TRANSFORMERS 439 



circular shape which has many advantages over the former, in 

 that they can be more easily insulated and supported to with- 

 stand the mechanical stresses due to short-circuits. 



With shell-type windings the coils are assembled into several 

 primary and secondary groups so mixed as to obtain the proper 

 compromise between voltage regulation and a desirable reactance, 

 the spacings being furthermore dependent on the required dielec- 

 tric and the oil flow necessary for cooling. 



With the core-type design, the following three different wind- 

 ing arrangements are in use : 



1. Interleaved disc coils, for low and moderate voltages. 



2. Concentric cylinder coils, for intermediate voltages. 



3. Concentric disc cylinder coils, for high voltages. 



With the interleaved construction the coils are assembled 

 horizontally over an insulating cylinder around the core, the 

 primary and secondary coils being interleaved in symmetrical 

 groups with insulating oil ducts and barriers between them 

 (see Figs. 276 and 277). They are usually wound with rectangular 

 conductor, one turn per layer. The whole structure is securely 

 braced at each end by plates rigidly engaging with the steel channel 

 core clamps. There are usually four or more groups in the wind- 

 ings, depending upon the capacity, voltage and the required react- 

 ance. 



The concentric cylinder type involves a construction in which 

 all the coils are in the form of cylinders assembled concentrically 

 around the core legs, insulated from each other and from the core 

 by insulating cylinders (Fig. 278). The low-voltage coils are 

 placed nearest the core and may be wound with rectangular strip 

 on edge or flat depending upon the number of turns and the size of 

 conductor required. The high-voltage coils may be single- or 

 double-cylinder edge-wound coils or, if the size of conductor is 

 small, the winding may be broken up into a number of small sec- 

 tions and wound with round wire in layers. 



The concentric disc cylinder type is a combination of the 

 above, the high-voltage coils being of the disc form and wound 

 the same as the coils for the interleaved disc type, while the low 

 voltage coils are cylindrical the same as in the concentric cylinder 

 type (Fig. 279). The high-voltage coil is placed outside and the 

 low voltage inside, next to the core, cylindrical insulations being 



