A Novel Structure Design of Iron Core for Power Transformers Considering Joints Forms and Magnetostriction

Abstract

Body: Power transformers and reactors are important devices in power transmission systems. But their cores and windings could produce vibration and acoustic noise by magnetostrictive effect, Maxwell electromagnetic stress and Lorentz force. Noise has become a bad environmental pollution source in some situations, and its vibration and noise reduction has become an urgent problem need to be solved. The shapes, materials, magnetostriction and joints forms of iron cores, the limitations of test, modeling, and experimental measurement of vibration and noise characteristic, are all the difficult reasons to analyze the vibration and acoustic noise of transformers and reactors. 1. The magnetostrictive modeling can be divided into two steps based on the magnetic field force and materials deformation, so the magnetic field force will be fed into the structural finite element with Maxwell electromagnetic force produced on the interface between the iron core and the air to calculate the deformation of the magnetic materials and to make the model more general and consistent. In order to fully verify the correctness of the model in the magnetostrictive analysis, and to ensure the simplicity of test and avoid unnecessary interference factors, the most simple magnetostriction tests will be carried out by using a single closed circle or rectangular electrical steel sheet, as shown in Fig. 1(a). The tests will measure the relationship between magnetostriction and magnetic field of different magnetic materials, different saturations, different sizes and different shapes. 2. The joint is a necessary form for the stacking of large transformer iron cores, and the main joint form is the step jiont at present. The introduction of the joints causes the magnetic force lines to flow through the different laminations and to make the local magnetic field complicated. From simple to complex process is adopted in this paper, and the verification is gradually complicated. Based on the study of a simple sheet with no joint showed in Fig. 1(a)and (e), a sheet with joint will be used to analyze the vibration and modeling calculation, as shown in Fig. 1(b). Maxwell electromagnetic force caused by the the joints and magnetostrictive force due to the magnetostriction of sheet are all actually included in the model. Later, further complicated two or more pieces of sheets are designed up and down on the joint of the simple circle or rectangular sheet, as shown in Fig. 1(c) and (d) to simulate the joints forms in the iron cores of transformers with the magnetic flux transition circulation though the joints. Meanwhile, the separation of the Maxwell electromagnetic force and the magnetostrictive force from the overall analysis can be worked out by using mathematical method, and the separated curve will be given in the end. 3. In order to improve the effect of vibration and acoustic noise deduction sharply, a novel structure design of iron core for power transformer is put forward, with the core and winding position interchange, compared to conventional power transformers, as shown in Fig. 2. Compared with traditional transformers, this structure has many advantages: firstly, the short magnetic circuit reduces the deformation caused by magnetostriction; secondly, the circular ring core does not have joints to avoid the vibration and acoustic noise caused by the joints; thirdly, the radial deformation of circular ring is greatly reduced; the last, the long cantilever structure that causes deformation in the traditional iron core can be eliminated. These advantages together will realize the new low vibration and low acoustic noise transformers. 4. The test prototype of power transformer with laminated core is processed in different stacking modes. The vibration acceleration and displacement of power transformer iron core are measured with high precision laser acceleration sensors, and the modal test is carried out by using Finite Element (FE) software. Based on the comparison between simulation and test data, the effect of vibration and noise reduction is analyzed and established to verify the novel iron core structure design of the power transformers, and to give further reference and guidance to the engineering applications.