| With the popularization and application of distributed generation technology,the influence of distributed generation system on the power system becomes more and more significant.On the one hand,the LCL grid-connected inverter is a key interface between the distributed generation system and the utility grid.It also introduces high-frequency switching ripple while achieving efficient power transmission.On the other hand,due to the distributed generation system and here is a dynamic interaction of impedance between power systems,which may cause a series of harmonic and oscillation problems in the system,which threatens the stable operation of the entire power system.Therefore,it is necessary to design an excellent control strategy to enable the grid-connected inverter to quickly respond to system harmonics and oscillations to improve the reliability of the grid-connected operation of the distributed generation system.This paper takes three-phase LCL grid-connected inverter as the research object.The main contents and innovations are as follows:(1)The positive and negative sequence impedance model of three-phase LCL grid-connected inverter is obtained by harmonic linearization modeling method,and the positive and negative sequence impedance characteristics of the inverter are analyzed in detail.From the obtained impedance analysis expression,it can be known that the output impedance of the inverter is not only related to the topology of the main circuit and the control strategy adopted,but also th e system parameters such as the bandwidth of the current controller,the bandwidth of the phase-locked loop and the LCL filter parameters will be applied to the inverter.The output impedance has an effect.The correctness of the analytical expression of t he positive and negative sequence impedance of the three-phase LCL grid-connected inverter is verified by MATLAB simulation platform.The relationship between the parameters of each system and the output impedance of the inverter is analyzed in depth.(2)The impedance stability criterion of interconnected systems is analyzed from the perspective of sequence impedance and the impedance stability criterion is extended to the three-phase LCL grid-connected inverter system.The Nyquist diagram and impedance of the system under different grid impedance conditions are studied.The comparative analysis diagram reveals the dynamic interaction process between inverter impedance and grid impedance and the oscillation mechanism of the system.The effects of different current controller parameters and phase-locked loop bandwidth and LCL filter inductance on system stability are analyzed from the perspective of sequence impedance.It provides a theoretical guide for the parameter selection of the grid-connected system.The correctness of the theoretical analysis of impedance stability is verified by simulation.(3)Aiming at the harmonic problem and oscillation phenomenon caused by the dynamic coupling of inverter impedance and grid impedance in the distributed generation grid-connected system,a method of damping resonance suppression and fast power adjustment of LCL inverter is proposed.The method is mainly composed of synchronous reference system proportional proportional integral control and robust grid-connected current feedback active damping control.The working principle of the control method is analyzed in detail,and the steady state and dynamic performance of the system are considered comprehensively.The parameter optimization design method of the controller is proposed.Compared with the traditional active damping control method,the proposed control method can suppress the system oscillation phenomenon more effectively and improve the power adjustment capability of the system.The superiority of the control strategy proposed in this paper compared with the traditional active damping control strategy is verified by the MATLAB simulation platform and the three-phase LCL photovoltaic grid-connected experimental platform. |
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