Online non-intrusive grid impedance estimation method for energy storage converters

Abstract

Grid impedance has an important influence on the synchronization stability of grid-connected energy storage converters and their adaptability to weak grids. Conventional intrusive estimation methods require additional perturbations to be injected, which reduces power quality and complicates practical applications. This paper proposes a non-intrusive online grid impedance estimation method. The excitation source is the power regulation dynamics of the energy storage converter itself. The sliding-window discrete Fourier transform is used to extract the fundamental components of the point of common coupling (PCC) voltage and the converter output current from two adjacent single-cycle windows. By combining the voltage equations of the two windows, the unknown equivalent grid voltage components are eliminated, and the grid resistance and reactance can be calculated without injecting additional perturbations or using external phasor measurement equipment. Under constant-current conditions, a simulation with a sudden change in grid impedance is used to verify the effectiveness of the method. Experiments are carried out on a three-phase three-wire energy storage converter under current-step and impedance-switching conditions. The results show that the estimated grid resistance and reactance have small differences from the preset values. The estimation errors of the resistance and reactance on the α-axis are 5.78% and 5.27%, respectively, while those on the β-axis are 0.27% and 1.60%, respectively. This shows that the proposed method can achieve accurate online grid impedance estimation without interfering with the normal operation of the converter, and can improve the stability and adaptive control capability of the energy storage converter.