The electric wave of an AC distribution network has a fundamental frequency essential to the proper functioning of electrical equipment (60 Hz for most networks in North America). However, many equipment connected to power grids have non-linear loads that emit harmonic currents. Equipment such as variable frequency drives, AC / DC converters, arc or plasma welders, are likely to emit such significant harmonic currents.

These harmonics are sinusoidal electric waves whose frequency is a multiple of the fundamental frequency (120 Hz, 180 Hz, 240 Hz, etc.). These electric waves increase energy losses and noise on the network. They can even damage some equipment. In addition, the emission of harmonic currents causes the appearance of voltage harmonics which can affect, according to their importance, the equipment of the other customers connected to the same distribution network.

When setting up a new electrical installation, Hydro-Québec sets a limit for the emission of harmonic currents according to the electrical power of the installation. This limit is established for each of the first 50 harmonic ranks as well as for the total harmonic distortion and this, for 3 load scenarios of the HQ network. A harmonic current emission study aims to demonstrate that the installation will not exceed the emission limits of these harmonic currents. The calculation is performed according to the requirements and demands of HQ C.25-01 standard.