Underground power lines are commonly referred to as underground cables or underground transmission lines. These essential components of the electrical grid are laid beneath the ground, distinguishing them from traditional overhead lines. While generally known by these terms, more specific terminology is used depending on their construction and purpose.
Modern underground lines predominantly utilize advanced insulation technology. For instance, the cross-linked polyethylene (XLPE) underground transmission line is a prominent example often called a solid dielectric cable. This technology represents a significant advancement as it employs a solid dielectric material, replacing the pressurized liquid or gas insulation found in older pipe-type cables. XLPE cable has become the national standard for underground electric transmission lines operating at less than 200 kilovolts (kV) due to its efficiency and reliability.
Key Characteristics and Types of Underground Cables
Underground power lines come in various forms, each with distinct characteristics and applications:
| Cable Type | Description | Common Usage/Notes |
|---|---|---|
| Solid Dielectric | These cables utilize solid insulation materials, such as cross-linked polyethylene (XLPE), wrapped around the conductors. They are highly reliable, require less maintenance, and are simpler to install compared to older technologies. XLPE's solid insulation avoids the complexities and environmental concerns associated with pressurized fluids. | Modern standard, widely adopted for new installations and replacements. XLPE cables are a prime example and have become the national standard for lines under 200 kV. Found in residential, commercial, and industrial areas. |
| Pipe-Type | An older technology where multiple insulated conductors are pulled into a large steel pipe that is then filled with a pressurized insulating fluid, typically oil or nitrogen gas. The pressure helps to suppress partial discharges and improve insulation properties. | Primarily found in older, high-voltage transmission systems in urban areas. These are gradually being replaced by more efficient solid dielectric technologies due to their complexity, cost, and potential environmental risks. |
| Direct Buried | These cables are laid directly into trenches in the ground without the use of protective conduits. They often feature robust outer sheaths and/or armor to protect against physical damage and moisture. This method is generally simpler and less expensive for certain applications. | Common in rural areas, residential subdivisions, and for lower-voltage distribution lines where future access and replacement are less critical concerns. Requires careful site preparation and backfilling. |
| Conduit/Duct | Cables are pulled through pre-installed protective pipes or ducts underground. These conduits can be made from various materials like PVC, polyethylene, or concrete. This method allows for easier future upgrades, replacements, or repairs without requiring extensive re-excavation of the entire trench. It also provides an additional layer of physical protection to the cables. | Often used in urban environments, dense commercial areas, and critical infrastructure projects where future maintenance, expansion, and protection from ground disturbances are important considerations. |
Advantages of Underground Power Lines
The adoption of underground power lines offers several significant benefits:
- Aesthetics: They eliminate the visual clutter of overhead poles and wires, preserving natural landscapes and enhancing urban appearances.
- Increased Reliability: Underground lines are less susceptible to damage from severe weather events (e.g., high winds, ice storms, lightning), falling trees, or vehicular accidents, leading to fewer outages.
- Enhanced Safety: The risk of electrocution or accidents from downed power lines is significantly reduced, improving public safety.
- Reduced Maintenance from External Factors: While initial installation is complex, long-term maintenance related to external environmental factors can be lower due to the lines being protected from direct exposure.
- Flexible Land Use: By removing overhead infrastructure, ground space above the lines can be used more flexibly for parks, buildings, or other developments.
Considerations for Underground Power Lines
Despite their advantages, underground power lines also present certain challenges:
- Higher Installation Cost: Burying lines is significantly more expensive than installing overhead lines, primarily due to the extensive excavation, specialized equipment, and skilled labor required.
- Difficult to Locate and Repair Faults: Pinpointing the exact location of a fault and subsequently repairing it can be more time-consuming and costly, often requiring precise excavation and specialized diagnostic tools.
- Heat Dissipation: Dissipating the heat generated by the current can be a challenge underground, as the surrounding soil is a less efficient cooling medium than air. This can sometimes limit the power capacity of underground lines compared to air-cooled overhead lines.
