Harmonic distortion is one of the disturbances that most affects the quality of the electrical system. The widespread use of power electronic systems, especially power converters, has increased harmonic and interharmonic emission in a wide range of frequencies. Therefore, there are new needs in the measurement of harmonic distortion in modern electrical systems, such as measurement in the supra-harmonic range (>2 kHz) and the measurement of interharmonics. The International Electrotechnical Commission (IEC) standards define new total harmonic distortion (THD) rates based on the concept of frequency groupings. However, the rates defined in the IEC standards have shortcomings when measuring signals such as those present in the outputs of power systems with abundant interharmonic content and presence of components in the supra-harmonic range. Therefore, in this work, a comparison is made between the different THD factors currently defined, both in the literature and in the standards, to show which of them are the most suitable for assessing harmonic and interharmonic contamination in power system signals such as those present at the output of inverters.

The increasing incorporation of power electronics and other non-linear loads, in addition to their energy advantages, also implies a poor power quality, especially as regards harmonic pollution. Different solutions have been proposed to measure harmonic content, taking the International Electrotechnical Commission (IEC) standards as a reference. However, there are still some issues related to the measurement of the harmonic, and especially, interharmonic content. Some of those questions are addressed in this work, such as the problem derived from the instability of the values obtained by applying the discrete Fourier transform to each sampling window, or the appearance of local peaks when there are tones separated by multiples of the resolution. Solutions were proposed based on time aggregation and the overlapping of windows. The results demonstrate that aggregation time, window type, and overlapping can improve the accuracy in harmonic measurement using Fourier transform-based methods, as defined in the standards. The paper shows the need to consider spectral and time groupings together, improving results by using an appropriate percentage of overlap and an adaptation of the aggregation time to the harmonic content.

Frequency converters controlling electric motors are widely used as they increase the functionality of the equipment and improve energy efficiency. However, it is inevitable that the quality of the power delivered to the motor does not have the same characteristics as when it is supplied from the mains. Due to the impact of the power quality on the operation of the motor and even on its possible diagnosis, it is desirable to characterise the power quality, preferably by means of rates or indices that reflect the quality as completely as possible. However, the rates specified in the standards and in the literature are designed to characterise the quality delivered from the grid and are not well adjusted to the features of the signal at the output of the converter with a high interharmonic and harmonic content at high and low frequencies, and with variable fundamental frequencies. For this reason, this work proposes a set of distortion rates and the appropriate procedures for calculating them to reflect as reliably as possible the quality of the energy delivered by the converter to the motor. To verify the validity of the proposal, several practical examples are presented with induction motors fed from different frequency converters.