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:
- Distortion factor
- Crest factor
- Notch area
- Recovery time
- Displacement power factor
- Total power factor
- K 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.
- Vh 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 (THDVn) and Total Demand Distortion (TDD), defined as follows:
- 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
The ratio of the peak value of a periodic function to the RMS value, i.e.:
- 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.
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:
- An is the notch area in volt-microseconds
- t is the notch time duration in microseconds
- d is the notch depth in volts
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.
The ratio of the active power of the fundamental wave, in watts, to the apparent power of the fundamental wave, in volt-amperes.
The ratio of the total input power, in watts, to the total volt-ampere input. This includes the effects of harmonics.
A measure of a transformer’s ability to serve non-sinusoidal loads. The K factor is defined as:
- 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
Reference: Power Quality Considerations- Bill Brown, P.E., Square D Engineering Services