A Complex Coefficient Filter-Based Grid Impedance Identification Method Considering the Effects of the Grid Background Harmonics

To identify the grid impedance accurately, high frequency non-characteristic harmonic voltages and currents are often injected into the grid by the power converter firstly. Then, digital signal filters are used to extract the high frequency voltages and currents from the sampled voltages and currents at the point of common coupling. Finally, the grid impedance can be calculated. Due to effects of the grid background harmonics, such as 5th and 7th harmonic, the grid impedance identification precision is usually poor. In this paper, to improve the grid impedance identification precision, a new complex coefficient filter-based grid impedance identification method is proposed considering the effects of the grid background harmonics. To reduce the influences of the grid background harmonics, such as 5th and 7th harmonic, an improved complex coefficient filter is proposed by adding a 5th and 7th harmonic suppression module to the conventional complex coefficient filter. The filter characteristic of the proposed complex coefficient filter is analyzed in detail to show its properties. Additionally, a simplified implementation method of the proposed complex coefficient filter is also proposed to reduce the calculation burden. Finally, simulation studies are carried out to further show the effectiveness of the proposed grid impedance identification method.