输电线路的防雷英文文献翻译

The Lightning of Transmission LineOvervoltages on power systems are traceable to three basic causes, lightning, switching, and contact with circuits of higher voltage rating. The power system designer seeks to minimize the number of these occurrences ,to limit the magnitude of the voltages produced,and to control their effects on operating equipment. Lightning results from the presence o clouds which have become charged by the action of falling rain and vertical air currents, a condition commonly found in cumulus cloudsVoltages may be set up on overhead lines due to direct strokes and due to indirect strokes . In a direct stroke, the lightning current path is directly from the cloud to the subject equipment-an overhead line. From the llne, the current path may be over the insulators and down the pole to ground. The voltages setup on the line may be that necessary to flash over this path to ground. In the direct stroke, the lightning current path is to some nearby object, such as the tree shown In Fig. 10 lb. The voltage appearing on the line is explained as follows As the cloud comes over the line, the positive charges it carries draw negative charges from distant points and hold them bound on the line under the cloud in position as shown. The voltage on the ine is zero assuming that the line is not energized, IF the cloud is assumed to discharge on the occurrence of the stroke in zero time, the positive charges suddenly disappear, leaving the negative charges unbound. Their presence on the llne implies a negative voltage with respect to ground. On the occurrence of a stroke, lightning clouds do not discharge in zero time. Instead,the stroke current rises from zero value to maximum value (perhaps 50, 000 amperes) in a few microseconds and is completed in a few hundred microseconds.Direct lightning strokes to lines as shown in Fig. lO-la are of concern on lines of all voltage class ,as the voltage that may be set up is in most instances limited by the flashover of the path to ground, Increasing the length of insulator strings merely permits a higher voltage before flashover occurs. The most generally accepted method of protection against direct strokes is by use of the overhead ground wire For simplification only one ground wire and one power conductor are shown. The ground wire is placed above the power conductor at such a position theractically all lightning-stroke paths will be to it instead of to the power conductor. Stroke current then flows to the ground most of it passing through the tower footing ground resistance Rwhde a smaller part goes down the line and to ground through the adjacent tower footings. The tower rises in voltage due to the current I1 through the resistance R1 to a value which is Approximately this voltage appears between the tower and the power conductor (which was not struck). If this voltage is less than that required to cause insulator flashover, no trouble results. Protection by this method is improved by using two carefully placed ground wires and by making tower footing ground resistance of low value.The lightning record of lines supported on towers 80 to 90 feet tall substantiates the simple theory of line protection just presented. The poorer record of lines on towers over 100 ft in height indicates that other factors, perhaps the inductance of the path down the tower, should be considered. low-voltage lines supported on small insulators. They are of little importance on high-volt-age lines whose insulators can withstand