CURRENT-LIMITING REACTORS 459 



current into a short circuit with a view to protect the apparatus 

 from overheating as well as failure from destructive mechanical 

 forces; also protecting the system as a whole against shut-down 

 by maintaining the voltage on part of the system while the short 

 circuit is being cleared. 



Rating. Reactors are generally spoken of as introducing a 

 certain per cent reactance in a circuit. This is the ratio of the 

 voltage drop across the reactor (when the rated current of the cir- 

 cuit at rated frequency is flowing through the reactor), to the 

 voltage between line and neutral on three-phase circuits, or the 

 voltage between the lines on single-phase circuits. The reactance 

 is, therefore, expressed as being single-phase in either case. 



The kilovolt-ampere (Kv.A.) rating of the reactor is the product 

 of the voltage drop across the reactor and the rated current. For 

 generator, transformer and feeder reactors the rated current is 

 usually taken as equal to the current-carrying capacity of the 

 apparatus, while, for bus sectionalizing reactances, it is determined 

 by the power which must be transferred over the reactor. This is 

 very often chosen so as to correspond to the capacity of one of the 

 generators. 



Current-limiting reactors should furthermore be designed for 

 the maximum load current they will have to carry. Being self- 

 cooled and having neither iron nor oil to provide thermal storage 

 they reach their maximum temperature very quickly. Therefore, 

 in cases where the apparatus or circuits must carry overloads for 

 two hours or more, this overload current should be considered the 

 rated current of the reactor, and the capacity should be selected 

 on this basis. Under this assumption, a temperature rise of 85 C. 

 represents common practice, the rise being based on an ambient 

 room temperature of 40 C. 



As reactors, as a rule, do not have an iron core to become 

 magnetically saturated, the reactive drop will be proportional to 

 the current. That is, if a circuit having a 5 per cent reactor were 

 to be short-circuited at the reactor terminal on the load side and 

 having full sustained voltage on the supply side, the sustained 

 current would be limited to 100 -r- 5 or twenty times normal. It 

 should be remembered that transformers and generators in cir- 

 cuit with the reactor also have definite values of reactance which, 

 when expressed in terms of the current of the circuit (per cent 

 reactive drop with normal current flowing) may be added directly 



