30 CONSTANT-VOLTAGE TRANSMISSION 



case is where the expense of rebuilding existing parts 

 of the line prohibits the use of as high a voltage as would 

 be used for a new project. If, for example, it is desired 

 to extend a 6o,ooo-volt, 6o-mile line 20 miles farther, it 

 can be done more cheaply and quickly by installing 

 phase modifiers than by building a higher voltage line 

 the entire distance. This is also true at much lower 

 voltages and shorter distances. In the same way, the 

 use of phase modifiers enables overloads to be carried 

 by transmission lines without impairing the quality of 

 service, though the efficiency is reduced. On the other 

 hand, if phase modifiers are not used, overloads mean 

 either that the voltage variation is increased, thus giving 

 poorer service, or else duplicate lines must be installed. 



In city work, the voltage is limited, and the cost of 

 lines is sometimes excessive, due to voltage variation, 

 though heating of the conductors is often the factor to 

 be reckoned with. 



In long-distance transmission, the power loss at 

 very high voltage due to corona, that is, electric dis- 

 charge through the air between the conductors, puts a 

 limit on the voltage. (See Table V.) An extreme case 

 of this has been described by L. P. Jorgensen,* and it 

 forms an interesting example of how the difficulty of a 

 necessarily low transmission voltage was overcome at 

 small expense by the constant-voltage method. Quoting 

 from his description, "The lowest point on the line is 

 the power-house, located at 12,200 feet above sea. level, 

 and the highest substation is located at nearly 16,000 

 feet elevation. The line runs for the greater part of 



* Proc. A. I. E. E., p. 510, March, 1914. 



