Research On Grid-connected Control Based On Three-phase LCL Filtered Of Quasi-Z Source Photovoltaic Inverter

Realizing "carbon peaking and carbon neutralization" is the most important task at this stage.If we want to do this work well,we must first start from the root and source,determine the industrial structure policy,effectively implement the rectification strategy and optimiza-tion scheme of energy structure,and strengthen the development of new energy.In the new energy development system,photovoltaic grid connected inverter,as the connecting device from new energy power generation device to power grid energy conversion,plays a key role in the photovoltaic power generation system,so that the system can operate safely and stably.Therefore,the selection and design of inverter is very important.The characteristic of quasi Z-source inverter is that the upper and lower power tubes can be connected directly without dead zone.It only needs to change the direct state to complete the voltage rise and fall,which greatly reduces the cost of photovoltaic power generation and has better output power quality.However,the quasi-z-source inverter also has its own problems,such as high starting impulse current and large steady-state current stress.In addition,the LCL filter is connected to the AC side of the grid connected inverter to filter out harmonics.Although the filtering effect is remarkable,there is resonance,which leads to the instability of the system.Therefore,this pa-per calculates the current stress of the three-phase LCL filter quasi-z-source inverter.Finally,when the power grid voltage is unbalanced,the LCL filter quasi-z-source inverter is incorpo-rated into the power grid system,and the corresponding control strategy is designed to realize friendly grid connection.(1)Firstly,the working principle and circuit analysis of the traditional quasi-z-source in-verter in the two states of through and non-hrough are carried out.On this basis,according to its topology,the mathematical models of each impedance network element and power switch are established,the analytical formulas of the average value and effective value of each ele-ment current are listed,and the current stress is analyzed,It provides a theoretical basis for the design and setting of impedance source Z network parameters and the separation of positive and negative sequence currents in unbalanced power grid.(2)Secondly,through the topology analysis of the AC side and DC side of the three-phase LCL filter Quasi-Z source inverter photovoltaic grid connected system,because the LCL filter has resonance,and DQ in the two-phase static coordinate system,the active damping control of capacitive current feedback is adopted,and the proportional resonance regulators cooperate with each other to suppress the resonance of the filter and weaken the coupling relationship between the two components.Simulation results show that under the propose control strategy,in the photovoltaic grid connected system of LCL quasi-z-source inverter,the incoming current is controlled and the resonance of LCL filter is suppressed,so that the incoming current quality is better,the waveform is consistent with the sine wave,and the coupling relationship between modulation ratio and through duty cycle is weakened to a great extent.(3)Thirdly,the expression of instantaneous power and reactive power pulsation in the grid can be used as the reference to keep the target voltage and reactive power in the case of unbalanced grid connected current,and then the power can be calculated according to the ex-pression of unbalanced grid connected current,The angular frequency feedback reaches the regulator to achieve the purpose that the final current amplitude is constrained.(4)Finally,aiming at the problems of serious harmonic and excessive fault current in the grid connected reference current under the three control objectives,an improved model pre-dictive current control is proposed,which has a good function of restraining the current.By calculating the Z-source impedance,the network inductance current,cost function and the selection of through state are determined by the cost function,so as to reduce the current cal-culation amount of the model predictive control algorithm.The simulation experiment shows that the model predictive control can not only make the network power and negative sequence current cooperate with each other,but also reduce the calculation amount of model prediction to a great extent,and solve the problem of fault overcurrent.