CHAPTER X 

 TRANSMISSION LINE 



254. Transmission Line. The transmission line carries the 

 electrical energy from the generating station to the receiving 

 station or substation, where it is either transformed into mechanical 

 energy or distributed to the customers throughout the district. 



The most important characteristics of a transmission line are 

 (1) reliability, (2) regulation and (3) efficiency. 



(1) To insure reliability of service lines should, wherever 

 possible, be installed in duplicate and all the necessary protective 

 devices applied. 



(2) For good regulation the reactance of the line should be as 

 small as possible and therefore the frequency should be low. The 

 capacity of a line draws a leading current, which partially counter- 

 acts the drop in voltage due to reactance and so improves the 

 regulation. 



(3) The power losses in a line are the resistance loss, which varies 

 as the square of the current, and the comparatively small losses 

 due to leakage over the insulators and to the formation of corona 

 around the conductors. 



To reduce the power loss the resistance of the line should be 

 made as low as possible. This can be done by increasing the cross 

 section of the conductors, but the increased cost of the material 

 required soon overcomes the saving due to the increase in efficiency 



For a given loss and a given voltage between lines power can 

 be transmitted with a smaller amount of conducting material 

 three-phase than either single-phase or two-phase. 



255. Relative Amounts of Conducting Material for Single-, 

 Two- and Three-phase Transmission Lines. 



Let P = power input to the line in watts. 



p per cent loss of power in the line resistance due to 



full-load current. 

 I = full-load current, 

 cos 6 = power factor. 



r = resistance of each conductor. 

 n = number of conductors in the system. 

 397 



