Power quality has become an important issue to electricity consumers at all levels of consumption. This has especially been intensified by the common use of sensitive equipment and non-linear loads in both industrial and the domestic environs. In this article we look at some of the common terms typically used when describing power quality problems.
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A voltage dip is a reduction in the rms voltage in the range of 0.1 to 0.9 p.u. (retained) for duration greater than half a mains cycle and less than 1 minute; frequently referred to as ‘sag’. It is caused by faults, increased load demand and transitional events such as large motor starting.
A voltage swell is an increase in the rms voltage in the range of 1.1 to 1.8 p.u. for duration greater than half a mains cycle and less than 1 minute. It is caused by system faults, load switching and capacitor switching.
The magnitude of a voltage swell is dependent on the fault location relative to the point of measurement, the system impedance and system earthing. An earthed system with delta-star connected substation transformer will ensure little change in the un-faulted phase’s voltages due to a low impedance path from fault to transformers.
An interruption is a reduction in the supply voltage or load current, to a level less than 0.1 p.u. for a time of not more than 1 minute. Interruptions can be caused by system faults, system equipment failures or control and protection malfunctions.
Since the magnitude of the supply voltage drops below 0.1 p.u. during an interruption, the magnitude of the interruption is considered to be zero; the interruption is defined only by duration. The length of short interruption is generally governed by the type of protection equipment utilized on the system such as auto-reclosers.
A transient is an undesirable momentary deviation of the supply voltage or load current. Transients in general can be categorized into two types: impulsive and oscillatory. An impulsive transient is an abrupt change in the steady state voltage and/or current, that occurs outside the power frequency range and is unidirectional in polarity. Impulsive transients are characterized by the rate of change of voltage or current magnitude i.e. the rise and fall times. Oscillatory transients are typically characterized by their frequency content and duration. Transients are often due to the network response to an impulsive transient.
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Harmonics are periodic sinusoidal distortions of the supply voltage or load current caused by non-linear loads. Harmonics are measured in integer multiples of the fundamental supply frequency.
By using Fourier series analysis, the individual frequency components of the distorted waveform can be described in terms of the harmonic order, magnitude and phase of each component. The quantity total harmonic distortion (THD) is commonly used; it represents a summation of all the harmonic components present in a waveform.
These are distorted voltage or current waveforms containing periodic distortions of a sinusoidal nature that are not integer multiples of the fundamental supply frequency.
Flicker is a term used to describe the visual effect of small voltage variations on electrical lighting equipment (particularly tungsten-filament lamps). The frequency range of disturbances affecting lighting appliances, which are detectable by the human eye, is in the range of 1 to 30 Hz.
This is the deviation in the magnitude and/or phase of one or more of the phases of a three-phase supply, with respect to the magnitude of the other phases, and the normal phase angle (120°). The voltage imbalance may happen where one, two, phase(s) of a three-phase supply is/are being loaded more than the other(s).
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This is a variation in frequency from the nominal supply frequency above/below preset level normally ±0.1%.
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