8/10/2019 The Most Typical Indices for Measuring Power Quality Disturbances

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The Most Typical Indices For Measuring Power Quality

Disturbances

The Most Typical Indices For Measuring Power Quality Disturbances (photo credit: ktiengineering.com)

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 

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.

where:

Vh is the RMS harmonic voltage (or current) value at afrequency of n times the fundamental frequency

V1 is the RMS fundamental-frequency voltage or current

 Alternate forms for the distortion factor are given as

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percentages of the nominal voltage or demand load current for the system under consideration, for use inevaluation 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:

where:

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.:

where:

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 thesewould be √2.

Go back to Index ↑

3. Notch Area

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

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

where:

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

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Figure 1 – Voltage (or current) notch illustration

This is the time needed for theoutput voltage or current to returnto a value within theregulation specification after a stepload or line change.

Go back to Index ↑

5. Displacement Power 

Factor 

The ratio of the active power of thefundamental wave, in watts, to theapparent power of the fundamentalwave, 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:

where:

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

About Author //

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MVswitchgears. Professional in AutoCAD programming and web-design. Present on

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One Comment

1.Jimmy Valeriano

Dec 27, 2014

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

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