Research On Magnetic Saturation And Excitation Inrush Current Suppression Strategy Of Series Transformer Between Microgrid And Distribution Network

Construction of AC-DC hybrid microgrid is an effective way to improve the energy efficiency of distributed utilization of new energy.However,when AC/DC hybrid microgrid is connected to the grid,it will be affected by large grid conditions,in which large grid voltage fluctuations will not only affect the power supply quality of the load in the microgrid,but also threaten the safe operation of its power electronic devices.In order to solve this problem when AC-DC hybrid microgrid is connected to the grid and improve the power quality and power supply reliability of the microgrid,a voltage compensator is introduced between the distribution network and the AC-DC hybrid microgrid.Voltage compensation must have a high response speed,and the rapid output voltages,the voltage compensator series transformer connected to the electricity grid in the susceptible to dc bias phenomenon,when accumulated to a certain level of the dc bias magnetic saturation phenomenon will lead to the transformer core,resulting in a lot of excitation inrush current,which not only affects the normal operation of the voltage compensation link,Moreover,it will increase the power dissipation of the transformer itself and lead to the temperature rise of the transformer,accelerate the insulation aging of the transformer windings,and even burn down the transformer due to thermal breakdown.Therefore,how to restrain the magnetic saturation phenomenon of series transformer during compensation has become an important problem of this device.In this paper,the series compensation link between large power grid and AC/DC hybrid microgrid is taken as the research object,and a control strategy with transformer flux as the state variable is proposed to suppress the DC bias and magnetic saturation phenomenon in the transformer during voltage compensation.The main research contents are as follows:Firstly,the research status of AC-DC hybrid microgrid at home and abroad,the damage of voltage sags between microgrid and distribution network,the damage of magnetic saturation of core in transformer when compensating voltage sags,and the research status of DC magnetic bias detection and suppression in transformer at home and abroad are introduced.The concrete structure and operation mode of the series voltage compensation link are introduced,the appropriate filter structure is selected to filter the output compensation voltage,and the operation mode and principle of the series voltage compensation link are analyzed.Secondly,several current commonly used voltage detection methods were investigated,and the advantages and disadvantages of different voltage detection methods in real-time detection of voltage changes were compared and analyzed.Based on the instantaneous reactive power theory,the instantaneous voltage D-Q detection method was deduced in detail.The voltage and current in the three-phase static coordinate system are converted into the voltage and current in the two-phase rotating coordinate system by coordinate transformation,and the formula to reflect the voltage phase change and voltage amplitude change in real time is derived,and the method to detect voltage imbalance by establishing the virtual three-phase voltage is analyzed.Two kinds of voltage fluctuation compensation methods are compared and analyzed.Then,based on the double closed-loop voltage compensation strategy,the causes of magnetic saturation and excitation inrush current in the transformer are analyzed according to the magnetization curve of ferromagnetic materials,and the size of magnetic flux in the transformer under two critical conditions is analyzed.Based on T equivalent circuit of transformer in series with the voltage compensation link mathematical model is established,based on this model,this paper proposes a direct inhibition of transformer magnetic flux as state variables transformer dc bias magnetic and magnetic saturation of the control method,analyzes the control strategy of different links in the process of the whole control effect and influence.Finally,the proposed method was validated in ANSYS Simplorer/ MATLAB/ANSYS Maxwell simulation environment.The effectiveness of the proposed strategy for the DC magnetic bias suppression of the series compensating transformer between the micro-grid and the distribution network is verified by the different voltage sag depth and sag time length caused by the faults on the side of the distribution network.