DISTANCE PROTECTION SCHEMETYPES OF PROTECTION SIGNALLINGDistance scheme comprises of several auxiliary relays, which perform functions such as flag indications, trippings, signaling, alarm etc.Power Swing blocking relay, VT fuse failure relay, Switch onto fault relay, auto-reclosing scheme. Carrier communication scheme.Power Swing blocking relay:Distance relay which respond to balanced 3phase changes in the impedance will be affected by power swings. These swings or oscillations occur following a system disturbance such as major load change or a dip in voltage due to delayed fault clearance the enroute distance relays on the line may see these conditions as three phase fault and falsely operte CB. In order to avoid power swing blocking relay used.VT fuse failure relay: The distance relays being voltage restraint O/C relays, loss of voltage due to main PT fuse failure or inadvertent removal of fuse in one or more phases will cause the relay operation. The fuse failure relay will sense such condition by the presence of residual voltage without residual current and blocks the relay.Switch onto fault relayUnder normal service conditions, a close-up 3-phase fault will be seen by the relays and clear the fault instantaneously. But when the line is switched on to a The voltage applied to the relay is low and this condition occurring simultaneously will cause instantaneous trip by SOTF relay. This SOTF feature will be effective only for about 1-2 seconds after the line is charged. Signaling scheme:the purpose of this scheme is transmit the system condition to other remote end of protected line to initiate or prevent tripping of breaker1) Transfer trip scheme2) Blocking schemePermissive Trip commands are always monitored by a protection relay. The circuit breaker is trippedwhen receipt of the command coincides with operation of the protection relay at the receiving end responding to a system fault.

Blocking commands are initiated by a protection element that detects faults external to the protectedzone. Detection of an external fault at the local end of a protected circuit results in a blocking signal beingtransmitted to the remote end. At the remote end, receipt of the blocking signal prevents the remoteend protection operating if it had detected the external fault.

The direct under-reach transfer tripping scheme described above is made more secure by supervising the received signal with the operation of the Zone 2 relay element before allowing an instantaneous trip. The scheme is then known as a Permissive Under-reach Transfer Tripping Scheme PUTT ' or PUP Z2scheme or Permissive Under-reach Distance Protection, as both relays must detect a fault, before the remote end relay is permitted to trip in Zone 1 time.

DIRECT TRIPPING PERMISSIVE TRIPPING BLOCKING

DIRECT TRIPPINGIn direct tripping applications, inter-trip signals are sent directly to the master trip relay. Receipt of the command causes circuit breaker operation. The method of communication must be reliable and secure because any signal detected at the receiving end will cause a trip of the circuit Breaker at that end.The communications system design must be such that interference on the communication circuit does not cause spurious trips.

PERMISSIVE TRIPPINGPermissive Trip commands are always monitored by a protection relay. The circuit breaker is tripped when receipt of the command coincides with operation of the protection relay at the receiving end responding to a system fault. Requirements for the communications channel are less onerous than for direct tripping schemes, since receipt of an incorrect signal must coincide with operation of the receiving end protection for a trip operation to take place. The intention of these schemes is to speed upTripping for faults occurring within the protected zone.

BLOCKING SCHEMEBlocking commands are initiated by a protection element that detects faults external to the protected zone. Detection of an external fault at the local end of a protected circuit results in a blocking signal being transmitted to the remote end. At the remote end, receipt of the blocking signal prevents the remote end protection operating if it had detected the external fault.Loss of the communications channel is less serious for this scheme than in others as loss of the channel does not result in a failure to trip when required. However, the risk of a spurious trip is higher.

DIRECT UNDERREACH TRANSFER TRIP SCHEME (DUTT)The simplest way of reducing the fault clearance time at the terminal that clears an end zone fault in Zone 2 time is to adopt a direct transfer trip or inter-trip technique. A contact operated by the Zone 1 relay element is arranged to send a signal to the remote relay requesting a trip. The scheme may be called a Direct Under-reach Transfer Tripping Scheme (DUTT) or Transfer Trip Under-reaching Scheme' or Inter-tripping Under-reach Distance Protection Scheme, as theZone 1 relay elements do not cover the whole of the line. The disadvantage of this scheme is the possibility of undesired tripping by accidental operation or mal-operation of signaling equipment, or interference on the communications channel. As a result, it is not commonly used.

PERMISSIVE UNDERREACH TRANSFER TRIP SCHEME ( PUTT)The direct under-reach transfer tripping scheme described above is made more secure by supervising the received signal with the operation of the Zone 2 relay element before allowing an instantaneous trip. The scheme is then known as a PermissiveUnder-reach Transfer Tripping Scheme PUTT ' or PUP Z2 scheme or Permissive Under-reach Distance Protection, as both relays must detect a fault, before the remote end relay is permitted to trip in Zone 1 time. A variant of this scheme, found on some relays, allows tripping by Zone 3 element operation as well as Zone 2, provided the fault is in the forward direction. This is sometimes called the PUTT-Fwd scheme.

PERMISSIVE OVER-REACH TRANSFER TRIPPING SCHEME (POTT) In this scheme, a distance relay element set to reach beyond the remote end of the protected line is used to send an inter-tripping signal to the remote end. However, it is essential that the receiverelay contact is monitored by a directional relay contact to ensure that tripping does not take place unless the fault is within the protected section. The instantaneous contacts of the Zone 2 unit are arrangedto send the signal, and the received signal, supervised by Zone 2 operation, is used to energise the trip circuit. The scheme is then known as a Permissive Over-reach Transfer Tripping Scheme -POTT' or Directional Comparison Scheme', or Permissive Overreach Distance Protection Scheme. Since the signalling channel is keyed by over-reaching Zone2 elements, the scheme requires duplexcommunication channels - one frequency for each direction of signalling.