hundreds of kilovolts without flashover. Insulation is required to keep electrical conductors separated from each other and from other nearby objects. Ideally, insulation should be totally nonconducting, for then currents are totally restricted to the intended conductors. However, insulation does conduct some current and so must be regarded as a material of very high resistivity. In many applieatlons, the current flow due to conduction through the insulation is so small that it may be entirely neglected. In some instances the conduction currents, measured by very sensitive instruments, serve as a test to determine the suitability of the insulation for use in service. Although insulating materials are very stable under ordinary circumstances, they may change radically in characteristics under extreme conditions of voltage stress or temperature or under the action of certain chemicals. Such changes may, in local regions, result in the insulating material becoming highly conductive. Unwanted current flow brings about intense heating and the rapid destruction of the insulating material. These insulation failures account for a high percentage of the equipment troubles on electric power systems. The selection of proper materials, the choice of proper shapes and dimensions and the control of destructive agencies are some of the problems of the insulation-system designer. Many different materials are used as inaulation on eIectrle-power systems. The choice of material is dictated by the requirements of the particular application and by cost. In residences, the conductors used m branch ctrcults and m the cords to appIlances may be insulated with rubber or plastics of several different kinds. Such materials can withstand necessary bending, are relatively low electrical stress. High-voltage cables are subjected to extreme voltage stress;in some cases several hundred kilovolts are impressed across a few centimeters of insulation. They must be manufactured in long sections, and must be sufficiently flexible as to permit pulling into duets of small cross seetion. Tbe insulation may be oil-impregnated paper, varnished cambric, or synthetic materials such as polyethylene. The coils of generators and motors may he insulated with tapes of various kinds. Some of these are made of thin sheets of mica held together by a binder, and others are of fiber glass impregnated with insulating varnish. This insulation must be capable of withstanding quite high operating temperatures, extreme mechanical forces, and vibration. The insulation on power-transformer windings is commonly paper tape and pressboard operated under oil, The oil saturates the paper, greatly increasing its insulation strength, and, by circulating through ducts, serves as an agent for carrying a way the heat generated due to IZR losses and core losses in the transformer. IThe transformer insulation is subjected to high electric stress and lo large mechanical forces, The shape and arrangemert of conducting metal parts is of particular concern in transformer design. Overhead lines are sulaoorted on porcelain insulators. Between the suooorts air servesThe insulation of an electric power system is of critical importance from the standpoint of service continuity. Probably more major equipment troubles are traceable to insulation failure than to any other cause. It might be argued that equipment should he overinsulated in terms of present practice. There are, however, other factors in addition to direct cost that argue against the