Comparison Between Overhead & Underground Transmission Systems

05/30/2025

Key Differences and Comparison Between Underground and Overhead Power Transmission & Distribution
Public Safety

In terms of public safety, underground systems outperform overhead transmission systems. With all transmission and distribution components buried, underground setups minimize risks from obstacles and external interference. Additionally, they are less susceptible to environmental factors like wind, storms, and heavy rain, making them inherently more secure.

Initial Cost

Underground systems incur significantly higher initial costs than overhead systems. Excavation, conduits, specialized cables, manholes, and other transmission equipment drive expenses, with underground installations costing 5 to 10 times more than overhead counterparts.

Flexibility

Overhead systems offer greater flexibility for modifications. Wires, poles, and transformers are easily accessible, allowing quick adjustments to meet changing load demands. Underground systems, however, rely on permanent manholes and duct lines. Adding capacity or modifying the system requires installing new duct lines, making adjustments far more difficult.

Faults

Underground systems have a low fault risk due to buried cables with robust insulation. Overhead systems, exposed to environmental factors (e.g., severe weather), face higher chances of power faults and external accidents.

Fault Location & Repair

While underground faults are rare, locating and repairing them is challenging due to buried infrastructure. Overhead systems, with exposed conductors, enable rapid fault detection and repair.

Current Carrying Capacity & Voltage Drop

Current Capacity: Overhead systems can carry significantly more current than underground systems with the same conductor material and cross - sectional area.
Voltage Range: Underground systems are typically used for voltages below 150 kV, while overhead transmission supports extra - high voltages (EHV/UHV), ranging from 380 kV to 800 kV and above.
Reactance Characteristics: Underground cables have lower inductive reactance (due to closer spacing) but higher capacitive reactance, increasing charging currents. This limits their suitability for very long - distance transmission.

Interference with Communication Circuits

Overhead systems can interfere with telephone lines, causing unwanted potential increases and noise in communication networks. Underground systems eliminate such interference.

Maintenance Cost

Underground systems have lower routine maintenance costs due to reduced exposure to wind, snow, and lightning. However, fault repairs are time - consuming and expensive. Overhead systems, despite higher fault probabilities, allow faster and cheaper repairs.

Appearance

Underground systems preserve the visual appeal of an area by burying all infrastructure, avoiding interference with buildings. Overhead power lines, by contrast, can detract from the landscape.

Useful Life

Underground systems typically have a lifespan twice that of overhead systems. While an overhead system may last 25 years, an underground setup can operate for approximately 50 years.

Comparison between underground cables and overhead lines