Research On Control Technology Of LCL Grid-connected Inverter Under Weak Power Gri

The stability and current quality of LCL grid-connected inverter are seriously challenged when it is connected to weak grid because of the wide range of varying grid impedance and the large amount of grid voltage harmonics.In this paper,emphasis is placed on enhancing the stability of grid-connected inverters under weak grid and proposing corresponding control strategies to ensure that the grid-connected system operates well under weak grid and that the background harmonics of the grid voltage are effectively suppressed.The topology and control system diagram of the digitally controlled single-phase LCL grid-connected inverter are investigated.Based on the inferred grid-connected current expression,it is found that the grid-connected current is affected by the grid voltage harmonics,and a capacitor voltage full feedback strategy that completely eliminate the grid voltage harmonics is then introduced based on the grid voltage full feedforward control strategy.The harmonic suppression capability of the capacitor voltage feedback control is evaluated with the ratio of the output impedance and the ratio of the feedback function under different feedback controls to determine the magnitude of the effect of the digital control delay on the capacitor voltage feedback control,and it is concluded that the capacitor voltage feedback amplifies the high frequency harmonics while suppressing the low frequency harmonics in the gridconnected current.The stability of LCL-type grid-connected inverters with capacitor voltage feedback control under weak grids is reasoned based on impedance analysis.It is derived that due to the introduction of the capacitor voltage feedback loop,the digital control delay leads to a phase frequency curve of the inverter output impedance with approximately 90° hysteresis,which affects the ability of the grid-connected inverter to adapt to weak grids in a certain extent.The starting point of this paper to strengthen the stability of grid-connected systems under weak grids is obtained: correction of the inverter phase characteristics at the intersection frequency of grid impedance and inverter output impedance.This paper proposes a capacitor voltage weighted full feedback strategy based on the mechanism of the feedback loop causing delay to the system.By introducing suitable weighting factors in different frequency bands to correct the inverter output impedance in order to guarantee that the ratio of inverter output impedance with grid impedance satisfies the Nyquist stability criterion,thus improving the resilience of the grid-connected system under weak grid.Next,the harmonic suppression capability of the proposed capacitor voltage weighted full feedback strategy is analyzed.It is found that although the capacitor voltage weighted full feedback amplifies the 33 rd and higher harmonics,the proposed control strategy still effectively suppress the grid voltage harmonics due to the correct design of the inverter parameters.A hardware-in-the-loop experimental platform is established in the laboratory to verify the theoretical analysis.The experimental results demonstrate that the proposed capacitor voltage weighted full feedback strategy can not only adapt to the weak grid situation without grid voltage distortion,but also effectively suppress the grid voltage harmonics under the weak grid condition.