Short circuit electrodynamic force of the hottest

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ANSYS Maxwell transformer short-circuit electrodynamic simulation

0 introduction

with the increase of single transformer capacity and energy density, the requirements of various performance indicators of the transformer are also higher and higher, in order to deal with the impact of short-circuit accidents on the safe operation of the whole power system and people's lives and properties. How to improve the anti short circuit ability of transformer itself? In addition to reducing the short-circuit current of the transformer according to the percentage criterion of impedance voltage proposed by the national standard, the design can also optimize the transformer structure by analyzing the leakage magnetic field distribution of the transformer through simulation according to the decisive factors of electric power, so as to further reduce the size of short-circuit electric force

1 qualitative analysis of short-circuit electrodynamics

when there is current flowing through the transformer winding, due to the joint action of current and leakage magnetic field, Ampere force will be generated in the winding, and its unit length. This discovery can greatly improve the plastic material in automobile or other fields. The degree of plastic material depends on the product I of magnetic induction intensity of leakage magnetic field and short-circuit current in the conductor × b. The direction is determined by the left-hand rule. When the transformer is short circuited, a large amount of short-circuit current passing through the winding of the transformer will produce great electric power. Once the anti short circuit ability of the transformer is not enough, it will lead to winding deformation, resulting in violent movement between windings and turns, leading to insulation failure and internal short circuit. At the same time, when the short-circuit current flows through the winding, the winding loss is great, and the heating is serious, resulting in insulation aging. At least, the insulation is damaged, and at worst, the wire is fused. In this regard, the dynamic and thermal stability of transformers used in the national standard and IEC are regulated accordingly. Therefore, transformer manufacturers must take various measures in design, raw materials and technology to improve the short-circuit resistance of transformers

1.1 dynamic stability of transformer

the dynamic stability of transformer in short circuit is usually decomposed into axial force and radial force, which are studied respectively, so that measures are taken to solve the stability problem under the action of these two forces in the structural design

according to the fact that current carrying conductors attract and repel in the same direction, it can be qualitatively judged that the force of conductors between transformer windings is repulsion. Therefore, the radial inner winding will be subject to inward compression force, and the outer winding will be subject to outward tension; Axially, they are subject to inward compressive force, as shown in the schematic diagram of figure (2). The leakage field is related to the current, the arrangement of windings, the geometric size of windings, ampere turn distribution, iron core structure, etc. For the magnetic line of force as shown in figure (1), the winding is of equal height and balanced along the axial ampere turn, but due to the actual design, manufacturing, drying process and other factors, the leakage magnetic field is usually asymmetrically distributed, and the axial force will increase rapidly in case of short circuit. When the mechanical strength of parts and components is insufficient, except for the coil, the axial force is transmitted to the iron core clamp and other places through yoke, pressing plate and other devices, which may eventually lead to the axial deformation of the transformer

similar to axial force, radial force is mainly caused by axial leakage magnetic field. Radial electromagnetic force makes the inner diameter of inner winding smaller and the inner diameter of outer winding larger. Under asymmetric conditions, the force on the circumference of the winding is uneven, which is prone to local instability and warping deformation. Excessive tensile stress will also produce permanent deformation, which will further cause destructive effects such as insulation damage and inter turn short circuit

1.2 thermal stability of transformer

when the transformer has a short circuit, the huge short-circuit current will increase the temperature of the winding. When the temperature of the wire in the winding rises and exceeds a certain temperature, the mechanical strength of the wire decreases significantly compared with that at room temperature, softening occurs, and the inter turn insulation is damaged, resulting in internal faults of the transformer. For the double winding transformer, the three-phase symmetrical short circuit on the low-voltage side is the most serious form of short circuit. Therefore, in the calculation, it is necessary to ensure the short-circuit resistance of the critical value of the maximum short-circuit force under the worst short-circuit conditions

1.3 measures to improve the short-circuit resistance of transformer

according to the qualitative analysis of dynamic stability and thermal stability of transformer, it can be seen that improving the short-circuit resistance can be started from three aspects: reducing short-circuit electrodynamic force, reducing short-circuit temperature rise, and improving dynamic stability intensity. According to these three physical quantities, it can be seen that reducing the short-circuit current, reducing the leakage magnetic field, and using conductors with greater allowable stress can improve the short-circuit resistance of the transformer. Therefore, to improve the distribution of the leakage magnetic field of the transformer, analyze the axial and radial magnetic fields caused by the leakage magnetic field, and improve the window structure and ampere turn distribution, theoretically, we can find opportunities to optimize the structure and coil layout, greatly improve the distribution of the leakage magnetic field and improve the short-circuit resistance on the basis of improving the short-circuit current

figure (1) distribution of magnetic lines of force

figure (2) schematic diagram of transformer winding stress

2 short circuit electrodynamic simulation analysis

the simulation and analysis software used in this paper is the latest version of ANSYS Maxwell 2019 r1.ansys Maxwell is a finite element electromagnetic field simulation and analysis software widely used in the design of various electromagnetic components based on solving Maxwell differential equations. The design setting and solution process are shown in figure (3). Through electromagnetic field simulation, the visual dynamic field distribution diagram, force, torque, inductance, coupling coefficient and other electromagnetic parameters can be obtained. Further analysis of strength, noise, heat and so on can be carried out in ANSYS mechanical and fluent, and the ontology can be further optimized in combination with multiple physical fields. Maxwell can also automatically generate ROM model and optimize the system design in Simplorer considering the influence of ontology

