繼電保護(hù)選擇的應(yīng)用指導(dǎo)畢業(yè)外文翻譯-其他專(zhuān)業(yè)-全文預(yù)覽
【正文】 circuit level is very high, or when it is likely to vary widely at a given point (for example when a work is supplied from small generators whose shortcircuit decrement curve falls off rapidly). As a basic rule, however, there is a general tendency to use independent time relays in Continental Europe and dependent time in AngloSaxon countries. Timedelayed overcurrent relays lend themselves to chronological selectivity. This however is not without a certain inconvenience, in that fault clearance time increases closest to the source, where shortcircuit levels are the highest. It is thus necessary to minimise the grading interval . The grading interval normally used for electronic protection relays is 400 ms, which is obtained by summating the following: ? Breaker fault clearance time, ? Summation of the time errors of the two relays, ? Overshoot of the upstream relay, ? Safety margin of approximately 100 ms. When transformer feeder protection is being considered it is advisable to use relays having an instantaneous highset unit. This is set above the secondary shortcircuit level and transformer inrush current, which then allows the operating time of upstream relays to be reduced thereby reducing the cable shortcircuit withstand requirements. The grading interval may be reduced if required, for times less than about 1 sec, to 250 ms, using relays with highstability timer circuits (which is the case with ITG 7000 or RMS 700/7000 relays). Accelerated selective protection In cases where the number of grading steps at one voltage level would lead to fault clearance times which are either too long for the withstand of the equipment in the work, or inpatible with the time allowed by the power distribution authority, it is possible to use an accelerated system. This consists of reducing the relay timedelay to a predetermined minimum value when the fault is on the section immediately downstream. The information required to achieve this is transmitted from the downstream relay to the upstream relay via wiring. The fault clearance time is thus independent of its location, the relay minimum operating time being a function only of the speed with which the downstream information can be transmitted (the operating time of the “instantaneous” unit of the downstream relay). Relay types ITG 7172 and ITG 7173 (and RMSR relays) with a twolevel definite time characteristic for phase faults and a single level for earth faults operate in accordance with this principle. The phase fault highset unit and the earthfault unit are associated with the acceleration logic incorporated inside the relay, the lowset unit being used to protect against overloads and is totally independent of the logic. In order to maintain a high degree of security, selective (or graded) backup timedelays are incorporated, which trip under fault conditions even in the event of a fault occurring within the wiring connecting the relays. Differential protection This type of protection operates on the principle of current parison in the same phase but at the two extremities of the protected equipment, (cable: SOLKORRF, transformer: DMS7002, DTT7031, rotating machines: DMS 7001, DTM 7033, IAG 7034 or bus bars: IAG 7034) and has two main advantages: ? It may be instantaneous, because it only reacts to faults inside the protected zone, ? It will operate for the transfer of energy in either direction, which is particularly important in the event of multiple sources. It may also allow a grading step to be eliminated, thus red
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