212 ALTERNATING CURRENTS 



be supplied by primary coil 2. This primary coil cannot supply 

 the power because it is in series with the primaries 1 and 3 whose 

 secondaries are open-circuited. The two primaries 1 and 3 under 

 these conditions act as very high impedances so that the primary 

 2 can obtain but very little current through them from the line. 

 Therefore, transformer 2 can supply no appreciable power. In 

 fact, the secondary of 2 may be short-circuited and only a small 

 current will flow. The short-circuit merely pulls the primary 

 and secondary neutrals over to wire 2. 



Primaries Secondaries 



FIG. 202. Delta-delta connection of transformers. 



This difficulty of the "floating" neutral may be obviated by 

 connecting the primary neutral back to the generator so that 

 the primary of transformer 2 can take its power from between 

 its line and the neutral. Another objection to Y-Y connection 

 is the fact that the secondary coil voltages contain large third 

 harmonics. 



Primaries 



Secondaries 



FIG. 203. Delta-Y connection of transformers. 



The delta-delta bank shown in Fig. 202 is often used, especially 

 for moderate voltages. Its chief advantage is that if one trans- 

 former becomes disabled, the system may operate in "V" or 

 open delta. In both the Y-Y and the delta-delta connections, 

 the ratios between the primary and secondary line voltages are 

 the same as the individual transformer ratios. 



