| Transmission line thermal limits |
| A large current flow increases the losses in the form of heat. This results in increased conductor temperatures. Excessive temperature may result in expansion and resultant sag of conductors causing decreased clearance to ground. Temperature extremes have an "annealing effect" causing reduced mechanical strength of aluminum. |
Since thermal time constants are large, it is useful to distinguish between steady state and transient thermal ratings.
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Thermal Capability is a function of 1) ambient temperature 2) Wind conditions 3) Condition of conductor 4) Conductor type and 5) ground clearance.
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Typical steady state thermal specifications of conductors used for 400 kV overhead transmission:
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ACSR (aluminum conductor steel reinforced) Moose Conductor (520 sq mm): For an ambient temperature of 40º C, and a maximum conductor temperature of 75º C, ampacity is 700 A (approx.). Typically, two moose conductors form a bundle of 400 kV line so that the total ampacity is 1400A for such a line. |
If AAAC (all aluminum alloy conductor) of 520 sq mm is used, higher conductor temperatures as compared to ACSR are possible. For an ambient temperature of 40º C, and a maximum conductor temperature of 85º C, ampacity is 850 A (approx.). |
| The thermal ratings are a strong function of ambient conditions (wind flows, solar radiation) and absorption and emmisivity of the conductor material ; it makes sense to evaluate the actual ampacity during real time operation (season to season, day to day or hour to hour). |
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