390 



ALTERNATING CURRENTS 



ing end is E . In order to determine the line characteristics, one 

 phase is removed, Fig. 354(6), and its characteristics determined. 

 Under the condition of balanced load, which is assumed, the 

 relations in all three phases are similar, so that the results ob- 

 tained with one phase may be applied to the other two. As 

 each pair of wires is the common return of the third wire, no 

 current returns through the ground under the balanced conditions 

 assumed. As the voltage drop between the load neutral and 

 the generator neutral is zero, the ground may be considered as a 

 return conductor of zero resistance and of zero reactance, as was 



1 R X 



(a) Three-phase transmission line having 

 resistance and reactance. 



R X 



-vwwv ^Rnnnnno 



(6) One phase of 3-phase line. 

 FIG. 354. Three-phase line having resistance and reactance. 



done in the single-phase case. The load need not necessarily be 

 Y-connected, as indicated in Fig. 354 (a). The same method is 

 used even if the load be delta-connected and there be no neutral. 

 The delta-load is replaced by an equivalent Y-load and the com- 

 putations are made for one phase only. 



Example. Solve the problem of Par. 163, assuming three-phase trans- 

 mission, other conditions remaining the same. Power to be delivered, 4,000 

 kw.; load voltage, 33,000 between conductors; distance, 25 miles; frequency, 

 60 cycles; load power-factor, 0.85; spacing of conductors, 48 in.; allowable 

 line loss 10 per cent of power delivered. Find (a), (6), (c), (d), and (e), 

 Par. 163. 



(a) The power per phase = 4,000/3 = 1^330 kw. 



The voltage to neutral E R = 33,000/\/3 = 19,070 volts. 



