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EU12 - Transmission and distribution efficiency

For additional information, please visit http://www.aep.com/about/transmission/

An energized transmission line carrying load incurs power losses due to heating and so-call "corona" effects. Heating (or resistive) losses increase linearly with line resistance and quadratically with loading. Corona losses result from undesirable discharge of electric energy, which can be visible and/or audible especially during rain, caused by air ionization around line conductors and hardware. Corona losses increase with voltage level and elevation above sea level of the line

The following statistics characterize EHV transmission lines operating at different voltages, in normal weather, carrying 1000 ME of power:

  Resistive Corona*
765 kV line @1000 MW LOAD:        
Original 4-conductor ("Rail") bundle 4.4 6.4 10.8 -1.10%
Newer 4-conductor ("Dipper") bundle 3.3 3.7 7.0 -0.70%
Current 6-conductor ("Tern") bundle 3.4 2.3 5.7 -0.60%
Planned 6-trapezoidal cond. ("Kettle") bundle 3.1 2.3 5.4 -0.50%
500 kV LINE @1000 MW LOAD"
Typical 2-conductor bundle 11.0 1.6 12.6 -1.30%
345 kV LINE @1000 MW LOAD:
Typical 2-conductor bundle 41.9 0.6 42.5 -4.20%

*Yearly average corona loss at sea level based on 20%/2%/78% rain/snow/fair weather conditions, respectively.

The markedly superior transmission efficiency of 765 kV transmission is attributable to its higher operating voltage and thermal capacity/low resistance compared to 500 kV and 345 kV. Furthermore, by unloading the underlying, lower-voltage systems with higher resistance, overall system losses are reduced.