figure (3) Maxwell simulation process

take a three-phase transformer as an example, using Maxwell 2019 R1 transient solver, the model is shown in figure (4). Inner coil low voltage, outer coil high voltage

figure (4) transformer short-circuit model

2.1 transient solution setting

winding connection mode setting y, Y0 has three closed-loop control modes of stress, strain and displacement, and the winding excitation is the power frequency sine function under short-circuit current. The set winding excitation is shown in the following figure (5), and the solution setting is shown in the following figure (6)

figure (5) transformer winding excitation

figure (6) solution setting

2.2 result analysis

first check the grid and whether the input is correct, as shown in figure (7), figure (8) and figure (9), and the results are available. The distribution of leakage magnetic field and electrodynamic force density is shown in figure (10) and figure (11). The saturation magnetic induction intensity of the iron core material is 1.95t. It can be seen that at the time of T = 0s, the current of phase A is the largest, the core column of phase A has reached saturation, and the magnetic leakage between the high and low voltage coils of phase A is the largest. The magnetic leakage cloud diagram is shown in figure (12), and the radial magnetic leakage and axial magnetic leakage are shown in figure (13) and figure (14) respectively. Similarly, the saturation of other B/C columns at other times can be analyzed, which is consistent with the theory. The simulation setting steps of axial and radial magnetic flux leakage are shown in the following figure (15), figure (16) and figure (17). The specific steps are as follows: open the caculator field calculator, select quantity in the input column, then select magnetic induction intensity B, and then select scal in the winding vector column?, Select scalarx (radial magnetic induction component) and scalary (axial magnetic induction component) respectively, and write the expression of radial magnetic induction component respectively, save it as named expression, and give a name such as BX for output. In the same way, the axial magnetic induction intensity can be output

figure (7) grid division

figure (8) input current at high voltage side

figure (9) input current at low voltage side

figure (10) t = distribution of magnetic induction intensity at 0s

figure (11) t = density of electromagnetic force at 0s

figure (12) amplitude magnetic induction intensity figure (13) radial magnetic induction intensity figure (14) axial magnetic induction intensity

figure (15) calculate the magnetic induction vector x (radial) component

figure(16) Calculate the X (radial) component of the magnetic induction vector

figure (17) output radial magnetic induction intensity

2.3 qualitative analysis and simulation comparison

in the eddycurrent solver, the radial magnetic induction intensity and axial magnetic induction intensity of the leakage field in the window can be output, and the changes with space are shown in the following figure (18) and figure (19). Figure (18) and figure (19) are the magnetic induction intensity values taken out by drawing a line in the window on the original side (as shown in figure (20), the blue line in the red wireframe). Figure (18) shows that the radial magnetic induction intensity is large at both ends and small in the middle. Figure (19) shows that the axial magnetic induction intensity is small at both ends and large in the middle. The qualitative analysis is carried out at the center of the winding. According to the ampere turn law, the changes of axial and radial magnetic induction intensity are consistent with the simulation. It can be seen from figure (18) that the radial magnetic induction intensity is small, and it can be seen that the transverse magnetic leakage is small. It can be seen from figure (19) that the axial magnetic induction intensity is large, and the appropriate optimization design can be carried out from the design point of view

figure (18) output radial magnetic induction intensity

figure (19) output axial magnetic induction intensity

figure (20) value line of magnetic induction intensity in the middle of the primary and secondary windings

3 mechanical short-circuit resistance check

radial force is inward pressure (compression stress) on the inner winding, and outward tension (tensile stress) on the outer winding. The failure mode of radial force combines the difference between inward and outward forces. The outward tensile stress is judged by the elastic limit of the conductor material, and the inward compressive force depends on the elastic modulus and geometric structure of the material. There are several types of winding faults under the action of axial force, and corresponding criteria need to be adopted in combination with product design. As mentioned above, electromagnetic analysis can obtain the distribution of electrodynamics in space and time domain. In the simulation process, Maxwell and mechanical are coupled under the workbench, as shown in figure (21). The coupling method is the same. Mechanical simulation obtains physical quantities such as deformation nephogram and maximum stress point, which are generally spiral, curly and curved in criterion; The concentrated appearance of such particles is the manifestation of serious wear process, and the corresponding analysis and judgment are carried out in combination with dynamic stability and thermal stability

figure (21) ANSYS Workbench platform

figure (22) import electromagnetic force

figure (23) outward bulge trend of high voltage coil

figure (24) inward concave trend of low voltage coil

according to the calculation, the maximum stress of low voltage winding appears at the upper end of the outermost ring, which is 30kg/cm ^ 2, the maximum stress of high voltage winding appears at the lower end of the innermost ring, which is 87kg/cm ^ 2, and the axial force of high voltage winding is 250N, The axial force of low-voltage winding is 3707n, which meets the design requirements

4 summary

in this paper, the distribution of electrodynamic density is obtained through three-dimensional simulation. The axial and radial magnetic induction intensity distributions are obtained through the simulation of two-dimensional leakage magnetic field. The simulation conclusion is consistent with the qualitative analysis, so the design engineer can refer to the leakage magnetic field to improve the design and further improve the reliability of the design

CAE Department of Shanghai Pairui Information Technology Co., Ltd

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