Power quality determines the fitness of electric power to consumer devices.
Synchronization of the voltage frequency and phase allows electrical systems to
function in their intended manner without significant loss of performance
or life. The term is used to describe electric power that drives an electrical
load and the load's ability to function properly. Without the proper power, an
electrical device 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, electric power transmission and ultimately electric
power 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. Introduction
The quality of electrical power may be described as a set of values of
parameters, such as: Continuity of service
Variation in voltage magnitude Transient voltages and currents
Harmonic content in the waveforms for AC power
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. The tolerance of data-processing
equipment to voltage variations is often characterized by the CBEMA curve, which
give the duration and magnitude of voltage variations that can be
tolerated. Ideally, AC voltage is supplied by a
utility as sinusoidal having an amplitude and frequency given by
national standards or system specifications with an impedance of zero
ohms at all frequencies. No real-life power source is ideal and
generally can deviate in at least the following ways:
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" or a "sag" 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 rapid visible changes of light level. Definition of the
characteristics of voltage fluctuations that produce objectionable light flicker
has been the subject of ongoing research.
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 and a blackout. 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 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. Nonzero high-frequency impedance.
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; the higher the level of the
fault, the greater the number affected. A problem on one customer’s site may
cause a transient that affects all other customers on the same subsystem.
Problems, such as harmonics, arise within the customer’s own installation
and may propagate onto the network and affect other customers. Harmonic
problems can be dealt with by a combination of good design practice and
well proven reduction equipment. 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 condition on the line. However, cheaper UPS units create
poor-quality power themselves, akin to imposing a higher-frequency and
lower-amplitude square wave atop the sine wave. High-quality UPS units
utilize a double conversion topology which breaks down incoming AC power into
DC, charges the batteries, then remanufactures an AC sine wave. This
remanufactured sine wave is of higher quality than the original AC power feed.
A surge protector or simple capacitor or varistor can protect against most
overvoltage conditions, while a lightning arrester protects against
severe spikes. Electronic filters can remove harmonics.
Smart grids and power quality Modern systems use sensors called phasor
measurement units distributed throughout their network to monitor power quality
and in some cases respond automatically to them. Using such smart grids features
of rapid sensing and automated self healing of anomalies in the network
promises to bring higher quality power and less downtime while simultaneously
supporting power from intermittent power sources and distributed generation,
which would if unchecked degrade power quality.
Notes References
Dugan, Roger C.; Mark McGranaghan; Surya Santoso; H. Wayne Beaty. Electrical
Power Systems Quality. McGraw-Hill Companies, Inc. ISBN 0-07-138622-X.
Meier, Alexandra von. Electric Power Systems: A Conceptual Introduction. John
Wiley & Sons, Inc. ISBN 978-0471178590. Heydt, G.T.. Electric Power Quality.
Stars in a Circle Publications. Library Of Congress 621.3191.
Bollen, Math H.J.. Understanding Power Quality Problems: Voltage Sags and
Interruptions. New York: IEEE Press. ISBN 0-7803-4713-7.
Sankaran, C.. Power Quality. CRC Press LLC. ISBN 0-8493-1040-7.
Baggini, A.. Handbook of Power Quality. Wiley. ISBN 978-0-470-06561-7.
Kusko, Alex; Marc Thompson. Power Quality in Electrical Systems. McGraw
Hill. ISBN 978-0-07-147075-9. IEEE Standard 519 Recommended Practices
and Requirements for Harmonic Control in Electrical Power Systems section 10.5
Flicker