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

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:

• V_h is the RMS harmonic voltage (or current) value at a frequency of n times the fundamental frequency
• V₁ is the RMS fundamental-frequency voltage or current

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

where:

• V_h is the RMS value of the n^th harmonic component of the voltage
• V_n is the RMS nominal fundamental voltage value
• I_h is the RMS value of the n^th harmonic component of the current
• I_L 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:

• y_peak is the peak value of a periodic function
• y_rms 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.

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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:

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

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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.

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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.

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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.

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7. K Factor

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

where:

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

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