Faults in Power Systems 电力系统故障.doc

一 英语原文Faults in Power SystemsPart 1 Faults and its DamageEach year new designs of power equipment bring about increased reliability of operation. Nevertheless, equipment failures and interference by outside sources occasionally result in faults in electric power systems. On the occurrence of a fault, current and voltage conditions become abnormal, the delivery of power from the generating stations 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 fault 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 may be through ab 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 currents will usually be much greater than the designed thermal ability of the conductors 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 to 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 power system desiger must anticipate points at which faults may occur, be able to calculate conditions that exist during a fault, and provide equipment properly adjusted to open the switches necessary to disconnect the faulted equipment from the remainder of the system. Ordinarily it is desirable that no other switches on the system are opened, as such behavior would result in unnecessary modification of the system circuits.Part 2 OverloadA distinction must be made between a fault and an overload. An overload implies only that loads greater than the designed values have been imposed in system. Under such a cir-cumstance the voltage at overload point may be low,but not zero. This undervoltage condition may extend for some distance beyond the overload point into the remainder of the system. The currents in the overloaded equipment are high and may 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 kitchen circuit during a neighborhood party. Such an overload, if 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 above specified values persist. Distribution transformers are sometimes overloaded as customers install more and more appliances. The continuous monitoring of distribution circuits is necessary to be certain that transformer sizes are increased as load grows.Part 3 Various FaultsFaults of many types and causes may appear in elecric 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, for the most part, are constructed of bare conductors.These are sometimes accidentally brought togther by action of wind, sleet, tress, cranes, airplanes, or damage to supporting structures. Overvolages due to lightning or switching may cause flashover 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 other and from ground by solid insulation, which may be oil-impregnated paper or a plastic such as polyethylene.These materials undergo some deterioraion with age, particularly if overloads on the cables have resulted in their operation at elevated temperature. Any small void present in the body of the insulating material will result in ionization of the gas contained therein, the products of which react unfavorably with the insulation. Deterioration 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 if lightning or switching produces transient voltage of abnormally high values between the conductors.Transformer failures may be the result of insulation deterioration combined with overvoltages due to lightning or switching transients. Short circuits due to insulation failure between adjacent turns of the same winding may result from suddenly applied overvoltages. Major insulation may fail, permitting arcs to be established between primary and secondary windings or between a winding and grounded metal parts such as the core or tank.Generators may fail due to breakdown of the insulation between adjacent turns in the same slot, resulting in a short circuit in a single turn of the generator. Insulation breakdown may also occur between one of the windings 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 sections of machine.Balanced three-phase faults, like balanced three-phase loads, may be handled on a lineto-neutral basis or on an equivalent single-phase basis. Problems may be solved either in terms of volts, amperes, and ohms. The handling of faults on single-phase lines is of course identical to the method of handing three-phase faults on an equivalent single-phase basis.Part 4 Permanent Faults and Temporary FaultsFaults may be classified as permanent or temporary. Permanent 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 lines frequently are of this nature. High winds may cause two or more conductors to swing together momentarily. During the short period of contact, an arc is formed which may continue as long as the line remains energized. However, if automatic equipment can be brought into operation to deenergize the line quickly, little physical damage may result and the line may be restored to service as soon as the arc is extinguished. Arcs across insulators due to overvoltages from lightning or switching transients usually can be cleared by automatic circuit-bteaker operation before significant structure damage occurs.Because of this characteristic of faults on lines, many companies operate following a procedure known as high-speed reclosing.On 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 course, if structure damage has occurred and the fault persists, it is necessary for the breakers to reopen and lock open. 二 原文翻译第二章电力系统故障第一节 故障及其危害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、电压变得不正常，从电厂到用户的送电在相当大的区域内不令人满意。此时若故障设备不立即从系统中切除，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相接触而造成的。导体之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，其结果是，温度的升高导致导体的烧毁或绝缘体焦化。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，提供调解好的设备，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。第二节 过负荷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越过过负荷处蔓延一定距离，进而影响系统的其他部分。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，供电虽往往能维持，但电压较低。过负荷的情况经常发生在家里，例如请街坊邻居聚会时，女主人可能将五个华夫饼烘烤器的插头同时插入厨房的插座，诸如此类的过负荷倘若不能迅速处理，就会造成电力线发热甚至酿成火灾。为了避免此类情况发生，须采用保险丝或断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。第三节 各类故障电力系统会发生各种类型、由各种因为引起的故障。我们在家里看到过破损的照明灯电线其两根导线相触，并发出弧光。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线架设的，有时由于风、雨、雪、大树、起重机、飞机