Research On Control Strategy Of Three-level Grid-connected Inverter Under Non-ideal Grid Conditions

With the continuous innovation of energy technology,large-scale development of new energy has gradually become a world trend of development.The main way to develop new energy is distributed generation based on renewable energy.Distributed generation system is a decentralized power supply system based on load center.Inverter plays a very important role as the interface between power generation system and power grid.Three-level grid-connected inverters are more and more widely used in power generation system.Because there are a lot of harmonics in the power grid or there may be grid faults and other phenomena,with the continuous access of distributed generation system to the power grid,its power quality requirements are also getting higher and higher.Therefore,under the condition of non-ideal power grid,in order to meet the relevant requirements of power grid operation,it is very important to study the control strategy of three-level grid-connected inverters.Firstly,the three-level switching function is introduced.In the dq coordinate system,the mathematical model of LCL NPC three-level inverters under the ideal condition of power grid is established.Because there are coupling terms in the system model under the dq coordinate system,the control becomes complicated.Therefore,the decoupling analysis is carried out by using the equivalent principle,taking the d-axis as an example.When the power grid is in an imperfect condition,there are a lot of harmonics in the power grid,and three-phase voltage contains positive and negative sequence components.In this paper,the method of separating positive and negative sequence is used to establish a mathematical model under the DQ axis.Secondly,the traditional SVPWM method for three-level inverters is studied in this paper.Because the traditional method is very complex in judging the size of the synthesized vector sector,in order to solve the problem of complex calculation,this paper adopts the SVPWM method based on non-orthogonal g-h coordinate system,which makes the judgment of the size of the sector more simple and simplifies the related calculation.At the same time,the memory of NPC inverters is saved.The corresponding control scheme is given for the neutral point potential problem,and a 7-segment modulation algorithm is adopted.Because the traditional phase-locked loop can not accurately lock the phase in the non-ideal state of the power grid,this paper adopts a phase-locked loop based on decoupled double synchronous reference frame(DDSRF-SPLL),which can accurately synchronize the power grid by decoupling the negative sequence voltage component on the positive sequence component.In order to suppress the resonance problem of LCL system,the active damping method based on state variables is used to derive the equivalent block diagram and mathematical model of the active damping of parallel capacitors of LCL inverters.On the basis of the previous related control strategies,in order to restrain the influence of harmonic and unbalanced components of grid-connected current on grid-connected current,this paper adopts the full feed-forward control strategy of grid voltage,and derives the equivalent block diagram and mathematical model of the full feed-forward strategy of grid voltage.Finally,this paper builds a simulation model on MATLAB/Simulink and carries out simulation experiments.And then designs an experimental prototype of 5 kW grid-connected inverters with DSP28335 as the main control chip on the power electronics experimental platform.The simulation results and experimental results verify the effectiveness of the relevant control strategies.