/images/TDUpdate2.gif/images/southwire_100x70.jpgRevising ACSR Capacity AssumptionsRevisingACSRCapacityAssumptions.htmChanging design parameters to acheive more capacity could damage ACSR conductors
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Revising ACSR Capacity Assumptions

Changing design parameters to acheive more capacity could damage ACSR conductors

Conductor temperature dictates transmission line capacity. In the quest for increasing grid throughput, it's tempting to change the limiting assumptions in the conductor temperature rating calculations. This should be done with caution as this can increase the risk of overheating the conductor.

"Excessive conductor operating temperatures can damage the zinc-galvanized (GA) coating on conductor cores used in ACSR," says Mark Lancaster, manager of overhead transmission engineering for Southwire's Energy Division. This can lead to conductor failure, ultimately cancelling out any benefits gained from increasing the capacity.

IEEE revisiting conductor temperature standards

ACSR is not a high-temperature conductor: it is rated for 75°C continuous operation and 100°C emergency operation for a total of 1,500 hours over the conductor life. These ratings specify the conductor surface temperature. However, there is much discussion in the industry as to how hot the conductor core gets compared to the conductor surface. Currently, there is no agreed on temperature difference.

Work is being done by the Towers, Poles and Conductors (TP&C) subcommittee of the IEEE to address this issue. One TP&C working group is revising the IEEE 1283, Guide for Determining the Effects of High-Temperature Operation on Conductors, Connectors, and Accessories which addresses the effects of higher conductor operating temperatures. This guide should be considered by anyone considering operating conductors at temperatures higher than those recommended by the conductor manufacturers.

Another TP&C working group is currently revising a related standard, IEEE 738, the Standard for Calculating the Current-Temperature of Bare Overhead Conductors. While the basic calculation method isn't likely to change, but there is significant discussion about underlying assumptions like wind velocity and AC resistance values.

Starting place for calculation assumptions

Whether you are considering operating conductors at elevated temperatures, or changing rating parameters to increase line ratings, Lancaster recommends, "CIGRE - the International Council On Large Electric Systems - is a good source for understanding initial assumptions when calculating conductor temperature." CIGRE Technical Bulletin TB299 covers the selection of weather parameters, and TB345 covers the calculation of AC resistance. To join CIGRE and get access to their technical bulletins, go to: www.cigre.org/gb/organization/Membership.asp.

As an alternative to running existing lines at risky temperatures, Lancaster suggests reconductoring with ACSS, specifically Southwire HS285 or other high temperature low sag conductor. For more information on ACSR capacity assumption or reconductoring options, contact the Southwire Energy Division.