Download Power Quality Issues, Disturbance Sources, Financial Impacts

Power Quality Issues, Disturbance Sources, Financial
Impacts, Control, Monitoring & Remedies
Brief power quality tutorials for engineers






Sags, dips, and swells: Introduction to the most common disturbance on AC mains
Transient overvoltages: Introduction to capacitor-switching and high-frequency transients
Harmonics: Introduction to voltage and current harmonics issues
Flicker: Introduction to voltage flicker
Voltage regulation: Introduction to voltage regulation issues
Other disturbances: Frequency variations, noise bursts, and other less common problems
Industry Standards

IEEE power quality standards
o
o
o
o
o
o
o
o
o
IEEE SCC-22: Power Quality Standards Coordinating Committee
IEEE 1159:Monitoring Electric Power Quality
 IEEE 1159.1: Guide For Recorder and Data Acquisition Requirements
 IEEE 1159.2: Power Quality Event Characterization
 IEEE 1159.3: Data File Format for Power Quality Data Interchange
IEEE P1564:Voltage Sag Indices
IEEE 1346:Power System Compatibility with Process Equipment
IEEE P1100: Power and Grounding Electronic Equipment (Emerald Book)
IEEE 1433: Power Quality Definitions
IEEE P1453: Voltage flicker
IEEE 519: Harmonic Control in Electrical Power Systems
IEEE Harmonics Working Group
 Single-phase Harmonics Task Force
 IEEE P519A Guide for Applying Harmonic Limits on Power Systems




o
Interharmonics Task Force
Harmonics Modeling and Simulation Task Force
Probabilistic Aspects of Harmonics Task Force
Surge Protective Devices Committee
Seventeen sub-committee links can be found at the "Sub-committee pages" link...
o
o
o


IEEE P446: Emergency and standby power
IEEE P1409: Distribution Custom Power
IEEE P1547: Distributed Resources and Electric Power Systems Interconnection
IEC 61000-4-11 - voltage sag immunity - 16 amps or less
IEC 61000-4-34 - voltage sag immunity - more than 16 amps

IEC 61000-4-30 - Power quality measurement methods
General IEC power quality standards
o
o
IEC Power quality standards - numbering system
 61000-1-X - Definitions and methodology
 61000-2-X - Environment (e.g. 61000-2-4 is compatibility levels in industrial
plants)
 61000-3-X - Limits (e.g. 61000-3-4 is limits on harmonics emissions)
 61000-4-X - Tests and measurements (e.g. 61000-4-30 is power quality
measurements)
 61000-5-X - Installation and mitigation
 61000-6-X - Generic immunity & emmissions standards
IEC SC77A: Low frequency EMC Phenomena -- essentially equivalent of "power quality"
in American terminology
 TC 77/WG 1: Terminology (part of the parent Technical Committee)
 SC 77A/WG 1: Harmonics and other low-frequency disturbances
 SC 77A/WG 6: Low frequency immunity tests
 SC 77A/WG 2: Voltage fluctuations and other low-frequency disturbances
 SC 77A/WG 8: Electromagnetic interference related to the network frequency
 SC 77A/WG 9: Power Quality measurement methods
 SC 77A/PT 61000-3-1: Electromagnetic Compatibility (EMC) - Part 3-1: Limits Overview of emission standards and guides. Technical Report
Common Power Problems
Here are some of the most common power supply problems and their likely effect on sensitive
equipment:
Power Surges
A power surge takes place when the voltage is 110% or more above normal. The most common cause is
heavy electrical equipment being turned off. Under these conditions, computer systems and other high
tech equipment can experience flickering lights, equipment shutoff, errors or memory loss.
Possibile Solutions: Surge Suppressors, Voltage Regulators, Uninterruptable Power Supplies, Power
Conditioners
High-Voltage Spikes
High-voltage spikes occur when there is a sudden voltage peak of up to 6,000 volts. These spikes are
usually the result of nearby lightning strikes, but there can be other causes as well. The effects on
vulnerable electronic systems can include loss of data and burned circuit boards.
Possibile Solutions: Surge Suppressors,Voltage Regulators, Uninterruptable Power Supplies, Power
Conditioners
Transients
Transients are potentially the most damaging type of power quality disturbance that you may
encounter. Transients fall into 2 categories.


