Research On The Controlof Grid-connected Three-level PV Inverters Under Unbalanced Grid Voltage

The PV grid-connected power system plays an important role in thefield of new energy because of its unique advantages. In this thesis,control method of the PV inverter is studied in detail. Currently, most ofstudies about control method of the PV inverter have a basic premise thatthe grid voltage is balanced. However, when the grid voltage is unbalanced,inverter AC’s side current and DC’s side voltage will have harmonic. Inthis respect, do research about inverter’s control method under unbalancedgird voltage is great of significance for the PV grid-connected powersystem security and stability.In this thesis, it introduces inverter’s work principle take three-levelNPC inverter for example firstly, and then it establishs the mathematicalmodel of three-level inverter under unbalanced gird voltage. Based on the analysis of PV system’s power flow the control method is designed in thepositive and negative sequence independently rotating coordinates. Twocontrol methods are compared and analyzed: considering the averagepower control method and considering the average power and its secondharmonic component control method.In addition, several key issues are studied: there is a brief descriptionof inverter’s SVPWM control and VSVPWM control which suppressthree-level inverter’s neutral point voltage fluctuation; base on the analysisand comparison of Notch Method and Delay Method, a new method calledMode Detection Method is proposed in order to simplify the calculation ofthe positive and negative sequence component.At the end, the control methods are simulated by Matlab/Simulink.Simulation results show that: using unbalanced control method,harmonics of inverter AC side’s current and DC side’s voltage are greatlyreduced and achieve the desired control objectives.