【正文】 外文翻譯 1 LINE PROTECTION WITH DISTANCE RELAYS Distance relaying should be considered when overcurrent relaying is too slow or is not selective. Distance relays are generally used for phasefault primary and backup protection on subtransmission lines, and on transmission lines where highspeed automatic reclosing is not necessary to maintain stability and where the short time delay for endzone faults can be tolerated. Overcurrent relays have been used generally for groundfault primary and backup protection, but there is a growing trend toward distance relays for ground faults also. Singlestep distance relays are used for phasefault backup protection at the terminals of generators. Also, singlestep distance relays might be used with advantage for backup protection at powertransformer tanks, but at the present such protection is generally provided by inversetime overcurrent relays. Distance relays are preferred to overcurrent relays because they are not nearly so much affected by changes in shortcircuitcurrent magnitude 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. Act