406 ELECTRICAL EQUIPMENT 



more units originally installed. It is not, however, generally 

 realized that such an arrangement will, in general, prove either 

 uneconomical as to capacity, if all the units are kept to rated 

 currents, or disastrous to the units on the legs having the smaller 

 numbers, if it be attempted to work all units at overloads guar- 

 anteed for single-phase operation. Not only is this from the addi- 

 tional 15| per cent capacity required on units for open-delta 

 service, but a further increase in current takes place in the V-con- 

 nected transformers due to change in phase relation, and for this 

 reason when delta and V groups are operated in parallel the result- 

 ant capacity is not the sum of the individual delta and V ratings. 

 More than one V group cannot be used advantageously with a 



TABLE XLVI 



86.6 

 100 



80 



86.6 



82 

 100 



73 

 88 



delta group of transformers nor with two or more paralleled delta 

 groups. Three delta-connected transformers when added to 

 another delta group will give more capacity than if four trans- 

 formers, connected in two V groups, were added to the same delta 

 group. This is because the four transformers, which would form 

 two V groups, can be rearranged to form a delta group (one trans- 

 former remaining idle), and the delta group will have the capacity 

 of three transformers while the two V groups will add the capacity 

 of only two transformers. The addition of two transformers, 

 connected in V, in parallel with a delta group adds the capacity 

 of only one transformer to the capacity of the total group. 

 Although two V-connected groups should never be used in parallel 



