3/8 – Electrical equipment: power harmonics

What are Power Harmonics ?

The presence of power harmonics in electrical systems means that current and voltage are distorted and deviate from sinusoidal waveforms.

Harmonic currents are caused by non-linear loads connected to the distribution system. A load is said to be non-linear when the current it draws does not have the same waveform as the supply voltage. The flow of harmonic currents through system impedances in turn creates voltage harmonics, which distort the supply voltage.

On figure this figure are presented typical current waveforms for single-phase (top) and three-phase non-linear loads (bottom).

Typical distorted current waveforms

Harmonic distorsion: typical distorted current waveforms

Electrical Equipment comprising power electronics circuits are typical non-linear loads. Such loads are increasingly frequent in all industrial, commercial and even residential installations and their percentage in overall electrical consumption is growing steadily.

Electrical equipment examples:
–    Industrial equipment (welding machines, arc and induction furnaces, battery chargers),
–    Variable Speed Drives for AC or DC motors,
–    Uninterruptible Power Supplies,
–    Office equipment (PCs, printers, servers, etc.),
–    Household appliances (TV sets, microwave ovens, fluorescent lighting, light dimmers).

The total harmonic distortion THD is the usual parameter to evaluate the level of distortion of an alternating signal. The voltage distortion THDu is usually considered at the installation level, while the current distortion THDi is usually considered at the non-linear equipment level.

Jacques Schonek


  • Topic is gives good basic, can we achieve 0% harmonics for particular order by using passive filters. say tuned or detuned. Also can any body share more about detuned filter function.

  • The explanations are good but are there any statistics on how much THD different equipments produce.

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