An Improved Impedance Measurement Method for Grid-Connected Inverter Systems Considering the Background Harmonics and Frequency Deviation

The impedance of inverter and grid is to determine the stability of grid-connected inverter systems. Therefore, it is of great importance to obtain accurate grid impedance and inverter impedance. However, the previous studies on impedance measurement are easily subject to background harmonics and grid frequency deviation, which brings to nonunique identification and inaccurate extraction of current or voltage perturbations. In this article, an improved impedance measurement method is investigated for the grid-connected inverter systems. First, a novel selection principle of frequency for injected perturbation is proposed to avoid the impact of background harmonics on impedance measurement. Moreover, a two-stage series multiple-complex coefficient filter (TSMCCF) is also proposed to accurately extract these perturbations considering system may subject to harmonics and grid frequency deviation. Compared with the conventional MCCF, the proposed TSMCCF improves the dynamic performance and accuracy of impedance measurement without increasing the complexity. It is worth noting that the proposed method is not affected by background harmonics and grid frequency deviation, and could be applied in both three-phase and single-phase systems for impedance measurement. To validate the effectiveness and feasibility of the proposed impedance measurement method, real-time digital simulation results are presented based on the RT-LAB platform.