When transmission lines extend from protected substations into heavy-duty direct-burial or long-distance distribution environments, electrical performance specifications alone are insufficient. The cables must withstand immense mechanical stresses, including tensile loads during installation, crushing forces from heavy vehicle traffic, and the impacts of geological shifts.
This report on durability performance analyzes the structural engineering characteristics of YJV62 and YJV72 cables (rated 35kV and above), detailing the mechanical and physical principles that establish them as core components of medium-voltage distribution and substation networks.
Structural Analysis: Mechanical Characteristics of Double-Layer Armoring
A distinctive feature of YJV62/YJV72 cables (see structural cross-section) is their double-layer armor protection system. This design is not redundant; each armor layer serves a specific mechanical function.
Inner Layer: Coarse Steel Wire (Tensile Protection)
Primary Function: Acts as the main load-bearing element for tensile forces.
Engineering Principle: Cables are subjected to immense tensile forces during installation in deep trenches or vertical shafts. The coarse steel wire layer bears this axial load, thereby preventing tensile deformation of the copper conductors; such deformation would otherwise increase electrical resistance and cause permanent damage to the cable.
Application criticality: A mandatory requirement for long-distance horizontal pulling and vertical installations (risers) involving significant elevation changes.
Outer structure: Steel tape (lateral protection)
Primary function: Resistance to lateral pressure.
Engineering principle: Once buried, the cable is subjected to both static soil pressure and dynamic loads (such as heavy trucks driving over shallow trenches). The steel tape layer provides radial protection, distributing external pressure around the cable's circumference to prevent deformation of the XLPE insulation—deformation that could otherwise lead to insulation breakdown at the 35kV voltage level.
Application criticality: An essential feature for direct-buried installations beneath roads, at railway crossings, and in heavy industrial zones.
Application Scenario: Matching Mechanical Characteristics with Infrastructure Requirements
YJV62/YJV72 cables are specifically designed for high-stress environments. Using unsuitable cables in these scenarios could lead to catastrophic failure.
A. Substation Incoming and Outgoing Lines
Substations are characterized by dense, complex cable layouts and a risk of high fault currents. The dual-layer metal armor structure provides critical electromagnetic shielding (radial electric field confinement) and physical protection within crowded cable trenches, ensuring that immense short-circuit electromagnetic forces do not cause physical deformation or rupture of the cables during a fault event.
B. Heavy-Duty Direct Burial / Long-Distance Power Transmission
Using concrete conduits is often not economically feasible when laying 35kV lines in open terrain. Direct burial requires cables to withstand soil subsidence, root penetration, and rodent damage. The outer steel-tape structure provides the necessary armor protection, enabling the cable to endure direct contact with rocky backfill and resist continuous bending stresses caused by ground settlement.
C. Road and Bridge Crossings
Dynamic traffic loads generate intense, cyclic lateral pressure. A double-layer armor design (using coarse steel wire or steel tape) represents the minimum technical standard for Horizontal Directional Drilling (HDD) installations and road-crossing applications.
Core Engineering Principles: Thermal and Electrodynamic Characteristics at 35kV
Operation at 35kV generates electrical stresses far exceeding those found in low-voltage systems. The YJV62/YJV72 system addresses this challenge through advanced materials science:
- XLPE Dielectric Strength: The cross-linked polyethylene (XLPE) insulation is specifically engineered to withstand high electric field gradients without experiencing partial discharge. At 35kV, even minute air gaps within the insulation can trigger "electrical treeing," ultimately leading to insulation breakdown. These cables utilize a precision extrusion process to create a thick insulation layer, thereby effectively eliminating such air gaps.
- Semi-conductive layer: The diagram shows a layer of black material situated above the copper conductor and below the metallic shield; this is the semi-conductive shielding layer. Its function is to homogenize the electric field and prevent points of electric field stress concentration; otherwise, such stress concentration points could lead to air ionization, subsequently causing the XLPE (cross-linked polyethylene) insulation to degrade from the inside out.
- Capacitive charging current: At a voltage of 35 kV, the cable behaves like a large capacitor. Engineers must account for capacitive charging current because, during long-distance power transmission, this current can be substantial, thereby reducing the cable's effective current-carrying capacity. The precise geometric design of the XLPE insulation and semi-conductive layers aims to minimize this dielectric loss.
On-Site Installation and Geological Contingency Manual
Laying 35kV armored cables requires the use of heavy machinery and strict adherence to physical installation specifications. Site engineers must ensure the following requirements are met:
- Tension Calculation: Exceeding the maximum allowable pulling tension is strictly prohibited. For 35kV armored cables, this limit is typically calculated based on the conductor cross-sectional area and the tensile strength of the steel wire; exceeding it can cause permanent tensile deformation of the copper conductor.
- Bending Radius Limits: Due to the use of thick steel wires and steel tapes in the armor layer, the minimum bending radius must be strictly maintained at no less than 20 times the cable diameter (and at least 25 times during the pulling process). Forcing a bend with a smaller radius can cause the steel tape to buckle inward, crushing the XLPE insulation layer and rendering the cable unable to meet its rated 35kV performance specifications.
- Trench Bedding and Thermally Stable Backfill:
- Direct laying of cables on rock is strictly prohibited; a bedding layer of sand or screened soil at least 100 mm thick must be installed.
- In high-voltage transmission projects, the thermal resistivity of the surrounding soil is critical. Thermally stable backfill materials should be used to prevent heat accumulation (which can lead to increased electrical resistance and reduced cable service life).
- Install concrete protection slabs or warning tapes above the cables.
- Criticality of Cable Joints: For 35 kV systems, field joints (cable connection points) represent the weakest link. Environmental conditions must be strictly controlled during installation, and humidity levels during joint assembly must remain below specified limits. Stress cones (or stress relief cones) must be installed by qualified personnel to effectively control the electric field distribution at the cable stripping point.
- Sheath Connection and Grounding: The coarse steel wires and steel tapes must be reliably connected and grounded at the terminations. Circulating currents induced in the armor by the 35kV conductors must be properly managed to prevent overheating.
Engineering FAQ
- Q: Why are thick steel wires used instead of just steel tapes?
A: While steel tapes offer excellent resistance to lateral pressure (crushing), their tensile strength is very low. 35kV cables are heavy and often require long-distance pulling during installation. Thick steel wires serve as a structural framework, bearing the cable's weight during installation and preventing damage to the internal cable cores. - Q: Can YJV62/YJV72 cables be installed in vertical shafts?
A: Yes, but such installations involve significant tensile loads. The steel wire armor structure is specifically designed for this purpose. However, the cables must be properly secured (clamped) at regular intervals to support their own weight. The clamps must not damage the outer PVC sheath; otherwise, moisture could seep in and reach the steel tapes, causing rapid corrosion. - How does double-layer armoring affect heat dissipation?
Answer: The metal layers possess good thermal conductivity, facilitating radial heat dissipation. However, if multiple armored cables are laid in close proximity—such as when tightly bundled within cable ducts—mutual heating effects occur. Consequently, the current-carrying capacity (rated current) must be derated based on the number of cables in the group and the thermal resistivity of the surrounding soil.
