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1、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
2、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 type
3、s of lines for primary phase fault protection are as follows:1. 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 0.9.2.
4、Set zone 2 for a value greater than the largest impedance, actual or apparent, to the remote terminals.3. 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 lineFor som
5、e 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 . T
6、hus 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 lin
7、e 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 generator or source connected to R, large enough to maintain a fault on the line, but not large enough to support
8、 fault-detecting relays. In other words, the impedance to a line fault from bus R is very large and approaches infinity.Fig. 2.2 Multiterminal line where fault current can flow out at one terminal for internal faults.For the case of fig. 2.2, current can flow out R terminal for an internal line faul
9、t near the H bus. Thus distance or directional relays at breaker 3 see the internal fault as external for no operation until after breaker 2 has opened. Thus protection of tapped and multiterminal lines is more complex and requires specific data on the line impedance, location and type of tap or ter
10、minal, and fault data with current distributions for the various system and operating conditions. Most often, except for small transformer load taps, these types of lines are protected best by pilot relaying.Three-phase voltage is required and provides reference quantities to which the currents are
11、compared. For phase distance relays, either open-delta or wye-wye voltage transformers (VTs) or coupling capacitor voltage devices (CCVTs) can be utilized and connected either to the bus or to the line being protected. Both are widely used and the decision is economic as well as involving use of lin
12、e-side CCVTs for radio-frequency coupling for pilot and/or transfer trip relaying.These voltage sources involve fuses/primary and secondary for VTs and secondary for CCVTs. These fuses should be generously sized, carefully installed, and well maintained, as a loss of one or more phase voltages may r
13、esult in an undesired, unwanted relay operation. Where this is of great convern, overcurrent fault detectors can be added to supervise the trip circuit of the distance relays. For loss of voltage in the absence of an actual fault, the overcurrent units would not operate. The disadvantages are the ne
14、ed for additional equipment and the loss of the feature of distance relays operating for fault levels less than maximum load.Recently, detectors measuring but no have been used to supervise the relays. This requires wye grounded-wye grounded voltage sources. Useful Expressions in Passage A1) Multite
15、rminal lines 多端线路2) apparent impedance 视在阻抗3) remote terminals 远程终端4) misoperation 误动5) a breaker 断路器6) current distributions 电流分布7) line-side 铁路沿线8) voltage sources 电压源B Differential ProtectionTransformers are everywhere-in all parts of the power system, between all voltage levels, and existing in
16、many different sizes, types, and connections. Small transformers of about 3 to 200 kVA can be observed mounted on power distribution poles in many areas. Usually, circuit breakers or other disconnection means are available at or near the winding terminals of the transformer banks. However, economics sometimes dictates omission of a breaker. Thus transformer banks can be connected directly to a bus, line, or generato
