Research On The Stability Of Grid-Connected Inverter Under Different Grid Conditions

With the rapid development of distributed power generation systems (DG), the application ofgrid-connected inverter which attracts more attention for its stability is more widely used. Longdistance transmission line will introduce an equivalent impedance to grid which be made a weakpower grid characteristic especially for those DG located in remote areas. For this type of interactivesystems, harmonic resonance will be produced by the interaction between grid impedance and inverteroutput impedance, and thus will pollute the quality of the power gird. This harmonic pollutionrepresents a new source of power quality problems in DG and affects the stability of inverter isdifferent from conventional power system harmonics caused by nonlinear loads.This paper presents a research on the influence of grid-parallel inverter stability by consideringharmonic resonance problems and phase-locked loop (PLL) performance under different conditions.Firstly, based on existing impedance-based system stability criterion，a new stability criterion ispresented and the molding of inverter output impedance with different filter and controller is depictedas well. The influence on LCL filter resulting from harmonic resonance caused by dynamic interactionand grid impedance is researched. In addition, a new type of active damping algorithm based oncapacitor in parallel with a virtual resistance is used to suppress LCL filter resonance. Secondly, amethod of single-phase software PLL (SPLL) is presented in this paper by combining second ordergeneralized integrator (SOGI) with d-q transformation. The performance of SPLL is analyzed undervarious conditions, including harmonics, dc bias and the phase difference of quadrature signalfluctuating in the range of90°. Besides, by using a low-pass filter, an improved strategy is proposedin accordance with the influence of dc bias.Finally, a500W single phase full bridge grid-parallel inverter is designed based onTMS320F28335. Experimental results demonstrate the following conclusions: Low-order harmonic ofthe grid current may be increased due to impedance interaction. LCL filter resonance is effectivelysuppressed by a new type of active damping algorithm. An error exists in phase-locked angle undervarious conditions. In summary, the results show the correctness of theoretical analysis and thefeasibility of improving strategies.