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).
1 comment:
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