电力系统故障—毕业设计翻译完整版.docx

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1、Faults on power system Each year new designs of equipment bring about increased reliability of operation. Nevertheless, equipment failures and interference by outside sources occasionally result in faults on electric power system. On the occurrence of power from the generating stations to the loads

2、may be unsatisfactory over a considerable area, and if the faults 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 ordinar

3、ily operate with a difference of potential between them. The connection between the conductions 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 t

4、he connection is through an arc. Currents of abnormally high magnitude flow 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 m

5、ay 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 year vicinity of the fault will be so low that utilization equipment will be inoperative. It is apparent that the power syste

6、m designer must anticipate points at which fault may occur, be able to calculate conditions that exist during a fault, and provide equipment properly adjusted to open the switches necessary to disconnect faulted equipment from the remainder of the system. Ordinarily it is desirable that no other swi

7、tches on the system are opened, as such behavior would result in unnecessary modification of the system circuits. A distinction must be made between a fault and overload. An overload implies only that loads greater than the designed value have been imposed on system. Under such a circumstance the vo

8、ltage 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 currents in the overload equipment are high and may exceed the thermal design limits. Nevertheless, such current are substan

9、tially lower than in the case of a fault. Service frequently may be maintained, but at below-standard voltage. Overloads are rather common occurrence in homes. For example, a housewife might plug five waffle irons into the kitchen circuit during a neighborhood party. Such an over-load, if permitted

10、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 fuse or circuit breakers which open quickly when currents above specified values persist. Distribution transformers are sometimes ove

11、rloaded as customers install more and more appliances. The continuous monitoring of distribution circuits is necessary to be certain that transformer size are increased as load grows. Faults of many types and cause may appear on electric power systems. Many of us in our homes have seen frayed lamp c

12、ords which permitted the tow conductors of the cord to come in contact with each other. With 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

13、 accidentally brought together by action of wing, sleet, trees, cranes, airplanes, or damage to supporting or from conductor to conductor. Contamination on insulators sometimes results in flashover even during normal voltage conditions. The conductors of under-ground cables are separated from each o

14、ther and from ground by solid insulation, which may be oil-impregnated paper or a plastic such as 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 o

15、f the insulting 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 circuit will develop between the cable co

16、nductors. The possibility of cable failure is increased if lighting or switching produces transient voltage of abnormally high value between the conductors.Transformer failures may be the result of insulation deterioration combined with over-voltages due to lighting or switching transients. Short ci

17、rcuits due to insulation failure between adjacent turns of the same winding may result from suddenly applied over-voltages. Major insulation may fail, permitting arcs to be established between primary and secondary windings or between a winging and ground metal parts such as the core or tank. Genera

18、tors 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

19、 different windings lying in the same slot results in shorts in short-circuiting extensive sections of machine.Balanced three-phase faults, like balanced three-phase loads, may be handled on a line-to-neutral basis or on an equivalent single-phase basis. Problem may be solved either in terms of volt

20、s, amperes, and ohms . The handling of faults on single-phase lines is of course identical to the method of handling three-phase faults on an equivalent single-phase basis. Voltage transformers Voltage transformers are used with voltmeters, watt-meters, watt-hour meters, power-factor meters, frequen

21、cy meters, synchroscopes and 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

22、 that for which the transformers is designed and compensated. Voltage 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 con

23、stant 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. Voltage

24、 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

25、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,

26、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

27、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 r

28、elays, 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.

29、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 instrument

30、s 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 hi

31、gher 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 circui

32、t 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 any

33、one 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 th

34、at 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

35、 may be used for this reason: active gap (SiC) and gapless (ZnO) metal oxide surge arresters.Active gap (SiC) arresterThe 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 g

36、aps, 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

37、 the distribution of voltage across the spark gaps.ZnO oxide arrestersThe 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

38、 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 overvoltage protect system or, an arrester has to meet and fulfill the following requirements.(1) It must withstand the normal phase to earth voltage

39、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 disch

40、arge 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 therm

41、al 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

42、，设备的使用不当以及一些偶然的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会降低为零；若为后者，则电压变得更低，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，温度的升高会导致导体烧毁或绝缘体焦化。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

43、行。显然，系统设计者必须事先考虑到故障发生在什么地方，能够推测出故障期期间的各种情况，提供调节好的设备，以便取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越过过负荷处蔓延一定距离，进而影响系统其他部分。过负荷设备的其他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。过负荷的情况经常在家里发生，例如请街坊邻居聚会时，女主人可能会将五个华夫饼

44、干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。为了避免这种情况发生，需采用保险丝或断路器来保护住宅区电路免受损坏。断路器会在电流超过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，因此有必要不时的监控配电线路以确保在负荷增加时变压器的容量也相应的增加。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，使得其两根导线接触，并会发生弧光。如果此时断路器或保险丝能够正常工作，见分晓电路能被自动切断。大部分架空明线是用裸线架设的，有时由于风、雪、大树、起重机、飞机或支撑物的损坏等因素会使导线

45、偶尔碰到一起。由雷电或开关瞬变过程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。通常采用油侵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。这些绝缘材料会将随着时间的流失而老化，尤其是在过负荷引起高温下运行的时候更是如此。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于绝缘老化、加上雷电、开关瞬变过程导致的过电压造成的。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电

46、压所致。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。绝缘破坏也可能发生在某一绕组与定子铁芯的接地钢结构之间。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。可能通过电压、电流和电阻的规律来解决问题。当然，单相线路上故障的处理方法也可用于在单相等效下三相故障的处理中。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

47、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200VA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%

48、。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2040A，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