378 



ALTERNATING CURRENTS 



requires only 75 per cent, as much copper as either the single- 

 phase or the two-phase system. 



The single-phase system is used in railroad electrification, 

 where single-phase power is supplied at the trolley. The most 

 notable examples of this are the New York, New Haven and 

 Hartford Railroad and the Norfolk and Western Railway. 



When the voltage is so high as to make transformers necessary, 

 the power is usually generated at 6,600 volts. This voltage is 

 not so high as to make difficult the proper insulation of the gener- 

 ators, and at the same time the armature conductors and the 

 leads running to the switchboard do not become too large. 



6600-V. Generators 

 6600-V. Bus-bars 

 Delta-Y 



Transformer Bank 

 110,000- V. Bus-bars 



110,000-V. 

 Transmission Line 



110,000-V. Bus-bars 

 Y-Y Trans. Bank 



13,200-V. Bus-bars 



13,200-V. 



Underground Cable 



13,200-V. Bus-bars 



Delta-Y 



Transformer Bank 



4000,2300-V. 



8-Ph. 4-Wire Bus-bars 



Distributing Lines 

 (Overhead or underground* 



Consumers Service 

 230-115 V s-Wire Lighting Mains 

 2300-230 V. Lighting Trans. 

 550-V. 3-Ph. Line 

 000 550-V. 



Connected Power 



Transformers 



FIG. 342. Typical connections of a power system. 



The transmission voltage is largely determined by economic 

 considerations. Although a high voltage reduces the conductor 

 cross-section, the saving in copper may be offset by the increased 

 cost of insulating the line, by the increased size of transmission- 

 line structures and by the increased size of generating and sub- 

 stations, due to the large clearances required by the high-voltage 

 leads and bus-bars. A rough basis for determining the trans- 

 mission voltage is to use 1,000 volts per mile of line. 



Because of the danger involved, it is not usually permissible 

 to carry high-voltage transmission lines through thickly popu- 

 lated districts in order to reach the distributing sub-stations. 

 The voltage is usually stepped down to about 13,200 volts at 

 sub-stations located at the outskirts of the city and thence carried 



