Kung Paunsa ang Epekto sa Pagkawala sa Langis sa Kinatibuk-ang SF6 Relay?

1. Pagsangay sa SF6 ug mga Problema sa Pagdumal sa Langis sa Density Relays sa SF6

Ang mga kagamitan sa elektrisidad nga gipangangga og SF6 kasagaran na gamiton sa mga kompanya sa pagsuministro sa kuryente ug sa industriya, nagpadako kaayo sa pag-abli sa industriya sa kuryente. Ang medium nga gamiton sa pagpatay sa ark ug insulasyon niini mao ang gas sa sulfur hexafluoride (SF6), nga dili dapat magdumal. Bisan unsa nga pagdumal mosukol sa reliable ug safe nga operasyon sa kagamitan, kini nga nagkinahanglan og pag-monitor sa density sa gas sa SF6. Sa karon, ang mga mekanikal nga pointer-type density relays kasagaran gamiton alang niining proposito. Kini makapadala og alarm ug lockout signals kung adunay pagdumal sa gas, ug usab naka-provide og on-site density indication. Aron mapadako ang pagtakda sa vibration, kini nga mga relays kasagaran gipuno og silicone oil.

Pero, sa praktikal, ang pagdumal sa langis gikan sa SF6 gas density relays usa ka kasagaran nga problema. Kini nga problema kasagaran—tanang power supply bureau sa nasod naka-eksperyensya niini. Ang uban nga mga relays naka-develop og pagdumal sa langis sa wala pa isang tuig sa operasyon. Sa ikatulo, ang pagdumal sa langis sa mga oil-filled density relays usa ka kasagaran ug persistente nga problema.

2. Mga Bahandi sa Pagdumal sa Langis sa Density Relays

Bisan unsa, ang SF6 density relays kasagaran gigamit og spring-type electrical contact, na-enhance pinaagi sa magnetic assist mechanism aron masiguro ang reliable nga closure sa contact. Pero, ang force sa contact (para sa alarm o lockout) kasagaran depende sa weak force sa spring. Bisan may magnetic assistance, ang force labi na gamay, nagresulta sa contacts nga highly sensitive sa vibration. Aron mapadako ang pagtakda sa vibration, ang silicone oil kasagaran gipuno sa relay. Kung adunay pagdumal sa langis, kini naghatag og potential nga bahandi sa SF6 electrical equipment.

Bahandi 1: Kung ang anti-vibration oil total nga nadumal, ang damping effect mawala, drastic nga pagbawas sa pagtakda sa vibration sa relay. Human sa strong mechanical shocks sa circuit breaker switching operations, ang pointer mahimong mag-stuck, ang contacts mahimong mag-fail permanent (wala mag-actuate o mag-stay actuated), o ang measurement deviations mahimong mogamay sa acceptable limits.

Bahandi 2: Tungod kay ang contacts sa relay magnetic assist kasagaran may inherently low contact force, ang prolonged exposure mahimong mag-lead sa oxidation sa surfaces sa contact. Para sa mga relays nga nawala ang tanang langis, ang magnetically assisted contacts direkta nga exposed sa hangin, making them prone to oxidation or dust accumulation, resulting in poor contact or complete failure.

Sumala sa mga ulohan: Sa tulo ka tuig nga usa ka utility intensify ang ilang testing sa SF6 density relays, 196 units gin-inspect, ug 6 (about 3%) nakit-an nga may unreliable contact conduction. Tanang mga defective relays nawala ang tanang damping oil. Kung ang density relay mag-suffer sa stuck pointer, failed contacts, o unreliable conduction, kini mahimong severe nga compromise grid safety. Consider the scenario where an SF6 circuit breaker leaks gas and loses its insulating medium, but the density relay fails to trigger an alarm due to a stuck pointer or faulty contact. If the breaker then attempts to interrupt a fault current, the consequences could be catastrophic.

Bisan unsa, ang leaked oil mahimong maka-contaminate sa uban pang components sa switchgear, attracting dust and further endangering safe operation. Some units resort to wrapping the leaking relay in plastic bags to prevent oil from spreading and causing dust buildup. Moreover, modern substations are designed to be oil-free; thus, oil leakage is considered a defect that must be rectified.

3. Root Cause Analysis of Oil Leakage

Ang primary leakage points sa density relays mao ang seals sa pagitan sa terminal block ug case, ang glass window ug case, ug cracks sa glass mismo. Sumala sa disassembly sa daghan nga leaking relays, natukod nami nga ang main cause sa oil leakage mao ang seal failure sa interface sa terminal block-to-case ug glass-to-case. Ang sumusunod mao ang preliminary identified reasons for seal failure.

