TRANSFORMERS 



423 



There are four other combinations possible for these two 

 banks of transformers, but these combinations will not operate 

 in parallel. These are as follows: 



TABLE XLIX 

 INOPERATIVE PARALLEL CONNECTIONS 



For example, consider case No. 2 low-voltage sides in delta 

 and high-voltage sides in Y and delta respectively. Then as- 

 suming the low-voltage sides already paralleled and high-voltage 

 sides open, the phase diagrams are as follows where A, B, C, a, 

 6, c represent one bank 

 and X, Y, Z, x, y, z the 

 second bank. (See Fig. 

 267A). 



Then if b ajid y be 

 joined on the low-voltage 

 side, serious displacement 

 voltages occur between a 

 and x and c and z (see 



Fig. 267s), and if these terminals are connected, these displace- 

 ment voltages will cause heavy short-circuit currents and destroy 

 the transformers. 



The reversal of two leads of either the high-and-low voltage 

 windings will reverse the polarity, this being identical with re- 

 versing one winding. Reversing the line leads of a delta- or T- 

 connected combination will, however, not reverse the polarity, 

 since the transformer leads themselves must be changed in order 

 to make the change in polarity. 



With delta-delta connection, the reversal of one or two 

 high-voltage windings will immediately produce a short-circuit 



FIG. 267A. 

 Low-voltage Side. 



FIG. 267s. 

 High-voltage Side. 



