34 CONSTANT-VOLTAGE TRANSMISSION 



power system is well known, and consists in the reduc- 

 tion of short-circuit currents. Where a large number 

 of generators are connected in parallel, the current flow- 

 ing when a short circuit occurs is enormous, and has 

 great destructive effects in distorting the ends of gener- 

 ator coils, in wrecking transformers, and in blowing up 

 circuit breakers. The value of this current may be cal- 

 culated,* and it depends on the amount of reactance 

 in its path. It has been shown by oscilligraph records, 

 and proved by the resulting immunity from accidents, 

 that high reactances give valuable protection when short 

 circuits occur. 



One of the main reasons for the adoption of the fre- 

 quency of 25 cycles to any great extent in this country 

 was because the low reactance pertaining to it allowed 

 larger loads to be transmitted than could be done with 

 the same voltage variation at the higher frequency of 

 60 cycles. In other words, line costs were lower at 25 

 cycles. Other reasons for the lower frequency were 

 sometimes the inherently slow speed of low head water 

 wheels, which made the 25-cycle generators more eco- 

 nomical, and often the desirability of using 25-cycle 

 current for synchronous converters. The last reason is 

 disappearing of late years, since 6o-cycle synchronous 

 converters are now manufactured which give practically 

 as reliable and satisfactory operation as 25-cycle conver- 

 ters. With the constant-voltage system of transmission, 

 the first reason for low frequency also largely disappears, 

 because low reactance, as stated above, is scarcely an 

 advantage with this new method of transmission. It 



* See " Current Limiting Reactances," Proc. A. I. E. E., February, 

 1914. 



