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1 spiral winding torsion electromagnetic force
When the transformer is short-circuited, the short-circuit power is divided into axial force, radial force and tangential force. The tangential force is related to the axial current component and the radiating leakage magnetic field, which is caused by the joint action of the two. The radial leakage magnetic field is caused by the axial current component, the end leakage flux bending, and the unbalanced ampoule distribution. Since the axial current component has little influence on the radial leakage magnetic field, the radial leakage magnetic field is mainly due to the bending of the magnetic field at the end of the winding and the unbalanced distribution of the ampoule.
For spiral windings, the strands rise spirally at a small angle.
Therefore, not only the current in the circumferential direction, but also the current component in the axial direction around the resistance, ie, J=JQ+JZ, in the transformer winding, the radial magnetic flux interacts with the axial current component, resulting in a spiral along the spiral Torsional forces in the circumferential direction of the windings generate torsional moments. In particular, the 840 MVA large-capacity power transformer required by the Three Gorges Project has a low-voltage winding rated current of up to 25 kA. Therefore, the influence of the axial current component on the short-circuit strength must not be overlooked.
In the upper and lower parts of the winding, due to the opposite direction of radial magnetic flux, the torsional moment generated is also opposite. For the adjacent line cake, when the friction between the pad and the line cake is less than the difference between the torsional forces of the two line cakes, the relative movement occurs, and the line cake and the pad are twisted in the circumferential direction, and the entire winding follows the spiral The direction produces a tight twisting deformation. There are enlarged oil passages at the ends of the windings. The value of radial flux density is large, and it is in a serious ampere-turn unbalanced state. Therefore, a large radial leakage magnetic field will be generated, and the effect of the axial current component will be greater. The tangential force has a great influence on the torsional deformation of the low-voltage spiral winding.
In the calculation of the leakage magnetic field, the axial component JZZ of the current can generally be ignored, but in the calculation of the torsional force, this current component cannot be ignored.
The calculation of the torsional force density on the unit length line cake has f=B ri(t)sin and thus the torsional force on the unit length line cake is F=2 r/cos 0 f dl=2 r/cos 0 B ri(t)sin Dl and thus the torque per unit length of the line cake is M = F r = r 2 r/cos 0 B ri(t) sin dl
2 Influence of transformer material parameters
2 1 Effect on Torsional Electromagnetic Force of Low-Voltage Helical Windings The radial leakage magnetic field is greatest at the ends, causing the torsional forces to be greatest at the ends. The pressure plate is pressed against the end of the winding through the pad, so that the pressure plate material has a greater influence on the magnetic leakage at the end. Pressboard generally has the following materials: special cardboard, carbon steel and low-magnetic steel. In this paper, the influence of the transformer platen material on the torsional electromagnetic force of the low voltage spiral winding is analyzed and calculated. The transformer plate material has a great influence on the torsional electromagnetic force of the low-voltage spiral winding, which is mainly manifested in the end region of the winding. The low magnetic steel material pressure plate and the carbon steel material pressure plate increase the torsional electromagnetic force per unit length of the spiral winding, which is mainly due to the effect of the pressure plate vortex. Compared with the special cardboard material pressure plate, the difference is about 8 41% 15 72%. The pressure plate has a greater influence on the magnetic flux leakage at the ends of the windings, which significantly reduces the torsional electromagnetic force at the ends. Compared with the special cardboard material plate, the difference is about 30%. Through the above calculation and analysis can be seen: From the perspective of reducing the torque of the winding end of the torsion, the choice of magnetic material plate is better.
2 2 Influence of the end of the low-voltage spiral winding on the torsional electromagnetic force The ampere-turn given in the design calculation of the transformer is the average ampere-turn and is calculated according to the average ampere-turn in the numerical calculation of the electromagnetic field. Therefore, the radial flux leakage caused by the unbalance of the winding ampere can be calculated more accurately by numerical calculation. The end windings of the end windings will generate an additional radial leakage magnetic field, and the torsional electromagnetic force will be the largest at the end of the windings. Therefore, the additional torsional electromagnetic force caused by the end of the wire pulling out must be analyzed. This article has carried on the computational analysis to this.
In the calculation, a small ampoule section is attached to the end of the winding according to the different heights at the end of the winding.
From the results of the calculation, it can be seen that the end windings of the end windings will generate an additional radial leakage magnetic field to cancel out the original radial magnetic field, so that the end winding tangential electromagnetic force can be reduced by considering the end winding out of the end winding. However, from the calculation results, the impact is not great.
From the analysis of the effect of the head of the low-voltage spiral winding on the torsional electromagnetic force, if the height of the low-voltage spiral winding is increased, the torsional electromagnetic force at the end can be reduced. In this paper, the influence of the height of the low-voltage spiral winding on the torsional electromagnetic force is calculated and analyzed. From the results of the calculations, it was found that the height increased by 4%, the torsional force at the end decreased by 2 62%, the height increased by 8%, and the torsional force at the end decreased by 98.9%. From this point of view, it is considered that the torsional deformation of the low-voltage spiral winding is reduced. The height of the low-voltage spiral winding can be appropriately increased to generate an additional radial leakage magnetic field, thereby reducing the torsional electromagnetic force at the end, thereby reducing the torsional deformation of the low-voltage spiral winding.
3 conclusions
The torsional deformation of the spiral coil is mainly caused by the combined action of the axial current component and the radial leakage magnetic field. From the analysis of this article, we can see that the effect of simple torsional deformation is not enough to make the winding damaged. However, due to the slight misalignment of the position of the pad, there is a potential risk of instability and other strength problems of the winding. Therefore, this dissertation systematically studies the torsional deformation of transformer low-voltage spiral windings.
1) Analyze and calculate the influence of the structural material parameters of the transformer on the torsional electromagnetic force. From the perspective of the bending of the magnetic flux at the end of the winding, it is considered that the selection of the magnetic material pressure plate can reduce the radial leakage magnetic flux at the end of the winding.
2) From the perspective of additional ampoules caused by the end of the winding head, the influence of the size of the head ampoules on the radial magnetic flux density at the ends was calculated and analyzed, and the height variation of the low-voltage spiral winding on the torsional electromagnetic force at the end was calculated and analyzed. The effect of proposing to raise the height of the low-voltage winding appropriately can reduce the torsional electromagnetic force at the end.
3) From the perspective of reducing the axial current component of the end, this paper believes that the spiral angle of the end is reduced in process and gradually increases along the middle direction of the winding. This reduces the torsional electromagnetic force of the end without affecting the middle of the winding.
4) For the structure of low-voltage windings with 4 helices, this paper considers the use of the structure in the middle of the winding, with the upper and lower ends connected in parallel, so that the low-voltage spiral winding of the transformer can be changed from a 4 helix structure to a 2 helix structure, and the helix angle can be reduced by half, reducing The axial current component of the entire winding reduces the tangential electromagnetic force of the entire winding.
5) The distribution law of the torsional electromagnetic force of the spiral winding of the transformer is given, and the corresponding restraint measures are proposed, which provides a new reference for the transformer winding strength design.