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'Power quality' is a term used to discuss events on electric power grids that can damage or disrupt sensitive electronic devices. There are many ways in which a power feed can be poor quality, and so no single figure can completely quantify the quality of a power feed. Although power equals the flow of useful energy over time, the relation between the various conceptions of energy quality and power quality are unknown.

Contents

Introduction

Power conditioning

Introduction

It is often useful to think of power quality as a compatibility problem: is the equipment connected to the grid compatible with the events on the grid, and is the power delivered by the grid, including the events, compatible with the equipment that is connected? Compatibility problems always have at least two solutions: in this case, either clean up the power, or make the equipment tougher.
Ideally electric power would be supplied as a sine wave with the amplitude and frequency given by national standards (in the case of mains) or system specifications (in the case of a power feed not directly attached to the mains) with an impedance of zero ohms at all frequencies.
No real life power feed will ever meet this ideal. It can deviate from it in the following ways (among others):

★ Variations in the peak or RMS voltage (both these figures are important to different types of equipment). When the RMS voltage exceeds the nominal voltage by a certain margin, the event is called a "swell". A "dip" (in British English) or a "sag" (in American English - the two terms are equivalent) is the opposite situation: the RMS volage is below the nominal voltage by a certain margin.

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★ Quick and repetitive variations in the rms voltage. This produces flicker in lighting equipment.

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★ Flicker is the impression of unsteadiness of visual sensation induced by a light stimulus, the luminance or spectral distribution of which fluctuates with time.

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★ Abrupt, very brief increases in voltage, called "spikes", "impulses", or "surges", generally caused by large inductive loads being turned off, or more severely by lightning.

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★ An "undervoltage" or brownout occurs when the low voltage persists over a longer time period

★ Variations in the frequency

★ Variations in the wave shape - usually described as harmonics

★ Nonzero low-frequency impedance (when a load draws more power, the voltage drops)

★ Nonzero high-frequency impedance (when a load demands a large amount of current, then stops demanding it suddenly, there will be a dip or spike in the voltage due to the inductances in the power supply line)

Power conditioning

'Power conditioning' refers to conditioning the power to improve its quality.
An uninterruptible power supply can be used to switch off of mains power if there is a transient (temporary) condition on the line. However, cheaper UPS units create poor-quality power themselves, akin to imposing a higher-frequency and lower-amplitude sawtooth wave atop the sine wave.
A surge protector or simple capacitor or varistor can protect against most overvoltage conditions, while a lightning arrestor protects against severe spikes.
Electronic filters can remove harmonics.