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Home / Technical Articles / The most typical indices for measuring power quality disturbances

Power quality metrics

There are various methods for categorizing the severity of power disturbances. The most typical indices for measuring power quality disturbances are listed and explained below:

  1. Distortion factor
  2. Crest factor
  3. Notch area
  4. Recovery time
  5. Displacement power factor
  6. Total power factor
  7. K factor
The most typical indices for measuring power quality disturbances
The most typical indices for measuring power quality disturbances (photo credit:

1. Distortion Factor

The ratio of the root square value of the harmonic content to the root square value of the fundamental quantity, expressed as a percentage of the fundamental, also known as total harmonic distortion.

Distortion Factor (THD)


Alternate forms for the distortion factor are given as percentages of the nominal voltage or demand load current for the system under consideration, for use in evaluation of the harmonic content of the system voltage or current.

These are referred to as Total Harmonic Distortion (THDVn) and Total Demand Distortion (TDD), defined as follows:

Total Harmonic Distortion (THD) and Total Demand Distortion (TDD)


  • Vh is the RMS value of the nth harmonic component of the voltage
  • Vn is the RMS nominal fundamental voltage value
  • Ih is the RMS value of the nth harmonic component of the current
  • IL is the maximum demand load current, typically the average maximum monthly demand over a 12-month period

Go back to Index ↑

2. Crest Factor

The ratio of the peak value of a periodic function to the RMS value, i.e.:

Crest Factor


  • ypeak is the peak value of a periodic function
  • yrms is the RMS value of the function

Because power system voltages and currents are nominally sinusoidal, the nominal crest factor for these would be √2.

Go back to Index ↑

3. Notch Area

A notch in the power system voltage (or current) is illustrated in figure 1 below:

Voltage (or current) notch illustration
Figure 1 – Voltage (or current) notch illustration

The notch area for the notch as illustrated in figure above is defined as:

The notch area formula


  • An is the notch area in volt-microseconds
  • t is the notch time duration in microseconds
  • d is the notch depth in volts

Go back to Index ↑

4. Recovery time

This is the time needed for the output voltage or current to return to a value within the regulation specification after a step load or line change.

Go back to Index ↑

5. Displacement Power Factor

The ratio of the active power of the fundamental wave, in watts, to the apparent power of the fundamental wave, in volt-amperes.

This is the traditional definition of power factor.

Go back to Index ↑

6. Total Power Factor

The ratio of the total input power, in watts, to the total volt-ampere input. This includes the effects of harmonics.

Go back to Index ↑

7. K Factor

A measure of a transformer’s ability to serve non-sinusoidal loads. The K factor is defined as:

K factor formula


  • Ih is the harmonic component at h times the fundamental frequency
  • h is the harmonic order of Ih in multiples of the fundamental frequency
  • hmax is maximum harmonic order present

Go back to Index ↑

Reference: Power Quality Considerations- Bill Brown, P.E., Square D Engineering Services

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More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Umeshan. V. V
    Dec 05, 2019

    Thank you sir

  2. Aurelio Mayorca
    Nov 24, 2019

    I want more articles about harmonic distorcion. Could you help me, I will thanks.

  3. Santosh Patil
    Nov 24, 2019

    Thank you sir good knowledge and good explained

  4. Bernabe ocampo
    Dec 13, 2017

    one the sign is the increment of the loos en the transformer and the increment in the temperature in the motors and the power supply,the armonic made the system more inefficient-

  5. Apolo Fumbuja
    Aug 26, 2016

    If I have not a power analyzer, how can i measure or estimate those parameters?

  6. Jim
    Nov 11, 2015

    Can i ask you something? Why true power doenst change (theoretically speaking right, as long as in actual) and there is a big change in Active energy in the result when i install power quality in the mdp.Thank you and im looking forward for your response.

    • Bernabe ocampo
      Dec 13, 2017

      Really the first harmonic is only that do for active Power the other only ,made distortion and loss in the equipment ,for this reason,that idea would be get only the first harmonic,

      there are harmonics thah are inevitables for example the fluxes generated in the core of transformes ,the current magnetization have important 3 harmonic-

  7. Emad Eddin
    Feb 18, 2015

    thank you so much for all these useful information , but plz would you like to write some practical examples

  8. James
    Jan 26, 2015

    I really am interested in that particular section of electrical engineering.

  9. Jimmy Valeriano
    Dec 27, 2014

    Can you discuss how to interpret the grahical test result of power analyzer…

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