A New Thermal Rating Approach: The Real Time Thermal Rating System for Strategic Overhead Conductor Transmission Lines Part III Steady State Thermal Rating Program Continued-Solar Radiation Considerations

For the first time, equations and computer programs are developed to show how the solar radiation received by an overhead transmission line is dependent on the conductor sag or slope. In the past, the overhead line has been assumed to lie in a horizontal plane with respect to the earth for the entire span length. Mathematical relationships for determining the sun´s position in terms of solar azimuth and solar altitude, as a function of latitude, declination and hour angle are formulated and a polar plot is constructed to show the apparent path of the sun during the day for each month. Two different methods are presented to determine the angle of incidence between the sun´s ray and the normal to any sloped cylindrical surface oriented at any azimuth angle. Plots are provided to indicate the variation in the angle of incidence and the direct component of solar radiation absorbed as a function of solar time for four general transmission line azimuths and five slopes at one latitude and declination. In addition, plots are drawn for the total solar radiation received by a conductor on a typical clear day as a function of both the span length and any sag or slope for both minimum and maximum diffuse radiation conditions for four general transmission line azimuths at one latitude and declination. It is shown that the conductor slope can have as much of an effect on the total radiation as does the solar adsorption constant.