Ideally, an electrical wave should have a perfectly sinusoidal shape over time, with a frequency of 60 Hz. This is the ideal regime for the proper operation of electrical machinery and equipment. However, in reality, the wave is never perfect and can be more or less deformed compared to the ideal sine wave. When the deformation repeats itself uniformly with each cycle, which is generally the case, it is said that there is harmonic distortion of the wave. The distortion of the wave can then be represented as the sum of a series of perfectly sinusoidal waves and an integral multiple frequency of the fundamental frequency at 60 Hz.

When the distortion is minor, the wave is essentially characterized by a fundamental wave (harmonic rank 1) with a very small presence of the multiples of this fundamental (harmonic ranks 2 and more). On the other hand, for a wave with more distortion, the harmonic ranks 2 and more will become more significant. In general, the importance of the harmonics will decrease with their rank and the even ranks (2, 4, 6, etc.) will be less present than the odd ranks (3, 5, 7, etc.). Also, in three-phase power systems, the odd multiple harmonic ranks of 3 (3, 9, 15, etc.) are naturally less present.

Total harmonic distortion is calculated by averaging the rms value of all 2nd and higher harmonics and dividing by the rms value of harmonic rank 1 (the fundamental). When the total harmonic distortion becomes too high, it can cause problems of premature heating, malfunction and tripping of electrical equipment. It is therefore important to keep the total harmonic distortion below a certain threshold, determined in various standards. Thresholds also exist for each individual harmonic rank.

These harmonic distortions of the voltage wave are directly related to the strong harmonic distortions of currents caused by certain electrical equipment. These harmonics of currents then cause harmonic voltage drops in the electrical network, which in turn causes the distortion of the voltage. The equipment in question may be single or three-phase voltage rectification units, variable speed drives, lighting systems, welding equipment, etc.

When the total harmonic distortion becomes or may become too great, the situation must be remedied so as not to disturb the operation of the surrounding electrical equipment. The electricity distribution company may also require a customer to limit its harmonic current emissions so as not to disturb other customers around. In most cases, correction solutions often require the installation of harmonic filters to better control the harmonic currents generated. Calibration and sizing of these filters can be quite complex. Network modeling and simulation is often required to achieve this. It is then important to call on a firm specialized in this field to carry out these calculations. Replacing some equipment with less disruptive ones can also be a solution. Again, a specialized firm will determine the optimal solution to implement.