Research On Magnetostrictive Properties Of Grain-Oriented Electrical Steel Sheet And Laminated Structure Under Non-Standard Magnetization

Nowadays,the system of power grid develops incredibly quickly,which results in some serious problems.For example,the vibration and noise resulted from operation of power transformer endanger its lifespan and surrounding residents’ health.The magnetostrictive effect caused by electrical steel material is one of the main sources of vibration and noise of transformers.At present,the research on magnetostrictive properties of electrical steel mainly focuses on the test and modeling of single sheet magnetostrictive properties under standard alternating magnetization.However,in the actual operation condition of transformers,the core may be affected by high frequency,harmonic,DC bias,rotating magnetic flux and another nonstandard magnetization.At the same time,the clamping force,the lamination form and other factors also cause the magnetostrictive characteristics of electrical steel lamination structure to be different from that of single electrical steel sheet.Therefore,the magnetostrictive characteristics of laminated core should be investigated intensively.In order to realize the fine modeling of magnetostrictive characteristics of laminated core under non-standard magnetization and improve the prediction accuracy of electromagnetic vibration noise of electrical equipment products such as transformers.This thesis focuses on the measurement method and set-up of testing platform,the establishment of the characterization model of the magnetostrictive characteristics under non-standard magnetization,and the numerical analysis of transformer core strain utilizing the suggested magnetostrictive model.The main contents are summarized as follows:Firstly,under the non-standard magnetization condition,the measurement and analysis of magnetostrictive properties of single sheet grain-oriented electrical steel were carried out.Based on improving the measuring devices for 1D and 2D magnetic characteristics of electrical steel sheet in the laboratory.On this foundation,the measurement system used for non-standard magnetization was developed,which realized measurement of magnetostrictive characteristics of single sheet steel under alternating,DC bias and rotating magnetization conditions.The analysis of measurement results indicates that the magnitude and direction variations of magnetostrictive principal strain may be affected by the magnetic field intensity of DC bias and the magnetization direction.The dynamic distribution characteristics of magnetostrictive strain under different elliptical rotating magnetization trajectories were analyzed,and the vector and hysteresis characteristics between magnetostrictive principal strain and flux density were pointed out.The results show that the appearance of DC bias leads to the formation of highorder harmonics of magnetostriction,which aggravates the magnetostrictive strain characteristics of electrical steel sheet;while the rotational magnetostriction presents more complex dynamic strain characteristics,and the strain value is significantly greater than the alternating characteristics.Secondly,for the laminated electrical steel,the local magnetostrictive characteristics were studied.In order to further master the magnetostrictive deformation characteristics of laminated structure of electrical steel,one research method of directly testing the local magnetic and magnetostrictive strain of laminated core was suggested.This thesis developed and set up the magnetostrictive measurement system,which consisted of single-phase double-column and three-phase three-column laminated core.Utilizing the developed system,the vector magnetostrictive characteristics in critical areas of laminated core under different magnetizations were discussed.The influences of DC bias magnetic field on the magnetostrictive performance of yoke and limb were discussed.The influences of clamping position and clamping force on the magnetostrictive performance of transformer core was analyzed.Then,the B-H vector sensor was used to measure the B-period trajectory of local region,the characteristics of different regions of local magnetostriction were analyzed.Then,the differences of magnetostrictive characteristics between single sheet and laminated core are compared and analyzed.Through a large amount of measurement,the database of magnetostrictive characteristics for laminated core is established.It supplies enough data for subsequent modeling of magnetostriction.Thirdly,the modeling method of magnetostrictive properties of laminated structure of electrical steel sheet was studied.Based on above constructed database,from the viewpoint of engineering application,the magnetostriction modelling of laminated core under DC bias was proposed utilizing back propagation neural networks(BPNN)is proposed.Compared with the experimental results,the accuracy of the model is good.Simultaneously,based on the vector magnetostriction characteristics of the laminated core under rotating magnetization,utilizing analogy with classical vector E-S magnetic properties model,the dynamic vector model and parameter calculation method of magnetostrictive properties of laminated core with rotational distortion were proposed and derived.The modelling method was validated by comparing the experimental test with the prediction model under the specific flux.The magnetostrictive model is closer to the actual magnetization environment of transformer core,and can model the service condition of transformer more effectively.Finally,based on the magnetostrictive characteristic model,the vibration and strain characteristics of dry-type transformer core were modeled.Based on the mathematical model of magnetostriction of laminated core,this thesis proposed the combination approach of mathematical magnetostriction model and numerical calculation of electromagnetic field.The magnetostrictive deformation of a dry-type transformer core was calculated.The magnetic field data of unit node of the core in one magnetization period were collected,the model parameters of each node were calculated based on two mathematical models.Then,the magnetostrictive strain of each node was calculated.The quantitative analysis of principle strain distribution and corresponding local structure deformation were implemented.The results show that the distribution of the magnetostrictive strain of the core is anisotropic due to the different characteristics of the magnetic field distribution and the local deformation caused by the rotating magnetization in the T-joint region is obviously larger than that caused by the alternating magnetic field at the yoke.This method lays a foundation for accurate calculation of transformer core vibration and noise.