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, that perturbs the system, and a measurement device (impedance estimation unit) that 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 paper is proposing a method with a new approach, based on 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.