چۆن دانەدانی ئەویل بڕواندنەوەی ڕێکخستگەر SF6-یە؟

1. Pêşkêşên Elektrîkî SF6 û Pirsgirêka Herêmdekirina Nafara di Relâyên Densîtîya SF6 de

Pêşkêşên elektrîkî SF6 yê hêj ji bo kompaniyên bêrdestkirina dema nîvendî û çarçoveyên endustreyî yên digeran hatin bikarhênerin, wekhevi serdemeyên endamgirtîna endustriya elektrîkî. Medyanê qe herêmdekirina sarkir û tenzîl di pêşkêşan de ye, gazê sulfur hexafluoride (SF6) ye, ku nekaribê herêmdebike. Herêmdekirina jêrîn bi taybetmendiya dixwazra û xweşdariya pêşkêşan derbas dide, lê kirin da ku densitî ya gazê SF6 bimarebin. Yana, relâyên densitîya şêwar ên mekanîkî piştî rastîn bikarhênerin. Relâyên wan dikarin alarmlar û signalên kilitkirin biguherînin lê heke herêmdekirina gasê bigere û navnîşa densitîya li ser sahre bibin. Ji bo virazkirina dayinê, relâyên wan li serê rengîn bi silikonîkê têne rengkirin.

Lê, di amûrdî de, herêmdekirina nafara di relâyên densitîya SF6 de pirsgirêkî û tevahî ye. Ev pirsgirêkî hemî serkomînên bêrdestkirina dema nîvendî yên Sûriyê destniha kir. Hesp relâyên ên bi nafar rengkirin herêmdekirina nafara li ser salên yekan gerdîn. Bi binbihêl, herêmdekirina nafara di relâyên bi nafar rengkirin de pirsgirêkî û tevahî ye.

2. Zararrê Herêmdekirina Nafara di Relâyên Densitî de

Bilîye, relâyên densitîya SF6 yê genîn ên elektrikî yên springî bikarhênerin, di navbera îro çavkaniyên magnektîkî anîn guherandin da ku têkilîn guhertinê veqetandî bibe. Lê, zorê têkilîn (ji bo alarm û kilitkirin) paqij ji zorê zevînî springê ye. Di navbera çavkaniyên magnektîkî de, zorê wê cihêr zevîn e, ku têkilîn bi rêjîn vêjîn hewceyên vibrasyonê. Ji bo virazkirina dayinê, silikonîkê li serê relây rengkirin. Heke herêmdekirina nafara bigere, ew potensiyel tênduyên neshtmanî yên SF6 pêşkêşan dibêje.

Zarar 1: Herêmdekirina nafara yekê, dayinê rengkirin werdigire, di navbera operasyonên switchînên circuit breaker ên zorî de, şêwar dikare were bistîn, têkilîn dikare were çalakkirin (ya ne aktîf bike û ya aktîf bibe), an deviyeşên pêwistîn dikare were kevirin.

Zarar 2: Ji ber ku têkilîn ên çavkaniyên magnektîkî yê zorê zevînî, di dema dirêj de, têkilîn dikare were oxîdekirin. Ji bo relâyên ên nafar rengkirin, têkilîn ên çavkaniyên magnektîkî direk li ser havê, ku têkilîn dikare were oxîdekirin an dust gotin, ku têkilîn bi rêjîn vêjîn hewceyên vibrasyonê.

Li gorî raporan: Li dema sê salan di navbera ku kompaniya yekê testkirina relâyên densitîya SF6 ên guhertin, 196 unitê hatin parwestin, û 6 (piştî 3%) dikarin têkilîn bi rêjîn vêjîn hewceyên vibrasyonê. Hemî relâyên ên têkilîn bi rêjîn vêjîn hewceyên vibrasyonê nafar rengkirin. Eger relây ên densitî şêwar bistîn, têkilîn çalakkirin, an têkilîn bi rêjîn vêjîn hewceyên vibrasyonê, ew divê kesetiya guhertinê were girîng. Peywandbike şopandina ku circuit breaker ên SF6 gasê bigere û medyanê tenzîl hilan, lê relây ên densitî alarm bidin, eger circuit breaker ên wê di navbera guhertinê de, mîna tênduyên neshtmanî dibêje.

Di navbera ku, nafar herêmdekirin dikare komponentên din ên switchgear bigirin, dust gotin û tênduyên neshtmanî yên xebatkerdina werdigirin. Kompaniyên ên din ên relâyên herêmdekirin di plastic bag de bikin da ku nafar nebigere û dust nebigirin. Navbera, substationên modern ên oil-free dizayn kirin, lê, herêmdekirina nafara pirsgirêkî ye ku parastîn dibêje.

3. Analîzê Rêzikê Herêmdekirina Nafara

Navberên herêmdekirina nafara yê rastîn di relâyên densitî de terminal block û case, glass window û case, û cracks in the glass itself. Li gorî disassembling numerous leaking relays, we have determined that the main cause of oil leakage is seal failure at the terminal block-to-case and glass-to-case interfaces. The following are the preliminary identified reasons for seal failure.

3.1 Rubber Seal Aging

Currently, most density relays use nitrile rubber (NBR) for the oil-sealing O-rings. NBR is a copolymer of butadiene (CH₂=CH–CH=CH₂) and acrylonitrile (CH₂=CH–CN), produced via emulsion polymerization. It is an unsaturated carbon-chain rubber. The acrylonitrile content significantly affects NBR properties: higher content improves oil, solvent, and chemical resistance, increases strength, hardness, wear resistance, and heat resistance, but reduces cold flexibility, elasticity, and air permeability.

Rubber degrades during processing, storage, and use due to various factors, exhibiting discoloration, stickiness, hardening, and cracking—phenomena collectively known as rubber aging.

Factors contributing to NBR seal aging include internal and 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.