382 ELECTRICAL EQUIPMENT 



flexible, as by different connections any combination can be 

 obtained. Economical considerations are, however, often the 

 determining factor in deciding on what type to use. 



Three-phase designs may be connected either in delta or Y, 

 and the units may be either of the shell or the core type con- 

 struction. In delta-connected shell-type transformers, should one 

 phase be damaged, it is possible to operate the remaining two 

 phases in open-delta at 58 per cent of the combined capacity, by 

 simply disconnecting the damaged unit of the three single-phase 

 transformers, or in the case of three-phase shell-type units by dis- 

 connecting and short-circuiting the damaged phase, both high- 

 and low-voltage. This will reduce the flux passing through the part 

 of the core surrounded by these windings and limit the current in 

 the damaged winding to a fraction of the normal full-load current. 



Y-connected shell-type transformers of both the single- and 

 three-phase types cannot be operated with one phase damaged, 

 except where the neutral is grounded, in which case they may be 

 operated at 58 per cent of their total capacity by short-circuiting 

 both the high- and low-voltage windings of the damaged phase. 

 Such a scheme is, nevertheless, not very satisfactory for motor 

 operations on account of the unbalancing of the phases and the 

 reduced voltage. Lights can, however, be operated successfully 

 by connecting them between the live single-phase wires and the 

 neutral. 



In the case of three-phase core-type transformers, even though 

 the windings are delta-connected, it is impossible to operate when 

 one phase becomes short-circuited. This is due to the fact that 

 the three phases are magnetically interlinked in such a manner that 

 any one phase is a return path for the fluxes in the other two 

 phases. This means that when one phase is short-circuited the 

 short circuit is transmitted magnetically to the other two phases 

 in such a manner that when the two phases are excited large 

 short-circuit currents flow, the short-circuit phase acting as sec- 

 ondary and the remaining phases as primary. In the three-phase 

 shell-type transformer this does not occur, because the fluxes in 

 the three phases are independent of each other, and, therefore, 

 the flux in one phase can be reduced to zero without affecting the 

 other. However, if the damaged winding can be open-circuited 

 or removed from the core, the transformer will operate satisfactory 

 connected open-delta. 



