Transmission Line Grounding Design: Tower Footing Resistance, Lightning Performance, and IEEE/CIGRE Standards

Transmission Line Grounding Design: Tower Footing Resistance, Lightning Performance, and IEEE/CIGRE Standards

An authoritative engineering guide covering tower grounding, soil resistivity, grounding techniques, backflashover reduction, and lightning performance optimization.

1. Purpose of Transmission Line Grounding

  • Control tower voltage rise during lightning
  • Minimize backflashover probability
  • Dissipate earth fault current
  • Ensure touch and step voltage safety

2. Soil Resistivity Measurement (IEEE 81)

The Wenner 4-pin method calculates soil resistivity:

ρ = 2πaR

Where a = pin spacing, R = measured resistance.

3. Soil Modeling

  • Single-layer soil
  • Two-layer soil
  • Multi-layer soil (CIGRE)

4. Tower Footing Resistance (TFR) Targets

VoltageTarget TFR
69 kV25 Ω
115–138 kV15–20 Ω
230 kV10 Ω
500 kV5–7 Ω

5. Tower Grounding Methods

  • Counterpoise wires
  • Ground rods (multiple)
  • Deep rods
  • Chemical-enhanced rods
  • Ground boosters

6. Lightning Current Distribution

Tower voltage rise:

V = I * Rt

7. Backflashover Reduction

  • Lower TFR
  • Increase insulator string length
  • Improve shielding angle
  • Install line arresters

8. CIGRE Methods for LLP

CIGRE defines statistical computations for backflashover and shielding failure probability.

9. Numerical Example (230-kV Tower)

  • Soil resistivity = 400 Ω·m
  • Single counterpoise 60 m
  • Final TFR achieved = 12 Ω

10. Measuring Footing Resistance

Use the Fall-of-Potential method:

Rt = V / I

11. Ground Boosting Techniques

  • Parallel counterpoise
  • Additional rods
  • Soil conditioning
  • Mesh grounding

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