CURRENT-LIMITING REACTORS 485 



Mechanical Design. Current-limiting reactors must be de- 

 signed so as not to saturate at short-circuit when the full-circuit 

 voltage comes across the reactance, and for that reason they are, 

 as a rule, built without an iron core. There is, however, no the- 

 oretical objection to the use of iron and if, for example, a reactor 

 for say 25 per cent were required, it would be feasible and pos- 

 sibly even economical to provide an iron core, which, in such a 

 case, would have to have a normal magnetic density of one- 

 fourth the saturation. For 3 to 10 per cent reactors, however, an 

 excessive amount of iron would be required to prevent saturation 

 at short-circuits, thus making an iron core highly uneconomical. 



The latest construction of reactors is shown in Fig. 302. It is 

 known as the " cast-in " type because of the fact that the winding 

 is cast and directly supported in the concrete structure. 



The conductor, which may consist of one or several cables in 

 multiple, is wound radially in conical layers, an ample factor of 

 safety being preserved between each and every turn. The adja- 

 cent layers are inclined in opposite directions with ample spacings 

 between the layers, the spacing varying with the voltage of the 

 circuit and the numbers of layers required. Ample spacing is 

 essential during short-circuit conditions since there is almost 

 always arcing at the point of short-circuit which may set up high- 

 frequency disturbances. Any two layers thus converge toward the 

 point where the interconnecting cross-over is made and where the 

 maximum voltage between the layers is consequently equal to 

 that between turns. 



The windings are held rigidly in their position by the vertical 

 coil supports which are cast around the turns after these have 

 been wound in a form. The concrete is thereafter cured under 

 high steam pressure which gives it a mechanical strength obtained 

 in no other way. 



8. SWITCHING EQUIPMENT 



The engineering problems in connection with the operation 

 of high-voltage hydro-electric transmission systems are very 

 largely those which have to do with preventing interruptions to 

 the service and which isolate and localize electrical disturbances 

 before they can become of a general nature. This resolves itself 

 not only into the general design of the apparatus but also to a 

 careful study of the best possible arrangement of the different 



