Overhead Power Line Design: Conductor Sizing, Wind Loads, and Grounding

Posted on Nov 10, 2024 in Technology

Overhead Power Line Design

Conductor Sizing and Materials

The minimum allowable conductor sizes are 16 mm2 for aluminum and 10 mm2 for copper. These values apply to bare, insulated conductors rated for voltages less than 0.6/1 kV.

Permanent Loads

Permanent loads include the vertical forces due to the weight of conductors, insulators, clamps, and supports.

Wind Load Considerations

Overload from wind pressure is calculated as follows:

  • Conductors: 50 daN/m2
  • Flat surfaces: 100 daN/m2
  • Cylindrical supports: 70 daN/m2

Wind action on conductors is not considered where terrain or buildings block wind in the line’s longitudinal direction.

Ice Load Zones

For ice load calculations, the region is divided into three zones:

  • Zone A (Below 500 m): No ice load is considered.
  • Zone B (500-1000 m): Bare conductors are subject to an ice sleeve of 180√d grams/meter, where d is the conductor diameter in mm. Bundled cables use 60√d grams/meter, where d is 2.5 times the phase conductor diameter.
  • Zone C (Above 1000 m): Bare conductors are subject to an ice sleeve of 360√d grams/meter, where d is the conductor diameter in mm. Bundled cables use 120√d grams/meter, where d is 2.5 times the phase conductor diameter.

Maximum Permissible Tension

2.2.1 Maximum Permissible Tension

Maximum conductor tension must not exceed the ultimate load divided by 2.5, considering the worst-case scenario:

  • Zone A: Tension due to weight and wind load at 15°C, or tension due to weight and one-third wind load at 0°C.
  • Zones B and C: Tension due to weight and wind load at 15°C, or tension due to weight and ice load at 0°C.

For spans less than 15 m, dead loads have minimal influence and can generally be omitted from calculations.

Tensioned Cables

3.1.2 Tensioned Cables

Neutral wires can be secured to supports, facades, or walls using appropriate hardware. Galvanized steel fasteners with a minimum tensile strength of 800 daN are used for tensioned cables, fixed with clamps or other suitable devices. Ground clearance is 4 m, except for crossings (see section 3.9).

Neutral Conductor Sizing and Grounding

Minimum neutral conductor cross-section:

  • Two or three conductors: Same as phase conductors.
  • Four conductors: Half the phase conductor cross-section, with a minimum of 10 mm2 for copper and 16 mm2 for aluminum.

The neutral conductor is grounded at the processing center at least every 500 m.

Support Installation

Concrete poles are set in monolithic concrete foundations. Metal supports are grounded in concrete or by other approved methods (bolts, etc.). Foundations should facilitate water drainage and cover any bolt heads. Poles are mounted directly on the ground or on metal/concrete bases for easy replacement, with a minimum ground clearance of 0.15 m.

Conductor Ampacity

  • 3 x 50 Al/54, 6 Alm: 150 A
  • 3 x 95 Al/54, 6 Alm: 230 A
  • 3 x 150 Al/80 Alm: 305 A

Line Crossings

Low-voltage lines must cross under high-voltage lines. The minimum vertical distance (d) between conductors under worst-case conditions is calculated based on the high-voltage line’s rated voltage (U in kV), the distance to the nearest high-voltage support (L1 in meters), and the distance to the nearest low-voltage support (L2 in meters). Precautions must be taken to prevent conductors, insulators, or hardware from falling during crossings.

Neutral Conductor Identification

3.5 Neutral Conductor Identification

The neutral conductor must be clearly identified. Identification is not required for bare conductors if the neutral has a different cross-section or distinct position.

Neutral Conductor Continuity

3.6 Continuity of the Neutral Conductor

The neutral conductor must be continuous, except when interrupted by:

  1. Multipolar switches acting simultaneously on all phases and the neutral, or connecting the neutral before phases and disconnecting it after phases.
  2. Detachable connections or isolating switches near the neutral, clearly marked and operable only with tools. These must connect/disconnect the neutral before/after the phases.

Neutral Grounding

3.7 Neutral Ground

The neutral conductor is grounded at the processing center and generating station, as per regulations. In TT and TN-C systems, the neutral must be grounded at least every 500 m, preferably at branching points.