电力英语9.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: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. 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 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 generator or source connected to R, large enough to maintain a fault on the line, but not large enough to support 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 fault 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 terminal, 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 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 line-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 result 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 need 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) Multiterminal 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 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 generator. Differential protection, where applicable, provides the best overall protection for both phase and ground faults, except in ungrounded systems or where the fault current is limited by high-impedance grounding. In these latter low-ground-fault current systems, differential provides only phase-fault protection.Generally, differential protection is applied to transformer banks of 10 MVA and above. The key is the importance of the transformer in the system, so differential may be desirable for smaller units in some cases.In applying differential protection, several factors must be considered:1. Magnetizing inrush current. This is a normal phenomenon which has the appearance of an internal fault (current into but not out of the transformers).2. Different voltage levels and hence the current transformers are of different types, ratios, and performance characteristics.3. Phase shifts in wye-delta-connected banks.4. Transformer taps for voltage control. 5. Phase shift and/or voltage taps in regulating transformers.For applications to transformers the differential relays are less sensitive and with typical percentage characteristics between 20 and 60%. This provides accommodation for the different CT ratios, types, and characteristics, different primary current energization levels, and for transformer taps where they exist. To avoid undesired operation on the magnetizing inrush current, the relays may (1) be designed with reduced sensitivity to the transient inrush current, (2) use the harmonics of the inrush current to prevent operation, or (3) desensitize or inhibit operation momentarily during the energization time. For smaller transformer banks, particularly in the lower-voltage subtransmission and distribution systems, the electromechanical induction disk transformer differential relays with typically 50% characteristics and operating about 0.08 to 0.10 s (five to six cycles on a 60-Hz system) are used widely with good immunity to inrush. Generally in these areas the inrush is not too severe and there is sufficient resistance in the system to damp the transient rapidly. Also, the induction disk unit does not operate very efficiently on this high distorted offset wave, and the unit does not operate on dc. The advantage is the relative simple rugged design and low cost. However, it is not possible to assure that the relay will never operate on inrush, although experience can show it to be quite immune. Someone has observed that this type of relay “has a long and enviable record” of good performance, which accounts for its continued use. Typical pickup current is 2.5 to 3.0 A.For larger banks, banks in the higher-voltage systems and/or where it is important to assure no operation on inrush，harmonic restrained differential relays should be used. For faults the harmonics are of very low values, so the “handle” of harmonics provides an effective means to distinguish the inrush current from fault currents. The second harmonic, which is a major component in the inrush wave, is used to restrain or prevent relay operation. In some designs it is used alone; in others, in combination with the other harmonics. Typical pickup current for these types varies from about 0.75 to 2.5 A with operating times of 0.015 to 0.03s.Momentary desensitizing or disconnecting differential relays is not recommended, as it limits protection. It is used occasionally to solve a problem until a more satisfactory solution can be effected. A slip contact on the breaker control switch can insert a resistor in the operation coil to desensitize the relay or momentarily short the operating coil while the breaker is being closed. A high-set instantaneous trip unit can be used to operate for a heavy current internal fault occurring on energization. Useful Expressions in Passage B1) differential protection 差动保护2) power distribution 功率分布3) magnetize 使磁化4) inrush current