## 1 Introduction
Besides, time aggregation also compresses the information [15] and reduces the effects of noise present in the signal. These standards are based on the application of the DFT transform with a rectangular window (RW) and a resolution or basic frequency of 5 Hz, for obtaining the RMS values of the harmonic and interharmonic groupings. The spectral leakage generated by the interharmonic tones (and, in general, by the tones not synchronized with the basic frequency) is its primary disadvantage. The main means to reduce this leakage are the correct synchronization of the acquisition windows to the fundamental, and the use of Hann windows (HW) only in the case of loss of synchronism. The performance of RW versus HW to minimize the effect of leakage is discussed in [15; 16; 17; 18; 19; 20], where the results are compared to each of the frequency groupings defined in standard 61000-4 7. However, they do not present the development of the complete standard, taking into account the time grouping also described in standard 61000-4 30 and considering spectral and time groupings altogether, as it is done in this article. Many other authors studied and use methods based on IEC standards. In [21; 22; 23], various DFT calculation methods are compared in the framework of IEC standards. In other articles, the IEC standard is used as a single method of measuring frequencies above 2 kHz [6; 24; 25] or as a reference technique to compare with other new high-frequency measurement methods [26; 27; 28; 29]. Additionally, based on the IEC standards, in [30], the harmonic and interharmonic subgroups generated at the input of variable speed drives are analyzed; a fixed window width (_tw_) of 0.2 s and a fixed total aggregation time (_Tw_) of 3 s are used. [PERSON], [PERSON], et al. study in [31] a methodology to identify the most appropriate sampling window to quantify harmonic and interharmonic emissions, considering variable signals in time, based on the calculation of harmonic and interharmonic distortions using the DFT according to IEC 61000-4-7. [PERSON] and [PERSON] in [32] compare the results obtainedwith different total aggregation times, testing from windows of 0.2 s without time grouping to a total aggregation time of 3 s, as indicated in standard 61000-4-30. Despite the works previously mentioned, the selection of an adequate aggregation time is still a challenge, as it is necessary to determine an appropriate value for the total aggregation time, which, according to the harmonic content of the analyzed signal, stabilizes and improves the accuracy of the results obtained with the DFT along the successive analysis windows. This is one of the main contributions of this paper: to demonstrate the improvement that involves an adequate time aggregation obtaining more reliable and accurate measures in all the RMS values calculated with the DFT for each spectral bar. This improvement can be extended to all the harmonic groupings and distortion rates defined in the standard and formed by those spectral bars [6; 25; 29; 33], and to the distortion rates proposed by other authors and based on the same standard [6; 33; 34; 35; 36; 37]. The signal processing recommended in the standards is aimed at standardizing, simplifying, and unifying the way of measuring signals from the supply network [21]. Nevertheless, there are many practical situations where the harmonic content differs significantly from the one in the supply network. A widespread example is a measurement in the output of frequency converters for controlling electric motors, whose harmonic contents differ in the low, and mainly, in the high-frequency range due to the modulation techniques used in the equipment [30; 38; 39]. These motor power signals present rich harmonic content, with multiple interharmonic tones, and in many cases, with amplitudes similar to those of their closest harmonics.