Research On Control Strategy Of LCL Three-phase Grid-connected Inverter In Non-ideal Grid

Due to the intensification of traditional non-renewable energy consumption and environmental pollution,the demand for clean new energy and environmental protection is increasing.Distributed power generation has gradually become new power supply method due to its low consumption,high efficiency,and flexible application of clean energy.The gridconnected inverter is the connection unit between clean energy and the grid.When the distributed power is connected to the public grid through the inverter,the voltage disturbance of the grid will affect the current quality of the system output;in addition,weak grid conditions also affect the stability of the converter.Therefore,the research on the current loop control strategy when the grid-connected inverter is connected to the non-ideal grid is an indispensable link to ensure its efficient and stable operation.This article takes LCL type three-phase grid-connected inverter as the research object.The mathematical models of the grid-connected inverter are established in the abc three-phase stationary coordinate system,the αβ two-phase stationary coordinate system and the dq synchronous rotating coordinate system,respectively.The principle of feedback active damping resonance suppression.The interference of control delay on resonance suppression and inverter stability is described,and the theoretical method for the selection of active damping coefficient and controller parameters is given.It’s simulation proves the effectiveness of active damping’s resonance suppression and system control parameter design.In order to improve the output power quality of the grid-connected inverter in the presence of harmonics in the grid voltage,from the perspective of reducing input disturbance.Analyze the dual second-order generalized integral phase-locked loop(DSOGI-PLL),and propose a frequency adaptive synchronization detection method based on multiple generalized integrators.It can effectively obtain the grid angle to achieve precise phase lock in the case of a grid failure,and detect the positive and negative sequence components of any low-order harmonic voltage.Aiming at the traditional grid voltage proportional feedforward method,the control method based on the grid voltage full feedforward is derived.It is pointed out that its essence is to increase the amplitude of the inverter output impedance and reduce the influence of grid disturbance on the grid current.At the same time,it is pointed out that due to the existence of digital control delay,the inverter output impedance cannot be infinite in the whole frequency range,and may have a certain amplification effect on high-frequency harmonics.However,the background harmonics of the grid voltage are usually concentrated in the low frequency band.Full feedforward still has good practical application significance.Under weak grid conditions,the impedance of the grid cannot be ignored.The output impedance characteristics of the inverter under the full feedforward control mode of the grid voltage are analyzed.Based on impedance analysis,the stability of the grid-connected inverter using grid voltage feedforward is compared with that without feedforward control.It is pointed out that in the case of a weak grid,the inverter may not be able to work stably when adopting the full feedforward control of the grid voltage.Therefore,in order to satisfy the high-quality power output of the three-phase grid-connected inverter under non-ideal grid conditions,a weighted feedforward control method based on specific sub-components of the grid voltage is proposed,which can not only suppress the inverter output current harmonics,but also improve the robustness of grid-connected inverters under weak grid conditions.In order to prove the feasibility of the narrative control theory,the simulation and the establishment of an experimental platform are used to verify the theoretical analysis of this article.