Enerji iletim hatlarında yorulma olayları
Fatigue phenomenon on electric power transmission lines
- Tez No: 22063
- Danışmanlar: PROF. DR. NESRİN TARKAN
- Tez Türü: Yüksek Lisans
- Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1992
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 109
Özet
ÖZET“Enerji iletim Hatlarında Yorulma Olaylara”adı altında hasırlanan bu tez çalışması, elektrik enerjisinin hava hatlarıyla iletimi esnasında yorulmadan kaynaklanan arızalar ve bu arızaların giderilmesi ile ilgilidir. Malzemenin ömrünü etkileyen temel etkenlerden olan“Yorulma Olayı”, metalurjik açıdan yorumlanarak, yorulma ömrünü etkileyen faktörler açıklanmıştır. Enerji iletim hatlarının temel elemanı olan iletkenler incelenmiş ve bu malzemelerin seçiminde gözönünde tutulması gereken unsurlara değinilmiştir. Tezde üzerinde en çok durulan konu, enerji iletim hatlarını etkileyen kuvvetler olmuştur, özellikle sıcaklık değişiminin ve gerilmenin yorulmaya etkisi genişçe işlenmiştir. Bu konudaki deney sonuçları ekte verilmiş tir. Ayıcıca, enerji iletim hatlarında yorulma olayına marus kalan kısımların yapısı incelenmiş ve Türkiye şartlarında alüminyum-çelik iletkenler için, yorulma etkisi altında iletkenin dayanma yükü belirlenmiştir. Giriş bölümünde T.E.K. K.E.A. Şebeke Müdürlüğün den elde edilen istatistikler verilmiştir. Ek'te ise Rabak tarafından yapılan yorulma deneyi ne ait sonuçlar verilmiştir. (vi)
Özet (Çeviri)
SUMMARY FATIGUE PHENOMENON ON ELECTRIC POWER TRANSMISSION LINES As titled“Fatigue Phenomenon on Electric Power Transmission Lines”in this thesis, the fatigue pheno menon with its metallurgical explanation its causes, results, the places it occurs and some measures helping reducing the fatigue life are investigated in detail. In power transmission, overhead lines are gene rally preferred over underground lines, especially from costal considerations. However, there is an important problem commonly encountered with overhead lines: fatigue. The faults due to the fatigue on power transmis sion lines may occur on poles, insulators, hardwares and conductors. Therefore the“fatigue”phenomenon must be investigated from the metallurgical viewpoint. Fracture, because of repeated rather than undirec- tional or static loading, is known as fatigue and is the commonest cause of service failures. Such fractures can occur at stress levels well below the tensile strength, even in normally ductile low strength materials. Since it was first recognised more than a century ago, fatigue has been intensively investigated in order to understand the phenomenon and guard against its occurrence. These studies indicate that it is convenient to subdivide the over-all fatigue process into three main stages; crack initiation, including a consideration of the effects of cyclic loading on bulk properties, crack propagation to critical size, and unstable rupture of the remaining section. It is now recognised that fatigue occurs as the result of plastic deformation, both in the initiation and propagation of cracks. Up to the terminal fracture fatigue is a form of ductile (stable) rupture, although often of an extremely localised nature. Because fatigue failure involves the cumulative effect of small-scale events taking place over perhaps millions of cycles, it is extremely difficult to make an a priori prediction of the fatigue lifetime. However, certain aspects of fatigue such as low cycle fatigue (failure in less than 1000 cycles) and fatigue crack propagation can be treated quantitatively on a semiemprical basis. (vii)The fatigue of power transmission lines is in close relation especially to the environment and tempera ture. The fatigue properties of a metal can be greatly- influenced by the nature of the environment, especially in long time tests at low stresses and at low frequenci es. Normal air has a deleterious effect on fatigue pro perties. The principal adverse components of air are moisture and oxygen. The additional presence of moisture further reduces the fatigue life by a factor of three. For lead, as with copper, oxygen alone has been found to have an adverse effect. Moisture decreases the fatigue strength of copper, aluminum, magnesium, and iron by 5 to 15 %. Investigators haye generally agreed that the main effect of a normal atmosphere on fatigue is on the propagation of cracks rather than in their initiation. This seems to be a reasonable interpretation, for the initially present oxide films will not be greatly a altered by testing at low pressure. The principal effect of the environment will be in its reaction to the clean metal exposed at the tip of a fatigue crack. Cracks that would have been of a nonpropagating variety in vacuum, propagate in the presence of oxygen or moisture and thereby the fatigue resistance is lessened. In environments that are more corrosive than air and that substantially reduce the resistance of the sur face layers to fatigue crack initiation, both initiation as well as propagation may be affected. Thermal fatigue refers to cracking or fracture due to stresses induced by thermal cycling rather than load cycling. It can be a serious problem; for example, thermal-stress fatigue, the predominant cause of failure of military jet aircraft engines, has occurred in a form of low cycle fatigue in steam powerplant components operated on an intermittent basis. Â temperature change can lead to the development of stress if external con straints are present to prevent free expansion or con traction or if nonlinear thermal gradients develop. However, the other factors having effect on the fatigue lifetime are also important. Fundamentals of power transmission lines base on conductor materials. A conductor of electricity is any substance or material which will afford continuous passage to an electric current when subjected to a difference of electric potential. The greater the density of current for a given potential difference, the more efficient the conductor is said to be. Virtually all substances in (viii)solid or liquid state possess the property of electric conductivity in some degree, but certain substances are relatively efficient conductors, while others are almost totally devoid of this property. The metals, for example are the best conductors, while many other substances, such as metal oxides and salts, minerals, and fibrous materials, are relatively poor conductors, but their con ductivity is beneficially affected by the absorption of moisture. Some of the less effecient conducting materials, such as carbon and certain metal alloys, as well as the efficient conductors such as copper and aluminum have very useful applications in the electrical arts. Certain other substances possess so little conduc tivity that they are classed as nonconductors, a better term being insulators or dielectrics. In general, all materials which are used commercially for conducting electricity for any purpose are classed as conductors. Electric circuits in general possess four fun damental electrical properties, consisting of resistance, inductance, capacitance, and leakance. That portion of a circuit which is represented by its conductors will also