Directional Comparison Carrier Current Protection:

The protection operates on the basis of comparison of the fault power flow directions at the two ends of the protected line. Operation takes place only when the flow of power at both ends of the line is in the bus-to-line direction, a condition which will evidently only arise in event of a fault on the protected section of the line. With Directional comparison carrier current protection, the carrier pilot informs the equipment at one end of the line how a directional relay at the other end responds to a short circuit.

The conditions for internal and external faults are illustrated in Fig. 14.17. The relays at both ends of the protected section respond to fault power flowing away from the bus (tripping direction). For faults in the protected section, power flows in the tripping direction at both ends. For external faults power flow will be in opposite directions. A simple signal through carrier pilot is transmitted from one end to the other during faults.

The pilot scheme can be employed for transmitting either blocking, or permitting signals. Thus possible carrier protections are of two types viz, carrier blocking scheme and carrier permitting scheme.

In a carrier blocking protection scheme, the presence of carrier prevents or blocks operation of the protection. Carrier is, therefore, transmitted only upon the occurrence of a fault and is employed to prevent tripping in the event of an external fault. In carrier permitting scheme the presence of carrier permits operation of the protection. The carrier blocking scheme is more reliable than carrier permitting scheme. This is because a failure in the carrier permitting signal equipment will mean a failure in isolating the fault, where as a failure in carrier blocking signal equipment isolates the section on which no fault exists. However, such false operation is preferable to the failure to clear a faulted section.

In a carrier blocking protection scheme, normally no pilot signal is transmitted from any terminal. Should a short circuit occur in an immediately adjacent line section, a pilot signal is transmitted from any terminal where short-circuit current flows out of the line (i.e., in the non-tripping direction). While any station is transmitting a pilot signal, tripping is blocked at all other stations. But should a short circuit occur on the protected section of the line, no pilot signal is transmitted and tripping occurs at any terminal where short-circuit current flows. Therefore, the pilot is blocking pilot, since the reception of a pilot signal is not required of permit tripping.

Directional comparison carrier current protection scheme (carrier blocking type) is illustrated in Fig. 14.18. The operation of the directional element provided on each breaker is indicated by the arrow and the non-operation by the letter O. Occurrence of fault activates relays on each of the breakers near the fault. These relays unless blocked from operation, cause tripping of breakers. The blocking signal is controlled by the directional relays on each breaker, and is transmitted from one end of a protected section to the other by carrier. If a directional element determines that the fault is external to the protected section, a signal is transmitted blocking the operation of breakers at both ends of the section. In case the directional elements at both ends determine that fault is in the protected section, no blocking signal is transmitted from either end, and both breakers trip. The sequence of event for a fault at F is made clear by illustration in Fig. 14.18.

At breaker 1 the directional element shows that the fault may be in the section 1-2. This breaker trips if no blocking signal is received. No blocking signal is transmitted to breaker 2. At breaker 2 the directional element shows that the fault is not in section 1-2. A carrier signal is transmitted that blocks tripping of both the breakers 1 and 2.

At breakers 3 and 4 the directional elements show that the fault may be in section 3-4. No blocking signal is transmitted and after a very short time delay (1 to 3 cycles), both the breakers 3 and 4 trip.