390 



ELECTRICAL EQUIPMENT 



first step, and leads 1, 3, 5, 10, 11, and 12 for the second step. 

 Leads 2, 4, and 6 are for the final or full- voltage step. Leads 



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Collector 

 Rings 



Collector 

 Rings 



FIG. 228. 



123456 



FIG. 229. 



1, 3, and 5 are connected directly to the converter and the starting 

 is done by two triple-pole, double-throw switches, as shown. 



Recent improvements in the design of synchronous converters 

 have, however, made possible the elimination of the second start- 

 ing tap, and it is now general practice to use only one partial 

 starting voltage, requiring one three-pole, double-throw switch 

 for six-phase converters. 



Number and Size of Units. The number and size of the trans- 

 former units and whether they should be single- or three-phase 

 depends entirely on the nature of the development and on the con- 

 ditions to be met. With moderate voltage developments it has 

 in the past been the general practice to install one transformer 

 bank for each generator and of equal capacity to the same, even if 

 the size was not the most economical. With a large number of 

 units it was then naturally more advantageous to install three- 

 phase transformers, while in plants consisting of one or two gen- 

 erating units, where the cost of a spare three-phase unit was not 

 warranted, it was found preferable to install single-phase units. 



With present modern high-voltage systems where it is unde- 

 sirable to parallel the outgoing transmission lines on the high- 

 tension side of the transformers or to carry out any high tension 

 switching, which may cause surges, it has become a general 

 practice to install the transformers in groups, each having a 

 capacity corresponding to one line; the transformer group and 

 the line thus being considered as a unit. Transmission lines 



