TRANSMISSION LINE CONDUCTOR ICE LOADING BASICS AND TUTORIALS

When a conductor is covered with ice and/or is exposed to wind, the effective conductor weight per unit length increases. During occasions of heavy ice and/or wind load, the conductor catenary tension increases dramatically along with the loads on angle and deadend structures. Both the conductor and its supports can fail unless these high-tension conditions are considered in the line design.


Certain utilities in very heavy ice areas use glaze ice thickness of as much as 2 in (50 mm) to calculate iced conductor weight. Similarly, utilities in regions where hurricane winds occur may use wind loads as high as 34 lb/ft2 (1620 Pa).

As the NESC indicates, the degree of ice and wind loads varies by region. Some areas may have heavy icing, whereas some areas may have extremely high winds. The loads must be accounted for in the line design process to prevent a detrimental effect on the line.

Some of the effects of both the individual and combined components of ice and wind loads are discussed below:

Ice loading of overhead conductors may take several physical forms (glaze ice, rime ice, or wet snow). The impact of lower-density ice formation is usually considered in the design of line sections at high altitudes.

The formation of ice on overhead conductors has the following influence on line design:

• Ice loads determine the maximum vertical conductor loads that structures and foundations must withstand.
• In combination with simultaneous wind loads, ice loads also determine the maximum transverse loads on structures.
• In regions of heavy ice loads, the maximum sags and the permanent increase in sag with time (difference between initial and final sags) may be due to ice loadings.

Ice loads for use in designing lines are normally derived on the basis of past experience, code requirements, state regulations, and analysis of historical weather data. Mean recurrence intervals for heavy ice loadings are a function of local conditions along various routings.

The impact of varying assumptions concerning ice loading can be investigated with line design software. The calculation of ice loads on conductors is normally done with an assumed glaze ice density of 57 lb/ft3 (8950 N/m3).

The weight of ice per unit length is calculated with the following equation: Wice = 0.0281t(Dc +  t)  where
 t = radial thickness of ice, mm
Dc = conductor outside diameter, mm
Wice = resultant weight of ice, N/m

The ratio of iced weight to bare weight depends strongly on conductor diameter. As shown in Table 14-16, for three different conductors covered with 0.5-in radial glaze ice, this ratio ranges from 4.8 for No. 1/0 AWG wire to 1.6 for 1590-kcmil conductors.


As a result, small-diameter conductors may need to have a higher elastic modulus and higher tensile strength than do large conductors in heavy ice and wind loading areas to limit sag.

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