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
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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.
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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.
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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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