繼電保護(hù)選擇的應(yīng)用指導(dǎo)畢業(yè)外文翻譯-其他專業(yè)(存儲(chǔ)版)
【正文】 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 reducing the tripping time for faults on the upstream work. To counterbalance these advantages however, it requires pilot links (SOLKORRF) and matched class X current transformers are generally remended with stabilising resistors for the differential protection of transformers, rotating machines and busbars. The differential relays for cable protection, type SOLKORRF, operate on a special principle ? They do not pare individually the currents in each phase, but rather a bination of the three currents. ? The advantage of this system is that only one pair of pilot wires is required, but the operating levels are different for faults on different phases. Directional protection When a substation is supplied by two cables or two transformers in parallel, the protection on these two feeders (in the upstream substation) would operate simultaneously for a fault affecting one of them. To obtain selective protection it is necessary to use either differential or directional relays. In the latter case phase directional overcurrent relays type RMSD7921
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