494 ELECTRICAL EQUIPMENT 



reverse power relays in connection with overload relays which, 

 in a system of this kind, can be set for practically instantaneous 

 action. 



The time settings of the relays should, in general, be so arranged 

 that the substation circuit breaker trips out first and then the 

 breaker in the generating station, thus disconnecting only the 

 faulty line. The load is then shifted over to the other line which 

 remains in service and may overload the transformers of this line. 

 This will not cause any danger, as transformers can readily carry 

 up to 100 per cent overload for a few minutes until the operator 

 has had time to open the high-tension circuit breakers of the faulty 

 line and close the tie circuit breakers and the low-tension trans- 

 former breakers, thus again paralleling the transformers. The 

 overload relays on the generating station breakers should, there- 

 fore, be set sufficiently high so that they can carry the entire load 

 without tripping. 



The outgoing substation feeder circuit breakers should be 

 equipped with inverse time-limit relays set proportionally lower 

 than the overload relays in the generating station. In a system 

 of this kind the time element may be very short, which is an 

 important item, as previously mentioned. The substation line 

 relays can, therefore, be set for instantaneous action on reversal 

 and, in such a case, the generating station overload relays need 

 only be set for a second at the most. The feeder relays may also 

 be set for nearly instantaneous action in order to have them trip 

 before the overload line relays in the generating station. 



Example II: This refers to a somewhat more complicated 

 system, as illustrated in the diagram, Fig. 304. It consists of 

 one large generating station still feeding one main substation from 

 which several distribution systems are supplied. 



The main substation in this case is fed over three parallel 

 transmission lines and as far as the relay protection for these is 

 concerned, it may be done in the same manner as explained in 

 Example I, but interconnected reverse power relays may also be 

 used in either case. 



The first consideration in relaying a system of this kind is to 

 keep the power on the main substation bus, no matter what hap- 

 pens, and in protecting the various circuits beyond, the sub- 

 station bus may be treated just as if it were the generating sta- 

 tion bus. 



