## 1 Introduction
These near-tone situations, whose leakage may interact, occur especially in frequency converters that use modulations, such as closed-loop [40] or random-type [41; 42], or when connected to faulty motors which cause additional harmonic content [43; 44; 45]. This paper analyzes a problem caused by spectral leakage when IEC-based groups are obtained, especially when these are obtained from measurements on signals such as the output of electronic converters. The variability and unreliability of the RMS values obtained with the DFT in the successive sampling windows is the problem. This problem is caused by the interaction of the leakage produced by nearby tones when some of them are interharmonic tones not synchronized with the sampling window and of relatively large magnitude. It will be demonstrated how the time grouping and the choice of appropriate aggregation times specific to the type of signals measured at the output of inverters, and therefore adapting well those proposed in the regulations--will make it possible to reduce the effect of these variations due to leakage. A second problem, related to the previous one, is the appearance of singularities or local peaks, due to interharmonic tones separated from each other by multiples of the spectral resolution and not affected by time aggregation as a way of obtaining a reliable RMS value. To correct this problem, this study developed a solution by aggregating with overlapping windows, that being the second main contribution of this paper. The method proposed in this article improves harmonic and interharmonic identification using DFT-based methods, such as those defined in IEC standards, reducing spectral leakage effects by selecting a proper aggregation time. The proposed method also eliminates local peaks that appear in sensitivity curves by using overlapping windows. The proposed method was validated using synthetic signals and real signals coming from an induction motor, showing the advantages of this method over the one window DFT. Results demonstrate that, through the selection of an adequate aggregation time with an appropriate percentage of overlap, a response closer to the ideal can be achieved in all the frequency groupings defined in the standards; thus, more reliable and accurate measurements can be obtained in their related distortion rates. The rest of this paper is organized as follows: Section 2 presents the problems of calculating amplitudes with DFT; these problems are caused by the interaction between several nearby tones, where at least one is not synchronized with the sampling window. First, a concrete case is examined, based on one of the examples described in the annex of standard 61000-4-7. General cases with any positions of frequencies are considered later, using the rectangular window. Next, in Section 3, a solution that minimizes these problems is studied using a simple and easy-to-implement technique,such as time aggregation. This study is expanded in Section 4 by comparing the use of rectangular and Hann windows when several nearby tones interact. Then, in Section 5, particular cases not solved by time aggregation are studied, such as local peaks due to interharmonic tones separated by multiples of the frequency resolution. A solution to this new problem is developed based on time aggregation with overlapping between successive analyzed windows. Section 6 presents and analyzes experimental results, testing an induction, motor-fed with inverters with different harmonic contents, verifying that the aggregation time needed to stabilize the measurements depends on the harmonic content. Finally, the discussion and conclusions reached in this study are addressed in Section 7.