use of higher insulation levels1. In cables, insulation is operated at very high stress. If insulation thickness were increased, more material would be required. In addition, larger-diameter cables would require more lead for covering, would be more difficuh to handle, and lengths that could be put on reels would be reduced. In addition, electrical insulation is also good thermal insulation. Increased insulation thickness increase the problem of heat removal irom the power conductors and requires a reduction ot their current ratingz. Increased thickness of insulation in transormers increases the size of coils and cores and increases copper and iron losses. The larger spacing between coils results in increased per unit impedance. Increasing the number of suspension units in transmission line insulators necessitates an increase in cross-arm length, which in turn requires heavier structures and perhaps wider rights of way. Similar statements could be made regarding other equipment, such as generators, in-stru-ment transformers, and circuit breakers3. An arbitrary increase in insulation strength results in increased costs of associated parts and, in many instances, less satisfactory operating characteristics. Because of the problems associated with equipment designs that attempt to utilize overly generous insulation, efforts are made in other directions. Manufactures attempt to produce insulation of uniformly high quality, operators attempt to maintain the insulation with minimum deterioratlon, and designers atempt to plan systems in which overvoltages due to transient conditions are limited to values only slightly above the System operating voltage. The conductors of overhead transmission lines are supported by porcelain insulators and are insulated from each other by air between the points of attachment. Modern porcelain insulators are designed and manufactured in such a fashion that in themselves they are almost perfect in operation. Very seldom is porous of cracked porcelain found, Flashover of line insulators is almost always traceable to the breakdown of the air around them due to overvoltage from lightning or other causes. Insulators whose surfaces are contaminated and then moistened by light rain of frog may flash over even under norreal-operating-voltage conditions. If an insulator is cracked or porous and permits lightning or power-frequency current to pass through ,he body of the insulator, it may be shattered, with the resultant dropping of the line. The air between the conductors of a high-voltage transmission line is under electrical stress. This stress is relatively great immediately adjacent to the conductors and very low midway between tbem. Wben the stress in the air exceeds about 30 kilovolts/era, breakdown occurs within that area where the high stress exists. Hence on a transmission line it is possible to have dielectric breakdown of the air around the conductors without total breakdown between conductors. This condition is termed corona. Corona on transmission lines produces power loss, generates ozone and acid corn pounds of nitrogen, and produces radio interference and audible noise. Tbese effects are easily tolerated if of low level but can become very annoying if excessive. A great amount of experimental work has been done to study these effects, for they present limiting gactors in the voltage at which lines may be operatedI. Present day designs permit these effects hut attempt to control their levels to point where they are relatively