Comparison between Three Phase Overhead and Underground cables

🔌 Power Transmission: Overhead vs Underground Cables

· Overhead Lines (OHTL): Inductance is predominant.

· Underground Cables (UGC): Capacitance is predominant.

1. Lower Conductor Cost:

o Overhead conductors run cooler in open air → smaller cross-section needed compared to underground.

o Results in cheaper conductor material.

2. Cheaper Insulation:

o OHTL uses bare conductors, spaced by air (a natural insulator).

o UGC requires costly insulation: paper tapes, metal sheath, oil/gas filling, storage vessels.

3. Lower Installation Cost:

o Poles/towers are cheaper to install than trenching and laying underground cables.

4. Easy Capacity Addition:

o OHTL can be upgraded (adding more circuits or reconductoring) faster and at lower cost.

1. Public Safety:

o No risk of electrocution from exposed conductors.

o Less chance of accidental contact.

2. No Interference & Better Aesthetics:

o No electromagnetic interference to nearby communication lines.

o Cityscape remains uncluttered → better appearance in urban areas.

Feature	Overhead Lines (OHTL)	Underground Cables (UGC)
Electrical Property	Inductance predominant	Capacitance predominant
Conductor Cost	Lower	Higher
Insulation Requirement	Minimal (air insulation)	Complex (paper, oil, sheath, vessels)
Installation Cost	Lower	Very high
Capacity Upgradation	Easy	Difficult
Public Safety	Exposed conductors, safety concern	Safer (no exposure)
Interference	May interfere with nearby comm. lines	No interference
Aesthetics	Visible structures, cluttered look	Hidden, better appearance

Aspect	Overhead (OHTL)	Underground (UGC)
Initial Cost	Baseline—e.g., ~$285k/mile (69 kV)	Approximately 3× to 14×, typically ~5× or 4.5× higher
Reliability	Vulnerable to weather, vegetation, etc.	Highly reliable—e.g., 95% SAIDI improvement, 4% outage vs 24%
Maintenance & Repairs	Quick repairs, lower cost	Slower, more expensive maintenance
Lifetime Costs	Lower overall	Higher—12–28% more in distribution; 2–4× for transmission
Urban Use Case	Less preferred—visible infrastructure	Preferred—saves ₹500 cr/year in Hyderabad example
Real-World Costs	~$40 m/15 km UK, or AU$700M+	~$330 m/15 km UK; AU$2–4.5 b; DKK 30M/km in Denmark