Test And Weakening Of Residual Magnetism In Transformer Iron Core

When the large no-load transformer is switched to connect with the power grid,magnetizing inrush current may be resulted for the residual flux exists in the iron core,and thus the transformer perhaps fail to connect with the power grid.In order to eliminate the influence of remanence on transformer,a new method for analyzing and measuring residual flux in the closed iron core is proposed in this paper,based on the positive and negative DC voltage excitation method.The magnitude and direction of remanence in the closed iron core can be obtained by using this method without changing the initial remanence.In addition,the existing demagnetization method is improved according to this method,and the experimental results show that the method is time-saving and economical.Below are the main points in this thesis:(1)In this paper,the structure and basic working principle of transformer are introduced,and the equivalent circuit of transformer ring core is established.The harmfulness of remanence to transformer is analyzed,the relationship between remanence and inrush current is introduced in detail,and the protection measures against inrush current are presented.(2)Based on the introduce of the magnetization mechanism of ferromagnetic materials,this paper explains the principle of transformer remanence,and measured the magnetic characteristics of toroidal iron core through experiments.By introducing the concept of local hysteresis loop,a new method for measuring remanence based on the positive and negative excitation method is proposed,and the feasibility of the method is analyzed theoretically.The method transformed the magnetic field problem into a circuit problem,which is convenient for analysis and measurement.(3)In order to establish the simulation model of transformer toroidal iron core,the advantages and disadvantages of common hysteresis models are introduced.By comparison,the simulation model of toroidal transformer based on Jiles-Atherton hysteresis model is built in COMSOL software.The relationship between response current and remanence is found by applying two DC voltage with same magnitude but different polarity to the winding on the closed toroidal iron core and analyzing the field-circuit transient.Three key problems in the process of simulation and experiment are analyzed.The simulation results(4)show that this method can accurately measure the magnitude and direction of remanence without damaging the transformer itself and having little effect on remanence.(5)Through the toroidal iron core of transformer experimental platform,the remanence is measured according to the simulation steps,and the reliability of the results is verified.By changing the preset remanence and the experiment is carried out many times,the response current value is brought into the empirical formula,then the calculated remanence value is obtained.Compared with the preset remanence,the error is less than 6.5%,which verifies the accuracy of the simulation result.(6)The common demagnetization methods are introduced and simulated.Combined with the method of remanence measurement proposed in this paper,the original demagnetization method is improved,the remanence in iron core is measured first,then demagnetization is carried out according to the remanence value measured,and the demagnetization effect is compared.Finally,the experimental results show that the demagnetization method is more economical and time-saving.