TRANSFORMERS 



405 



designs it is possible to operate open-delta when the damaged 

 winding is open-circuited. With V-connected three-phase shell- 

 type transformers the damaged phase should be short-circuited 

 to prevent stray fluxes from the other phase from inducing volt- 

 ages in the damaged windings. 



With the V connection the current in each transformer is 30 

 out of phase with the transformer voltage, so that each trans- 

 former under non-inductive load operates at only 86.6 per cent 

 power-factor. Based on a three-phase load, the cutting out of 

 one transformer would therefore reduce the current-carrying 



M/WWA 



FIG. 248. 



capacity not to two-thirds of 100 per cent, which equals 66.6 per 

 cent, but to two-thirds of 86.6 per cent which equals 58 per cent. 

 Assuming that each transformer shall have a capacity of 



J?T 



= 1.5E7, it must be capable of carrying 1.73E7 kilovolt- 



amperes, because the transformer voltage is equal to the line 

 voltage E, and the transformer current equal to the line current 

 1.737. Therefore, the single-phase rating of each transformer 



1 73 

 must be -^ = 1.155 or 15| per cent greater than one-half the 



group rating. 



Sometimes it is desired to parallel a number of transformers 

 in such a way that certain of the transformers will form a delta 

 group while the others may be connected in open-delta or V. 

 Such a combination may be caused by the desire to increase the 

 capacity by adding spare transformers of insufficient number to 

 form a group of complete deltas or through the failure of one or 



