Güç transformatörlerinde kazan kayıplarının azaltılması
The Reducing of tank losses in large transformer
- Tez No: 39121
- Danışmanlar: PROF.DR. NURDAN GÜZELBEYOĞLU
- Tez Türü: Yüksek Lisans
- Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1993
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 87
Özet
ÖZET Güç transformatörlerinde, dağılmış akıların madeni parçalarda oluşturduğu kayıplar önemlidir. Bu kayıplar, tasarım sırasında, transformatör kaçak alan dağılımına göre sınırlandırılabilir. Bu çalışmada, transformatör demir kayıplarının bir kısmını oluşturan kazan kayıplarının azaltılması için gerekli konstrüktif tedbirler üzerinde durulmuştur. Demir kayıpları ve çekirdek kayıplarının azaltılmasına ilişkin bilgiler verilmiş, dağılmış alan kayıpları ve kaçak akının kazan üzerine olan etkilerinin önlenmesi ayrıntılı olarak incelenmiştir. Transformatör alan dağılımının belirlenmesinde kullanılan Sonlu Elemanlar Yöntemi tanıtıldıktan sonra, Sonlu Elemanlar Yöntemiyle yazılmış FLDll isimli paket program ile ekranlı ve ekransız durumda kazan kayıpları hesaplanmıştır. Kazanda oluşan demir kayıplarının ekran kullanılmasıyla, kazan kayıpları % 48 azaltılabileceği görülmüştür. Bu hesaplarla ilgili bilgisayar çıkışları ekte verilmiştir. - iv
Özet (Çeviri)
SUMMARY THE REDUCING OF TANK LOSSES.IN LARGE. TRANSFORMER One of the designer's major problems in to prevents the transformer from reaching too high a temperature. Ot her problems are to provide sufficient insulation and to orrange it, to produce, a transformer which will operate with satisfactorily low losses, and to keep the material used, and consequently the cost at min. The efficiency of a trans former is determined by the copper losses in its windings and the iron losses in its core and material parts. The losses, as is known, may be subdivided into ohmic losses and stray losses. The latter, which are due to the leakage flux depends on the element where they are located. These are winding losses, magnetic core losses, losses in the clamps and their accessories, if any, losses in the tank and shielding, if any. Hysteresis and eddy-currents in the tank, clamps and core plate, caused by the alternating leakage flux cutting them produce losses. The losses in clamps, tank and core are generally calculable. Only with the aim of empirical constants based an experience with specific types of constructions, on account of the great non unifor mity of the distribution of the leakage flux density in these members. When an iron is magnetized by alternating current, it's necessary to supply power continqusly to overcome the loss and maintain the magnetic flux. The amount of power required depends on the frequency, on the characteristics of the iron, and on the magnetic density at which it's worked. The losses that occur in the material arise form two causes (a) The tendency of the material to retain magnetism or to oppose a change in magnetism, of ten reffered.to as magnetic hysteresis; and (b) The heating which appears the material as a result of the time varia tion of flux. The first of these contributions to the energy dissipation is known- as hysteresis loss and the second as eddy-current loss. Both these losses appear as heat in the transformer. The sum of the hysteresis and eddy-current losses is called the total core loss. The hysteresis loss is the loss caused by the peri odic reversal of the direction of the magnetic flux n the care; and the result of the tendenciy for. the B(H) charac teristic of the material to involue a loop when the v -material is subjected to a cylic magnetizing force. The area of a closed hysteresis loop indicates how much energy is dissipated in the core per unit volume because of hys teresis. Therefore hysteresis loss depends on. closed hys teresis loop area. The total hysteresis loss per unit volume in which the flux density is everywhere uniform can then be expressed emprically as PH - Vf 'Bmax (1) 3 Where P is the hysteresis loss in W/m, f is the frequncy in Hz, Bmax“*”s t*ie peak value of flux density in T, cth and n have values that depend on the material. When ever the magnetic flux in a medium is changing an electric field appears within the medium as a result of the time variation of the flux. When the medium is conduct ing, a current is set up around this path by the induced electromotive force resulting from the line integral of the electric field | Chapter 2|. These currents are called eddy-currents. Their presence results in an energy loss in the material proportional to i2R, called eddy-current loss, the energy being absorbed from the circuit that sets up the field and being dissipated as heat in the medium. The average eddy-current power loss per unit volume when the flux density is varying sinuzoidally at a frequency f when can be given as 2 *2.2 2 p = max to) F 6p Uj 3 Where P" is the eddy-currents loss in W/m, BwjaX is the peak value of the flux density in T, p is tickness of individual laminations in m, p is the specific resistance of the lamination in Qm. Hysteresis is minimized by a proper composition of the iron and by proper annealing. It has been found that the addition of a small percentange of silicon to iron and the uses of cold-rolled sheets reduces the iron loss. The eddy-current loss can be reduced by the use of thin sheets of a material which has a high resistivity and insulating the sheets from each other. This breaks up the path of the circulating current and so reduces its effect. The flux distribution depends on the core material properties core joint design and assembly methods, the operating flux density, imperfections in the magnetic circuit and the presence of building stress. The magnetic characteristics and losses of sheets are also strongly dependent an the direction of magnetizing. The geometry of the construction (the direction of flux lines deviates from that of rolling and the machining of sheets. ) causes additional losses. Minimum losses occur when the rolling vi -direction coindides with that of the flux lines. The rota- tional flux in on average much greater in the stressed core and widespread in the T-joints. V-notched yoke lamina- tions might be used instead of the 45o-90° T-joint. The problem of stray-losses becames increasingly important with growing unit ratings. This can be easly be seen when the relation between stray field strength and transformer ratings is determined for transformers of dif- ferent ratings and voltage lovels. At'transformers of various sizes and rated voltages may be regorded as hav- ing cores and windings of geometrically similar shapes. Under such conditions the stray magnetic field strength is H = ~~ = constant. 