Research On Harmonic Current Optimization Control Of Three-phase Four-wire Grid Connected Inverter

Under the dual pressures of environmental protection and depletion of fossil fuels,distributed power generation technologies and micro-grid technologies have made great strides.However,the integration of large-scale new energy sources has brought a series of challenges to the traditional power grids.In addition,the extensive application of new power electronic devices,such as charging pile,inverter and single-phase non-linear load,the power quality problems faced by distribution networks are more severe.To solve this problem,a multi-inner-mode compound control strategy for three-phase four-wire inverter is proposed in this thesis,to enhance the controller bandwidth and optimize the inverter’s tracking characteristics of harmonic currents.At the same time,according to the principle of energy balance,the reference value of the grid-connected inverter current is calculated.The grid-connected inverter can not only comprehensively manage the power quality of the distribution network,but also can feed back the excess power on the DC side to the distribution network and supply power to the load.Firstly,this thesis confirms that the three-phase four-wire inverter is a four-leg inverter structure,analyzes its working principle and establishes a mathematical model under the dq0 coordinate system,and the large-signal model of inverter is given.In order to improve the compensation performance of harmonics,this thesis applies the non-harmonic detection method based on the energy balance principle to calculate the grid-connected reference current,and decouples the d axis and the q axis by using the feed-forward decoupling strategy.And the double closed-loop control structure diagram of the inverter is given.In addition,considering the distortion and imbalance of the grid voltage,the fundamental positive sequence voltage detector is adopted to extract the grid voltage fundamental positive sequence component,for getting the accurate current compensation command.Secondly,the compensation performance of the three-phase four-wire grid-connected inverter depends on the design of the current loop controller.To quickly and error-freely track the reference current command,the controller’s bandwidth must be improved.This article proposes a composite control strategy that combines the advantages of PI control,double resonance control and repetitive control.Using PI control to increase control speed,double resonance control and repetitive control to increase controller bandwidth and accuracy,and the simulation shows that the tracking characteristics of composite control strategy for harmonic current.Finally,a simulation model of three-phase four wire grid connected inverter is built by PSIM,and carried out in three cases under ideal and non ideal conditions and DG connection.The simulation results verify the effectiveness of the proposed hybrid control strategy in optimizing harmonic currents,and the flexibility of three-phase four-wire inverter.At the same time,the three-phase grid current can be balanced and the distortion rate can be reduced.The neutral current is approximately 0,and the DG can be connected to the grid.In addition,an experimental device is build and the compound control strategy is used to compensate the load current,the experimental results verify the ability of the compound control strategy to track and control the grid current.