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Page 277

| Figure 14-9 Alternating-current pilot wire scheme using HCB relays. (Courtesy, Westinghouse Electric Co.) |

| Carrier Pilot Relaying |

| In this type of pilot relaying, a high-frequency current is superimposed on a transmission line. The frequency employed, however, is confined to the lines and does not act as a radio broadcaster. A simplified diagram of an installation of one terminal is shown in Fig. 14-10. Power directional relays are installed at both ends. In a simplified explanation of operation, the carrier signal normally tends to block the operation of the relays. When the line is faulted, the carrier signal is interrupted and the relays operate to trip the breakers. Similarly, if the carrier unit fails for any reason, the breakers will open and the system fails safe. |

| The principal advantage of this type of pilot relaying is its ability to function positively and effectively over several hundred miles. Operation at frequencies from 50 to 150 kilocycles per second results in freedom from local inductive interference. Its first cost and operating expense are economically more favorable than the pilot wire schemes and are practically unaffected by the length of the lines. Moreover, the carrier channel can be used for other purposes, for example, for telemetering and supervisory controls that operate on coded impulses. |

| To transmit several functions simultaneously over the same carrier channel, the carrier is audio modulated with two or more frequencies, each of which is controlled by one of the functions (Fig. 14-11). Tone receivers (incorporating filters) are used at the receiving end to separate the incoming quantities. With this method, some ten separate quantities may be transmitted over a single-carrier channel. Although reception may not always be satisfactory, the carrier channel may also be used for telephonic communication for other line operations as well as for other nonrelated business. |

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