A Comprehensive Analysis of Breakdown Causes and Repair Techniques for 35kV XLPE Cables

1. Breakdowns Caused by Cable Termination Defects

Cable terminations and joints represent vulnerable points in insulation, and their installation quality directly impacts operational safety. Statistics from a power supply company's 2023 fault reports indicate that 72% of termination failures originated from the following issues:

(1) Stress Cone Installation Failure
Cold-shrink terminations utilize stress cone structures to reduce field intensity at the shield cut-off point by modifying electric field distribution. Common on-site construction errors include:
① Stress cone misalignment exceeding 5mm from the semicon cut-off point (specification requirement: ±2mm), causing electric field distortion and localized field intensity reaching 25kV/mm (normal: ≤12kV/mm).
② Inadequate smoothing of the copper shield cut-off point, leaving burrs that create tip discharge (measured field intensity at tips can increase 3-5 times).
③ Excessive pulling speed (>50mm/s) during installation, causing stress cone wrinkling and forming air gaps (partial discharge levels > 10pC).

(2) Improper Semicon Layer Handling
In a case involving a 35kV cable breakdown at a steel plant, dissection revealed 0.3mm deep scoring on the main insulation surface (see Fig.2). This was caused by using a utility knife at too steep an angle (>45°) during semicon layer removal. GB50150 explicitly requires the use of dedicated stripping tools for semicon removal, forming a 15° gentle slope at the cut-off, with scoring depth not exceeding 0.1mm.

(3) Sealing/Waterproofing Failure
Statistics from rainy regions show 43% of termination failures involve moisture ingress. Typical errors include:
① Failure to follow the "triple-seal" process (outer sheath, copper shield, semicon layer).
② Insufficient sealing compound thickness (<2mm) and inadequate compression of constant-force springs (compression should reach 1/3 of original length).
③ Gaps between cold-shrink tubing and cable body (feeler gauge inspection should show no gap ≥0.05mm).

2. Mechanism of Main Insulation Degradation

(1) Electrical Treeing Aging
XLPE insulation can initiate electrical tree growth when localized field strength exceeds 10kV/mm. Accelerated aging tests at a research base demonstrate:
① For every 10°C temperature increase, tree growth rate increases by 2.3 times.
② With moisture present, tree channel propagation accelerates by 3-5 times (significantly when moisture content > 0.02%).
③ Impurity particles (>50µm) cause field concentration and easily become tree initiation points.

(2) Thermal Aging Failure
When cables are continuously overloaded (current >120% of rating), causing insulation temperature to exceed 90°C, XLPE molecular chains break:
① Oxidation Induction Time (OIT) drops from 30min to below 5min (GB/T 11026.1 requires ≥20min).
② Dissipation Factor (tanδ) rises from 0.002 to above 0.01 (measured at 20°C).
③ Tensile strength decreases by over 25%, and elongation at break reduces by 40%.

(3) Cumulative Mechanical Damage
Common mechanical damages during installation include:

• Insufficient bending radius (single-core cables: ≥20× outer diameter; multi-core: ≥15×).

• Excessive pulling tension (copper cables: ≤3kN; aluminum: ≤2kN).

• Unrepaired damaged outer sheaths, allowing moisture ingress (radial moisture penetration rate: ~0.1mm/day).

3. Impact of External Environmental Factors

(1) Overvoltage Surges
Lightning and switching surges are significant triggers:

• Direct lightning strikes can generate up to 200kV, far exceeding the 35kV cable's rated impulse withstand voltage of 32kV.

• Current chopping by vacuum breakers can produce overvoltages reaching 3.5× phase voltage.

• During system single-phase-to-ground faults, healthy phase voltage rises to line voltage (for a 35kV system: 60.6kV).

(2) Chemical Corrosion
Measurements in industrial zones show, in soil with pH <4 or >9:

• Outer sheath corrosion rate reaches 0.2mm/year (normal soil: 0.05mm/year).

• Steel tape armoring can perforate within 5 years, allowing moisture direct access to insulation.

• Organic acids from microbial corrosion reduce XLPE dielectric strength by 5% annually.

(3) Temperature Cycling Stress
Outdoor terminations experience periodic thermal expansion/contraction with diurnal temperature differences >15°C:

• Shear stress at interfaces reaches 1.2MPa (exceeding the fatigue limit of EPDM material).

