Modern grid codes require validated impedance models to assess stability in converter-dominated systems. Since field measurements yield site-specific results, test benches with grid emulators are increasingly utilized. Nonetheless, grid emulators exhibit background harmonic noise. Annex E of IEC 61400-21-4 proposes a procedure to assess this influence and can also be used to determine the Thévenin equivalent of the device under test (DUT). However, practical experience with the method is still limited. This paper addresses this gap through investigations on a multi-megawatt Vestas wind turbine at the Dynamic Nacelle Testing Laboratory (DyNaLab) of Fraunhofer IWES. As Annex E provides sparse practical guidance, specific methods for implementation, outlier detection, and filtering are proposed. The results confirm that the method is capable of determining the impedance of the DUT but requires careful evaluation of the measurements to ensure sufficient spectral content by varying testing configurations. It proves particularly effective for estimating impedance trends in the high kHz range, where grid emulators often lack harmonic injection capability, and may thus serve as a valuable complement to classical impedance scans.