(完整版)电力系统故障_毕业论文中英文资料外文翻译_文献.doc

1、Faults on Power SystemsEach year new design of power equipment bring about increased reliability ofoperation. Nevertheless, equipment failures and interferenceby outside sourcesoccasionally result in faults on electric power systems. On the occurrence of a fault ,current an voltage conditions become

2、 abnormal, the delivery of power from thegenerating station to the loads may be unsatisfactory over a considerable area, and if the faulted equipment is not promptly disconnected from the remainder of the system, damage may result to other pieces of operating equipment.A faulty is the unintentional

3、or intentional connecting together of two or moreconductors which ordinarily operate with a difference of potential between them. Theconnection between the conductors may be by physical metallic contact or it may bethrough an arc. At the fault, the voltage between the two parts is reduced to zero in

4、the case of metal-to-metal contacts, or to a very low value in case the connection isthrough an arc. Currents of abnormally the designed thermal ability of the condition in the lines or machines feeding the fault . The resultant rise in temperature may cause damage by the annealing of conductors and

5、 by the charring of insulation. In the period during which the fault is permitted to exist, the voltage on the system inthe near vicinity of the fault will be so low that utilization equipment will be inoperative. It is apparent that the late conditions that exist during a fault, andprovide equipmen

6、t properly adjusted to open the switches necessary to disconnect the faulted equipment from the remanding of the system. Ordinarily it is desirable thatno other switches on the system are opened, as such behavior would result in unnecessary modification the system circuits.A distinction must be made

7、 between and an overload. An overload implies onlythat loads greater than the designed values imposed on system. Under such a circumstance the voltage at the overload point may be low, but not zero. This undervoltage condition may extend for some distance beyond the overload point into the remainder

8、 of the system. The current in the overload equipment are limits. Nevertheless, such currents are substantially lower than in the case of a fault. Service frequently may be maintained, but at below-standard voltage.Overloads are rather common occurrences incircuit during a neighborhoodpart. Such an

9、overload, if permitted to continue, would causequickly when currentsabove specified values persist. Distribution transformers are sometimes overloads ascustomers install more and more appliances. The continuous monitoring of distribution circuits is necessary to be certain that transformers sizes ar

10、e increased as load grows.Faults of many types and causes may appear on electric power systems. Many of us in our frayed lamp cords which permitted the two conductors of the cord to comein contact with each other. When this occurs, there is a resulting flash, and if breaker or fuse equipment functio

11、ns properly, the circuit is opened.Overhead lines, for the most part, are constructed of bare conductors. There aresometimes accidentally brought together by action of wind, sleets, trees, cranes,airplanes, or damage to supporting structures. Overvoltages due to lighting or switching nay cause flash

12、over of supporting or from conductor to conductor. Contamination on insulators sometimes results in flashover even during normal voltage conditions.The conductors of underground cables are separated from each and from groundby solid insulation, which nay be oil-impregnated paper or a plastic such po

13、lyethylene. These materials undergo some deterioration with age, particularly if overloads on thecables their operation at elevated temperature. Any small void present in the bodyof the insulating material will results in ionization of the gas contained therein,theproducts of which react unfavorably

14、with the insulation. Deterioration oftheinsulation may result in failure of the material to retain its insulating properties, andshort circuits will develop between thecable conductors. The possibility of cablefailure is increased if lightening or switching produces transient voltage of abnormally t

15、he conductors.Transformer failures may be the result of insulation deterioration combined with overvoltage due to lightning or switching transients. Short circuit due to insulation failure between adjacent turns of the same winding may result from suddenly applied overvoltage. Major insulation may f

16、ail, permitting arcs to be established between primary and secondary windings or between winding and grounded metal parts such as the core or tank.Generators may fail due to breakdown of the insulation between adjacent turnsin the same slot, resulting in a short circuit in a single turn of the gener

17、ator. Insulation breakdown may also occur between one of the winding and the grounded steel structure in which the coils are embedded. Breakdown between different windings lying in the same slot results in short-circuiting extensive section of machine.Balanced three-phase faults,likebalanced three-p

