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Pagsasara ng metal na sheath ng high-voltage cable

Felix Spark
Felix Spark
Larangan: Pagsasara at Pagsasainit
China

I. Mga Punsyon ng Metal na Sheath at Necessity ng Repair

Ang metal na sheath ng high-voltage cables ay isang metal shielding structure na inilalapat sa labas ng insulation layer, kasama ang mga uri tulad ng lead sheaths, aluminum sheaths, at steel wire armor. Ang mga pangunahing punsyon nito ay kinabibilangan ng mechanical protection (resistensya sa external impact at compression), electrochemical corrosion protection (paghihiwalay ng moisture at soil pollutants), electromagnetic shielding (reduksyon ng electromagnetic interference sa environment), at pagbibigay ng grounding path (pag-ensure ng safe discharge ng fault currents). Kapag nasira, maaaring magresulta ito sa pagpasok ng moisture sa insulation layer, distortion ng lokal na electric field, o kahit na serious na mga aksidente tulad ng cable breakdown at short circuits. Dahil dito, mahalagang maipakilala ang precise repair para sa iba't ibang uri ng pinsala upang matiyak ang long-term na ligtas na operasyon ng cable system.

high-voltage cable.jpg

II. Fault Diagnosis at Assessment Bago ang Repair

(A) Pag-identify ng Mga Uri ng Damage

  • Mechanical Damage: Karakterisado ng dents, cracks, o punctures sa sheath, karaniwang dulot ng mechanical rolling o sharp object punctures sa panahon ng installation, o tensile rupture dahil sa long-term na foundation settlement.

  • Electrochemical Corrosion: Ang stray currents sa soil o acidic/alkaline environments ay maaaring magdulot ng electrochemical corrosion sa sheath, na may ebidensiya ng localized bulging, rust, perforation, at white/green corrosion products (aluminum sheath) o black sulfides (lead sheath).

  • Thermal Aging Damage: Ang long-term overload operation ay nagdudulot ng embrittlement ng sheath material, na nagresulta sa cracking at delamination, karaniwang matatagpuan sa joints o areas na may mahina na heat dissipation.

(B) Application ng Detection Technologies

  • Visual Inspection: Gumamit ng endoscopes o infrared thermal imagers upang obserbahan ang surface ng sheath, nakatuon sa pag-identify ng obvious damage points at hot spots.

  • Sheath Withstand Voltage Test: I-apply ang DC withstand voltage (10 kV sa 1 minuto) upang suriin ang integrity ng sheath insulation. Ang abnormal increase sa leakage current (>10 μA) ay nag-indicate ng damage.

  • Partial Discharge Detection: Gumamit ng high-frequency current sensors (HFCT) upang i-capture ang partial discharge signals sa damage points, na may accuracy ng location na ±0.5 m.

  • Soil Corrosivity Assessment: I-collect ang soil samples mula sa cable laying environment upang suriin ang pH, chloride ion concentration, at stray current density, na nagbibigay ng pundamento para sa pagpili ng repair materials.

cable.jpg

III. Pagpili ng Repair Materials at Tools

(A) Core Repair Materials

  • Metallic Sheath Replacement Materials:

    • Aluminum Alloy Compression Sleeve: Angkop para sa aluminum sheath repair, may mabuting ductility at corrosion resistance. Dapat tumugon sa cable outer diameter (tolerance ≤ ±0.5 mm).

    • Lead-Tin Alloy Tape: Ginagamit para sa lead sheath repair, may mababang melting point (~183°C), madali para sa thermal welding, tumutugon sa GB/T 12706.2 purity requirements (lead content ≥ 99.9%).

    • Stainless Steel Corrugated Tube: Para sa steel armor damage, gawa ng 304 stainless steel, wall thickness ≥ 0.8 mm, may resistensya sa impact at soil stress.

