As the name describes, carrier aided protection operates when the power line carrier is received in the relay and after checking the other fault condition in a transmission line.
In distance protection of transmission lines, generally following 4 types of zones are used to protect the line.
- Zone-I: It will protect 80% of the line length and the operating delay time is zero.
- Zone-II: It will protect 120% of the line length and the operating delay time is 350 ms
- Zone-III: It will protect 100% of the line length + 100% of the longest line length of the connected substation. The operating delay time is 1000 ms
- Zone-IV: It is used for reverse zone protection. It will protect the 10% to 20% of the line length or a maximum of 50% of the connected shortage line in the substation. The operating time is 500 ms.
As shown in the picture, when the fault happened at a position which is in Zone-II for substation A and for zone-I for substation B. In this case, the distance relay of substation A will operates after a delay of 350 ms, and the distance relay of substation B will operate instantly.
The fault should be isolated as fast as possible for the stability of the power system and transmission line equipment. In this case, even after the opening of CB from substation B, the fault is persisting from substation A.
To avoid this delay time, after the zone-I operation from any substation, the distance relay will send the carrier signal through PLCC to operate the distance relay from the other end. After receiving of the carrier at substation B, it will operate after checking the operation of Zone-II protection of its own relay.
Fault in Zone-II + Received of Carrier = Carrier Aided protection