Repair of high-voltage cable metallic sheaths

I. Functions of Metallic Sheaths and Necessity of Repair

The metallic sheath of high-voltage cables is a metal shielding structure laid outside the insulation layer, including types such as lead sheaths, aluminum sheaths, and steel wire armor. Its core functions include mechanical protection (resisting external impact and compression), electrochemical corrosion protection (isolating moisture and soil pollutants), electromagnetic shielding (reducing electromagnetic interference to the environment), and providing a grounding path (ensuring safe discharge of fault currents). Once damaged, the metallic sheath may lead to insulation layer moisture ingress, local electric field distortion, or even serious accidents such as cable breakdown and short circuits. Therefore, precise repair targeting different types of damage is critical to ensuring the long-term safe operation of the cable system.

II. Fault Diagnosis and Assessment Prior to Repair

(A) Identification of Damage Types

• Mechanical Damage: Characterized by dents, cracks, or punctures in the sheath, commonly caused by mechanical rolling or sharp object punctures during installation, or tensile rupture due to long-term foundation settlement.

• Electrochemical Corrosion: Stray currents in soil or acidic/alkaline environments can cause electrochemical corrosion of the sheath, evidenced by localized bulging, rust, perforation, and white/green corrosion products (aluminum sheath) or black sulfides (lead sheath).

• Thermal Aging Damage: Long-term overload operation causes sheath material embrittlement, resulting in cracking and delamination, commonly found at joints or areas with poor heat dissipation.

(B) Application of Detection Technologies

• Visual Inspection: Use endoscopes or infrared thermal imagers to observe the sheath surface, focusing on identifying obvious damage points and hot spots.

• Sheath Withstand Voltage Test: Apply DC withstand voltage (10 kV for 1 minute) to test sheath insulation integrity. Abnormal increase in leakage current (>10 μA) indicates damage.

• Partial Discharge Detection: Use high-frequency current sensors (HFCT) to capture partial discharge signals at damage points, with location accuracy within ±0.5 m.

• Soil Corrosivity Assessment: Collect soil samples from the cable laying environment to test pH, chloride ion concentration, and stray current density, providing a basis for selecting repair materials.

III. Selection of Repair Materials and Tools

(A) Core Repair Materials

• Metallic Sheath Replacement Materials:

• Aluminum Alloy Compression Sleeve: Suitable for aluminum sheath repair, with good ductility and corrosion resistance. Must match cable outer diameter (tolerance ≤ ±0.5 mm).

• Lead-Tin Alloy Tape: Used for lead sheath repair, low melting point (~183°C), easy for thermal welding, meeting GB/T 12706.2 purity requirements (lead content ≥ 99.9%).

• Stainless Steel Corrugated Tube: For steel armor damage, made of 304 stainless steel, wall thickness ≥ 0.8 mm, with impact and soil stress resistance.

• Insulation and Sealing Materials:

• Cross-Linked Polyethylene (XLPE) Heat-Shrink Tubing: Shrinks at 120–140°C, shrink ratio ≥ 2:1, breakdown strength ≥ 25 kV/mm, requires hot-melt adhesive for sealing.

• Silicone Rubber Cold-Shrink Tubing: Relies on elastic recovery for sealing, no heating required, suitable for confined spaces, Shore hardness 60 ± 5 Shore A, tanδ ≤ 0.003 (20°C, 50 Hz).

• Butyl Rubber Sealing Tape: Used as 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: For 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).

IV. Detailed Repair Procedures by Damage Type

(A) Mechanical Damage Repair (Aluminum Sheath Example)

• Damage Area Preparation

• Use an angle grinder to cut open the damaged sheath axially, with cut length 5 times the damage diameter (minimum ≥ 100 mm), exposing clean insulation shield.

• Remove burrs from sheath edges with a stainless steel scraper, sand to metallic luster, clean oil with ethanol, and dry for ≥ 15 min.

• Metallic Sheath Restoration

• Select an aluminum alloy compression sleeve with inner diameter 1 mm larger than cable outer diameter, uniformly coat inner wall with conductive grease (nickel-based filler, volume resistivity ≤ 5×10⁻⁴ Ω·cm).

• Slide the sleeve over the damaged area, use a Staggered Crimping method, crimp from center to ends. After crimping, hexagonal opposite-side deviation ≤ ±0.1 mm, contact resistance ≤ 20 μΩ.

• Sealing and Anti-Corrosion Treatment

• Wrap butyl rubber sealing tape with 50% overlap, forming a sealing layer ≥ 3 mm thick, extending ≥ 50 mm beyond undamaged sheath on both ends.

• Install heat-shrink tubing, heat gradually from center to ends (120°C → 140°C) to avoid bubbles. After cooling, check uniform shrinkage (post-shrinkage wall thickness ≥ 2 mm).

• Wrap PVC corrosion-resistant tape externally in a half-overlap spiral, secure ends with stainless steel straps (torque 15–20 N·m).

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

• Corrosion Product Removal

• Use sandblasting (quartz sand 80–120 mesh, pressure 0.4–0.6 MPa) to remove rust from steel armor, exposing gray-white metal base, surface roughness Sa2.5 grade.

• Trim lead sheath corrosion area with a lead plane, forming a smooth transition with slope ≥ 1:5 to avoid stress concentration.

• Sacrificial Anode Protection

• Symmetrically install 2 zinc-aluminum alloy anodes (100 mm × 50 mm × 10 mm) on both sides of the repair section, connected to steel armor via copper braid (cross-section ≥ 16 mm²), welded (lap length ≥ 30 mm, welding current 120–150 A).

• Fill gap between anode and cable sheath with petroleum jelly, encapsulate externally with high-density polyethylene (HDPE) housing to ensure 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 and 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).