18、haseloads, may bealineto-neutral basis or on an equivalent single-phase basis. Problems may be solvedeither in terms of volts, amperes, and ohms. The single-phase lines is of course identical to the method of an equivalent single-phase basis.Faults may be classified as permanent or temporary. Perman

19、ent faults are those in which insulation failure or structure failure produces damage that makes operation of the equipment impossible and requires repairs to be made. Temporary faults are those which may be removed by deenergizing the equipment for a short period of time, short circuits on overhead

20、 lines frequently are of this nature. High winds may cause two or more conductions to swing together momentarily. During the short period of contact. An arc is formed which may continue as long as line remains energized. However, if automatic equipment can be brought into operation to service as soo

21、n as the are is extinguished. Arcs across insulators due to overvoltages from lighting or switching transients usually can be cleared by automatic circuit-breaker operation before significant structure damage occurs.Because of this characteristic of faults on lines, many companies operate following

22、a procedure known as the occurrence of a fault, the line is promptly deenergized by opening the circuit breakers at each end of the line. The breakers remain open long enough for the arc to clear, and then reclose automatically. In many instances service is restored in a fraction of a second. Of cou

23、rse, if structure damage and lock open.电力系统故障每年新设计的电力设备都使系统的可靠性不断提高，然而，设备的使用不当以及一些偶然遇到的外在因素均会导致系统故障的发生。发生故障时， 电流、 电压变化得不正常， 从电厂到用户的送点在相当大得内不令人满意。此时若故障设备不立即从系统中切除的话，则会造成其他运行设备的损坏。故障是由于有意或无意地使两个或更多的导体接触造成的。导体间本来是有电位存在的， 而这种接触可能是金属性接触，也可能是电弧引起的。如果是前者造成的故障，则两部分导体之间电压下降为零；若为后者，则电压变得很低，超常的大电流经过网络流至故障处。此短路

24、电流通常会大大超出导线以及供电发电机的热承受能力，其结果， 温度的升高会导致导体烧毁或绝缘焦化。 在允许的期限内， 最靠近故障处的电压会变得很低， 致使用电设备无法正常运行。 显然， 系统设计者必须事先考虑到故障可能发生在什么地方， 能够推测出故障期间的各种情况， 提供调节好的设备， 以便驱动为将故障设备切除所必须断开的开关能够跳闸。 通常希望此时系统无其他开关打开，否则会导致系统线路不必要的修改。过负荷与故障是两个概念。 过负荷仅指施加于系统的负荷超过了设计值。 发生这种情况时， 过负荷处的电压可能很低， 但并不等于零。 这种电压不足的情形可能会超过过负荷处蔓延一定距离， 进而影响系统其它部

25、分。 过负荷设备的电流变大而超过预定的热极限，但是这种情况比发生故障时的电流要小。此时，供电虽然往往能维持，但电压较低。过负荷的情况在家里发生， 例如请街坊邻居聚会时， 女主人可能将五个化夫饼干烘烤器的插头同时插入厨房的插座， 诸如次类的过负荷倘若不能迅速处理的话， 就会造成电力线发热甚至酿成火灾。 为了避免这种情况的发生， 须采用保险丝或短路器来保护住宅区电路免受损坏。 断路器会在电流超出预定值时迅速切断电路。 当用户安装的用电器增加时， 也会超过变压器负荷能力， 因此有必要不时地监视配电线路以确保在负荷增加时变压器的容量也相应增加。电力系统会发生各种类型， 由各种原因引起的故障。 我们在家里看到过破损的照明灯电线， 使得其两根导线相触， 并会发出弧光。 如果此时断路器或保险丝能够正常工作，则电路能被自动切断。大部分架空明线是用裸导体假设的，又是由于风、雨、雷、或大树、起重机，飞机及支撑物的损坏等因素会使导线偶然碰到一起。由雷电或开关瞬变过程中引起的过电压会在支撑物或导体之间产生电弧， 即使在电压正常的情况下， 绝缘材料的污染也会引起电弧。通常采用油浸电缆纸或聚乙烯一类固体塑料绝缘材料将埋地