INTRODUCTION 3 



place in which to make adjustments in voltage, either 

 by hand or by an automatic regulator. This is done with 

 the constant-voltage method, and, in addition, the same 

 steady voltage is produced in the same way at all other 

 parts of the line or system. 



The usual limit to the amount of power which a 

 given line can transmit is set by the voltage variation 

 produced by a large power load. With the constant- 

 voltage system, voltage variation is overcome, and the 

 amount of power can be increased until some other lim- 

 itation is encountered. This limitation will be imposed 

 by the cost of power for extra line losses at the heavier 

 load, or by the cost of the synchronous motors needed to 

 maintain constant voltage. In either case, the power 

 rating of a line will be found to be approximately twice 

 as great as when synchronous motors are not used. If 

 heavier conductors are used, which is often found to be 

 economical with the constant-voltage system, the power 

 rating of one line may be multiplied by three or more. 

 With lines of moderate or great length the cost of the 

 synchronous motors needed to double the rating of the 

 line is a small fraction of the cost of an extra transmission 

 line. Hence results the very great saving in money pro- 

 duced by the constant-voltage system, which is the 

 strongest reason for the installation of sychronous motors 

 in transmission systems. 



A noticeable feature of modern practice is the increas- 

 ing use of current-limiting reactances, in order to give 

 greater reliability. Ordinarily, the chief obstacle to their 

 use in transmission systems is the resulting extra voltage 

 variation, but with the constant-voltage system this 

 obstacle is overcome at small cost, as is shown by the 



