Key Takeaways
- Short-Term Viable: Pressure between alarm and lockout → arc-quenching & insulation still meet requirements → temporary loaded operation acceptable.
- Long-Term Prohibited: Continuous leakage → further pressure decline → eventual lockout or safety incident.
- Safety Pre-Control: Before outage: "Pull fuse → Block mechanism → Reinstall fuse" three-step sequence to ensure reliable lockout.
- Safety & Environment: Protect against toxic decomposition products; zero atmospheric SF6 release; monitor moisture concurrently.
- Mandatory Action: Enhance monitoring, report to dispatch, schedule outage for repair or replenishment promptly to eliminate hazard at source.
Edited From:Garca
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