Research On Resonance Control Strategy Of Photovoltaic Grid-connected Inverter Under Weak Grid Condition

The country’s vigorous promotion and policy support for the photovoltaic power generation industry have made photovoltaic grid-connected inverters more widely used.Due to the influence of geographic location,photovoltaic power generation often requires long lines and multi-stage transformers to connect to the large power grid to achieve spatial transfer of electrical energy.In this process,the impedance of the power grid cannot be ignored,which makes the power grid appear weak grid characteristics.Under the characteristics of weak grids,the wide-range changes of the grid impedance not only cause the instability of the grid-connected system,but also have a serious impact on the phaselocking link of the grid-connected system,which causes the grid-connected system to fail to operate normally.Therefore,the thesis focuses on the phase-locking problem of the photovoltaic grid-connected inverter system and the resonance of the LLCL filter under the characteristics of the weak grid,and the following researches are carried out:(1)The working principle of LLCL filter is studied.By comparing the filtering performance of LLCL filter with other filters,the unique advantages of LLCL filter in grid connected inverter system are analyzed,and its parameter setting principle is analyzed and studied in detail.The existence of the resonance peak of the LLCL filter will affect the stable operation of the grid-connected system.In order to study the phase-locked link and inherent resonance of the system,it is necessary to analyze the structure of the LLCL grid-connected system and establish its mathematical model.(2)Aiming at the influence of weak grid characteristics on the phase-locking performance of the phase-locked loop,a phase-locked loop structure combining a double complex coefficient filter and a delay signal elimination method is proposed.Firstly,the double complex coefficient filter is used to decouple the positive and negative sequence components of the three-phase fundamental voltage and filter the high-order harmonics;secondly,the delay signal elimination method is used to filter the low-order harmonics by setting its delay factor;Finally,a simulation platform was built through Matlab/Simulink software to verify that under the characteristics of weak grid,when the grid voltage distortion,unbalance,frequency and phase angle mutations and other unsatisfactory grid conditions occur due to grid impedance,it is compared to pure the phase-locked loop of the dual complex coefficient filter and the improved phase-locked loop can more effectively achieve precise phase-locking,which is beneficial to the stable operation of the grid-connected inverter.(3)Aiming at the grid-connected system’s instability and the resonance problem of the filter itself due to the grid impedance under the characteristics of weak grids,a control strategy that introduces capacitive current feedback based on the weighted current is proposed.First,according to the mathematical model of a single LLCL photovoltaic grid-connected inverter,analyze the resonance generation mechanism and the impact of grid impedance changes on the grid-connected system;secondly,apply the proposed control strategy to the photovoltaic grid-connected inverter system to suppress resonance and improve the damping performance of the system;Finally,simulations and experiments verify that the proposed control strategy can effectively enhance the damping performance of the inverter system,reduce the harmonic content of the grid-connected current,and improve the stability of the system in a weak grid.