SWITCHING EQUIPMENT 495 



Substations II and III are fed in tandem, the former by three 

 parallel transmission lines and the latter by only two, and the 

 relaying of these lines should be identical with the main trans- 

 mission lines with the exception, of course, that the time settings 

 have to be proportionally lower. This clearly demonstrates the 

 point of graded time settings, and it is evident that the circuit 

 breakers furthest away from the generating station should have the 

 lowest setting and the succeeding relays in each section, counting 

 towards the generating station, should each have an increase in 

 the time element of about half a second. This may put an 

 excessive time on the breaker nearest the generating station and, 

 in that case, by the use of inverse time limit relays without def- 

 inite minimum time setting, taking advantage of both the time 

 and current difference, it may be possible to considerably shorten 

 the time on all the relays. This usually involves a careful calcu- 

 lation of the actual short-circuit values to determine the required 

 settings. In certain cases where the time setting of the relays 

 nearest the generating station has become rather high, it has been 

 the practice to also install an instantaneous overload relay in 

 parallel with the time limit relay on the circuit breaker nearest the 

 generating station and to set this relay very high, the idea being 

 that, in case of a severe short-circuit, it should disconnect the cir- 

 cuit immediately. The use of such an arrangement is, however, 

 questionable as it often happens that the instantaneous relay 

 acts when it should not, thus crippling the entire service of all the 

 sections in the series. 



Substations IV to VIII are connected on the ring system prin- 

 ciple and the relaying can be done in several ways. One way 

 would be to provide reverse power and overload relays on the 

 incoming line circuit breakers in each substation and inverse time 

 limit relays on the outgoing line circuit breakers, this being, of 

 course, on the assumption that the power is being fed into station 

 VI over both lines. Circuit breakers a, c and e would then be pro- 

 vided with overload relays only and /, d and b with reverse power 

 relays in combination with overload relays. The settings of the 

 overload relays would be in the following order: a, c, e, f, d and 6; 

 a having the highest setting and b the lowest. 



Example III: This illustrates a system consisting of three 

 generating stations feeding a number of substations, the con- 

 nections, as illustrated on the diagram Fig. 305, being on the 



