电力英语9文档.doc

Unit 9 Distance Protection A Distance Relay Applications for Tapped and Multiterminal Lines A typical line of this type is shown in Fig.2.1. The tap at T may be a transformer at or near the line, so that would be the impedance from the tap plus the transformer bank impedance. Sometimes the tap ties through to a bus, as shown in the figure. The tap may serve a load, so that negligible fault current is supplied through it to line faults, or it may tie into a fault source at R, as shown dotted. Another variation is illustrated in Fig. 2.2. The fundamentals for setting distance relays on these types of lines for primary phase fault protection are as follows: Set zone 1 for k times the lowest actual impedance to any remote terminal for Fig. 2.1-type circuits, or for k times the lowest apparent impedance to any remote terminal for the special case of Fig. 2.2. k is less than 1, normal 0.9. Set zone 2 for a value greater than the largest impedance, actual or apparent, to the remote terminals. The zone 2 timer (T2) must be set so as not to cause misoperation when any terminal is out of service to cause the distance unit to overreach. Fig. 2.1 Typical tapped or multiterminal line For some arrangements of circuits, these requirements can make primary protection quite difficult and/or limited. In Fig. 2.1, consider that tap T is very near bus G, so is small and large with very small. Then zone 1 at breaker 1, bus G must be set at 90% of , which is a very small value compared with . Thus high-speed coverage of the line is almost negligible. On the other hand, If the tap is a load transformer where is high relative to , zone 1 at breakers 1 and 2 can be set for 90% of the line to provide good high-speed line protection. If R is a load tap in Fig. 2.1, with negligible current to line faults, distance relays (and overcurrent) are not applicable at breaker 3, and basically not necessary, as opening breakers 1 and 2 terminates the line fault. The worst case is a small ge