电力、电气专业论文外文翻译(本科)

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 Systems

Each 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 electric 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, 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 may 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 currents 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 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 late conditions that exist during a fault, and provide equipment properly adjusted to open the switches necessary to disconnect the faulted equipment 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 designed 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 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 part. 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 overloads as customers install more and more appliances. The

continuous monitoring of distribution circuits is necessary to be certain that transformers sizes are increased as load grows.

Faults of many types and causes may appear 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, 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 conductor. 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 deterioration 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 results 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 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 transients. Short circuit due to insulation failure between adjacent turns of the same winding may result from suddenly applied overvoltage. Major insulation may fail, 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 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 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, 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 handing of faults on single-phase lines is of course identical to the method of handing three-phase faults on an equivalent single-phase basis.

Faults 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 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 soon as the area is extinguished. Arcs across insulators due to over voltages 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 a procedure known as high-speed reclosing. On the occurrence of a fault, the line is promptly energized 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.

V oltage transformers

V oltage transformers are used with voltmeters, watt-meters, watt-hour meters, power-factor meters, frequency meters, synchronizing apparatus, protective and regulating relays, and the no-voltage and over-voltage trip coils of automatic circuit breakers. One transformer can be used for a number of instruments at the same time if the total current taken by the instruments dose not exceed that for which the transformers is designed and compensated.

V oltage transformers are generally designed for a capacity of about 00 volt-amp. These are two causes of errors in voltage transformers, namely, ratio error and phase-angle error. The part of these error due to the exciting current is constant for any particular voltage. It can be reduced to a minimum by choosing the best quality of iron and working it at a low magnetic density. The part of the error due to the load current varies directly with the load and can be minimized by making the resistance of the windings very slow.

V oltage transformers are compensated for their iron loses at rated voltage. When used on some other voltage, either higher or lower, an error is introduced. In general this error will not be more than 0.5 percent of rated voltage. A voltage transformer should never be used on a circuit whose is more than 0 percent above the rated voltage of the transformer.

The secondary terminals of a voltage transformer should never be short-circuit, a heavy current will flow which, if continue, will burn out the windings. In order to protect the system against sustained short circuits in the transformer circuit, it is generally recognized as good practice to introduce into the primary circuit a resister and fuse, these been connected in series. The resistors are designed to limit the current to about 20 to 40 amp. wile the fuses are designed to break such current. In normal operation the current which the resistor carries is only the very small primary current of the voltage transformer, and the drop in voltage that they cause is inappreciable.

Current transformers

Current transformers are used with ammeters, watt-meters, power-factor meters, watt-hour meters, compensators, protective and regulating relays, and the trip coil of circuit breakers. One current transformer can be used to operate not to exceed that for which the transformer is designed and compensated.

The current transformer is connected directly in series with the line, and usually ha a fixed number of instruments in the secondary. A rise or fall in the current requires a corresponding rise or fall in the secondary voltage to force the secondary current through the impedance of the meter load. The magnetic flux in the iron, which supplies the voltage, thus follows the rise and fall of the primary or line current.

The instruments connected in the secondary circuit of the transformer are placed in series, so that the secondary current will pass through each instrument. As the instrument are added, higher voltage is required to force the current through the instruments. This requires a high magnetic density in the iron. A higher magnetic density increases both the iron loss and the magnetizing current; hence both the ratio and the phase-angle errors are magnified. For the sake of accuracy, therefore, there is a limit to the number of instruments that should be placed on a single current transformer.

The secondary circuit of a current transformer should never be opened while the primary is carrying current. If it is necessary to disconnect instruments, the secondary should first be short-circuited. If the secondary circuit is opened, a difference of potential is developed between terminals which is dangerous to anyone coming in contact with the meters of leads. The cause of this high voltage I that with open secondary circuit all the primary ampere turns are effective in producing flux in the core, whereas normally but a small portion of the total performs this function. The danger is magnified by the fact that the wave form of this secondary voltage is peaked, produced in this way also permanently change the magnetic condition core, so that the accuracy of the transformer were be impaired.

