CURRENT-LIMITING REACTORS 473 



are made non-automatic, it would only be the sustained value of 

 the current, thus probably about two and one-half times the 

 normal rating or 75.000 Kv.A. With an automatic voltage 

 regulator holding up the excitation, this value would, however, 

 be greatly increased. 



If the inherent reactance had been less than the above, or the 

 capacity of the generators greater, it might have been necessary 

 to install external reactors in the generator leads to limit the 

 short-circuit current which the switch would have to rupture, as 

 shown in Fig. 289. This is, however, never done in hydro- 

 electric stations, and if such a condition should arise the bus is 

 generally sectionalized by means of reactors as shown in Fig. 290 

 and as explained in the following. 



As previously stated, the purpose of installing bus-bar reactors 

 is to limit the amount of current that can flow into a fault in one 

 section of the bus-bars, and so confine the disturbance to that part 

 of the system on which the fault occurs. Bus reactors should 

 have a reactance sufficiently high so that in case of a short-circuit 

 on one bus section the voltage of the adjoining sections is not 

 seriously disturbed by the current flowing from them over the 

 reactors into the short-circuit. On the other hand, it is highly 

 desirable to operate all the generators of the station in parallel, 

 and this necessitates a reactor of a low enough reactance to permit 

 the interchange current between the bus sections to take care of 

 the required distribution of the load along the bus. 



The amount of reactance to be installed involves a careful 

 study of the layout of the system. Probably a value allowing a 

 transfer of power equal to the capacity of one generator (one-half 

 from each adjacent section), may be considered sufficient. If 

 then each generator had a short-circuit current of eight times 

 normal full-load current, the value of the reactors would have to be 

 25 per cent, based on the full-load current of one generator, and 

 the current carrying capacity would have to correspond to one- 

 half of the full-load current of one generator, this being the full 

 load on the reactor. The displacement between the sections on 

 the above assumptions would be approximately 7J, a value at 

 which the generators of the sections could safely be maintained in 

 parallel. As a fact, this could be done safely at twice this angle 

 and they would probably not fall out of step until the displace- 

 ment was three or four times this value. 