• Micro-cracks form in sealing compound (observed depth up to 0.5mm under microscope).

• Gaps exceeding 0.1mm form between metal accessories and insulation, initiating partial discharge.

Breakdown Fault Diagnosis and Localization Technology

1. Fault Characterization and Assessment

1. (1) Insulation Resistance Testing
   Using a 2500V megohmmeter:
   • Inter-phase insulation resistance < 1000 MΩ or ground insulation resistance < 500 MΩ indicates severe defects.
   • Absorption ratio (R60s/R15s) < 1.3 suggests moisture ingress.
   • Polarization index (R10min/R1min) < 2.0 indicates insulation aging.

2. (2) Partial Discharge Detection
   Combined use of Ultra-High Frequency (UHF) and ultrasonic methods:
   • Discharge magnitude > 5 pC (at 1.73U₀) at terminations requires immediate action.
   • Typical discharge patterns: tip discharge shows scattered pulse amplitudes; void discharge exhibits regular pulse clusters.

3. (3) Dielectric Loss Factor (tan δ) Measurement
   At 10 kV test voltage:
   • Normal cable tan δ < 0.005; values > 0.01 indicate severe insulation degradation.
   • Significant increase in tan δ with rising voltage (Δtan δ > 0.002/kV) suggests the presence of defects.

2. Precise Localization Technologies

1. (1) Time Domain Reflectometry (TDR) Localization
   Using a pulse reflectometer (minimum resolution 0.5 m):
   • Fault distance formula: L = v × t / 2 (v = wave velocity, 172 m/μs for XLPE cables).
   • Low-resistance faults (< 100 Ω) use low-voltage pulse method; high-resistance faults use DC high-voltage flashover method.
   • Waveform characteristics: reflected pulse polarity opposite (low resistance) or same (high resistance) to incident pulse.

2. (2) Acoustic-Magnetic Synchronous Localization
   Applying impulse high voltage (3–5 times U₀) to the fault point:
   • Magnetic field signals detected via coil; acoustic signals received via piezoelectric sensor.
   • Time-difference localization: Δt = ΔS / v (v = 340 m/s), with localization error < 0.5 m.
   • Optimal under low ambient noise (< 40 dB at night); noise-cancelling headphones recommended.

3. (3) Distributed Optical Fiber Monitoring
   Using DTS (Distributed Temperature Sensing) systems:
   • Spatial resolution: 1 m; temperature accuracy: ±0.5°C.
   • Abnormal temperature rise at fault points (5–10°C higher than normal sections).
   • Combined with vibration-sensing fiber, can locate external damage points (alarm triggered at vibration frequency > 5 Hz).

35kV Cable Breakdown Repair Technical Specifications

1. Cable Termination Re-termination Procedure
Taking a 35kV cold-shrink termination (Model WLS-35/1×300) as an example, the key steps are as follows:

(1) Pre-treatment Stage

• Cable Straightening: Use a dedicated straightening machine (applying 2-3 kN tension) to ensure straightness error < 1‰.

• Outer Sheath Stripping: Ring-cut at 550mm from the end, retain 30mm of armor, roughen a 50mm area from the sheath cut-off using 80-grit sandpaper.

• Copper Shield Treatment: Retain 20mm of copper shield, polish the cut-off into a smooth arc transition (R ≥ 2mm) using No. 0 sandcloth.

(2) Semicon Layer Treatment

• Stripping Length: Retain 15mm of outer semicon layer, ring-cut using a dedicated stripping knife (15° angle), strictly avoid damaging the main insulation.

• Chamfering: Chamfer the main insulation end to a 45° angle (depth 0.5mm), round the semicon cut-off edge (R = 1mm).

• Cleaning Process: Wipe unidirectionally using a lint-free cloth soaked in anhydrous ethanol (purity ≥ 99.7%), change cloth every 100mm.

(3) Stress Cone Installation

• Silicone Grease Application: Apply dedicated silicone grease (tan δ < 0.001) within 5mm of the semicon cut-off, thickness 0.2mm.

• Positioning Mark: Wrap a positioning tape (width 10mm) at 75mm from the semicon cut-off.

• Cold-shrink Operation: Pull the core liner at a constant speed of 50mm/s, avoid rotating the termination during shrinking.

(4) Sealing Process

• Triple Waterproofing: Sequentially wrap semi-conductive water-blocking tape (25% overlap), sealing compound (thickness ≥ 2mm), and stainless-steel housing.

