26 CONSTANT-VOLTAGE TRANSMISSION 



In water-power plants of moderate size, sudden 

 changes of load not only produce the variations in volt- 

 age which have been described, but they alter the speed 

 of the water-wheels during the intervals of time before 

 the water-wheel governors can act. These changes in 

 frequency aggravate the voltage variation, and are det- 

 rimental to the satisfactory operation of motors. A 

 large flywheel effect of the system tends to minimize the 

 variations in frequency. Synchronous motors add their 

 fly-wheel effect directly to that of the system, and in 

 this way they improve the regulation of both frequency 

 and voltage of medium-sized power systems. 



The greatest advantage of the constant- voltage sys- 

 tem is, as has been mentioned, that it allows the load of 

 a transmission line to be made two or more times as 

 great as could ordinarily be carried, thus greatly reducing 

 line costs. Where the limit of the load is set by the volt- 

 age variation, the efficiency is usually higher than would 

 be needed for economic reasons. When voltage varia- 

 tion is eliminated by means of voltage control by syn- 

 chronous motors, the load can be increased, and the 

 limit that will be met in most lines will be the cost of 

 power for extra line losses, since the efficiency of a trans- 

 mission line is always reduced as the load is increased. 

 If we assume that the load of a varying-voltage line is 

 limited by a regulation of 15 per cent due to line impe- 

 dance only, and that the load of a constant-voltage line 

 is limited by an efficiency of 85 per cent, then the rela- 

 tive power capacity of lines controlled according to the 

 two methods can be compared, as in Figs. 5 and 6.* 



* See " Constant Voltage Transmission," Proc. A. I. E. E., 

 June, 1913, p. 1362. 



