Power quality
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In its broadest sense, power quality is a set of boundaries that allows electrical systems to function in their intended manner without significant loss of performance or life (Sankaran 2002). The term is used to describe electric power that drives an electrical load and the load's ability to function properly with that electric power. Without the proper power, an electrical device (or load) may malfunction, fail prematurely or not operate at all. There are many ways in which electric power can be of poor quality and many more causes of such poor quality power.
The electric power industry comprises electricity generation (AC power), electric power transmission and ultimately electricity distribution to an electricity meter located at the premises of the end user of the electric power. The electricity then moves through the wiring system of the end user until it reaches the load. The complexity of the system to move electric energy from the point of production to the point of consumption combined with variations in weather, generation, demand and other factors provide many opportunities for the quality of supply to be compromised.
While "power quality" is a convenient term for many, it is the quality of the voltage - rather than power or electric current - that is actually described by the term. Power is simply the flow of energy and the current demanded by a load is largely uncontrollable.
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[edit] 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 are both important to different types of equipment.
- When the RMS voltage exceeds the nominal voltage by 10 to 80% for 0.5 cycle to 1 minute, 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 voltage is below the nominal voltage by 10 to 90% for 0.5 cycle to 1 minute.
- Random or repetitive variations in the RMS voltage between 90 and 110% of nominal can produce a phenomenon known as "flicker" in lighting equipment. Flicker is the impression of unsteadiness of visual sensation induced by a light stimulus on the human eye. A precise definition of such voltage fluctuations that produce flickers have been subject to ongoing debate in more than one scientific community for many years.
- 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.
- "Undervoltage" occurs when the nominal voltage drops below 90% for more than 1 minute. The term "brownout" is an apt description for voltage drops somewhere between full power (bright lights) and a blackout (no power - no light). It comes from the noticeable to significant dimming of regular incandescent lights, during system faults or overloading etc., when insufficient power is available to achieve full brightness in (usually) domestic lighting. This term is in common usage has no formal definition but is commonly used to describe a reduction in system voltage by the utility or system operator to decrease demand or to increase system operating margins.
- "Overvoltage" occurs when the nominal voltage rises above 110% for more than 1 minute.
- 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)
Each of these power quality problems has a different cause. Some problems are a result of the shared infrastructure. For example, a fault on the network may cause a dip that will affect some customers and the higher the level of the fault, the greater the number affected, or a problem on one customer’s site may cause a transient that affects all other customers on the same subsystem. Other problems, such as harmonics, arise within the customer’s own installation and may or may not propagate onto the network and so affect other customers. Harmonic problems can be dealt with by a combination of good design practice and well proven reduction equipment.
[edit] Power conditioning
Power conditioning is modifying 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.
[edit] See also
[edit] References
- Dugan, Roger C.; Mark McGranaghan, Surya Santoso, H. Wayne Beaty (2003). Electrical Power Systems Quality. McGraw-Hill Companies, Inc.. ISBN 0-07-138622-X.
- Meier, Alexandra von (2006). Electric Power Systems: A Conceptual Introduction. John Wiley & Sons, Inc.. ISBN 978-0-471-17859.
- Heyden, G.T. (1991)). Electric Power Quality. Stars in a Circle Publications. Library Of Congress 621.3191.
- Bollen, Math H.J. (2000). Understanding Power Quality Problems: Voltage Sags and Interruptions. New York: IEEE Press. ISBN 0-7803-4713-7.
- Sankaran, C. (2002). Power Quality. CRC Press LLC. ISBN 0-8493-1040-7.
- Baggini, A. (2008). Handbook of Power Quality. Wiley. ISBN 978-0470065617.
