Usefulness of the various indicators of Harmonic distortion

From Electrical Installation Guide

THDu is an indicator of the distortion of the voltage wave.

Below are given indicative values of THDu and the corresponding consequences in an installation:

  • ≤ 5%: normal situation, no risk of malfunctions,
  • 5 to 8%: significant harmonic distortion, some malfunctions are possible,
  • ≥ 8%: major harmonic distortion, malfunctions are probable. In-depth analysis and the installation of mitigation devices are required.

THDi is an indicator of the distortion of the current wave.

The current distortion can be different in the different parts of an installation. The origin of possible disturbances can be detected by measuring the THDi of different circuits.

Below are given indicative values of THDi and the corresponding phenomena for a whole installation:

  • ≤ 10%: normal situation, no risk of malfunctions,
  • 10 to 50%: significant harmonic distortion with a risk of temperature rise and the resulting need to oversize cables and sources,
  • ≥ 50%: major harmonic distortion, malfunctions are probable. In-depth analysis and the installation of mitigation devices are required.

Power factor λ is used to determine the rating for the different devices of the installation.

Crest factor is 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.

Harmonic spectrum provides a different representation of electrical signals and can be used to evaluate their distortion.

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