Simulation, Measurement And Demagnetization Of The Residual Flux In The Iron Core Of Power Transformer

The objective of this research is to study the impact of the residual flux of the powertransformer iron core to the inrush current and the security issue of power transformers. Anovel approach for residual flux simulation, measurement and weaken was proposed.Firstly, the simulation model of a power transformer core was established, on the basis ofanalysis of the generation principle of the residual magnetization of the ferromagneticmaterial and power transformers. Then the test detection circuit was built to do the residualflux measurement and by testing and analyzing, the direction and value of the residual fluxwere determined. By comparing the preset residual flux and the calculations of simulationformula, the correctness of the calculation results of the residual flux was verified. Throughexploring the principles and methods of demagnetization, the DC demagnetization methodwas identified by simulation and experiment on the basis of the known residual flux. Thisarticle contains a series of analyses, testing and weakening the research method of theresidual flux of the electrical equipment with closed magnetic core structure, such as powertransformers. Below are the main points of discussions in this thesis:A brief description of the principles and the structure of the power transformer wereproposed, Different types of transformer, design methods and theories were discussed. Thesingle-phase power transformer is the focus of this research. The influence of the residualflux to the inrush current was analyzed using the Magnetic Simulation software.The mathematical analysis of the equivalent circuit on the toroidal transformer modelwas conducted. The size of the closed magnetic core structure parameters and the materialcharacteristics were defined. Change of the DC source loaded on the winding of thesimulation model that may induce magnetic flux was regarded as the residual flux. Thefixed DC voltage energization which loaded on the measurement winding of the simulationmodel with positive and negative direction are respectively applied to the transformersimulation model under multiple sets of residual magnetic flux density. Software Magnetwas utilized to do the transient simulation analysis. Therefore the variation of magnetizingcurrent can be measured at the transient switch-on process. The corresponding relationshipof residual flux density and the average value of the transient magnetizing current is obtained by curve-fitting.A toroidal transformer magnetic core was used for experimental testing. Results showthat the relative error of the measurement results and the preset residual flux is within7%.This figure suggested that the detection method of the residual magnetism in this study ishighly accurate.Different demagnetization methods were studied. On the basis of the known residualflux, the optimal demagnetization method was determined by simulation and validated byexperimentation. Findings of the measurement results and the simulation demonstrated thatthe detection method proposed in this study is feasible and effective.