Arresters

One of the means of protecting transmission equipment is the surge arrester. Two types of surge arresters may be used for this reason: active gap (SiC) and gapless (ZnO) metal oxide surge arresters.

Active gap (SiC) arrester

The two principal components of active gap surge arresters (diverters) are the spark gap and the non-liner resister. One of the earlier designs was the lighting arrester with plate gaps, which I still used today in some medium voltage network. At still higher voltages, arresters with magnetically blow spark gaps are more commonly used, in particular in EHV networks(300-750kV). These consist mainly of here parts: spark gaps, discharge resistors and a grading system that monitors the distribution of voltage across the spark gaps.

ZnO oxide arresters

The materials used for ZnO arrester are uniformly mixed, formed into grains, and sintered through special processes at temperatures between 1100 and 3500℃ . The gapless surge arrester obtained using ZnO element has the property hat its resistance decreases sharply as the voltage across it increases.

In order to keep the stress on the system insulation as low as possible, a good over voltage protect system or, an arrester has to meet and fulfill the following requirements.

(1)It must withstand the normal phase to earth voltage of the system for the whole of its operating life, even in the presence of pollution and after repeated discharges of high energy, such as are expected in a network;

(2)It must be withstand, without damage, temporary over-voltage caused by earth faults and other system transient conditions and discharge these over-voltages to earth without causing an earth fault;

(3)Interruption of the following current;

(4)The energy absorption capability must be such that, even after the most severe switching surge and temporary over-voltage, the temperature of the blocks dose not rise to a point where thermal runaway sets;

(5)Protection level must be maintained s low s possible.

The newly developed ZnO surge arrester wit its excellent high non-linearity characteristic, energy capability and protective performance can meet these conditions and fulfill these requirements.

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，从电厂到用户的电力输送状况在很大程度上会令人不满意。此时故障设备若不立即从系统中切除的话，则会造成其他运行设备的损坏。

故障是由于两个或更多的导体有意或无意地接触造成的。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，其结果就是,温度的升高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，以便使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变故。

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，女主人可能将五个曲奇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电缆中的导线与导线和导线与地隔开。这些绝缘会随着时间的流逝而老化，尤其是在过负荷引起高温下运行时候更是如此。绝缘材料内的空隙会造成气体的电离，其生成物对绝缘不利。绝缘材料老化会引起绝缘性能下降而导致导线短路。电缆故障的可能性会因雷电或开关瞬间引起的导线的电压骤然变高而增加。

变压器故障可能是由绝缘老化、加上雷电、开关瞬变过程导致的过高压造成的。同一绕组相邻线圈之间由于绝缘问题造成的短路可能是由于突然遇到外加高压电所致。绝缘失败会在一次绕组与二次绕组之间或绕组与接地金属部件如铁芯或变压器外壳之间产生电弧。

发电机故障可能是由于同一槽中相邻线圈之间绝缘被破坏而造成的，其结果会导致发电机匝内短路。绝缘损坏也可能发生在某一绕组与定子铁芯的接地钢架构之间。同一槽内不同绕组之间的绝缘损坏会导致电机大范围短路。

像处理平衡三相负荷一样，处理平衡三相故障也是依照基于由火线到零线的电路或等效单相电路的原则进行。可以通过电压、电流和电阻的规律求解问题。当然，单相线路上故障的处理方法也可以用于在单相等效电路下三相故障的处理中。

故障有永久性故障及暂时性故障之分。永久性故障指绝缘或结构上的损坏，致使设备不能维修则无法运行。暂时性故障指通过给设备临时断电即可排出的故障，架空线路往往就有这个特点。大风可能会使两根导线瞬时间碰在一起，并产生电弧。只要线路通电，此电弧会一直存在。然而如果能借助自动化设备使导线迅速断电的话，就不会造成任何损失了，一旦电弧熄灭，线路即可自行恢复。有雷电及开关切换产生的过电压引起的绝缘材料飞狐则可通过自动化开关设备动作，在严重的结构损坏发生之前便得到排除。

