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1、青岛大学毕业论文（设计）科技文献翻译院系：自动化工程学院电气工程系专业：电气工程及其自动化班 级：2010级电气1班姓 名：尹睿指导教师：袁博强2014年5月15日Faults on Power SystemsEach year new design of power equipment bring about increased reliability of operation. Nevertheless, equipment failures and interference by outside sources occasionally result in faults on elect
2、ric power systems. On the occurrence of a fault, current and voltage conditions become abnormal, the delivery of power from the generating 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,
3、damage may result to other pieces of operating equipment.A faulty is the unintentional or intentional connecting together of two or more conductors which ordinarily operate with a difference of potential between them. The connection between the conductors may be by physical metallic contact or it ma
4、y be through an arc. At the fault, the voltage between the two parts is reduced to zero in the case of metal-to-metal contacts, or to a very low value in case the connection is through an arc. Currents of abnormally high magnitude flow through the network to the point of fault. These short-circuit c
5、urrents will usually be much greater than 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 by the charring of insulation. In the period during which the fault is permitted t
6、o exist, the voltage on the system in the 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, and provide equipment properly adjusted to open the switches necessary to disconnect the faulted equi
7、pment from the remanding of the system. Ordinarily it is desirable that no other switches on the system are opened, as such behavior would result in unnecessary modification the system circuits.A distinction must be made between and an overload. An overload implies only that loads greater than the d
8、esigned values have been imposed on system. Under such a circumstance the voltage at the overload point may be low, but not zero. This under voltage condition may extend for some distance beyond the overload point into the remainder of the system. The current in the overload equipment are high and m
9、ay exceed the thermal design 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 in homes. For example, a housewife might plug five waffle irons into the k
10、itchen circuit during a neighborhood part. Such an overload,讦 permitted to continue, would cause heating of the wires from the power center and might eventually start a fire. To prevent such trouble, residential circuits are protected by fuses or circuit breakers which open quickly when currents abo
11、ve specified values persist. Distribution transformers are sometimes overloads as customers install more and more appliances. The丫丫TTcontinuous monitoring of distribution circuits is necessary to be certain that transformers sizes are increased as load grows.Faults of many types and causes may appea
12、r on electric power systems. Many of us in our homes have seen frayed lamp cords which permitted the two conductors of the cord to come in contact with each other. When this occurs, there is a resulting flash, and if breaker or fuse equipment functions properly, the circuit is opened.Overhead lines,
13、 for the most part, are constructed of bare conductors. There are sometimes accidentally brought together by action of wind, sleets, trees, cranes, airplanes, or damage to supporting structures. Over voltages due to lighting or switching nay cause flashover of supporting or from conductor to conduct
14、or. Contamination on insulators sometimes results in flashover even during normal voltage conditions.The conductors of underground cables are separated from each and from ground by solid insulation, which nay be oil-impregnated paper or a plastic such polyethylene. These materials undergo some deter
15、ioration with age, particularly 讦 overloads on the cables have resulted in their operation at elevated temperature. Any small void present in the body of the insulating material will results in ionization of the gas contained therein, the products of which react unfavorably with the insulation. Dete
16、rioration of the insulation may result in failure of the material to retain its insulating properties, and short circuits will develop between the cable conductors. The possibility of cable failure is increased 讦 lightening or switching produces transient voltage of abnormally high values between the conductors.Transformer failures may be the result of insulation deterioration combined with overvoltage due to lightning or switching