  • Insulation and Sealing Materials:

    • Cross-Linked Polyethylene (XLPE) Heat-Shrink Tubing: Nagshrink sa 120–140°C, shrink ratio ≥ 2:1, breakdown strength ≥ 25 kV/mm, nangangailangan ng hot-melt adhesive para sa sealing.

    • Silicone Rubber Cold-Shrink Tubing: Umaasa sa elastic recovery para sa sealing, walang heating required, angkop para sa confined spaces, Shore hardness 60 ± 5 Shore A, tanδ ≤ 0.003 (20°C, 50 Hz).

    • Butyl Rubber Sealing Tape: Ginagamit bilang auxiliary sealing layer, tensile strength ≥ 3 MPa, elongation at break ≥ 400%, aging resistance retains ≥ 80% performance after 100°C × 168 h thermal aging.

  • Corrosion Protection Materials:

    • Zinc-Aluminum Alloy Sacrificial Anode: Para sa high-corrosion soil environments, anode purity ≥ 99.5%, current density ≥ 15 mA/m², design life ≥ 20 years.

    • Polyvinyl Chloride (PVC) Corrosion-Resistant Tape: Thickness ≥ 0.4 mm, tensile strength ≥ 18 MPa, environmental stress cracking resistance (ESCR) ≥ 1000 h.

(B) Specialized Tools

  • Preparation Tools: Angle grinder (with 80-grit alumina grinding wheel), wire brush, anhydrous ethanol cleaner, stainless steel scraper (for removing corrosion products).

  • Forming Tools: Hydraulic crimping tool (crimping range 60–200 mm²), heat gun (temperature range 50–600°C), lead sealing dedicated torch (flame temperature ≤ 300°C).

  • Testing Tools: Megohmmeter (2500 V, range 0–10000 MΩ), double-arm bridge (contact resistance measurement, accuracy ±0.1 μΩ), ultrasonic thickness gauge (resolution 0.01 mm).

cable.jpg

IV. Detailed Repair Procedures by Damage Type

(A) Mechanical Damage Repair (Aluminum Sheath Example)

  • Damage Area Preparation

    • Gumamit ng angle grinder upang buksan ang nasirang sheath axially, na may cut length na 5 beses ang damage diameter (minimum ≥ 100 mm), na nagpapakita ng malinis na insulation shield.

    • Alisin ang burrs mula sa edges ng sheath gamit ang stainless steel scraper, sand to metallic luster, linisin ang oil gamit ang ethanol, at dry for ≥ 15 min.

  • Metallic Sheath Restoration

    • Piliin ang aluminum alloy compression sleeve na may inner diameter na 1 mm mas malaki kaysa sa cable outer diameter, pabigyan ng uniform coat ang inner wall ng conductive grease (nickel-based filler, volume resistivity ≤ 5×10⁻⁴ Ω·cm).

    • Islide ang sleeve sa damaged area, gumamit ng Staggered Crimping method, crimp mula sa center hanggang sa ends. Matapos ang crimping, hexagonal opposite-side deviation ≤ ±0.1 mm, contact resistance ≤ 20 μΩ.

  • Sealing and Anti-Corrosion Treatment

    • I-wrap ang butyl rubber sealing tape na may 50% overlap, bumuo ng sealing layer ≥ 3 mm thick, na umaabot ≥ 50 mm beyond undamaged sheath sa parehong dulo.

    • I-install ang heat-shrink tubing, i-heat gradually mula sa center hanggang sa ends (120°C → 140°C) upang iwasan ang bubbles. Matapos ang cooling, suriin ang uniform shrinkage (post-shrinkage wall thickness ≥ 2 mm).

    • I-wrap ang PVC corrosion-resistant tape externally sa half-overlap spiral, secure ang ends gamit ang stainless steel straps (torque 15–20 N·m).

(B) Electrochemical Corrosion Repair (Steel Armor + Lead Sheath Composite Sheath)

  • Corrosion Product Removal

    • Gumamit ng sandblasting (quartz sand 80–120 mesh, pressure 0.4–0.6 MPa) upang alisin ang rust mula sa steel armor, na nagpapakita ng gray-white metal base, surface roughness Sa2.5 grade.

