Research On Grid-connected Inverter Stability Based On Current Error Signal Compensation In Weak Grid

In recent years,among new energy generation technologies,photovoltaic power generation,which is rich in resources,clean and environmentally friendly,and has large-scale of commercial development value,has received extensive attention and has been vigorously developed and applied in the power industry.In actual PV grid-connected system,the equivalent impedance of the grid cannot be ignored due to the effect of long-distance transmission and multi-stage low-power transformers,and there are a large number of low-order background harmonics in the grid,resulting in weak grid characteristics.The weak grid characteristics will have a serious impact on the grid current quality and system stability of the grid-connected inverter.In addition,as a output filter device of the inverter,the LCL-type filter has been widely used in grid-connected inverter due to its superior filtering characteristics in high-power system,as well as it has the advantage of small size and low cost.However,due to LCL-type filters has its own inherent resonant characteristics,it is necessary to take related resonance suppression measures.In this thesis,the three-phase LCL-type grid-connected inverter in the weak grid is the research object.Based on the traditional grid current feedback combined with the grid voltage proportional feed-forward control method,a closed-loop model of the system is established to clarify the resonance mechanism of the system.In order to achieve the purpose of resonance suppression and improve the system stability in weak grid,according to the comparison of frequency characteristics between LCL-type and L-type filter with equal inductance,this paper proposes a new control strategy based on the current error signal compensation,that is,feedback error signal of the actual grid current and the estimated grid current to the control loop to suppress resonance.Simultaneously,this paper introduces the design of the control parameters of the system under the proposed strategy,and from the perspective of frequency characteristics and impedance-based stability criteria,respectively,the control performance and stability of the system in weak grid are analyzed.In this thesis,the MATLAB software is used to verify the resonance suppression effect of the proposed control strategy based on the feedback of current error signal in ideal grid and weak grid.Simultaneously,in order to further verify the practical feasibility of this control strategy,A three-phase LCL-type grid-connected inverter experimental platform with the rated capacity of 5kW is set up in the laboratory.The control chip used in the experiment is TMS320F28335,and a three-phase voltage regulator connects a group of magnetic inductances in series is used to simulate weak grid.The system stability improvement effect of the proposed control strategy in different grid environments is verified by experiments.The experimental results prove the correctness and practical feasibility of theoretical analysis and simulation verification of this paper.