Usefulness of the various indicators of Harmonic distortion

The THDu characterises the distortion of the voltage wave.
Below are a number of THDu values and the corresponding phenomena in the installation:

• THDu under 5% - normal situation, no risk of malfunctions
• 5 to 8% - significant harmonic pollution, some malfunctions are possible
• Higher than 8% - major harmonic pollution, malfunctions are probable. In-depth analysis and the installation of attenuation devices are required

The THDi characterises the distortion of the current wave.The disturbing device is located by measuring the THDi on the incomer and each outgoer of the various circuits and thus following the harmonic trail.
Below are a number of THDi values and the corresponding phenomena in the installation:

• THDi under 10% - normal situation, no risk of malfunctions
• 10 to 50% - significant harmonic pollution with a risk of temperature rise and the resulting need to oversize cables and sources
• Higher than 50% - major harmonic pollution, malfunctions are probable. In-depth analysis and the installation of attenuation devices are required

Power factor PF
Used to evaluate the necessary oversizing for the power source of the installation.

Crest factor
Used to characterise the aptitude of a generator (or UPS) to supply high instantaneous currents. For example, computer equipment draws highly distorted current for which the crest factor can reach 3 to 5.

Spectrum (decomposition of the signal into frequencies)
It provides a different representation of electrical signals and can be used to evaluate their distortion.