Research On Stable Control Performance Of Grid-connected Inverter Under Weak Grid

With the vigorous development of photovoltaic power generation,the application of photovoltaic grid-connected inverters has become more widespread.The long-distance transmission lines and the variable voltage links required for power supply in remote areas are not negligible,which leads to the weak grid characteristics of the power grid.At the same time,the increase of the grid-connected capacity of the inverter in the multi-inverter grid-connected system will also increase the equivalent impedance of the grid side,and the grid will also behave as a weak grid.Large grid impedance under weak grids will have a certain impact on the stable control performance of PV grid-connected inverters.Based on the mathematical model of grid-connected inverter,the control method of grid-connected inverter under weak grid and the harmonic resonance characteristics of multi-inverter system are studied.Based on the most commonly used three-phase LCL type photovoltaic grid-connected inverter,the mathematical model of grid-connected inverter under weak grid is established firstly.Based on this,the grid resistance and grid inductance in the grid impedance are analyzed.Sexual influence.At the same time,aiming at the phenomenon that the resonant frequency decreases when multiple inverters are operated in parallel in a weak grid,the mechanism of the increase in the number of inverters causing the resonance low frequency and the equivalent grid impedance to increase and the system stability is reduced is studied.The stability criterion of impedance-based inverter is introduced and the output impedance model of the inverter is given.Based on this,the influence of grid impedance variation on the stability margin of the inverter is analyzed.In order to ensure the stability of the inverter in the process of grid impedance change,a phase margin compensation method based on lead correction is proposed.The phase of the output impedance of the inverter inverter is compensated by the grid voltage feedforward method to ensure the inverter.The device has sufficient phase margin.The design process of the lead correction link is given and the dynamic control scheme of phase margin compensation when the grid impedance is increased is proposed.Finally,the effectiveness of the proposed control method is verified by simulation.A multi-inverter parallel system is proposed to study the resonant frequency and resonance characteristics of multi-inverter systems based on modal analysis.Firstly,the basic concept of modal analysis method and the calculation method of participation factor are given.The impedance model of single inverter is derived.The node admittance matrix of multi-inverter with the same parameters and multi-inverter system with different parameters is established.The modal resonance frequency of the system is obtained by obtaining the eigenvalues of the node admittance matrix and the modal analysis characteristic curve i s drawn.The correctness of the modal analysis results is verified by the transfer function analysis method and simulation.