possess these four properties, but only two of them are related to the properties of the conductor considered by itself. Capacitance and leakance depend in part upon the external dimensions of the conductors and their distances from one another and from other conducting bodies and in part upon the dielectric properties of the materials employed for insulating purposes. The inductance is a function of the magnetic field established by the current in a conductor, but this field as a whole is divisible into two parts, one being wholly external to the conduc tor and the other being wholly within. the conductor; only the latter portion can be regarded as corresponding to the magnetic properties of the conductor material. The resistance is strictly a property of the conductor itself. Both the resistance and the internal inductance of conductors change in effective values when the current changes with great rapidity, as in the case of high- frequency alternating currents; this is termed the“skin effect.”In certain cases, conductors are subjected to various mechanical stresses. Consequently their weight, tensile strength, and elastic properties require con sideration in all applications of this character. Con ductor materials as a class are affected by changes in temperature and by the conditions of mechanical stress to which they are subjected in service. They are. also af fected by the nature of the mechanical working and the heat- treatment which they receive in the course of manufacture or fabrication into finished products. (ix)The type of material from which conductors are made is highly important as well as the manufacturing way. The material used in overhead lines conductors is desired to have mechanical endurance beside electrical conductivity. The materials satifying these conditions are copper, bronse, aluminium, steel cored aluminium, cadmium copper, copper-clad steel and steel. The mechanical and the electrical properties should be con- s i dered together. Besides, in selecting the conductor material some other dominant factors such as conductivity, conductor diameter, weight of conductor (spesific gravity), sag, corona, propensity to vibration and thermal endurance. In order for the power transmission be economical transmission must be made on highly voltage. However, in this case the corona problem arises, and it should be taken into account. The increasing use of aluminum conductors in extra-high voltage transmission lines, both ac and dc, and the presence of corona even at transmission voltages not regarded as“extra-high,”point to a need of under standing the phenomenon. Corona occurs when the poten tial of the conductor is raised so that the dielectric strength of the surrounding air is exceeded. The air be comes ionised and bluish illuminated gaseous tufts or streamers appear around the conductor, being more pronounced where there are irregularities of the conduc tor surface. The discharge is accompanied by the odor of ozone, and there may be a hissing sound. The effects of corona discharge from a bare con ductor power line may interfere with radio and TV reception, or adjacent carrier and signal circuits. Bundled conductors are frequently used to effect lower voltage stress on the air insulation, which is one of the reasons why bundled conductors are much used in extra-high voltage lines, 300 kV and up. On the other hand the overhead line must be capable of operating in safety anâ must have high ther mal endurance. As a final point, vibrations occur ing on the over head conductors and the earth wires should be considered. These vibrations are of four types: i) Swinging of the conductors caused by changes in wind pressure. This is harmless provided that sufficient clearance is left between conductors to prevent flashover. Larger clearances will be required on long spans such as river crossings. (x)ii) Dancing of the conductors caused by irregular coat ings of sleet in wind. Parts of the conductor are tem porarily changed from round to oval section and the wind causes varying amounts of aerodynamic lift throughout the conductor length. The conductors may dance vartically or horisontally with an amplitude great enough to cause them to touch each other. The use of bundle conductors in creases the tendency to dance because of torsional oscil lations of the bundle and the use of rigid spacers. iii) Jumping of the conductors caused by the sudden shedding of ice loads. The worst jumping occurs when ice melts from the centre span of a section after it has fal len from the other spans. Serious jumping also takes place when ice slips down the conductor towards roidspan. Attempts have been made to inhibit conductor jumping by fitting special insulator assemblies at the suspension points and by increasing the mass per unit length of line at roidspan. It has not been possible, however, to achieve a sufficient reduction in jump height to prevent the conductors from clashing. iv) High frequency vibrations caused by the formation of eddies on the leeward side of a conductor. The frequency of the oscillations is of the order of 5 to 100 Hz and their amplitude is about 1.3 centimetres. These vibrations may cause failure of the conductor at the sup porting clamps due to metal fatigue. This is less likely when the conductor clamps have been carefully designed or when the conductor has been reinforced for a few metres on either side of the clamp. A more effective method, however, is to prevent the oscillations from reaching the clamps by suspending a Stockbridge damper from each con ductor near each point of entry into a clamp. Power transmission lines are subject to some dif ferent kinds of forces. These forces can be classified as dynamic and static forces in general. Static forces acting upon transmission lines can appear due to internal oi- external factors. Among the external factors“temperature change”deserves to be pointed out especially. Within the temperature ranges of ordinary service there is no appreciable change in the properties of con ductor materials, except in electrical resistance and physical dimensions. The change in resistance with change in temperature is sufficient to require considera tion in many engineering calculations. The change in physical dimensions with change in temperature is also important in certain cases, such as in overhead spans and in large units of apparatus or equipment. The variation of conductor voltage with the tem perature or conductor weight is calculated through state equations.“The critical temperature”can be found from these equations. (xi)Moreover, the effects of short-circuit currents causing dynamical and thermic straints should be investigated. As well known, the resistance vary with the temperature. This variation characteristic and the cor responding temperature coefficients could be calculated. Besides, the direct current resistance value and its variation with temperature can be found, as well as the alternative current resistance. The factors having ef fect on the resistance such as skin, proximity, spriality and so on, are also considered. (xii)
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