unobjectionable. Lightning arresters are devices put on electric power equipment to limit overvoltages to a value less than they would be if the attesters were not present. Ideally a lightning ar rester should be off the line under normal operation, switch onto the line when the voltage The basic form of a lightning arrester is shown in. A spark gap is connected in series with a reals tot. The gap is set at a sparkover value greater than normal line voltage, hence the gap is normally non-conducting. Onthe occurrence of an overvoltage, the gap sparks over, and then the voltage across the arrester terminals is determined by the 1R drop in the arrester. The resistor limits the current flow, avoiding the effect of a short circuit. When the over voltage condition has passed, the are in the gap should cease, thus disconnecting the arrester from the circuit. If the arc does not go out, current continues to flow through the resistor, and both the resistor and the gap may be destroyed. Arresters must be placed very near the equipment to be protected. In many instances arresters are mounted directly on the tanks of large power transformers. If placed at a distance from the equipment to be protected, traveling-wave conditions may result in a voltage at the equipment much higher than that permitted at the arrester.Is perhaps so percent atove normal value, llratt the voltage to this value regaraless nature or source of the overvohage, and switch off of the line when the disturbance and normal voltage has been restored. Circuits are grounded in order to prevent high voltages from building up on the eondue-tots, while equipment grounding aims at preventing enclosures rom reaching voltages above ground. Grounding thus improves system protection and reliability and provides safety to people standing by. Grounding every circuit, however, makes the system susceptible to excessive currentsshould a short circuit develop between a llve conductor and groundI. Thus, not all neutrals of wye-connected loads (especially large motors) should be grounded. Grounding should then be practiced selectively, especially on the primary distribution system, Inpart (a), diseonnectionof motors M1 and M3 for maintenance of repair dePrives the 2400-volt system of a ground. It is preferable to ground the system at the source, that is, at the transformer neutral . Metal enclosures, raceways, and fixed equipments are normally grounded. However, motors and generators well insulated from ground ,and metal enclosures used to protect cables or equipments from physical damage, may be left ungrounded2. Also, portable tools and home appliances, such as refrigerators and air conditioners, need not he grounded if constructed with double insulation. Some ac circuits are required to be ungrounded as, for instance, in anesthesizing locations in hospitals. In fact, line isolation monitors are installed in such cases, capable of sounding warning signals. High-voltage services (IO00V) are not necessarily grounded, but they must be so if they supply portable equipment.Metal underground water pipes are normally used for grounding. If their length is judged inadequate, they may be complemented by other means, such as a building metal frame or some underground pipe of tank. 输电线路的防雷 过电压在电力系统中已知的三个基本原因：雷击、开关、以更高的电压等级与电路接触. 电力系统的设计者尽量尽量减少这种情况,以限制过电压电压产生,并控制其对作业设备的影响。 雷电是由于那些因降雨而带电的云层，以及通常存在于积云中的垂直气流而引起的。在架空线上可能会由于直接和间接的雷击而建立起的过电压。在直接霄击中，雷电电流的路径是直接从云朵到设备。通过架空线，雷电产生的电流可以越过绝缘子，然后顺着线杆人地。架空线上产生电压或许会在此路径上发生闪络然后接地。在直接雷击中，雷电产生的电流流经一些附近的物体。