Transmission Lines Around the World: Design, Standards, Technologies, and Engineering Fundamentals

A comprehensive guide for electrical and power systems engineers

Table of Contents

• 1. Introduction
• 2. Transmission Line Components
• 3. Electrical Design
• 4. Mechanical Design
• 5. Right-of-Way & Clearances
• 6. Insulation & Lightning Protection
• 7. Substation Interface
• 8. Protection Systems
• 9. HVDC Transmission
• 10. Full Transmission Line Design Workflow
• 11. Global Transmission Line Trends

1. Introduction to Transmission Lines

Transmission lines are critical infrastructure that deliver bulk electrical energy from generation plants to substations. They operate at high, extra-high, and ultra-high voltage levels such as:

• High Voltage (HV): 69–230 kV
• Extra High Voltage (EHV): 345–765 kV
• Ultra High Voltage (UHV): 800–1200 kV AC / ±500–1100 kV DC

Major global transmission achievements include:

Country	Voltage Level	Notes
China	±1100 kV HVDC	Highest and longest HVDC lines globally
India	1200 kV AC	Highest AC transmission system in the world
Brazil	±800 kV HVDC	Massive Amazon–load center projects
USA	765 kV AC	Key backbone for Midwest and East Coast
Europe	400 kV AC	Highly meshed interconnection grid
Philippines	500 kV AC, ±350 kV HVDC	Luzon backbone and HVDC Leyte–Luzon

2. Transmission Line Components

2.1 Conductors

Common conductor types include:

• ACSR (Aluminum Conductor Steel Reinforced)
• AAAC (All-Aluminum Alloy Conductor)
• ACCC/ACCR (Composite core, high-temperature)

2.2 Insulators

Insulators provide mechanical support and electrical isolation.

• Suspension type
• Tension type
• Post insulators
• Polymer/composite insulators

2.3 Towers

Towers may be:

• Lattice steel towers
• Monopole structures
• Guyed towers
• Portal structures

2.4 Shield Wires

Shield (ground) wires protect phase conductors from lightning by providing a low-impedance path.

2.5 Line Hardware

Includes armor rods, vibration dampers, spacers, suspension clamps, and tension clamps.

3. Electrical Design of Transmission Lines

3.1 Line Parameters

Resistance (R): Depends on conductor size and temperature.

Inductance (L):

L = 2 × 10⁻⁷ ln(Dm / r')

Capacitance (C):

C = (2πϵ₀) / ln(Dm / r)

3.2 Transmission Line Models

• Short line (< 80 km) – R and L only
• Medium line (80–250 km) – π-model
• Long line (> 250 km) – Distributed parameters

3.3 Surge Impedance & SIL

Z₀ = √(L / C)
SIL = V² / Z₀

3.4 Corona Performance

Corona depends on conductor diameter, surface condition, voltage gradient, and bundling configuration.

4. Mechanical Design

4.1 Sag and Tension

Sag is calculated using:

S = (wL²) / (8T)

• w = conductor weight per unit length
• L = span length
• T = horizontal tension

4.2 Tower Loading

Loads include:

• Vertical loads
• Transverse loads
• Longitudinal loads
• Wind load on conductors and towers
• Ice load (in cold climates)

Standards used:

• ASCE 74
• IEC 60826
• NESC
• PGES / NGCP TLDS Manual

5. Right-of-Way (ROW) and Clearances

Voltage Level	ROW Width
115 kV	15–25 m
230 kV	30–35 m
500 kV	50–60 m
±800 kV HVDC	70–100 m

6. Insulation & Lightning Protection

6.1 BIL (Basic Insulation Level)

Determines the insulation strength required for lightning and switching surges.

6.2 Shielding Angle

Typical shielding angle = 25°–30°

6.3 Surge Arresters

Used at line terminations and high-lightning-density areas.

7. Substation Interface

Transmission lines must match substation equipment ratings:

• Bus configuration
• CT/PT class
• Breaker interrupting capacity
• Line protection panels
• Communication channel (OPGW, PLCC, microwave)

8. Protection of Transmission Lines

• Distance (21) relays
• Line Differential (87L)
• Pilot protection (using OPGW or PLCC)
• Backup overcurrent protection

9. HVDC Transmission Systems

HVDC is ideal for long-distance bulk power transfer, underwater connections, and asynchronous grid interconnections.

• Overhead bipolar transmission
• Monopolar with ground return
• Metallic return configurations

10. Complete Transmission Line Design Workflow

1. Requirement definition (voltage, MW, route)
2. Route selection (GIS, environmental, ROW)
3. Electrical calculations (RLC, corona, SIL)
4. Mechanical design (sag-tension, wind load)
5. Tower structural design and spotting
6. Protection and communication planning
7. Construction planning and commissioning

11. Global Trends in Transmission Technology

• High-temperature low-sag (HTLS) conductors
• Composite-core conductors
• UHVAC and UHVDC expansion
• Dynamic Line Rating (DLR)
• Drone-based line inspection
• LiDAR-based route optimization
• PLS-CADD for advanced modeling