TRANSFORMERS 



407 



with a delta group, they may be paralleled with one another and 

 in this case will give a greater capacity than three units con- 

 nected in delta. The capacity of the two V groups would be 

 0.866 times four or 3.46 as against three, the corresponding 

 rating of three transformers connected in delta. 



Table XLVI gives the transformer capacities available with 

 various combinations of open and closed delta groups. 



T-T. JAs with the open-delta arrangement, the T-T con- 

 nection requires only two single-phase transformers, Fig. 249, 

 representing the diagram of connections: , A is called the main 

 transformer and is provided with a 50 per cent voltage tap to 



a 6 c Id 



VVWVWV VWVWW 



B 



/WsAA/WN 



k <. 



AAA/SAAAA 



#" 



w^ v 



* C 2 



4 



&, 



d, 



a, 



FIG. 249. 



which the teaser transformer J5 is connected. This transformer 

 may be designed for only 86.6 per cent of the line or main trans- 

 former voltage, but generally it is made identical with the main 

 transformer and operated at reduced flux density. It should be 

 noted that although the teaser operates at 86.6 per cent of line 

 voltage it is unnecessary to provide an 86.6 per cent tap as is 

 often supposed. On this account it is possible to operate two 

 identical transformers connected T-T as well as open delta, when 

 one transformer of a delta-delta bank burns out, the only require- 

 ment for the T-T connection being a 50 per cent tap. Although 

 interlacing is not required between halves of the main winding 

 nevertheless each half of the primary winding must be properly 

 wound with respect to the corresponding half of the secondary 