架空线上出现电压可做如下解释：当云朵飘到架空线上空时，它所带的正电荷吸日I远处的负电荷，并将这些负电荷附在如图所示的云层下的架空线上。假设架空线未通电，那么线上的电压就为零。假设云层在发生闪电那一刻放电，正电荷突然消失，留下负电荷未被释放。那么架空线上的负电荷就会对地产生负电压。 在发生闪电时，闪电云层立刻完成放电的。相反，产生的电流在几微秒内从零值增长到最大值(大约50kA)。并在几百微秒内全部释放。作用于架空线的直接雷击影响到架空线上所有电压等级。在许多情况下，可能产生的电压通过闳络接地而得以限制。增加绝缘子串的长度只能在闳络发生前允许建立较高的电压。大多情况下可采用架设架空接地线的方法来防止直接雷击。为了简化起见，只画出一条接地线和一条电力导线。 接地线放置在电力导线上，使得每一次雷击都要通过接地线而不是电力导线，雷击产生的电流就会流到地。之后，电流的大部分都经过塔脚地面电阻R。人地，而少部分沿着传输线通过塔脚人地。塔架的电压升高到一个值，该值是由电流，和经过的电阻R。确立的即u一f1R1。大致上。这个电压出现在塔架和(没有被击穿)电力导线之间。如果该电压低于所能引起闶络的值，则不会引起任何麻烦。用两条精心放置的接地线和减少塔脚接地电阻值的方法可改善这种保护方法。从支撑于8090英尺高的塔架上的传输线的雷击报告中，可以证明上述线路保护的简单原理。但是从100英足以上的铁塔的不太理想的记录表明，或许还必须考虑到其他因素，如从塔架到地的电癔等。 间接雷击在传转线上产生相对较低的电压，这些过电压只在电压等级较低并且绝缘水平不高时才真正需要考虑。而对绝缘水平较高的高压线路就不太重要，这些线路可承受几百千伏的电压而不会发生闪络。 为了使得导线相互隔离并且不接触到其他物体，必须采取绝缘措施。在理想条件下，绝缘应做到完全不导电，那样电流才能完全束缚在计划的导体内。但事实上即使采用绝缘措施t也会有少许漏电流，因此绝缘材料应当当作具有高电阻率的材料看待。在许多情形下，由于绝缘材料的导电性造成的漏电流很小，以致于可以忽略不计。在一些情况下，采用非常敏感的仪器铡量的漏电流可以用来确定某种绝缘措施是否合适。 尽管在正常情形下绝缘材料的性能会锟稳定，但在极端情况下，比如电压过高、高温或在某种化学物质的作用下，绝缘材料的性能会遭到完全的破坏。绝缘材料性能的改变会导致其成为良导体，由此而造成的不应有的电流会使得绝缘材料被强烈地加热导致其迅速损坏。在电力系统中有相当比例的设备损坏是由于绝缘故障引起的。绝缘设计人员面临的问题包括选择合适的材料、合适的形状和尺寸，以及控制破坏性因素等。 有许多种绝缘材料应用于电力系统中，绝缘材料的选择取决于具体应用的需要和费用。在居民小区，分支线和电线中的导体可以采用橡胶或各种塑料材料绝缘。这些材料可以承受必要的弯曲，但其电气应力较低。高压电缆适用于较大的电气应力的场合，在某些情况下，几百千伏的电压只采用几厘米厚的绝缘处理。这些绝缘材料必须作成长形的，并且足够柔软，以便于将其穿进小口径的管道。绝缘材料可以是油浸纸质、浸漆麻纱或合成材料，如聚乙烯。 发电机和电动机的绕组可以采用绝缘带绝缘。一些绝缘带是通过用胶将云母片粘在一起制成的另一些是采用绝缘漶浸的玻璃纤维制成的。这些绝缘方式必须能够承受很高的温度、很强的机械力和振动。 电力变压器的绕组的绝缘通常采用油授纸带和压制纸板。纸在油的授泡下，其绝缘强度大大增强，而且令油在管道中循环，还可带走变压器中的欧姆损耗和铁蕊损耗引起的热量。变压器绝缘要能经受高的电气应力和强的机械应力。在变压器的设计中，导电的金属部件的形状和布置必须特别考虑。架空线是通过瓷绝缘子绝缘的。在支撑件之间是采用空气绝缘的。选用陶瓷材料是由于它在暴露于环境中时，具有抗变质的能力，并且具有高的介电强度而且在雨中能够得到清洗。从连续供电的角度，电力系统的绝缘是至关重要的。由于绝缘问题引起的设备故障可可能远多于其他故障。按照目前习惯的做法，人们认为对设备的绝缘应留有较的余量。但是这样做除了会带来较大的费用外还会引起其他的问题。 在电缆中，绝缘的强度是很高的。如果绝缘的厚度增加了，就需要更多的绝缘材料。此外，较大直径的电缆将需要更多的铅来包裹，并且会很难处理，而且绕在卷轴上的长度也将受到限制。此外，电气绝缘的材料也具有良好的隔热性，因此增加绝缘材料的厚度会防碍传输电能的导体的散热，使得其额定电流降低。 在变压器中增加绝缘的厚度会增大绕组和铁芯的尺寸，从而使得铜损和铁损加剧。 绕组间距的增大会使得其归一化阻抗增加。 增加传输线的悬挂绝缘子的数量，就必须增加铁塔横臂的长度，从而使得铁塔的结构更笨重并且需要占用更宽的空间。 对于其他一些电气设备，如发电机、互感器以及断路器也会具有类似的问题。盲目的提高绝缘强度会导致相关部分造价增加，并且往往使得其使用特性不能令人满意。 由于在设备设计中倾向于采用过度的绝缘措施，人们做了大量的努力以其他的方式(不单是增加厚度)来达到这个目的。制造厂家试图生产出具有均匀的高绝缘性能的材料，使用者试图尽可能地减少绝缘材料的老化变质，设计者试图将系统设计成为在瞬态过程中的过电压能够被限制在略微高于系统工作电压的范围。 架空传输线是采用瓷绝缘子支撑的，它们彼此之间是通过空气绝缘的。现代瓷绝缘子在设计和制造上都彼此使得它们自身具有完善的使用性能。很少发现有气孔或有裂缝的绝缘子。线路上绝缘予的飞弧闪络大都可以归结为由于雷电或其他原因造成的过电压导致绝缘子周围的空气击穿造成的。表面污染了或被雨和霉弄湿了的绝缘子甚至在正常操作电压下也会闪络。如果一个绝缘子有气孔或有裂缝并且未能阻挡雷电或功率高频电流通过，绝缘子将会破碎，导致线路跌落下来。 高压传输线的导体之间的空气承受着电气应力。这个应力在邻近导体处相对很大，而在各导体的中间位置都很低。当空气中的应力超过30kVcm时，则在具有高应力的地方将发生击穿。因此在传输线上有可能发生导线周围的空气击穿，而不是导线间的空气全部击穿。这种情况被称为电晕。 传输线上的电晕产生的功率损耗，产生臭氧和氯的酸性混和物，并且产生射颡干扰和可听觅的噪声。这些影响在较低水平下尚可容忍，但当其越过一定限度后就非常讨厌。为了研究这些效应已作了大量的实验，因为它们限制了线路上所能工作的电压。目前从设计上是允许这些效应发生的，但应尽力降低其量级，使之控制在不令人讨厌的水平。 避雷器是一种安放在电力设备上用以限制其过电压的装置。在理想情况下，在正常时，港雷器廊脱离电力线而在电压高于诈常信的20蟛时棺凡线路从而无论县什么原因措成的过电压都会得到抑制，并且一旦干扰过去电压恢复正常以后，避雷器卫会脱离电力线。避雷器的基本形式，由一只电阻串联一个火花放电闯蹋c组成。这个间隙整定在高于正常线路电压的火花放电值，因此在正常情况下这个间隙是不导通的。当过电压发生时，通过火花放电使问隙导通，此时避雷器端的电压就由其，尺(电流和电阻)引起的压降决定。电阻起到了限制电流，避免了短路的影响的作用。当过电压过去后，间隙中的电弧就毁灭了，从而使避雷器脱离线路。如果电弧不熄灭，电阻中将仍会有电流通过，导致电阻和气隙均被烧毁避臂器必须安装在非常接近被保护的设备处。在许多情形下，避雷器直接放在大型变压器的油箱上，如果避雷器安放在了远离待保护设备处，由于行渡会导致设备上的电压较避雷器允许的电压高得多。 线路接地是为了避免在导线上建立高电压，设备的接地是为了避免让设备遭遇高于地电平的电压。因此接地提高了系统的保护性和可靠性，并且对设备附近的人员提供安全将每一回路接地，即使是在带电导体与地面之同发生短路对，仍髂便系统对超电流保持敏感。因此并不是所有星型连接的负载(尤其是大电机)的中性线均接地的。应该有选择性地进行接地，尤其是配电系统的一次，为了维修要使电机jlf，和M。脱离联接，从而使该240。V系统失去接地。将接地在源端实现更可取，接地放在变压器的中性线上。 金属附件、铁路，以及固定设备通常接地，但是和地面绝缘良好的电动机和发电机，以受用于保护电缆及设备以防其碰毁的金属外壳可不必接地。便携式工具和家用电器，如电冰箱和空调器，如果采用了双层绝缘，则也不用接地。 一些交流电路需要不接地，如医院中的麻醉设备。实际上，在这些情形下均安装了线路绝缘监视器，用于提供告警信号。 大于1000V的高电压不一定要接地，但它们如果用于便携设备，则必须接地。 金属地下水管道通常用作接地体，如果它们的长度被认为不舍适，则需要采用其他方式补偿，如建筑物的金属框架或其他接地管道或箱体等。