TRANSFORMERS 421 



Figs. 262 to 265 are the connections of single-phase trans- 

 formers used for six-phase operation, and they do not apply to 

 three-phase units. 



Parallel Operation. In order that two or more transformers 

 or groups of transformers shall operate successfully in parallel it 

 is necessary that they are connected so that their polarity is the 

 same, that their voltages and voltage ratios are identical, and that 

 their impedances in ohrns are inversely proportional to the ratings. 



The polarity expresses the phase relation between the high and 

 low voltages as measured at the terminals. When the trans- 

 formers are of the same manufacture, they usually have the same 

 polarity, while if of different makes some may have the high- and 

 low-voltage windings in phase and others 180 apart. 



Effect of Polarity on Parallel Operation It is" easy to deter- 

 mine the right polarity of two single-phase transformers which are 

 to operate in parallel. Fig. 266 represents such a case in which 

 all connections are made except 61. 

 If now the voltage between 61 and d\ 

 is zero it indicates that the two trans- 

 formers have the same polarity, while 

 if the polarities were opposite the 

 voltage from 61 to d\ would be the 

 sum of the two transformers, and the 

 joining of the two leads would cause 

 a short-circuit. When testing for F 266 



polarity the two terminals should, 



therefore, be joined through a fuse or automatic switch. If the 

 fuse does not blow, the connection may be made permanent, 

 while, if the fuse blows the two leads of one transformer must be 

 reversed. 



With three-phase transformer banks operating in parallel it is 

 also necessary that the phase relation of the voltages in the two 

 banks is the same, both as to direction and position. It is, there- 

 fore, not possible to parallel a group of transformers which is 

 connected in delta on both high- and low-voltage sides with a 

 group connected in delta on the high-voltage side and Y on the 

 low-voltage side or vice versa. On the other hand, it is possible 

 to parallel a delta-delta connection with a Y-Y connection, and 

 also a delta-Y connection with a Y-delta connection. 



Three-phase transformer banks divide themselves into three 