鉴于线路故障的这些特征，许多公司都使用一种叫做高速重合器的装置。故障发生时，线路两端的断路器跳闸，电流即被切断，经过一定的时间间隔，待电弧熄灭后，断路器又自动进行再次合闸，大多数情况下，不到一秒种即可恢复正常供电。当然，如果因结构损坏，故障不能很快排除的话，则断路器必须再次跳闸且保持这种跳闸状态。

由于电力系统故障多是随机发生的，而且很多故障超出了系统工程人员的控制能力，因此一般说绝对的毫不中断地连续供电实际上是不可能的。为了尽量减少由于系统元件随机故障对系统供电造成的影响，在电力规划时，采用增加机组的办法，但是经济性和可靠性是相互制约的，增加投资可以提高可靠性，然而过高的投资违反了经济性的约束。

为了摆脱经济性和可靠性约束之间相抗衡的困境，多年来，设计规划人员一直在探索确实可行的判据和分析技术。早期的判据和方法是以确定性为基础的。四十年代，应用概率理论来定量描述和计算工程系统的可靠性技术引入了电力工程领域，但是当时这种方法一直没有得到广泛应用，主要原因是缺乏数据、受到计算工具的限制，而且工程人员对这种方法存在偏见。六十年代中期，由于电网结构的不合理导致无法及时适应新的情况，许多国家的大电网相继发生了重大的事故，引起大面积长时间的停电，这些停电事件不但造成了巨大的经济损失，而且危及社会秩序，对整个社会的影响非常深刻，同时也给从事电力系统规划和运行的人员以极大的教训。规划过程中过多考虑经济性，而不相应提高安全可靠性要求，将可能造成更大的经济损失。

电力系统的根本任务是尽可能经济而可靠地将电能供给各种用户。用户对供电的要求，一是保证供电的连续性；二是保证电能的质量。由于系统内元件的随机故障，且这些故障又超出运行调度人员的控制能力，因此完全不间断的连续供电实际上是不可能的。随着人们对供电质量的要求越来越高，促使电力部门寻求提高供电可靠性的途径。

电力系统可靠性评估有两方面的目的：一是为电力系统的发展规划进行长期可靠性评估；二是为制定短期的运行调度计划进行短期可靠性预测。从国内外的总体发展水平来看，长期可靠性评估研究比较成熟，不仅取得了不少理论成果，并且达到实用阶段，而短期可靠性评估则正处于理论探索阶段，仍有大量问题需要解决。在继电保护故障的观察与处理中，直观法是一种较为简单有效的处理措施一般情况下，直观法主要应用于以下继电保护故障的处理：无法使用专业电子仪器进行测试和检查的故障；当继电保护系统中某一插件发生故障时，因暂时缺少备用的产品，而采取的一种临时性处理措施目前，在国内的继电保护故障分析与处理中，直观法主要应用于开关拒分拒合的处理例如：在开关柜控制系统发出操作命令后，继电保护人员应注意观察合闸接触器的运行是否正常，以判定电气回路的实际运行情况。如果电气回路无明显的故障，则可初步判断继电保护故障发生与系统内部。同时，继电保护人员还可以通过观察继电器的颜色或气味，判定继电器是否出现元件故障，以便及时进行更换。比如IOKV开关柜分或拒合故障处理。在操作命令下发后，观察到合闸接触器或跳闸线圈能动作，说明电气回路正常，故障存在机构内部。到现场如直接观察到继电器内部明显发黄，或哪个元器件发出浓烈的焦味等便可快速确认故障所在，更换损坏的元件即可。

电压互感器

电压互感器与电压表、功率表、电能表、功率因数表、频率表、同步检测装置和同期设备、保护和调节继电器以及自动化断路器的失压和过压跳闸线圈一起使用。只要仪表的总电流不超过互感器的设计的补偿要求，一个互感器可以同时供多个仪表使用。

通常，，电压互感器容量设计为200V A电压互感器的误差有两个，称为变比误差和相角误差。对于任何电压，这些误差中由于励磁电流而引起的部分是恒定的。通过选择最佳质量的铁芯和低磁场强度下运行，可以将这个误差降到最小。这些误差中由于负荷电流引起的部分直接随着负荷变化，并且可以通过绕组电阻的减小来使其最小化。

