Construction, Laying, and Installation Techniques for Extruded and Self-Contained Fluid Filled Cable Systems
Extruded and self-contained fluid-filled (SCFF) cable systems are widely used for high-voltage power transmission, particularly in applications where high reliability and long-term performance are critical. These cables are often employed in underground and submarine installations for power distribution in urban areas, as well as in interconnecting power grids across regions or countries.
1. Construction of Cable Systems
A. Extruded Cable Systems
Extruded cables are made with solid insulation materials that are extruded around the conductor. The most common insulation materials are cross-linked polyethylene (XLPE) and ethylene propylene rubber (EPR).
- Conductor:
- The conductor is typically made of copper or aluminum, designed to offer high conductivity and flexibility. For high-voltage applications, the conductor may be stranded or segmental to reduce the skin effect and losses.
- Insulation Layer:
- The insulation layer is extruded around the conductor, forming a solid barrier that provides electrical insulation. XLPE is the preferred material for its excellent electrical properties, thermal resistance, and mechanical strength.
- Semiconducting Layers:
- Extruded cables feature inner and outer semiconducting layers around the insulation to smooth the electric field, preventing electrical stress concentration.
- Metallic Screen and Sheath:
- The metallic screen, made of copper tape or wires, provides a path for fault currents and protects the cable from electromagnetic interference.
- The outer sheath is usually made of polyethylene (PE) or polyvinyl chloride (PVC), providing mechanical protection and resistance to environmental factors.
B. Self-Contained Fluid Filled (SCFF) Cable Systems
SCFF cables are designed with a pressurized fluid (typically oil) that surrounds the insulation to maintain dielectric strength and prevent void formation. This design ensures consistent performance and reliability.
- Conductor:
- Similar to extruded cables, SCFF conductors are made of copper or aluminum, with stranded or segmental construction.
- Insulation:
- The insulation consists of multiple layers of impregnated paper or laminated paper-polypropylene. The insulation is impregnated with a low-viscosity dielectric fluid to enhance its dielectric properties.
- Fluid Channel:
- SCFF cables have an internal channel or a hollow conductor through which the dielectric fluid circulates. This helps in maintaining uniform pressure and temperature throughout the cable.
- Lead or Aluminum Sheath:
- A metallic sheath made of lead or aluminum encloses the cable, providing a barrier against fluid leakage and external moisture ingress.
- Outer Sheath and Armoring:
- The outer sheath is typically made of polyethylene or PVC, while additional armoring (such as steel tapes or wires) may be applied for mechanical protection, especially in submarine installations.
2. Laying Techniques
A. Trenching and Direct Burial
For both extruded and SCFF cables, direct burial is a common method, where the cable is laid directly in a trench excavated in the ground.
Preparation:
- The trench is excavated to a depth and width that meet local regulations and installation guidelines. A bedding layer of sand or fine soil is laid to protect the cable from sharp objects.
Laying:
- The cable is laid into the trench using specialized equipment, such as cable rollers or cable drums, to minimize tension and prevent damage.
- The trench is backfilled with a protective layer of sand or fine soil, followed by compacted earth.
Thermal Considerations:
- The thermal properties of the surrounding soil are considered to ensure the cable can dissipate heat effectively, preventing thermal degradation.
B. Cable Duct Systems
In urban areas or congested environments, cables are often laid in pre-installed ducts made of concrete, PVC, or HDPE.
Installation:
- Ducts are installed in trenches, and the cables are pulled through using pulling winches or hydraulic equipment.
- Lubricants are often used to reduce friction during pulling, minimizing stress on the cable.
Advantages:
- This method provides additional protection from mechanical damage, makes future repairs easier, and allows for easier upgrades or replacements.
C. Submarine Laying
For underwater installations, specialized cable-laying vessels are used.
Preparation:
- A detailed seabed survey is conducted to map the route and identify potential obstacles or challenging seabed conditions.
Laying:
- The cable is deployed from the vessel, which moves along the planned route while controlling the tension to prevent damage.
- For SCFF cables, maintaining consistent fluid pressure during laying is crucial to prevent dielectric fluid leakage and maintain insulation integrity.
Protection:
- The cable is often buried using jet trenching or plowing to protect it from anchors, fishing activities, and other mechanical damage. Additional protection, such as rock dumping or concrete mattresses, may be applied as needed.
3. Installation Techniques
A. Jointing and Termination
Jointing:
- Jointing involves connecting two cable ends securely. For extruded cables, this requires precise alignment of conductors and careful insulation of the joint area.
- For SCFF cables, jointing is more complex, involving sealing the fluid channels and maintaining fluid pressure to prevent air ingress.
Termination:
- The termination process involves connecting the cable to equipment like transformers or switchgear. This requires sealing the cable end and providing stress control using specialized termination kits.
- For SCFF cables, fluid pressure must be maintained throughout the process to prevent dielectric fluid loss and air entry.
B. Testing and Commissioning
Before energizing the cable system, rigorous testing is conducted to ensure reliability and performance:
- High-Potential (Hi-Pot) Testing:
- This test applies a voltage higher than the operating voltage to check for insulation integrity and detect potential weaknesses.
- Partial Discharge Testing:
- Partial discharge testing identifies areas of insulation that may have defects, allowing for corrective action before the cable is put into service.
- Fluid Pressure Testing (for SCFF Cables):
- The fluid pressure in SCFF cables is checked to ensure it remains within the specified range. Any drop in pressure could indicate a leak or issue with the fluid system.
4. Maintenance and Monitoring
Regular Inspections:
- Routine inspections using infrared thermography or online monitoring systems help detect hot spots, partial discharges, or fluid pressure drops.
Preventive Maintenance:
- SCFF systems require regular checks of fluid levels and pressure. Top-ups of dielectric fluid may be necessary to maintain the required pressure and prevent insulation breakdown.
Repair Techniques:
- For extruded cables, damaged sections can be cut and re-jointed. For SCFF cables, more complex repairs may be required to restore fluid integrity and pressure.
Conclusion
The construction, laying, and installation of extruded and self-contained fluid-filled cable systems demand specialized techniques to ensure long-term performance and reliability. While extruded cables are known for their ease of installation and low maintenance, SCFF cables offer excellent dielectric performance, making them suitable for ultra-high-voltage applications. Careful planning, rigorous testing, and regular maintenance are essential to the successful deployment and operation of these advanced cable systems in diverse environments.
