TRANSFORMERS 



401 



A' B' C' represents the Y-connected high-voltage winding of 

 the step-up transformers and a b c the high-voltage winding of 

 the step-down transformers, of which the phase c-d is out of ser- 

 vice. A three-phase open-delta connection a' b' c' is thus obtained 

 on the low-voltage side. 



The capacity is reduced to 57 per cent of the original value, and 



FIG. 245. 



care must be taken not to connect a' c' in the position a' c", since 

 this will not give a three-phase relation. The neutral connection 

 on the high-voltage side should preferably be made through a 

 wire, but can be made by solidly grounding the neutral of both 

 transformers. The system will, however, be electrostatically 

 and electro-magnetically unbalanced, and the usual disturbances 

 characteristic of such a condition will be observed, the severity 

 depending on the circuit characteristics. 



Synchronous converters are frequently installed in connection 

 with Edison systems, where three- wire direct-current is required. 



FIG. 246. 



The three-wire feature is readily obtained by connecting the neu- 

 tral wire directly to the neutral point of the low-voltage winding 

 of the step-down transformers. Care should, in such a case, be 

 taken in using only such connections, that the transformer will 

 act as an auto-transformer, that is, that the direct current in 

 each transformer divides into two branches of equal m.m.f. 



