ADVANTAGES AND DISADVANTAGES 37 



synchronous motors show small tendency to be un- 

 stable, and the phase modifiers, not being connected to 

 any mechanical load, would probably be especially 

 safe, and would remain in step at times of line dis- 

 turbances as tenaciously as a fully loaded induction 

 motor. 



Another disadvantage of synchronous machines is 

 that they tend to increase the current flowing into a 

 short circuit. In connection with this, it must be re- 

 membered that the synchronous phase modifiers are 

 scattered, and are a long distance from the generators. 

 The most dangerous short circuit is one close to the 

 largest station, and synchronous machines at the other 

 end of a transmission line cannot increase the short- 

 circuit current appreciably, owing to the large interven- 

 ing line reactance. Moreover, the increase in reliability 

 due to the greater reactance in all parts of the system, 

 made economical by the constant-voltage method, is 

 probably of more effect than the above-mentioned 

 tendencies. 



A large number of lines is always useful, since it 

 insures a good reserve in case of line trouble. The num- 

 ber of lines for any project must be reduced if the econ- 

 omies of the constant-voltage system are to be fully 

 utilized. On the other hand, reserve is becoming of 

 somewhat less importance because of the tendency to 

 interconnect power systems as much as possible, and 

 even to exchange power between independent companies. 

 Reserve is also provided and extra line construction 

 saved, by steam turbo-generators used as synchronous 

 motors maintaining constant line voltage, and also used 

 as reserve generators. 



