476 ELECTRICAL EQUIPMENT 



tion A is represented by the vector OE and that of the generators 

 on section B by OF. It will, therefore, be noted that the current 

 through the reactor increases the load on generator A and de- 

 creases that on B. Similarly, the power-factor of the load on A 

 has been increased and that on B decreased. The projection OF 

 on OB equals the projection OE on OA showing that the energy 

 delivered by the generators on each section is equal. 



The size of feeder reactors depends on the size of the feeders, 

 the relation of their capacity to that of the generators and the 

 capacity of the feeder circuit breakers, i.e., their safe rupturing 

 capacity. In general, the reactor required for an overhead cir- 

 cuit will be less than for an underground cable, because the former 

 usually has a higher reactance. 



As an example, assume a 100,000 Kv.A. station, the inherent 

 reactance of the generators being such as to limit the short- 

 circuit current to six times full-load current. In case of a short 

 circuit on one of the feeders close to the bus-bars, not less than 

 600,000 Kv.A. would pass into the fault, and if the capacity of 

 the feeders were 3000 Kv.A., this would be equal to two hundred 

 times the normal capacity of the feeders and the reactance of the 

 generators would, therefore, only be equivalent to one-half per 

 cent reactance in the feeders. 



If now a 3 per cent reactor is placed in each feeder the total 

 reactance will be equal to 3.5 per cent and the worst possible short 



3000 



circuit conditions would be equivalent to X 100 = 86,000 



o.o 



Kv.A., or 28.6 times the normal capacity. The voltage of the 

 bus instead of dropping to zero, would only be reduced to 28.6X3 

 or to approximately 86 per cent of its normal value. 



Besides the above, the problem must also be dealt with from 

 the economical point of view. For example, the cost of the 

 different types and sizes of reactors must be compared, the space 

 occupied thereby must be considered as well as the effect which 

 the introduction of reactors may have in permitting less expensive 

 switches and apparatus to be used. 



The magnitude and intricate connections of modern transmis- 

 sion systems makes the determination of the probable short-circuit 

 current at the various points a very tedious work, and, in order to 

 facilitate the calculations it is always desirable and almost neces- 

 sary to graphically represent the system in a diagram with the 



