Carrier Communication Schemes

Carrier Communication Schemes:-

The main disadvantage of conventional time-stepped distance protection is that the instantaneous Zone-1 of the protective scheme at each end of the protected line is set to cover 80% of the line and hence faults in the balance 20% of the line (at each end) are cleared in Zone-2 time, which is undesirable.

The desirable scheme is the one wherein the relays clear the faults on the 100% of the protected line instantaneously and also provide backup for uncleared faults on adjacent lines.  This can be achieved by interconnecting the distance relays are each end of the line by a signaling channel (which can be either pilots, a power line carrier communication channel, a radio link or a microwave channel).

The purpose of the signaling channel is to transmit the information about the system conditions at one end of the protected line to the other end and initiate or prevent tripping of the remote circuit breaker.  The former arrangement is referred to as a “Transfer trip scheme” while the latter is known as “Blocking scheme”

a) Transfer trip scheme:-

            In this scheme, the distance relay at one end of the protected lines sends a carrier signal to the relay at other end of the line for inter-tripping, thereby clearing the faults on entire line instantaneously.

 Transfer trip is of two types:-

i)                    Under-reaching scheme:

The scheme in which the Zone-1 relay (set to cover about 80% of ZL) is used to send a signal to the remote end of the feeder for inter-tripping is termed as transfer trip under-reaching scheme.  To avoid mal-operation due to receipt of false signal, the receiving end relay operation is inter-locked with its Zone-3/starter operation i.e. the scheme operates either by its own Zone-1 relay operation or by receipt of carried and its Zone-3/starter operation.

ii)                  Over-reaching scheme:-

This scheme is suitable for short lines where an underreaching Zone-1 would be too short to be of any practical use.  In this scheme the relay set to reach beyond  100% of the line, is used to send an inter-tripping signal to the remote end of the line.  It is essential that the receive relay contact be monitored by a directional relay to ensure that tripping does not take place unless the fault is within the protected section.  The disadvantage of this scheme is that there is no independent Zone-1 tripping.  The fast tripping therefore relies entirely on signaling channel.

The disadvantages of these schemes is that the signal is transmitted over the fault line section.  Distortion of the signal may occur due to attenuation introduced into the line by the fault.

b)         Blocking schemes:-

In this scheme, a blocking signal is sent by the reverse looking directional unit ZR to prevent instantaneous tripping for Zone-2 & Zone-3 faults, external to the protected line.  Here ZR must operate faster then forward looking Zone-3 units and the signaling channel must also be extremely fast is operation.

Though all the distance schemes with carrier inter-tripping/carrier blocking facility are procured, the same are yet to be commissioned.