需要对电压互感器在额定电压下的铁芯进行补偿。当运行在其他电压时，无论电压高低，都会产生误差。总的来讲，当使用电压为额定电压的50%～110%时，这些误差都不会超过0.15%。电压互感器不允许应用于电压超过其额定电压的10%的电路。

电压互感器的二次侧端子不允许短路。如果其二次侧持续短路的话，将在二次绕组中产生很大的电流，从而烧毁绕组。为了防止系统中电压互感器电路持续短路，一个认可的常用措施是在电压互感器的一次侧串连接入一个电阻器和熔断器（保险）。电阻器的选择是将电流限制到约20～40A，而熔断器的选择是按照能断开这样的电流来设计的。在正常运行情况下，流过电阻器的仅仅是电压互感器的小的一次侧电流，并且它们引起的电压降落是可忽略的。

电流互感器

电流互感器与电流表、功率表、电能表、功率因数表、电能表、补偿装置、保护和调节继电器以及断路器的跳闸线圈一起使用。一个电流互感器可在不超过其设计和补偿值的范围内运行。

电流互感器串联于电路，并且在二次侧连接仪表数量是固定的。线电流的增加或减小

需要二次侧电压降落相应的上升或下降，从而强制二次侧电流流过表计负荷的阻抗。因此，产生这个电压的铁芯中的磁通也将随着一次侧电流上升或下降。

连接与电流互感器二次侧电路的仪表是串联接入的，以便二次侧电流流过每一个仪表。随着仪表的增加，就需要较高的电压来强制电流流过这些仪表。这要求铁芯中具有较大的磁场密度。一个较高的磁场密度将增大铁芯损耗和励磁电流，因此造成变比误差和相角误差增大。所以，为了保证一定的精确度，需要对每一个电流互感器所允许带的仪表数设置一个极限。

一次侧负载运行时，电流互感器的二次侧电流不允许开路。如果必须要断开仪表的话，应首先将二次侧仪表断路。如果二次侧电路断开的话，在端子之间将产生电位差，这对于任何接近或接触表计和表头的人员来说都将是危险的。引起这个高电位差的原因是：当二次侧电路开路时，所有的一次侧安匝都有效的用于产生铁芯的磁通，而正常中只要总安匝中的小部分用于产生铁芯磁通。事实上，而磁通电压的波形上升达到波峰并产生最大值，危险被放大。在这种情况下所产生的大磁通还会永久性的改变磁状况和铁芯，从而损坏互感器的精确度。

避雷器

保护输电线路的一个方法就是避雷器，用于这个目的的避雷器有两种类型：有效间隙（碳化硅）避雷器和无间隙（氧化锌）金属氧化物避雷器。

氧化硅避雷器

有效间隙避雷器的两个主要部分是火花间隙和非线性电阻。最早的一种设计是平板间隙的避雷器，今天在一些中压供电网中还仍然得到使用，而在高压电网中，特别是在超高压电网中（300～750kV），通常更普通使用的是磁吹火花间隙的避雷器。它主要包括三个部分：火花间隙、放电电阻和一个能监测通过火花间隙的电压分布的分级系统。

氧化锌避雷器

这种避雷器的材料是被均匀混合，形成晶粒，经过特殊过程在温度1100～1350度时烧结；。使用氧化锌材料的无间隙避雷器的特性是：随着电压的增大其电阻值迅速减小。为了保持系统绝缘受到的应力尽可能的小，一个好的过电压保护系统或者一个避雷器应该满足下列要求：

（1）在它的运行寿命中，即使在污染的情况下，或在电网可能出现的大能量的重复放点后，它必须能承受系统的正常的相对地电压；

（2）它必须能承受由姐弟故障和其他的系统过渡状态造成的短时过电压而不被破坏，并且这些过电压对大地放电不会导致接地故障；

（3）能断开续流电流；

（4）它的能量吸收能力必须满足这种情况，即在最严重的操作过电压和短时过电压下，其部件的温度也不能升高到散热允许的设定值；

（5）它必须维持尽可能低的保护水平。

最新开发的氧化锌避雷器具有优异的非线性特性、能量吸收能力和保护功能，能满足上面的要求。