4/P(3) where I is the current flowing in the H.V. winding in A, N is the number of turns, ü is the effective length of the leakage field in m and P is the power of öne limb in VA. The stray flux intruducing into the structural parts gives rise to eddy-currents in them. in parts of mor e extensive size (such as in tank walls), the eddy- current losses caused by the stray flux may be considerab- le, thereby increasing the load losses of the transformer and lowering its efficiency. in smaller metallic parts the eddy-current losses are negligible as compared to other losses of the transformer. The loss density (eddy- current loss per unit volume) tends to attain excessive levels that may lead to hazardous local temperature rises. Designers of large transformer are reguired, to keep the losses cause by stray flux below an acceptable level, and on the other hand, to prevent höt spots from developing in metallic parts located in theastray field..The stray flux departing radially from the outer surface of ttindings gives rise to eddy-current losses in the transf ormer thank walls. The flux density in the tank wall is rather low, the tank losses may be considerable and represent by for the greater part of the losses caused by the stray flux because of the large size of the tank. The stray flux departing radially through the inner sur¬ face of windings intrudes into core and the fittings mount- ed on it. The stray flux departing axially from the winding, may induce eddy-currents in the damping structures of the vrindings/ in the yokes and in the yoke beams. The electro- magnetic field of conductors and internal connections within the transformers causes eddy-current losses in metallic parts situated in their vicinity. The loss caused in the cover by the field of current flowing in the bush- ing also belongs to this category. A characteristic fea-^ türe of these losses is that they are distributed along bar conductors and connections/ mainly in the cover in the - vii -tank walls and in the frome e lamping structure, i.e. in extensive sheets. These losses are, important as compared to overall losses of the transformer. Losses and temperature rises caused by the leakage flux may generally be reduced in the following three ways; (a) by deflecting the magnetic flux from endengered spots by electromagnetic shields or magnetic shunts, (b) by pro per dimensioning of constructional parts with respect to eddy-current losses, (c) by proper selection of construc tional materials. Shields and shunts are placed next to the inside surface of the tank to reduce the tank losses and there fore the tank is very close. to the coils and the leakage field. Shield prevents the stray flux from penetrating in to the tank by the reacting effect of eddy-currents induc ed in the said shield, whereas shunt diverts the flux from the endangered location. The problem can be thought in general. It's common practice to use as a magnetic shunt a core packet laminated parallel to the flux direction. Mag netic shunts are only applied to the tank wall, if the loss is higher than permissible- Although a magnetic shunt is usually a more effective method of protection against stray flux that shield, shield is used to avoid the addi tional weight caused by shunt shield is obviously effec tive the amount of overall losses can be reduced below the losses occur ing without the shield. Aluminium or cop per plate can be used as shield. The calculation of losses in structural parts of a trnasformer is a very complex task. These losses are dif ficult to calculate the orotically because of the complica ted geometries of parts. Solution of stray-field problems may become possible with the increasing capability of computers. However, there are no generally applicable com putation methods available for solving all. details with out resorting to approximations. For flat two dimensional field applications consi derable attention has been given in recent years to finite element- methods. The. field region is covered by a grid, and magnetic vector potential, are calculated at the nodes, giving a numerical solution for the flux densities so far these methods don't apperar to- have gained any wide acceptance among transformer designer. To calculate the losses in the tank and shield, it is necessary to know how much of the flux enters this parts, the surface areas of the same all affected perpen- dicullarly by the flux that penetrates- inside it or leaves from it. From the transformer leakage program based on the finite element method, the flux density at the tank sur face and the shield surface can be found, and then the - vm -losses in the tank and shield can be calculated from the formulas (1) and (2 ) Based on tests and on information from the program about how much flux enters the different parts, a reasonable estimate can be made. The calculation of transformer flux is a prerequist to not only the calculation of eddy-current losses, but also to the calculation of reactance short circuit forces it's therefore of fundamental importance for the transfor mer designer. - xx
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