CHAPTER XII 

 TRANSMISSION OF POWER BY ALTERNATING CURRENT 



157. Transmission Systems. To transmit power economically 

 over considerable distnnr< >, it is necessary that the voltage be 

 high. High voltages are readily obtainable with alternating 

 current. As high as 15,000 volts may be generated directly. 

 For voltages in excess of this it is desirable to UM- transformers, 

 as it is difficult to insulate the generators for these higher volt 

 The transmission volt.ii:> i- usually too high for commercial 

 uses, but for purposes of distribution it may be stepped down 

 d value by the use of transformers. 



Direct-current voltages for commercial power can be raised 

 and lowered only by machines having rotating commutators. 

 The efficiency of such apparatus is not high, and operating diffi- 

 culties are encountered in connection with the commutators, 

 even at comparatively low voltages. Hence, alternating current 

 is nearly always used for transmission purposes. (The one 

 exception is the Thury 1 System in Kurope.) Where considerable 

 power is involved, polyphase systems are used because of the 

 many advantages of polyphase over single-phase systems. For 

 example, polyphase motors are considerably cheaper and lighter 

 than -ingle-phase motors of equal rating and as a rule have better 

 operating characteristics. The output of generators when 

 operating polyphase is much greater than when operating single- 

 phase. (See page 76.) 



of the polyphase systems, the three-phase system is generally 

 used for transmission, although the employment of two-ph:. 



' -ut ion purposes is not uncommon. Tin- th KM '-phase system 

 has the advantage that it requires the least number of conduc- 

 tors of all the polyphase system-; the voltage unbalancing even 

 with unbalanced loads is not usually serious; and for a i 

 voltage between COfld \\\\\ u given power transmitted a 



given di-tam-i- with . the three-ph:- 



'Seo Vol. I. Page 303, and also 1 N.-mdlM.ok." Fifth Kdition, 



Srrtion XI. 



