| With the continuous consumption of coal,oil and other primary energy sources,the energy crisis and environmental pollution caused by a series of problems have forced people to pay more and more attention to the development and utilization of renewable energy.The grid-connected converter is the connection part between the renewable energy and the grid,and its working state determines the quality of the output current.In order to suppress the switching harmonics of the power devices of the gridconnected converter,it is necessary to add a filter on the output side of the converter.The LCL filter is widely used because of its low cost,small size,and good highfrequency harmonic attenuation ability.The magnetic powder core material has soft saturation characteristics.In order to improve the utilization of the magnetic core,the inductors designed based on this material usually have strong nonlinear characteristics,and the inductance value changes with the change of the operating current.For a three-phase grid-connected converter,the three-phase current changes sinusoidally over time,which in turn causes the corresponding inductance value to change in the same way.In the two-phase static coordinate system,the time-varying asymmetric characteristics of the filter inductance of the three-phase LCL grid-connected system will cause cross-coupling between the control system models,which will complicate the system resonance characteristics and stability analysis.The stability design of current loop control may fail.For this reason,considering the soft saturation characteristics of the magnetic powder core inductance,it is very important to study its influence on the stability and output power quality of the three-phase LCL grid-connected converter.When the filter inductor is a linear inductor,this article first establishes the mathematical model of the three-phase LCL grid-connected converter in different coordinate systems,and analyzes the use of the converter side current single loop feedback control and the grid side current single loop The design basis of the stability domain and current controller parameters of the system during feedback control is verified by simulation to verify the correctness of the theoretical analysis.Secondly,under the constraints of the soft saturation characteristics of the magnetic powder core inductance,since the three-phase inductance varies with the current interleaved,in the two-phase stationary coordinate system,the control system model changes from two independent single-input single-output models to multiple models with cross-coupling characteristics.Enter a multi-output model,and the resonance characteristics are also derived from a single resonance point to two different resonance points.Through the generalized Nyquist stability criterion,the new constraints of the filter resonance frequency design and the design principles of control parameters under the single-loop control of the grid side and converter side current are respectively given.Finally,a simulation model of magnetic powder core inductance with nonlinear characteristics is built,and the correctness of the theoretical analysis is verified through simulation and experiment.In the Matlab software,through data fitting,the relationship between the initial permeability and the inductor current is fitted into a fourth-order polynomial form,and the inductor current is expanded through the Fourier series to see that the output current of the grid-connected converter is Containing odd harmonics,theoretical analysis shows that the non-linearity of the inductor core is the cause of the low frequency odd harmonics in the output current of the grid-connected converter.A method of adding a harmonic compensator to the original current controller is proposed to suppress the harmonic components in the output current.Simulation and experimental results verify the effectiveness of the analysis and the proposed method. |
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