Transfer Trip or Inter Trip

❀The faults at both the ends (20 % on each side) of the transmission line which sums up to be 40 % of the line length fall in the second step of the distance protection.

❀This therefore receives a delayed protection service.

❀Thus only about 60% of the need length of the transmission line gets a high speed distance protection.

❀In case fault which is in the end 40 % section the nearest distance protection trips simultaneously but the remote end protection is delayed and there is a problem of speeding up the remote end distance protection.

❀As shown in the Fig. Z1, Za and Za are considered for the carrier signal communication during a fault condition.

❀If we consider a fault in the second zone of distance protection but not beyond end B as seen from the end A, then one can make use of first zone Z contact of the local relay at the end B to initiate a carrier and remotely operate a contact to close the trip circuit of the remote circuit breaker at the end A.

Acceleration of zone Diagram

➡️The logic of the protection scheme works on the following lines:

1. The Zi contact of the local relay at the end B operates and changes from its No position to CLOSED position.

2. This closure action of the contact is used to switch ON the carrier transmitter T. at the end B.

3. This carrier transmitter injects a carrier signal in to the line.

4. This carrier signal arrives at the remote end A at the speed of light after a very short delay and received by a carrier current receiver, R.

5. This carrier current receiver gives an output at the Carrier Receipt Relay, CRRA at the end A.

❀The contact of CRRA can be used to energies the trip coil of the circuit breaker at the remote end A in various alternative ways.

❀If the fault detector Fo contact is bypassed by connecting point P with P then the scheme is known as transfer trip or inter trip.

❀In case of transfer trip or inter trip the carrier signal is needed for tripping purposes and during the failure of carrier equipment or during the severe attenuation of the carrier signal due to fault the entire operation of the scheme is put at danger.

❀Therefore the tripping carrier scheme does not provide robustness. The carrier based scheme is expected ideally to be such that in case of failure of carrier it should automatically revert back to the three stepped distance scheme.

❀This type of logic can be built up in such a way that the carrier signal is not required for tripping but it is required for blocking the tripping.

❀If such schemes are used they obviously offer more robustness and such schemes are known as blocking carrier schemes.