SF6 Density Relay Oil Leakage: Causes, Risks & Oil-Free Solutions

1. Introduction
SF6 electrical equipment, renowned for its excellent arc-quenching and insulating properties, has been widely applied in power systems. To ensure safe operation, real-time monitoring of SF6 gas density is essential. Currently, mechanical pointer-type density relays are commonly used, providing functions such as alarm, lockout, and on-site display. To enhance vibration resistance, most of these relays are filled internally with silicone oil.

However, oil leakage from density relays is a common issue in practice, occurring in both domestic and imported products—though imported units generally exhibit longer oil retention periods and lower leakage rates. This problem has become a widespread challenge faced by power supply enterprises nationwide, significantly affecting the long-term stable operation of equipment.

2. Hazards of Oil Leakage in Density Relays

• Reduced Vibration Resistance:
       Silicone oil provides damping. Once it leaks completely, the relay becomes susceptible to pointer jamming, contact failure (non-operation or false triggering), and excessive measurement deviation under the impact of      switch operations.

• Contact Oxidation and Poor Contact:
       Most SF6 density relays use magnetic-assisted spiral spring contacts with low contact pressure, relying on silicone oil to isolate air. After oil leakage, the contacts are exposed to air, making them prone to oxidation      or dust accumulation, leading to poor contact or open circuits.

• Field Test Data:
       Among 196 density relays tested within three years, six exhibited unreliable contact conduction (approximately 3%), all of which were units that had lost their oil.

• Serious Safety Risks:
       If an SF6 circuit breaker leaks gas while the density relay fails due to oil leakage and cannot trigger alarm or lockout signals, major accidents may occur during arc interruption.

• Contamination of Equipment Components:
       Leaked silicone oil attracts dust, contaminating other components of the switchgear, thereby degrading overall insulation performance and operational safety.

3. Analysis of Oil Leakage Causes
Oil leakage primarily occurs at the following locations:

• Sealing interface between terminal base and case

• Sealing interface between glass window and case

• Cracking of the glass itself

3.1 Rubber Seal Aging
Most current seals use nitrile rubber (NBR), an unsaturated carbon-chain rubber highly susceptible to aging due to internal and external factors.

Internal Factors:

• Molecular Structure: The presence of double bonds makes the material vulnerable to oxidation, forming peroxides that lead to chain scission or cross-linking, resulting in hardening and embrittlement.

• Compound Ingredients: Excessive sulfur content in the vulcanization system accelerates aging.

External Factors:

• Oxygen and Ozone: Direct exposure to air or oxygen/ozone dissolved in oil initiates oxidative  reactions.

• Thermal Effects: For every  10°C increase in temperature, the oxidation rate approximately doubles.

• Mechanical Fatigue: Prolonged  compressive stress induces mechanical oxidation, accelerating the aging  process.

3.2 Improper Initial Compression of Seals

• Insufficient Compression:

• Design flaws: undersized seal cross-section or oversized  groove.

• Installation issues: reliance on manual tightening without  precise control.

• Low-temperature effects: rubber contracts more than metal when  cold, and hardens at low temperatures, reducing effective compression.

• Excessive Compression:

• Can cause permanent deformation or generate high Von Mises stress, leading to premature material failure.

3.3 Defects in Sealing Surfaces and Installation Issues

• Surface scratches, burrs, inappropriate surface roughness, or  unfavorable machining textures can create leakage paths.

• Seals damaged by sharp edges during installation, causing  hidden defects.

• Glass cracking causes:

• Uneven force application during installation;

• Cracking due to rapid changes in temperature or pressure.

4. Improvement Suggestions

Fundamental Solution: Use Oil-Free, Anti-Vibration SF6 Density Relays
This type eliminates the risk of oil leakage through structural innovation.

Technical Features:

• Vibration Isolation Pad: Installed between the connector and the case to absorb shock energy from switching  operations, achieving vibration resistance up to 20 m/s².

• Operating Principle: Utilizes  a Bourdon tube elastic element combined with a temperature compensation  bimetallic strip to accurately reflect changes in SF6 gas density.

• Signal Output: Employs  micro-switches actuated by the temperature compensation strip and Bourdon  tube, enhanced by the vibration isolation pad, offering strong  anti-interference capability and reduced risk of false operation.

Advantages:

• Completely eliminates the need for oil filling, thus preventing oil leakage at the source;

• Superior vibration resistance, suitable for high-vibration  environments;

• High structural reliability and low maintenance cost;

• Direct replacement for existing oil-filled models, enabling  "oil-free" upgrades.

Implementation Recommendations:

• Promptly replace any density relays exhibiting oil leakage;

• Prioritize oil-free, anti-vibration models during replacement;

• Conduct leak testing after replacement to ensure proper  sealing.

5. Conclusion

• SF6 gas density is a critical parameter for ensuring safe equipment operation and must be monitored via reliable density relays.

• Oil-filled density relays currently suffer from widespread oil  leakage, primarily due to rubber seal aging, improper compression control,  and substandard installation practices.

• Oil leakage leads to degraded vibration resistance and contact  failure, posing serious threats to grid safety.

• The adoption of oil-free, anti-vibration SF6 density relays is  recommended as a replacement solution, effectively eliminating oil leakage  and enhancing system reliability and economic efficiency.