電力系統(tǒng)中英文翻譯-展示頁(yè)
2024-11-12 23:38本頁(yè)面
【正文】 gnitude as overcurrent relays are, and , hence , are much less affected by changes in generating capacity and in system configuration. This is because, distance relays achieve selectivity on the basis of impedance rather than current. THE CHOICE BETWEEN IMPEDANCE, REACTANCE, OR MHO Because ground resistance can be so variable, a ground distance relay must be practically unaffected by large variations in fault resistance. Consequently, reactance relays are generally preferred for ground relaying. For phasefault relaying, each type has certain advantages and disadvantages. For very short line sections, the reactance type is preferred for the reason that more of the 外文翻譯 2 line can be protected at high speed. This is because the reactance relay is practically unaffected by arc resistance which may be large pared with the line impedance, as described elsewhere in this chapter. On the other hand, reactancetype distance relays at certain locations in a system are the most likely to operate undesirably on severe synchronizingpower surges unless additional relay equipment is provided to prevent such operation. The mho type is best suited for phasefault relaying for longer lines, and particularly where severe synchronizingpower surges may occur. It is the least likely to require additional equipment to prevent tripping on synchronizingpower surges. When mho relaying is adjusted to protect any given line section, its operating characteristic encloses the least space on the RX diagram, which means that it will be least affected by abnormal system conditions other than line faults。 in other words, it is the most selective of all distance relays. Because the mho relay is affected by arc resistance more than any other type, it is applied to longer lines. The fact that it bines both the directional and the distancemeasuring functions in one unit with one contact makes it very reliable. The impedance relay is better suited for phasefault relaying for lines of moderate length than for either very short or very long lines. Arcs affect an impedance relay more than a reactance relay but less than a mho relay. Synchronizingpower surges affect an impedance relay less than a reactance relay but more than a mho relay. If an impedancerelay characteristic is offset, so as to make it a modified relay, it can be made to resemble either a reactance relay or a mho relay but it will always require a separate directional unit. There is no sharp dividing line between areas of application where one or another type of distance relay is best suited. Actually, there is much overlapping of these areas. Also, changes that are made in systems, such as the addition of terminals to a line, can change the type of relay best suited to a particular location. Consequently, to realize 外文翻譯 3 the fullest capabilities of distance relaying, one should use the type best suited for each application. In some cases much better selectivity can be obtained between relays of the same type, but, if relays are used that are best suited to each line, different types on adjacent lines have no appreciable adverse effect on selectivity. THE ADJUSTMENT OF DISTANCE RELAYS Phase distance relays are adjusted on the basis of the positivephasesequence impedance between the relay location and the fault location beyond which operation of a given relay unit should stop. Ground distance relays are adjusted in the same way, although some types may respond to the zerophasesequence impedance. This impedance, or the corresponding distance, is called the reach of the relay or unit. For purposes of rough appro
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