Research On Vibration And Loss Characteristics Of Nanocrystalline Core Medium Frequency Transformer

At present,China’s wind power generation and photovoltaic power generation industry ranks first in the world,and wind farms and photovoltaic power stations are all over the inland and coastal areas of China.The DC power grid and large-scale distributed renewable energy power generation system put forward great demand for high-voltage and large-capacity DC-DC converters and solid-state transformers.High power medium frequency transformer is the core component of high voltage and large capacity DC-DC converter and solid state transformer,and its performance directly determines the performance of DC-DC converter and solid state transformer.With the increasing demand of large-scale megawatt DC source connected to the power grid and wind power plants connected to the grid,the power grid also puts forward higher requirements for transformers:high power density,miniaturization and light weight.Increasing frequency has become the most feasible way for transformers to meet these requirements,so the use of high-power medium frequency transformers in power systems is becoming more and more widely.With the increase of transformer working frequency,the vibration and loss problems will affect the surrounding environment,and long-term vibration will accelerate the aging of equipment and threaten the safe and stable operation of power system.Therefore,the problem of vibration and loss of transformer needs to be solved urgently.In this paper,firstly,an experimental measurement platform for vibration and loss of nanocrystalline iron core is built,and the vibration of magnetic ring under different excitation waveforms is analyzed experimentally.Secondly,the electromagnetic vibration mechanism between nanocrystalline tapes is analyzed by means of finite element method.Because the structure of a single magnetic ring core is simple,the influence of external clamps and windings on the vibration of transformer core is not considered,so this paper then designs two nanocrystalline core medium frequency transformers with different topological structures,considers the influence of clamps on vibration,and analyzes them based on the method of magnetic-structural field coupling numerical simulation and experimental verification.Finally,because the core will produce vibration and noise in the process of experimental measurement,its temperature rise changes obviously,and the main source of temperature rise is the loss of the core.Therefore,this paper combines the loss separation method with Steinmetz empirical formula and analyzes the loss of the core under non-sinusoidal excitation in detail.The specific research contents of this paper are as follows:(1)Aiming at the vibration problem of nanocrystalline magnetic ring under different waveform excitation.In this paper,an experimental platform for measuring the vibration and loss characteristics of nanocrystalline magnetic ring is built,and the vibration of nanocrystalline magnetic ring excited by sine wave,triangular wave,symmetric rectangular wave and asymmetric rectangular wave is measured respectively,and the vibration characteristics at different frequencies and different duty cycles are analyzed in detail.Due to the particularity of the laminated structure of nanocrystalline magnetic ring,the magnetostrictive coefficient is calculated by measuring the radial acceleration of nanocrystalline magnetic ring by magnetostrictive inversion method.The calculation results show that with the increase of excitation frequency,the skin depth will be reduced due to the eddy current effect,which will lead to the reduction of magnetostrictive coefficient.(2)Aiming at the vibration problem of medium frequency transformer with nanocrystalline core.In this paper,firstly,the microstructure of nanocrystalline ribbon is simulated by finite element method,and the electromagnetic vibration mechanism between layers and air gaps of nanocrystalline ribbon with and without air gaps is analyzed.Secondly,the three-dimensional transient magnetic-structural field coupling analysis and calculation model of medium frequency transformer is established,and the vibration acceleration and displacement of iron core under no-load condition are calculated by finite element method.Through the comparison of theoretical calculation and experimental results,it is confirmed that the Maxwell force at the air gap surface of the core is the main cause of vibration,and the maximum Maxwell force density of the shell-type medium frequency transformer(25257 N/m~2)is greater than that of the core-type medium frequency transformer(20936 N/m~2).The maximum acceleration of shell-type(40m/s~2)is greater than that of core-type(30m/s~2)and the error of numerical simulation is within 10%.(3)Aiming at the loss problem of nanocrystalline magnetic core under the condition of medium and high frequency non-sinusoidal excitation.In this paper,the hysteresis loops of sinusoidal and non-sinusoidal excitation waveforms at different frequencies and different duty cycles are measured by experimental equipment,and the corresponding losses are calculated according to the hysteresis loops.Secondly,the static hysteresis loss and dynamic loss are analyzed by the method of loss separation and Steinmetz empirical formula,and the static hysteresis loss is analyzed by the analytical Preisach hysteresis model based on Lorentz function considering reversible components.Finally,according to the magnetization rate of the rising and falling segments of the hysteresis loop,the equivalent relationship between the dynamic loss term under non-sinusoidal excitation including symmetric and asymmetric rectangular waves and sinusoidal excitation is established,and the calculation expression of the dynamic loss term under symmetric/asymmetric rectangular wave excitation is deduced and analyzed.