Underground cables

The trend of underground power grid systems is rapidly growing in major urban areas, marking the transition from traditional electrical systems to modern underground cable technology. Ngoc Lan Cable applies international IEC standards and TCVN in research and manufacturing of underground cables. This article will analyze:

• • Structure and operating principles of underground cable systems
  • Specialized product lines for different installation environments
  • Technical advantages in safety and economic efficiency
  • Practical applications in industrial and residential use

Whether you're an experienced engineer or new to the electrical industry, the information below will help you better understand the technology solution that's shaping the future of power transmission systems.

1. What is underground cable?

Underground cable is a specially designed cable for underground installation and plays a vital role in power transmission and distribution systems. This modern solution replaces traditional overhead power lines, ensuring safety and enhancing urban aesthetics. In terms of structure, underground cables typically consist of electrical conductors made from copper or aluminum, encased in high-grade insulation materials like XLPE (Cross-linked polyethylene) or PVC (Polyvinyl chloride). Metal armor or metallic screens surrounding the core protect against environmental factors like moisture, pressure, and underground temperatures.  

2. Types of underground cables

2.1 Low voltage underground cables

Structure

• • Conductor: Made from pure copper or aluminum (≈ 99.99%).
  • Insulation: XLPE insulation material encases the conductor core to prevent electrical leakage and ensure safety.
  • Metal armor: Usually armored with 2 layers of aluminum tape, 2 layers of steel tape or steel wires, installed to protect the conductor core and prevent environmental impacts on current flow.
  • Outer sheath: High flexibility PVC with good heat resistance and water resistance. Manufacturers often add flame retardant components to enhance heat resistance and safety for low voltage underground cables.

Standards and technical specifications

• • Applied standards: IEC 60502-1, IEC 60228, TCVN 5935-1, TCVN 6612
  • Rated voltage: 0.6/1kV
  • 5-minute power frequency test voltage at 50Hz: 3.5kV
  • Maximum rated conductor operating temperature: 90ºC
  • Maximum 5-second short circuit conductor temperature: 250ºC

2.2 Medium voltage underground cables

Structure

• • Conductor: Made from pure copper or aluminum (≈ 99.99%)
  • Conductor screen: Typically made from semiconducting polymer compounds, reduces electrical field at the conductor surface, preventing partial discharge and enhancing cable insulation strength
  • Insulation: XLPE insulation material encases the conductor core to prevent electrical leakage and ensure safety
  • Insulation screen: Made from semiconducting polymer compounds, reduces electrical field at the conductor surface, preventing partial discharge and enhancing cable insulation strength
  • Metallic screen: Usually aluminum or copper tape, wrapped around the insulation layer. Main function is reducing electromagnetic interference, ensuring stable signal transmission
  • Metal armor: Armored with steel wires, aluminum wires, steel tape or aluminum tape, enhancing mechanical strength and protecting cable from physical impacts
  • Outer sheath: Material usually PVC or HDPE, depending on installation environment conditions. This outermost layer protects the entire internal structure from environmental factors like water, moisture, chemicals...

Standards and technical characteristics

• • Applied standards: IEC 60502-2/ TCVN 5935-2; IEC 60228/ TCVN 6612
  • Rated voltage: 1.8/3(3.6)KV, 3.6/6(7.2)kV, 6/10(12)kV, 8.7/15(17.5)kV, 12/20(24)kV
  • Voltage test 50Hz - 5min: 6.5kV, 12.5kV, 21kV, 30.5kV, 42kV
  • Max. conductor temperature in normal operation: 90ºC.
  • Max. conductor temperature in short-circuit for 5s max duration: 250ºC.

2.3 High voltage underground cables

Structure

• • Conductor: Made from pure copper or aluminum (≈ 99.99%), stranded or compacted. For cables over 1000 mm², conductors are segmented to reduce skin effect.
  • Conductor screen: Made from semiconducting polymer compounds, reduces electrical field at conductor surface, preventing partial discharge and enhancing cable insulation strength.
  • Insulation: XLPE insulation material encases the conductor core to prevent electrical leakage and ensure safety.
  • Insulation screen: Made from semiconducting polymer compounds, reduces electrical field at conductor surface, preventing partial discharge and enhancing cable insulation strength.
  • Semiconducting tape: creates uniform electromagnetic field, protects insulation layer, reduces electrical stress and enhances safety.
  • Metallic screen: Copper wire screens, corrugated aluminum sheath or lead sheath. Aluminum and lead sheaths are used for environments with poor duct surface and high humidity.
  • Metal armor: Aluminum tape armoring enhances mechanical strength and protects cable from physical impacts.
  • Outer sheath: Typically HDPE, ensuring good mechanical and electrical durability. Additionally, PE is halogen-free and can be coated with fire-retardant paint to limit fire spread in buildings. For high fire resistance requirements, PVC sheathing is preferred, especially in buildings and tunnels with strict fire safety needs.

Standards and technical characteristics

• • Applied standards: IEC 60228, IEC 60287, IEC 60332, IEC 60502, IEC 60840, IEC 60853, IEC 61443, IEC 60949, IEC 62067, AS NZS 1429.2, IS 7098…
  • Rated voltage: 38/66(72.5)kV đến 64/110(123)kV.
  • Max. conductor temperature in normal operation: 90ºC.
  • Max. conductor temperature in short-circuit for 5s max duration: 250ºC.

3. Advantages and disadvantages of underground cables

Underground cables have distinct advantages and disadvantages that you need to consider before installation. Let's examine these factors carefully..

Advantages:

• • Superior safety compared to overhead cables, minimizing electrical accident risks for residents and surrounding activities.
  • Contributes to environmental protection by reducing artificial elements' impact on natural landscapes, which can cause aesthetic disruption and affect human information reception.
  • Enhances urban aesthetics by eliminating overhead power line networks.
  • High durability and excellent resistance to severe weather conditions.
  • Reduces power loss as cables are not affected by external environmental factors.

Disadvantages:

• • High initial investment costs, including materials, construction, and installation.
  • Maintenance and repair work is challenging due to underground installation.
  • Construction time is longer and more complex compared to overhead cables.
  • Difficulty in detecting and locating faults.
  • High repair costs when damage occurs.

4. Applications of underground cables

Underground cables are widely used across multiple sectors of modern urban infrastructure and residential electrical systems:

• • Power transmission and distribution to residential areas, industrial zones, and public facilities.
  • In telecommunications systems, underground cables transmit internet signals, telephone, and cable television.
  • Aesthetic and safety solutions for major construction projects such as new urban areas, shopping centers, high-rise buildings.
  • Applications in transportation infrastructure like tunnels and flyovers, where high reliability and safety in power supply are required.
  • Used in urban power distribution systems, connecting between distribution transformer stations.

Underground cables represent the modernization trend in electrical infrastructure, delivering superior value in safety and aesthetics. For students and technicians new to the electrical industry, understanding the structure, advantages, disadvantages, and applications of underground cables not only enhances technical knowledge but also provides crucial practical skills in selecting appropriate power transmission solutions for each project.