8/8 – Electrical equipment: know more about Power Harmonics

Basics on Power Harmonics

The following figure illustrates the typical waveforms of line current and line voltage for a three-phase variable speed drive.
The line current is distorted as a result of the non linearity of the input rectifier. The voltage distortion is the result of the circulation of the distorted current through the line impedance.
In this example, the Total Harmonic Current Distortion THD i is equal to 40%, and the Total Harmonic Voltage Distortion THD u is equal to 4.3%. (See definitions below).

The power harmonics current spectrum, representing the amplitude of individual harmonic currents Ih (%) is shown on the figure below.

The current distortion can be reduced by implementation of power harmonics mitigation. As an example, the improvement provided by a 12-pulse arrangement is presented below. THD i is reduced from 40 to 11%.

THD – Total Harmonic Distorsion

Total Harmonic Distortion (THD)
: ratio of the r.m.s. value of the sum of all the harmonic components of an alternating quantity up to a specified order (H), to the r.m.s. value of the fundamental component.

Q represents either current or voltage. H is generally equal to 40. Higher values (up to 50) are considered in some applications.
The current THD is commonly noted THD i and is equal to:

From this definition, we can obtain this very useful formula:

The voltage THD is commonly noted THDu and is equal to:

Jacques Schonek


  • i am an electrical student and i will to make friends with some of the e electrical engineers to help me in my studies.

    • If you got the answer then please tell me too, otherwise your question seems to be incomplete,and some additional info is required : what is the derating factor of the generator, what is the ammount of loading , what is the power factor of the load, then what type of generator are you taking in this case??

  • This post is very useful can you give us some case or example case for the formula.

  • The explanations and formulae are of great help but are there any statistics on how much THD different equipments produce.

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