SWITCHING EQUIPMENT 523 



transformers are connected in series in the circuit containing the 

 relay coil and, in such a manner, that, under normal conditions, the 

 current would simply circulate in the secondary circuit and not 

 enter the relay coil due to its higher impedance. If, however, 

 trouble should occur in the generator, there would be a reversal of 

 current through the current transformer nearest the oil circuit 

 breaker, and the two secondary currents would naturally oppose 

 each other, in which case both would take the path through the 

 relay coil. This would, therefore, receive the resultant of both 

 currents and trip out the oil circuit breaker and disconnect the 

 faulty generator unit from the system. 



If it should so happen that the two current transformer prima- 

 ries differ from that of the power transformer, which may easily 

 occur when tap connections are changed, the secondary currents 

 in the two current transformers would not be equal. This would 

 mean that there would be a resultant current or flux in the relay 

 which would be equivalent to that difference, and satisfactory 

 operation would be affected to some extent. It is, therefore, 

 important that, with normal load on the power transformer, the 

 unbalanced current, that is, the difference between the secondary 

 currents in the current transformers connected to the two sides 

 of the power transformer should be zero. Otherwise two coils 

 should be used, as shown in Fig. 327. These are wound on the 

 same core, the coils being connected separately to current trans- 

 formers in the primaries and secondaries of the power trans- 

 former. Normally the coils oppose each other, with resultant 

 zero flux in the relay core. When a winding of the power trans- 

 former is short-circuited, the other lines in parallel feed back into 

 the short, reversing the direction of one coil so that the flux in 

 the core becomes cumulative and the relay operates. When 

 used in connection with generators the neutral point must be 

 opened for the insertion of current transformers, as shown. 



Pilot Wire Relays. For a single tie line, over which energy 

 may normally be fed in either direction, reverse power relays at 

 each end of the circuit connected by means of pilot wires, will open 

 both ends of such a line whenever trouble exists on that line, and 

 under no other conditions. Energy may flow in either direction 

 so long as the energy in the two ends of the line shall flow in the 

 same direction. These relays are equipped with double-throw 

 contacts, the construction o/ the relays being such that so long as 



