Power quality
Encyclopedia
Power quality is the set of limits of electrical properties that allows electrical system
s 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 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.
etc.
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, voltage is supplied by a utility as sinusoidal having an 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 ohm
s at all frequencies.
No real-life power source is ideal and generally can deviate in at least the following ways:
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.
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
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 arrestor protects against severe spikes.
Electronic filter
s can remove harmonics.
, which would if unchecked degrade power quality.
Grid (electricity)
An electrical grid is a vast, interconnected network for delivering electricity from suppliers to consumers. It consists of three main components: 1) generating plants that produce electricity from combustible fuels or non-combustible fuels ; 2) transmission lines that carry electricity from power...
s to function in their intended manner without significant loss of performance or life. The term is used to describe electric power
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
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
Electricity generation
Electricity generation is the process of generating electric energy from other forms of energy.The fundamental principles of electricity generation were discovered during the 1820s and early 1830s by the British scientist Michael Faraday...
(AC power
AC power
Power in an electric circuit is the rate of flow of energy past a given point of the circuit. In alternating current circuits, energy storage elements such as inductance and capacitance may result in periodic reversals of the direction of energy flow...
), electric power transmission
Electric power transmission
Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to Electrical substations located near demand centers...
and ultimately electricity distribution
Electricity distribution
File:Electricity grid simple- North America.svg|thumb|380px|right|Simplified diagram of AC electricity distribution from generation stations to consumers...
to an electricity meter
Electricity meter
An electricity meter or energy meter is a device that measures the amount of electric energy consumed by a residence, business, or an electrically powered device....
located at the premises of the end user of the electric power. The electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
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
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
—rather than power or electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
—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 magnitudeAmplitudeAmplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
- TransientTransient (oscillation)A transient event is a short-lived burst of energy in a system caused by a sudden change of state.The source of the transient energy may be an internal event or a nearby event...
voltages and currents - HarmonicHarmonics (electrical power)Harmonics are electric voltages and currents that appear on the electric power system as a result of certain kinds of electric loads. Harmonic frequencies in the power grid are a frequent cause of power quality problems.-Causes:...
content in the waveforms
etc.
It is often useful to think of power quality as a compatibility
Compatibility
Compatibility may refer to:* Astrological compatibility* Compatibilism – a philosophical position* Compatibility * Compatibility * Compatibility * Electromagnetic compatibility* Interpersonal compatibility-Computing:...
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, voltage is supplied by a utility as sinusoidal having an amplitude and frequency given by national standards (in the case of mains
Mains electricity
Mains is the general-purpose alternating current electric power supply. In the US, electric power is referred to by several names including household power, household electricity, powerline, domestic power, wall power, line power, AC power, city power, street power, and grid power...
) or system specifications (in the case of a power feed not directly attached to the mains) with an impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
of zero ohm
Ohm
The ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...
s at all frequencies.
No real-life power source is ideal and generally can deviate in at least the following ways:
- Variations in the peakAmplitudeAmplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
or RMSRoot mean squareIn mathematics, the root mean square , also known as the quadratic mean, is a statistical measure of the magnitude of a varying quantity. It is especially useful when variates are positive and negative, e.g., sinusoids...
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 RMSRoot mean squareIn mathematics, the root mean square , also known as the quadratic mean, is a statistical measure of the magnitude of a varying quantity. It is especially useful when variates are positive and negative, e.g., sinusoids...
voltage between 90 and 110% of nominal can produce a phenomenon known as "flickerPower line flickerPower-line flicker is a visible change in brightness of a lamp due to rapid fluctuations in the voltage of the power supply. The source of this is the voltage drop generated over the source impedance of the grid by the changing load current of an equipment or facility. These fluctuations in time...
" 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 "spikesVoltage spikeIn electrical engineering, spikes are fast, short duration electrical transients in voltage , current , or transferred energy in an electrical circuit....
", "impulses", or "surges", generally caused by large inductive loadsElectric motorAn electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
being turned off, or more severely by lightningLightningLightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...
. - "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.
- "OvervoltageOvervoltageWhen the voltage in a circuit or part of it is raised above its upper design limit, this is known as overvoltage. The conditions may be hazardous...
" occurs when the nominal voltage rises above 110% for more than 1 minute. - Variations in the frequencyUtility frequencyThe utility frequency, line frequency or mains frequency is the frequency at which alternating current is transmitted from a power plant to the end-user. In most parts of the world this is 50 Hz, although in the Americas it is typically 60 Hz...
. - Variations in the wave shape - usually described as harmonics.
- Nonzero low-frequency impedanceElectrical impedanceElectrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
(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 dipDIPDip may refer to:* The Dip, in the 1988 film Who Framed Roger Rabbit, a substance which kills animated characters* Dip , an evil demonic dog that drinks people's blood* Dip , a partner dance move...
or spikeVoltage spikeIn electrical engineering, spikes are fast, short duration electrical transients in voltage , current , or transferred energy in an electrical circuit....
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; 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
Uninterruptible power supply
An uninterruptible power supply, also uninterruptible power source, UPS or battery/flywheel backup, is an electrical apparatus that provides emergency power to a load when the input power source, typically mains power, fails...
can be used to switch off of mains power if there is a transient
Transient (oscillation)
A transient event is a short-lived burst of energy in a system caused by a sudden change of state.The source of the transient energy may be an internal event or a nearby event...
(temporary) condition on the line. However, cheaper UPS units create poor-quality power themselves, akin to imposing a higher-frequency and lower-amplitude
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
square wave
Square wave
A square wave is a kind of non-sinusoidal waveform, most typically encountered in electronics and signal processing. An ideal square wave alternates regularly and instantaneously between two levels...
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
Surge protector
A surge protector is an appliance designed to protect electrical devices from voltage spikes. A surge protector attempts to limit the voltage supplied to an electric device by either blocking or by shorting to ground any unwanted voltages above a safe threshold...
or simple capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
or varistor
Varistor
A varistor is an electronic component with a "diode-like" nonlinear current–voltage characteristic. The name is a portmanteau of variable resistor...
can protect against most overvoltage conditions, while a lightning arrestor protects against severe spikes.
Electronic filter
Electronic filter
Electronic filters are electronic circuits which perform signal processing functions, specifically to remove unwanted frequency components from the signal, to enhance wanted ones, or both...
s can remove harmonics.
Smart grids and power quality
Modern systems use sensors called phasor measurement units (PMU) 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 generationDistributed generation
Distributed generation, also called on-site generation, dispersed generation, embedded generation, decentralized generation, decentralized energy or distributed energy, generates electricity from many small energy sources....
, which would if unchecked degrade power quality.