Bifurcation Control Of Three-phase Three-level NPC Grid-connected Converter Based On SPWM

Three-level grid-connected converter,as an important connection unit between new energy and large power grid,belongs to a typical power electronic device.The presence of the power switch makes the system exhibit strong nonlinear characteristics,causeing it to appear nonlinear phenomena such as bifurcation and chaos,which seriously affects the safe and stable operation of the system.Therefore,this paper deeply studies the bifurcation phenomenon of the three-level grid-connected converter and controls it,aiming to optimized the control parameters of system,avoid the bifurcation phenomenon of the system,and improve the reliability of the system.First of all,in this paper,the mathematical model of main circuit topology,sinusoidal pulse width modulation(SPWM)and current decoupling control of the Diode Neutral pointed Clamped(NPC)three-level grid-connected converter system are established by stroboscopic sampling method.According to the discrete mathematical model established in this paper,its numerical model program is designed.Based on the numerical model,the bifurcation boundary of system proportional controller coefficient kp,integral controller coefficient ki,and the grid-side filter inductance L,switching frequency fs in the circuit parameters is designed using the stability analysis method of bifurcation graph method and Jacobian matrix method.It is concluded that there are two bifurcation phenomena of high-frequency and low-frequency Hopf in the NPC three-level grid-connected converter system.In addition,this paper also analyzes the correctness of the designed bifurcation boundary through numerical simulation based on the folding graph method,and builds a circuit simulation model of the NPC three-level grid-connected converter system in PSIM software to verify the accuracy of the discrete mathematical model and the bifurcation boundary.According to this,the main circuit parameters and system controller parameters are optimized.Secondly,this paper designs a Time Delay Feedback Controller(TDFC)to eliminate the bifurcation phenomenon caused by the system due to environmental interference,improper parameter setting and other factors.By establishing the discrete mathematical model of the NPC three-level grid-connected converter system based on TDFC,the parameters of the TDFC controller are designed using the stability analysis method of bifurcation diagram and Jacobian matrix,and the numerical simulation based on the folding graph and Psim circuit simulation analysis are used.Verify the correctness and effectiveness of the parameters design of TDFC.Finally,on the experimental platform of the NPC three-level grid-connected converter with a power level of 7.5kW,a bifurcation control software program is designed to complete the bifurcation analysis of the NPC three-level grid-connected converter and the experiment of bifurcation control experimental results show the correctness of the control strategy in this paper.