## 1 Introduction  
In recent decades, attention has been directed towards the harmful effects of harmonics, interharmonics, and subharmonics in the distribution network, and so the number of standards and regulations concerning the quality of electrical energy has grown [1]. The IEC 61000-2-1:1990 standard [2] provides definitions for the frequency components of a signal, including interharmonics. In [3, 4], a precise definition of what constitutes a subharmonic component is formulated. Frequency component definitions are shown in Table 1. These disturbances cause operational problems in grid-connected systems, especially when dealing with microgrids [5], weak low-power grids, and in residential distribution networks [6]. Problems such as errors in metering equipment, malfunctioning of relays or differential protections, and vibrations in induction motors also occur. Interharmonics can also cause synchronization problems in equipment requiring grid synchronization. Grid-connected power electronic systems are the main cause of these disturbances [7; 8; 9; 10; 11], and this results in an overall decrease in the power quality of the electrical system. Power electronic converters are present in most domestic and industrial electrical and electronic equipment, and grid-connected photovoltaic generation systems can produce harmonics and interharmonics [8; 12] under different operating conditions [10]. In the same way, it has been demonstrated that wind-generation systems are also responsible for the generation of harmonics, interharmonics, and subharmonics [11]. In the case of industrial applications, AC/DC conversion [7], motor control [13], and arc furnaces [9] stand out among the major generators of interharmonics and subharmonics. Due to the occurrence of these phenomena, new power-quality measurement standards have been developed [1] to achieve better electrical power measurements. The IEEE Std. 1459-2010 [14] provides definitions of electrical power to quantify the flow of electrical energy in single-phase and three-phase circuits under sinusoidal, non-sinusoidal, balanced, and unbalanced conditions. IEC 61000-4-7 [15] and IEC 61000-4-30 [16] define specifications to be met by electrical measurement devices and instrumentation intended for measuring harmonics and interharmonics. In turn, new measurement systems employing different estimation methods and techniques have been developed [17]. Some of these systems are intended for renewable energy-generation systems, as in [12; 18; 19], while others are oriented to more general applications, as in [17; 20; 21]. Measurement systems based on the discrete Fourier transform (DFT) [8; 18], which is the method recommended in [15], have the disadvantage of being very sensitive to picket fence and spectral leakage effects [22; 23], and these are described in [24]. In addition, the error between the actual and the computed frequency value of a harmonic or interharmonic spectral component depends on the acquisition window. The IEC standard [15] does not identify the actual interharmonics existing between two consecutive harmonic frequencies. However, by grouping the spectral components between them, it provides a single rms value quantifying the interharmonic spectral information and indicating the presence of interharmonics in this range of frequencies. To improve the identification of interharmonics, new grouping techniques are proposed in [3] to achieve accurate measurements of interharmonic component values. These new techniques use longer times for the acquisition window [3], which makes them more sensitive to picket fence and leakage effects while also increasing the computational cost. However, these methods do not solve the main problem of providing accurate values for the frequency and rms values of the interharmonic components. Techniques based on the wavelet transform [25], hybrid models, including interharmonic estimation techniques and filter banks [26], or, more recently, those that incorporate artificial intelligence (AI) require considerable processing time [17; 27].