Research On Control Strategy Of Three-level Grid-connected Inverter Under Unbalanced Grid

With the gradual shortage of fossil energy and the continuous aggravation of environmental problems,people’s calls for clean energy and environmental protection have become more and more extensive.Distributed power generation has been widely used because it mainly adopts renewable energy.The distributed power source is connected to the grid through the inverter,but it is often unstable,so because the inverter connects a large number of distributed power sources and non-linear loads to the grid,the probability of grid unbalanced conditions increases significantly.The disturbance of the grid-connected current affects the grid-connected operation of the inverter.Therefore,under the unbalanced power grid,in order to ensure the stable operation of the grid-connected inverter under various working conditions,it is of great significance to study the grid-connected control strategy of the inverter.Firstly,this paper takes the diode-clamped three-level grid-connected circuit as the research object,and establishes the switching model of the LCL three-level grid-connected inverter.Considering the actual grid state,establishes a mathematical model of three-level grid connected inverter in three-phase abc static coordinate system,two-phase αβ static coordinate system and dq synchronous rotating coordinate system,and deduces decoupling control of dq synchronous rotating coordinate axis and no decoupling control model of two-phase αβ static coordinate axis.Then,the basic principle of three-level space vector width modulation(SVPWM)based on 60° coordinate system is deduced,and the optimization scheme of neutral point voltage fluctuation of NPC inverter is given on the basis of 7-segment time distributionSecondly,in order to successfully connect the inverter to the grid under the unbalanced grid,the system needs to be able to lock the phase quickly and accurately,and detect the positive and negative sequence components of the voltage.By analyzing the working principles of the traditional synchronous reference frame phase-locked loop(SRF-PLL)and the double second order generalized integrator phase-locked loop(DSOGI-PLL),the shortcomings of the two when the voltage is unbalanced are obtained.Accordingly,based on the structure of the secondorder generalized integrator,a new phase-locked loop method(HFESOGI-PLL)that can suppress DC offset,frequency fluctuation and harmonic distortion is further proposed in this paper,and the structure principle and parameter design of the method are analyzed in detail.Thirdly,when the grid voltage has negative sequence components or harmonic content,the grid voltage and grid-connected current will be distorted to a certain extent.At this time,the traditional control method in the current loop cannot eliminate the influence of voltage distortion.Therefore,the feedforward function of grid-connected inverter is derived in this paper,and the voltage disturbance is equalized by feedforward strategy.In order to further realize the non-error tracking of current signals,the grid-connected current is controlled by repetition control strategy,and the solution of compensator in repetition controller is simplified based on H∞ theory.Based on this,the standard H∞ structure of the current controller is constructed,and the stable constraint conditions of the closed-loop system are obtained by constructing H∞ norm.Finally,a three-level NPC inverter model under unbalanced power grid is built,and various power grid states are simulated and tested through Matlab/Simulink platform,which verifies the feasibility of the proposed synchronization signal extraction method under unbalanced power grid.At the same time,A comparative experiment is carried out with gridconnected current repetitive control strategy and the PR control strategy based on feedforward strategy under the balance condition and unbalance condition of grid voltage,which verifies the feasibility and effectiveness of the proposed control scheme.