**6. Conclusions**

This paper presented the validation and characterization of a wavelet based method for the accurate assessment of harmonic content in power systems under stationary and non-stationary conditions, using a WPD algorithm. Although other methods have been proposed in the last few years, some of them even using wavelets, Fourier analysis is still the preferred choice for power quality analyzers and the recommended approach according to IEC standards. It is however evident that Fourier analysis is inadequate for assessing harmonic content under non-stationary conditions, which is the most common situation in power systems. Even IEC 61000-4-7 recognizes this limitation, proposing a grouping strategy for adapting FFT to fluctuating harmonics. The IEC grouping strategy, however, can produce large errors in the case of fluctuating signals, as shown in this paper. The proposed method was therefore compared not only to standard FFT, but also to the IEC grouped FFT method, showing its superior performance with respect to both. After verifying compliance with the IEC precision requirements and validating the method using simulated waveforms with real harmonic

content, the method was employed to measure real voltage and current signals. This produced results similar to those obtained in the validation. The reliability of the method was clearly proven, and its performance was superior under fluctuating conditions. On the one hand, researchers are more and more transitioning to more reliable tools, while on the other hand, no methods have been shown to be able to comply with IEC standards and at the same time be precise enough to measure fluctuating harmonics. This paper intended to fill the gap between the research community and the application in the field, illustrating and validating a method whose performance was largely superior to the grouped FFT in assessing fluctuating harmonics. Moreover, the proposed method was shown to perform better than grouped FFT even under stationary conditions, and in the few cases when it did not perform better, the error was always within the IEC accuracy limits for steady-state signals. Lastly, the proposed frequency bands, time window, and bandwidth were, for the first time, compliant with the IEC 61000-4-7 and IEC 61000-4-30 requirements. This was achieved by an accurate selection of the decomposition tree and filter characteristics. The features of the proposed method made it suitable for implementation in Class I Power Quality analyzers, allowing a highly accurate estimation of harmonic content in power systems, both in stationary and non-stationary conditions.

**Author Contributions:** Conceptualization, J.J.M. and J.B.; methodology, J.B. and S.L.; validation, J.B., S.L., and J.J.M.; formal analysis, S.L.; investigation, J.B. and S.L; data curation, J.B. and S.L.; writing, original draft preparation, J.B. and S.L.; writing, review and editing, J.J.M. and J.F.S.; supervision, J.J.M. and J.F.S.

**Funding:** This paper has received funding from the European Union's Horizon 2020 research and innovation program under Marie Sklodowska-Curie Grant Agreement No. 676042.

**Conflicts of Interest:** The authors declare no conflict of interest.
