I recently chaired a task force under IEEE Subcommittee 15.11 (overhead lines) to revise Standard 738, IEEE’s standard for calculating the current-temperature relationship of bare overhead conductors. Since this standard is widely referenced in utility compliance documents, any changes are likely to impact many utilities and organizations. We were careful to avoid radical revisions to past practices. In 738-2012, we added a model for radial temperature calculation, revised the form of the convection equations, and made explicit that the method can be used to calculate dynamic ratings and to track conductor temperature where the weather conditions and line current vary over time.
We also removed some prior limited discussions regarding the selection of maximum conductor temperature limits and suitably conservative weather conditions. In particular, with regard to weather conditions, we chose to refer to CIGRÉ technical brochure 299, the guide for selection of weather parameters for bare overhead conductor ratings, published in 2006, which many of the task force members had helped to write. IEEE 738-2012 and CIGRÉ TB299-2006 should be used together to determine static line ratings (SLRs) and dynamic line ratings (DLRs).
Static (Book) Line Ratings
Power flow on overhead transmission lines is limited to keep the bare conductor temperature below the line’s maximum allowable conductor temperature (MACT), thereby limiting sag to maintain acceptable electrical clearances to avoiding excessive aging of the conductors and connectors. The system operator assumes that keeping the power flow less than the calculated line rating keeps the conductor temperature to an acceptable level.
SLRs are calculated using the 738 heat balance method given a MACT and suitably conservative weather conditions for the bare-stranded conductor. A utility may have multiple conductor types and sizes in various lines with varying MACTs; however, the weather conditions are used systemwide for all lines in the transmission system.
SLRs do not vary with actual weather conditions, though many utilities use a seasonal maximum air temperature rather than an annual maximum to produce seasonal ratings. CIGRÉ TB299 suggests using 2 ft/sec (0.6 m/sec) in the absence of field data but describes a reasonable method of field measurements that may justify the use of higher wind speeds.
Many utilities find it convenient and advantageous to use ambient-adjusted (AA) ratings for lines and substation terminal equipment. Since regional air temperature data is readily available in real time, it is relatively simple to adjust the thermal rating of both substation equipment and lines for air temperature. Changing the air temperature by 1°C (1.8°F) typically produces a circuit rating change on the order of 0.5% to 1%.
When based on predicted daily peak air temperature, AA ratings are typically applied systemwide, and produce ratings that are constant for the day and can be predicted days ahead. TB299 suggests the use of AA line ratings is safest when the rating wind speed is conservative (for example,
2 ft/sec [0.6 m/sec]), because the thermal rating of overhead lines is far more sensitive to wind speed and direction than to air temperature and solar heating. At a MACT of 125°C (257°F), the line rating of a Drake aluminum conductor steel reinforced (ACSR) at an air temperature of 30°C (86°F), 2 ft/sec (0.6 m/sec) is 100 A less than at a higher air temperature of 40°C (104°F) but with 4 ft/sec (1.2 m/sec) assumed.
At night, when wind speeds in line corridors are low, it is not correct to use higher ratings because of low air temperature and no solar; rather, TB299 suggests using a lower wind speed at night. If 2 ft/sec (0.6 m/sec) is used during the day, then the wind speed at night should be reduced to as low as zero. Some recent field studies seem to support the idea of improving AA line ratings by using line-specific wind statistics to allow higher daytime ratings and safer nighttime ratings.
Dynamic Line Ratings
IEEE 738-2012 specifically describes its use in calculating line-specific DLRs based on real-time field data. The use of DLR avoids guesswork about line corridor wind speeds but requires monitors and communications equipment be purchased, installed, calibrated and maintained. The volatility and poor predictability of DLR limits its use, but commercially available instruments such as ultrasonic anemometers, with essentially zero stall speeds and no inertia effects, are greatly improved. An IEEE working group paper on the subject has been submitted for publication in the IEEE transactions.
IEEE 738-2012 and CIGRÉ TB299-2006 offer significant opportunity for improvement in line rating calculation methods — for static, ambient adjusted and dynamic.
Dale Douglass is a principal engineer with Power Delivery Consultants Inc. He is an IEEE Fellow, a past chairman of IEEE Subcommittee 15.11 on overhead lines and the U.S. representative to CIGRÉ study committee B2 on overhead lines.