    • Trim ang lead sheath corrosion area gamit ang lead plane, bumuo ng smooth transition na may slope ≥ 1:5 upang iwasan ang stress concentration.

  • Sacrificial Anode Protection

    • Symmetrically install 2 zinc-aluminum alloy anodes (100 mm × 50 mm × 10 mm) sa parehong side ng repair section, konektado sa steel armor via copper braid (cross-section ≥ 16 mm²), welded (lap length ≥ 30 mm, welding current 120–150 A).

    • I-fill ang gap sa pagitan ng anode at cable sheath gamit ang petroleum jelly, encapsulate externally sa pamamagitan ng high-density polyethylene (HDPE) housing upang matiyak ang full soil contact.

  • Dual Sealing Structure

    • Inner layer: cold-shrink silicone rubber tubing, inner wall coated with semiconductive water-blocking gel (volume resistivity 10–100 Ω·cm). Maintain at room temperature for 24 h after recovery for curing.

    • Outer layer: pour epoxy resin (E-51 type, curing agent T-31, ratio 100:25), mold sealed at 0.2 MPa pressure, cure ≥ 48 h at 25°C. Post-cure Shore hardness ≥ 85 Shore D.

(C) Thermal Aging Cracking Repair (XLPE Insulated Cable)

  • Crack Area Reinforcement

    • Cut a V-groove along the crack direction (depth 1/3 of sheath thickness, angle 60°), fill with heat-resistant silicone rubber (temperature rating ≥ 180°C, tear strength ≥ 15 kN/m), level with scraper, cure at room temperature for 2 h.

    • Wrap glass fiber cloth (0.2 mm thick, 16×16 threads/cm), impregnate with epoxy resin (solid content ≥ 70%) to form a reinforcement layer. Tensile strength ≥ 200 MPa after curing.

  • Heat Dissipation Optimization

    • Install aluminum heat sinks (1.5 mm thick, fin spacing 5 mm) externally on the repair section, bonded to sheath with thermal grease (thermal conductivity ≥ 1.5 W/(m·K)) to reduce local temperature rise (≤ 5°C).

    • Monitor operating temperature with infrared thermal imager, ensuring temperature difference from undamaged section ≤ 2°C.

V. Quality Control at Acceptance Standards

(A) Key Parameter Testing

  • Electrical Performance: Sheath insulation resistance ≥ 1000 MΩ (2500 V megohmmeter), partial discharge ≤ 5 pC (at 1.73U₀ voltage).

  • Mechanical Performance: Impact resistance of repair section ≥ 10 J (at -30°C), bending radius ≥ 20 times cable outer diameter (no obvious deformation).

  • Sealing Performance: After immersion test (room temperature, 24 h), sheath leakage current change rate ≤ 10%, no bubbles or water ingress.

(B) Long-Term Reliability Verification

  • Accelerated Aging Test: Place repair sample in thermal aging chamber (135°C × 1000 h), after removal insulation resistance retention ≥ 80%, tensile strength degradation ≤ 20%.

  • Soil Burial Test: Simulate actual laying environment, buried depth 1 m, duration 1 year, check for no corrosion perforation or seal failure.

VI. Safety Operation Regulations

  • Power-Off Work Requirements: Follow "power-off - voltage test - grounding wire installation" procedure before repair. Hang "Do Not Close" warning signs on cable terminations, set safety barriers (distance ≥ 8 m) within work radius.

  • Hot Work Management: When using torches or welding, equip dry powder fire extinguishers (ABC type, capacity ≥ 4 kg), clear flammable materials within 3 m, assign a dedicated supervisor.

  • Personal Protective Equipment (PPE): Wear insulated gloves (35 kV rating), goggles, flame-resistant workwear. Use safety harness (static load test force 2205 N, hold 3 min without break) for work at heights (≥ 2 m).

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