Research On Finite Set Predictive Current Control For Non-isolated Three-level Grid-connected Inverter

With as a key interface device of solar photovoltaic panels and public power grids,the selection of photovoltaic control strategy and the performance of output filter will directly determine whether the output power of inverter meets the grid-connected standard.With the shift of solar PV grid-connected power generation technology toward large-scale,high-voltage and high-power,traditional two-level topologies are difficult to adapt to such grid-connected environment due to the limitation of device withstand voltage.Compared with the traditional two-level grid-connected inverter,the three-level inverter topological power device have the advantages of withstanding lower voltage,smaller distortion of output current,lower switching frequency and smaller electromagnetic interference.Under the conditions of high voltage,high power Grid occasion has been widely used.However,there are more switching devices and control targets in the three-level PV grid-connected inverter.In order to improve the performance and efficiency of the PV grid-connected inverter,an improved finite set model predictive control strategy for this topology is proposed.Compared with the traditional pulse width modulation strategy,this scheme does not need complicated controller parameter tuning,and easier to achieve multi-objective optimization control.The main contents of this paper are as follows:(1)Based on the existing literature,the paper is analyzed the main topological structure of three-level grid-connected inverter and their respective characteristics.By using the symmetrical component method and coordinate transformation,a mathematical model of T-type three-level photovoltaic inverter is established in two-phase stationary coordinate system.The commutation process and working principle are introduced in detail.(2)The main control technology of three-level photovoltaic grid-connected inverter is introduced.Based on the analysis of the basic principles of model predictive control,a discrete prediction model of T-type three-level photovoltaic inverter is established.An effective solution is provided to solve the delay and the conservation problems of traditional model predictive control(3)Aiming at the inherent midpoint potential imbalance of T-type three-level grid-connected inverter,the improved finite set model predictive control is analyzed.The system index function is improved,the switch state with the lowest deviation of the midpoint potential is preferentially selected to be combined with the inverter,and the schematic diagram and the flow chart of the integrated control system are given.(4)Non-isolated T-type three-level grid inverter will have a large common-mode voltage.The improved model predictive control is used to suppress the common mode voltage.On the basis of guaranteeing the neutral point potential balance control of the T-type three-levelgrid-connected inverter,the system index function is optimized,and the common mode voltage suppression target is added to the index function.The optimal output of the inverter is realized by adjusting the weight factors of the control targets in the comprehensive optimization index function.Through the simulation,the correctness and effectiveness of the proposed control strategy are verified.