Self-Synchronized Grid Impedance Estimation Unit Using Interpolated DFT Technique

Estimating the impedance of a grid-connected device or of a grid at the point of common coupling is important for evaluating the interaction between them. Impedance measurement involves a perturbation injection device, which perturbs the system, and a measurement device (impedance estimation unit), which acquires the grid voltages and currents to extract the impedance. When evaluating the grid impedance in the $dq$ reference frame, the reference frame transformation requires estimating the grid angle. For a self-synchronized impedance estimation unit, angle estimation is typically performed by a phase-locked loop, but the presence of low-frequency voltage perturbations limits the accuracy of the results. This article proposes a method with a new approach, based on an interpolated discrete Fourier transform technique, to extract the grid angle. Conventional phase-locked loop and proposed interpolated-discrete-Fourier-transform-based techniques are compared experimentally and the proposed technique shows more accurate impedance estimation in the low frequency range, for comparable frequency tracking performance.