ENVIRONMENTAL CONSIDERATION DURING TRANSMISSION LINES DESIGN

In order to match both the mechanical and electrical characteristics of the overhead line conductor to the environmental conditions climatic details must first be collected and analyzed.

• Temperature
The maximum, minimum and average ambient temperature influences conductor current rating and sag. For temperate conditions typically 20°C with 55°C temperature rise. For tropical conditions 35°C or 40°C with 40°C or 35°C temperature rise. Maximum conductor operating temperature should not exceed 75°C for bare conductors to prevent annealing of aluminium. Conductortemperatures up to 210°C are possible with ‘GAP’ conductor

• Wind velocity
Required for structure and conductor design. Electrical conductor ratings may be based on cross wind speeds of 0.5 m/s or longitudinal wind speeds of 1 m/s.

• Solar radiation
Required for conductor ratings but also for fittings such as composite insulators which may be affected by exposure to high thermal and ultraviolet (UV) radiation. Typical values of 850W/m2 and 1200W/m2 may be assumed for temperate and tropical conditions respectively.

• Rainfall Important in relation to flooding (necessity for extension legs on towers), corona discharge and associated electromagnetic interference, natural washing and insulator performance.

• Humidity
Effect on insulator design.

• Altitude
Effect on insulation and conductor voltage gradient.

• Ice and snow
Required for design of conductor sags and tensions. Build-up can also affect insulation as well as conductor aerodynamic stability.

• Atmospheric
Effect on insulation and choice of conductor pollution material (IEC 60815-1 and -2 – Guide for the selection of insulators in respect of polluted conditions – drafts for public comment).

• Soil
 Electrically affecting grounding requirements (soil characteristics resistivity) and structurally the foundation design (weights, cohesion and angle of repose).

• Lightning
Effect on insulation levels and also earth wire screening arrangements necessary to provide satisfactory outage performance.

• Seismic factor
Effect on tower and foundation design.

• General loadings
Refer also to IEC 60826 (Design criteria for overhead lines), EN50341 (Overhead lines exceeding AC45 kV –supersedes 60826 for European use) and BS8100 (Loading and strength of overhead transmission lines).

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