Research On Optimal Control Of Three-phase Photovoltaic Grid-connected Based On New Quasi-Z Source Inverter

With the reduction of petrochemical energy storage and the environmental pollution caused by petrochemical energy in the power generation process,new energy power generation plays an increasingly important role in modern power systems.It is one of the research hotspots of new energy power generation.In this thesis,the inverter,control loop and filter circuit of photovoltaic grid-connected system are deeply studied.The main research contents of the thesis are as follows:Firstly,in view of the low power level of the two-level Z-source inverter,many current and voltage harmonics,poor output power quality,and high capacitor voltage stress of the Z-source NPC three-level inverter,and its boost capability is insufficient.Therefore,a new type of quasi-Z source NPC three-level inverter is designed in this thesis,which can improve the boost capability of the inverter and reduce the capacitor voltage stress.Secondly,due to the influence of external environmental factors,the photovoltaic power generation system has the characteristics of nonlinearity,time variability and uncertainty,and its maximum power point tracking needs to be studied.Existing MPPT algorithms have certain deficiencies in convergence accuracy or convergence speed.In this thesis,the intelligent search for swimming bacteria foraging algorithm is used to conduct the maximum power point tracking research of photovoltaic cell arrays.The algorithm enables the bacteria to detect the fitness of the next position in advance.If it is better than the current moment,it will continue to swim;otherwise,it will stop swimming.This can avoid swimming to a bad environment,greatly speed up the running speed of the algorithm,improve the rapidity,and improve the stability of the running process.Thirdly,aiming at the response speed of the photovoltaic grid-connected system to changes in the external environment,the high harmonic distortion rate of the current inner loop control output,and the large steady-state error,a power feedforward BFOA-PCI control method.This method reduces the steady-state error through the PCI controller,and adds the bacterial foraging algorithm to optimize the proportional coefficient K_P and integral coefficient K_i,so that K_P and K_i can be optimal in any case,and the output power of the photovoltaic array is introduced into the in the voltage outer loop,its output is used as the reference value of the current inner loop of the photovoltaic grid-connected system,which can speed up the system’s response to changes in the external environment,reduce the harmonic distortion rate,improve the steady-state accuracy,and improve the output power quality.Finally,in the optimal control strategy of the photovoltaic grid-connected system,the control strategy based on power feedforward and bacterial foraging optimization proposed in this thesis is adopted,and the LCL filter is introduced to realize the grid connection.The harmonic distortion rate of the photovoltaic grid-connected system is greatly reduced.The grid-connected current is closer to a sine wave,which improves the grid-connected quality.