## 7 Discussion  
The correct measurement of the network harmonic content is important to characterizing the energy supplied, for example, at the output of frequency converters, as this type of equipment is the source of high harmonic and interharmonic content that makes analysis very challenging. The existing regulations establish procedures for adequate measurement, describing both the spectral groupings and the necessary time groupings, but problems may arise that make the measurements obtained unreliable. One of them is the instability and inaccuracy of the amplitude values obtained with DFTs in successive sampling windows, due to spectral leakage, and which affect the groupings and distortion rates based on IEC standards. A second problem is the possible occurrence of local peaks due to interharmonic tones that are distanced from each other by multiples of the spectral resolution, and therefore, are not affected by time aggregation, as a way of obtaining reliable RMS values. The evolution of the values of the groupings defined by the IEC standards has been studied to describe these problems and propose solutions. The study focused on the relative position between interharmonics and harmonics, and their relationship with the time of aggregation and type of window. Time aggregation is not necessary when a single frequency component is contained in the frequency grouping measured, since in that case, there is no interference between different components, and therefore, the RMS value obtained is stable and reliable. Examples of these frequency components are a tone present within the measured grouping, the spectral leakage received from another tone outside the grouping, or, if HW is used, a sideband to a tone. However, with more than one component within any of the groupings defined in the standards (such as several interharmonics whose leakages rotate at different speeds) each spectral bar will receive those leakages. Therefore, the total vector obtained in that bar will be the sum of the contributions of all the leakages, causing the variation of its amplitude in each successive analysis window. Therefore, it is convenient to perform time aggregation in all spectral groupings to minimize the effects of leakage on the resulting amplitudes, and to obtain a reliable and stable RMS value. Such time aggregation should be applied to all groups and subgroups, harmonics and interharmonics, and all distortion rates, as these rates are composed of those normalized groupings. That way, DFT amplitude variation caused by spectral leakage decreases, and the final aggregated value is closer to the correct RMS one. However, the ideal aggregation period _Tv_ is unknown in practical applications or differs depending on the area of the spectrum, type of signal (either from the network or from the output of a converter), and the rest of the conditions of each test. Besides, the 61000-4-30 standard proposes aggregation time values as distant as 3 s to 2 h, so the specific duration of this aggregation time for each particular case is open to modification. Time aggregation must be adapted to the thermal time constants of the connected equipment. When thermal time constants are short, as in industrial environments, it is preferable to use the 3 s aggregation interval; the 10 min aggregation interval is best used to evaluate systems with longer time constants, such as the public electric network. For this reason, even based on quality standards, the necessary modifications must be made to adapt the values recommended in these standards to the particular case of measurements on systems consisting of frequency converters supplying induction motors. For all the above reasons, a solution has been proposed to the problems of instability and imprecision in the RMS values, obtained with the DFT according to IEC standards: the use of time aggregation with an increase in the aggregation time with respect to the 3 s recommended by the regulations for signals in industrial environments.