page_287

< previous page | page_287 | next page >

Page 287

| one source (Fig. 14-1). The circuit may be sectionalized with fuses and reclosers that provide points for clearing faults without interrupting the entire circuit. Both fuses and reclosers may be coordinated with each other and with the protective relays operating the circuit breakers at the points of supply. |

| 4. Transmission supply protective relays must coordinate with protective devices on the circuits connected to them. This is more difficult as fault currents may flow from two or more sources. One solution is to assume only one source supplies the fault current; then assume each of the others does so one at a time. Then the probable magnitudes and direction of the fault currents may be found by combining all of the individual determinations and compromise settings can be made after the maximum and minimum fault currents at all locations are calculated (Fig. 14-2 A and B). |

| 5. The protective zones for generators, buses and circuit breakers may be accomplished by differential relaying (Fig. 14-6). Coordination with transmission lines may be accomplished by directional-overcurrent relaying. Radial feeders from these sources require only overcurrent relaying. |

| 6. Pilot protection employs separate protective circuits to operate breakers at both ends of a line simultaneously; the protective circuits are called pilot wires. The same general protection may also be accomplished by the use of carrier high-frequency currents using the transmission system in place of the pilot wires, or by the use of radio signals (Fig. 14-10). |

| 7. In three-phase wye connected lines, protection may be accomplished by measuring the current in the neutral or ground wire. An overcurrent relay, in this case called a ground relay (Fig. 14-12), accomplishes this purpose. |

| 8. Relays may be of the electromagnetic (Fig. 14-13), or induction type (Fig. 14-14), or electronic type. In all cases, the current magnitude and time settings to close or open the contacts must be checked periodically to retain their performance. The importance of clean (Fig. 14-16), smooth, properly aligned contacts and satisfactory operation of springs and moving parts cannot be overemphasized. |

| STUDY QUESTIONS |

| 1. Name several types of faults that may occur in a three-phase ac system. |

| 2. What is meant by the zones of protection in a relaying system? Describe them in a typical electrical power system. |

| 3. Where zones of protection overlap, how is the protection of the overlapped circuit or equipment accomplished? |

< previous page | page_287 | next page >