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1、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,
2、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 i
3、ndirect 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
4、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 t
5、hem 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
6、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 m
7、icroseconds.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
8、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 lightnin
9、g-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 curr
10、ent 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 ca
11、refully 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 oth
12、er 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
13、 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.
14、 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. Althou
15、gh 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
16、 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 dimens
17、ions 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 con
18、ductors 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 hu
19、ndred 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 p
20、olyethylene. 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 operati
21、ng 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 carryin
22、g 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 sul
23、aoorted 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
24、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
25、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 he
26、at 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 th
27、e 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
28、 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
29、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 Sys
30、tem 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
31、 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
32、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
33、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 li
34、ne 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
35、 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 h
36、ut 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 unde
37、r 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 ov
38、ervoltage, 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 disconnecti
39、ng 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 p
40、ower 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
41、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 thu
42、s 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 l
43、arge 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
44、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 a
45、ppliances, 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 soundi
46、ng 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
47、 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。
48、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
49、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