3.1 Rubber Seal Aging

Sa karon, daghang density relays gigamit og nitrile rubber (NBR) para sa oil-sealing O-rings. Ang NBR mao ang copolymer sa butadiene (CH₂=CH–CH=CH₂) ug acrylonitrile (CH₂=CH–CN), produced via emulsion polymerization. Kini mao ang unsaturated carbon-chain rubber. Ang acrylonitrile content significant nga affect NBR properties: higher content improves oil, solvent, ug chemical resistance, increases strength, hardness, wear resistance, ug heat resistance, pero reduces cold flexibility, elasticity, ug air permeability.

Ang rubber degrade during processing, storage, ug use tungod sa daghang factors, exhibiting discoloration, stickiness, hardening, ug cracking—phenomena collectively known as rubber aging.

Ang factors contributing to NBR seal aging include internal ug external causes.

3.2 Internal Causes

• Molecular Structure of NBR:
  NBR contains unsaturated double bonds in its polymer chain. Under heat and mechanical stress, oxygen reacts at these double bonds, forming peroxides that decompose into oxidative products, causing chain scission and cross-linking. This increases cross-link density, making the rubber harder and more brittle. Higher double bond content accelerates aging. Additionally, electron-donating substituents (e.g., –CH₃) in the molecular structure are easily oxidized.

• Effect of Rubber Compounding Agents:
  The choice of vulcanization system is critical. Higher sulfur content increases polysulfide cross-link concentration but accelerates aging.

3.3 External Causes

• Oxygen and Ozone:
  Oxygen is a primary aging factor, promoting chain scission and re-cross-linking. Ozone is even more reactive; it forms ozonides at double bonds, which decompose and break polymer chains. The seal is directly exposed to air, and trace amounts of oxygen and ozone dissolve into the oil, accelerating rubber aging.

• Heat:
  Heat accelerates oxidation—typically, a 10°C rise doubles the oxidation rate. It also accelerates reactions between rubber and additives or causes volatile components to evaporate, degrading performance and shortening service life.

• Mechanical Fatigue:
  Under constant stress (compression, torsion), rubber undergoes mechanical oxidation, accelerated by heat. Over time, elasticity diminishes—this is mechanical fatigue aging.

Aging of the rubber seal leads to seal failure, loss of sealing capability, and ultimately oil leakage.

3.4 Insufficient Initial Compression of the Seal

Rubber seals rely on compression deformation during installation to tightly fit against sealing surfaces and block leakage paths. Insufficient initial compression can lead to leaks. This can occur due to:

• Design issues: undersized seal cross-section or oversized groove;

• Installation issues: improper tightening of the cover (most relays rely on manual feel, making precise control difficult).
  Additionally, rubber has a cold-shrink coefficient over ten times that of metal. At low temperatures, the seal shrinks and hardens, further reducing compression.

3. Excessive Compression Rate

While compression is necessary for sealing, excessive compression is harmful. It may cause permanent deformation during installation or generate high von Mises stress, leading to material failure and reduced lifespan. Again, manual tightening often results in over-compression.

4. Surface Defects on Sealing Surfaces

Scratches, burrs, low surface roughness, or improper machining textures on sealing surfaces can create leakage paths.

5. Temperature Effects

At high temperatures, rubber softens and expands, potentially extruding and breaking the seal. At low temperatures, shrinkage and hardening can also cause leaks.

6. Improper Hardness Selection

If the rubber seal is too soft or too hard, it may fail to seal properly.

7. Rough Installation

Careless installation can damage the seal. For example, sharp edges or burrs may scratch the O-ring, creating invisible defects that lead to seal failure and oil leakage.Additionally, glass cracking can also cause oil leakage.

Causes include:
A) Uneven stress during installation, exacerbated by sudden changes in temperature or pressure;
B) Thermal shock causing the glass itself to crack. Cracks form leakage paths, resulting in oil loss.

Conclusion

In SF6 electrical equipment, SF6 gas serves as the primary insulating and arc-quenching medium. Its dielectric strength and arc-interrupting capability depend directly on gas density—higher density generally means better performance. However, due to manufacturing, operation, or maintenance issues, gas leakage is inevitable. A drop in density leads to two main risks: reduced dielectric strength and decreased circuit breaker interrupting capacity. Therefore, monitoring SF6 gas density is crucial for safe and reliable operation. This is typically achieved using SF6 density relays, which provide two-stage warnings—alarm and lockout signals—when density drops, enabling timely intervention.

Hence, on-site SF6 density relays must be reliable. Based on the above analysis, we conclude:

• Density relays exhibiting oil leakage must be promptly monitored and replaced.

• Newly installed relays should preferably be oil-free types with superior vibration resistance or improved gas-sealed designs.