Research On The Control Strategy Of Photovoltaic Grid-connected Inverter Under Grid Voltage Imbalance

The control problem of grid-connected inverter under the condition of grid voltage imbalance is one of the research hotspots in photovoltaic grid-connected technology.Since the grid-connected inverter control strategy under grid voltage imbalance cannot meet the control requirements under voltage imbalance,the inverter output current and voltage distortion will be caused,the harmonic content will increase sharply,and the inverter output will be the active power and reactive power produces a double frequency fluctuation.In this paper,the current distortion and output power fluctuation of the inverter under the voltage imbalance of the grid are studied.The corresponding control strategy is proposed,which finally achieves the effect of suppressing current distortion and power fluctuation.The main research contents are as follows:Firstly,the advantages and disadvantages of different topologies of inverters and engineering practice applications are analyzed.Two-stage three-phase full-bridge inverters are selected as the research object of this paper.Based on the topology,the mathematical model of the photovoltaic grid-connected system is established in three coordinate systems.Through comparative analysis,it is found that the mathematical model of the two-phase stationary coordinate system is more suitable for the control strategy under the grid voltage imbalance.Secondly,aiming at the problem of inverter output current distortion under grid voltage imbalance,a harmonic current suppression strategy based on proportional reduced-order resonance is proposed.The inverter output current harmonics under the grid voltage imbalance are analyzed under ?? stationary coordinates.The fundamental wave and harmonic control branch of the current signal are respectively constructed by the proportional reduced-order resonant controller,and the fundamental control branch realizes the tracking control of the power frequency fundamental wave.The harmonic control branch has no tracking control for 3th,5th and the 7th harmonic,so as to achieve the suppression of harmonic current;and verify its feasibility and effectiveness through simulation studies.Thirdly,aiming at the problem of output power fluctuation of grid-connected inverter under grid voltage imbalance,a power fluctuation mitigation strategy based on positive sequence voltage is proposed.By analyzing the output power of the inverter under the imbalance of grid voltage,the expression of power fluctuation component is derived,and the correlation law between negative sequence voltage and power fluctuation is obtained.The reduced-order resonant controller is used to eliminate the negative sequence component in the sampling voltage to achieve the effect of eliminating the fluctuation component in the power.The simulation results show the feasibility of the control strategy.