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Page 274

| Pilot Protection |

| So-called pilot protection is associated with the simultaneous tripping of the breakers at the terminals of transmission lines. When a fault occurs on such a line near one terminal, the greater part of the fault current will flow through the breaker of the nearest terminal, causing it to operate first and making the other end supply the fault current until the breaker there trips somewhat later. This relatively much longer period in which fault current flows may cause severe damage to the line and to the breaker furthest from the fault. It is desirable, therefore, that the breakers at both ends of a faulted transmission line trip as simultaneously as possible. |

| Pilot-relaying schemes compare conditions at the terminals of a transmission line by means of a communication channel. This communication link may consist of physically separate pilot wires, of a high-frequency carrier superimposed on the transmission lines, or of microwave radio communication. In many ways, pilot protection is similar to the differential protection of buses and equipment. |

| Two general methods of comparison are used. The first compares the magnitude and direction of current flow only; the second compares the direction of power flow. |

| Pilot Wire.. |

| There are several methods of employing pilot wires to trip the circuit breakers at each end of a transmission line simultaneously. Two schemes employing overcurrent relays at each end are shown in Figs. 14-8A and 14-8B. A scheme using differential relays at one end is shown in Fig. 14-8C. All of these employ from three to six pilot wires. Another scheme employing only two pilot wires, polyphase directional relays at each end, and a dc source is shown in Fig. 14-8D. |

| The most popular method, however, employs only two wires, an ac source, and special relays that combine the currents in each of the current transformers for any type of fault into a single-phase voltage that is compared to a similar quantity from the opposite end of the line. A simplified circuit is shown in Fig. 14-9. |

| For through-current conditions, the output voltages from the two special relays are in series and, by way of the pilot wires, circulate most of the current through the restraint coils of the relays and only a minimum of current in the operating coils. When a fault occurs on the transmission line, the output voltages buck each other and most of the current flows through the operating coils; the rest of it flows through the pilot wires and the restraint coils. Under these conditions, the relays trip and open the breakers at both ends of the line. |

| Beside the advantages of only two wires and an ac source, this scheme has the added advantage of tripping the breakers should the pilot wires be- |

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