TRANSFORMERS 



399 



The current and voltage relations in the delta side are the same 

 as in the delta-delta connection. On the Y-connected side, how- 

 ever, one end of each winding is connected to a common neutral 

 point and the other three ends to the lines. With this connection 

 the number of turns in a transformer winding is 58 per cent of 

 that required for delta-connected transformers, but the cross- 

 section of the conductors must be correspondingly greater for the 

 same output. For high voltages the currents are, however, gen- 

 erally so small that, in may cases, the size of wire in the high- 

 voltage winding must be governed by mechanical considerations, 



FIG. 243. 



and the size of wire may have to be the same for either system. 

 The delta connection is, therefore, sometimes somewhat more 

 expensive. 



If the neutral point of the Y-connected system is ungrounded, 

 the transformer insulation must be capable of standing the stress 

 of the full line voltage, since a ground on any line will throw full 

 voltage on parts of the transformers. With grounded Y the 

 stress is, of course, limited to the Y-voltage. This is, however, 

 only true for step-up transformers at the generating end of trans- 

 mission line, and also only when the neutral is solidly grounded. 

 When the neutral is grounded through a resistance the insulation 

 in transformer may be subjected to full voltage stress, and under 

 any conditions the step-down transformers may be subjected 

 to full voltage stress. 



For distributing service the transformers have, as previously 

 stated, often their low-voltage windings Y-connected and the 

 neutral brought out, forming a four-wire system, as shown in Fig. 

 244. The single-phase service is then obtained by tapping between 



