Research On The Control Strategy Of Three-phase Photovoltaic Grid-connected Inverter Under Unbalanced Grid Voltage

Most researches of three-phase grid-connected inverter are based on the idealgrid. However, the power grid will appear unbalanced fault in exceptionalcircumstances. At this time, the three-phase grid inverter will be affected. The outputcurrent harmonic content is high. The grid will be polluted. And in severe cases,grid-connected inverter will not work, even damage to the components. Therefore it isessential for the study of the three-phase grid-connected inverter control when gridunbalanced fault occurs.Firstly, the detailed study of the three-phase grid-connected inverter workingprinciple under the balanced grid has been done in this paper. Secondly, in-depthresearch of the three-phase inverter under the unbalanced grid has been discussed.Finally, the control strategies of three-phase grid-connected inverter under unbalancedgird are proposed, including the positive and negative sequence component separationcontrol algorithm, the proportion and resonance control (PR) algorithm controlalgorithm and quasi proportion resonance control (quasi-PR) algorithm. Unbalancedvoltage is decomposed into the balanced positive-sequence component andnegative-sequence component. So the unbalanced grid is equivalent to balanced grid,which makes the control algorithm become simply. The notch filter is designed and PIcontroller parameter expressions are given in the paper. Bode plots and frequencyresponses of the PR controller and quasi-PR controller have been analyzed carefully.The overall control strategies are given combined with the controllers.The mathematical model of the three-phase photovoltaic grid-connected inverteris created under the conditions of the balanced grid and unbalanced grid respectively.And the matlab\simulink model of the three-phase photovoltaic grid system based onthe mathematical model is built. The simulation of the three control algorithms forpurpose of eliminating the negative sequence current in grid side has been done.Simulation results show that the output currents are able to track the grid voltageeffectively under the three control strategies. And current total harmonic distortioncan meet the requirements. In the last, an experimental prototype has set up to achieve control algorithm program implementation. The experimental results verify theeffectiveness of control strategies. The output current THD is2.75%.