• Ground Connection: Use 25mm² copper cable, constant-force spring compression reaching 1/3 of original length, binding spacing ≤ 10mm.

• Phase Identification: Apply phase-color heat-shrink tubing (Yellow Phase A, Green Phase B, Red Phase C), length 100mm.

2. Main Insulation Repair Technology
For localized insulation breakdowns (area < 5 cm²), repair using patented technology (Patent No. ZL202210666205.8):

(1) Fault Point Treatment

• Ring-cut Insulation: Create a dumbbell-shaped groove (diameter 50mm, depth 20mm) centered on the breakdown point with a 1:5 slope.

• Surface Treatment: Sand circumferentially with 200-grit sandpaper until fresh insulation (no carbonized layer) is exposed.

• Cleanliness Check: Use a particle counter to ensure Class 100 cleanliness (< 3500 particles/m³ for particles ≥ 0.5μm).

(2) Nano-repair Liquid Infusion

• Material Ratio: 15% nano-SiO₂ (50nm particle size), 75% epoxy resin matrix, 10% curing agent (by weight).

• Vacuum Degassing: Process at -0.09 MPa for 30min to remove bubbles (bubble diameter < 5μm).

• Pressurized Infusion: Apply 0.3 MPa pressure for 2h to ensure repair liquid penetration depth ≥ 10mm.

(3) Curing and Finishing

• Step Curing: 60°C/2h + 80°C/4h + 100°C/2h, avoid local overheating (heating rate ≤ 5°C/min).

• Surface Finishing: Grind flush with the original insulation (deviation < 0.1mm) using a diamond grinding wheel (400-grit).

• Shield Restoration: Lap-wind 0.1mm thick copper tape (20% overlap), seal with solder (solder joint length ≥ 30mm).

3. Joint Replacement Procedure
When the breakdown point is in the middle of the cable, replace using a 35kV pre-fabricated straight joint (Model JLS-35/1×400):

(1) Cable Pre-treatment

• Cut Fault Section: Retain 1.5m of sound cable at each end, ensure insulation surface is free of scoring (check with eddy current flaw detector).

• Conductor Connection: Use a compression mold (hexagonal), compress from center outwards, compression factor ≥ 0.9.

• Insulation Stepping: Create a 1:10 tapered step (length 50mm), surface roughness Ra ≤ 0.8μm.

(2) Joint Assembly

• Semicon Shield Restoration: Wind semicon tape (width 50mm) to ensure reliable contact with original semicon (contact resistance < 50 mΩ).

• Insulation Component Installation: Heat the pre-fabricated insulator to 70°C, slide it on, apply 5kN axial pressure for 30min.

• Metal Housing Seal: Use dual O-rings (fluororubber material), control compression to 25%-30%.

(3) Shield and Grounding

• Copper Shield Bonding: Use 35mm² copper braid, bolt connection (torque 25 N·m).

• Grounding System: Adopt "double-end grounding", ground wire cross-section ≥ 50mm², ground resistance < 10 Ω.

• Anti-corrosion Treatment: Coat housing with epoxy primer (dry film thickness 80μm) + polyurethane topcoat (dry film thickness 120μm).

Prevention and remediation of 35kV XLPE cable breakdown faults must adhere to the principle of "prevention first, repair second." It is recommended to strengthen management in the following areas:

• Material Control: Establish a supplier whitelist system and implement incoming inspections for cold-shrink terminations (testing dielectric loss, partial discharge, and sealing performance).

• Process Optimization: Promote the use of intelligent construction robots to automate critical processes such as semicon layer stripping and stress cone installation.

• Condition Monitoring: Conduct insulation diagnostics (dielectric loss, tanδ, partial discharge) on cables in service for over 15 years to assess remaining lifespan.

• Emergency Response Capability: Form professional emergency repair teams equipped with acoustic-magnetic locators, high-voltage test vehicles, and other tools to ensure fault localization within 2 hours and restoration within 24 hours.

The technical methods outlined in this article can effectively reduce the 35kV cable breakdown failure rate. After implementing this technical system, a power grid company reduced the average cable fault repair time from 48 hours to 12 hours in 2024. In the future, with advancements in nano-repair materials and intelligent monitoring technologies, cable repair will evolve toward "precise localization, minimally invasive repair, and condition awareness."