458 ELECTRICAL EQUIPMENT 



7. CURRENT-LIMITING REACTORS 



Purpose of Reactors. Modern generating and transmission 

 systems have reached such magnitudes as to make it necessary to 

 very carefully analyze the abnormal conditions, which may take 

 place during short circuits on the system, with a view of pro- 

 viding such means as may be required for protection not only of 

 the apparatus involved, but also the service as a whole. This is 

 the function of a reactor by means of which the flow of current on a 

 short circuit may be limited to a safe value. It accomplishes this 

 purpose by reason of the voltage drop or back pressure which it 

 exerts in the circuit. 



By means of the proper installation of reactors the whole 

 station, or even several stations, may be operated in multiple while 

 at the same time the several sections may be protected from each 

 other and each section from the individual circuits which it feeds. 

 Troubles may be localized or isolated practically where they orig- 

 inate without communicating their disturbing effects. 



When a short-circuit occurs on a system the voltage will drop, 

 depending on the magnitude of the short circuit and the inherent 

 characteristics of the generators, i.e., their impedance. A severe 

 short-circuit, such as may occur when there are no reactors, will 

 cause the voltage to drop to a low value in a few cycles, whereas 

 on a less severe short-circuit, the time taken for the voltage to 

 drop to the same low value will be longer. Synchronous apparatus 

 will stand a complete loss of power for a few cycles only, but will 

 stand a 'reduction of voltage for a longer period. It is important 

 then that the value of short-circuit be small and that it be cleared 

 in the shortest possible time. Introducing reactors will limit the 

 maximum value of the current, and with the latest type of relays, 

 the time required for selective switch action is very short, so that 

 a trouble can be localized and cleared before the apparatus on the 

 rest of the system is affected. 



The protective and localizing functions of a reactor are, how- 

 ever, quite distinct. The former, since all the evil effects of heavy 

 current excessive mechanical stresses, heating, etc., are pro- 

 portional to the square of the current, is measured in terms involv- 

 ing the square of the total reactance, while the latter is measured 

 in terms of the first power of the reactance involved. 



The chief purpose of a reactor is, therefore, to limit the flow of 