Impulsive
Oscillatory
For more details about transients view our newsletter archive here.
Possibile Solutions: Surge Suppressors, Voltage Regulators, Uninterruptable Power Supplies, Power
Conditioners
Frequency Variation
A frequency variation involves a change in frequency from the normally stable utility frequency of 50 or
60 Hz, depending on your geographic location. This may be caused by erratic operation of emergency
generators or unstable frequency power sources. For sensitive equipment, the results can be data loss,
program failure, equipment lock-up or complete shut down.
Possibile Solutions: Voltage Regulators, Power Conditioners
Power Sag
A sag is the reduction of AC Voltage at a given frequency for the duration of 0.5 cycles to 1 minute’s
time. Sages are usually caused by system faults, and often the result of switching on loads with high
demand startup currents. For more details about power sags visit our newsletter archives.
Possibile Solutions: Voltage Regulators, Uninterruptable Power Supplies, Power Conditioners
Electrical Line Noise
Electrical line noise is defined as Radio Frequency Interference (RFI) and Electromagnetic Interference
(EMI) and causes unwanted effects in the circuits of computer systems. Sources of the problems include
motors, relays, motor control devices, broadcast transmissions, microwave radiation, and distant
electrical storms. RFI, EMI and other frequency problems can cause equipment to lock-up, and data
error or loss.
Possibile Solutions: Voltage Regulators, Uninterruptable Power Supplies, Power Conditioners
Brownouts
A brownout is a steady lower voltage state. An example of a brownout is what happens during peak
electrical demand in the summer, when utilities can't always meet the requirements and must lower the
voltage to limit maximum power. When this happens, systems can experience glitches, data loss and
equipment failure.
Possibile Solutions: Voltage Regulators, Uninterruptable Power Supplies, Power Conditioners
Blackouts
A power failure or blackout is a zero-voltage condition that lasts for more than two cycles. It may be
caused by tripping a circuit breaker, power distribution failure or utility power failure. A blackout can
cause data loss or corruption and equipment damage.
Voltage sags (dips) and swells
Voltage sags -- or dips which are the same thing -- are brief reductions in voltage, typically lasting from
a cycle to a second or so, or tens of milliseconds to hundreds of milliseconds. Voltage swells are brief
increases in voltage over the same time range.
(Longer periods of low or high voltage are referred to as "undervoltage" or "overvoltage".)
Voltage sags are caused by abrupt increases in loads such as short circuits or faults, motors starting,
or electric heaters turning on, or they are caused by abrupt increases in source impedance, typically
caused by a loose connection. Voltage swells are almost always caused by an abrupt reduction in load
on a circuit with a poor or damaged voltage regulator, although they can also be caused by a damaged
or loose neutral connection.
A typical voltage sag.
Voltage sags are the most common power disturbance. At a typical industrial site, it is not unusual to
see several sags per year at the service entrance, and far more at equipment terminals.
Voltage sags can arrive from the utility; however, in most cases, the majority of sags are generated
inside a building. For example, in residential wiring, the most common cause of voltage sags is the
starting current drawn by refrigerator and air conditioning motors.
Sags do not generally disturb incandescent or fluorescent lighting. motors, or heaters. However, some
electronic equipment lacks sufficient internal energy storage and, therefore, cannot ride through sags
in the supply voltage. Equipment may be able to ride through very brief, deep sags, or it may be able
to ride through longer but shallower sags.
1996 Version of the IT Industry Tolerance Curves (update from original CBEMA curve). The vertical axis
is percent of nominal voltage. "Well-designed" equipment should be able to tolerate any power event
that lies in the shaded area. Note that the curve includes sags, swells, and transient overvoltages.
The semiconductor industry developed a more recent specification (SEMI F47) for tools used in the
semiconductor industry in an effort to achieve better ride through of equipment for commonly
occurring voltage dips and therefore improving the overall process performance. It is basically the
same as the ITI Curve but specifies an improved ride through requirement down to 50% retained
voltage for the first 200 msec. Many short voltage dips are covered by this additional requirement. IEC
61000-4-11 and IEC 61000-4-34 provide similar voltage dip immunity standards.
Many utilities have benchmarked performance of the supply system for voltage dips but it has not
been the general practice to specify any required performance levels for the system. Performance is
often specified using the SARFI index that provides a count of all events with magnitudes and
durations outside of some specifications. For instance, SARFI-70 would provide a count of all voltage
dips with a retained voltage less than 70% (regardless of duration). SARFI-ITIC would provide a count
of all voltage dips that exceeded the ride through specifications of the ITI Curve.
The table below provides a summary of voltage dip performance levels from a few major
benchmarking efforts. Note that these are average performance levels and it would not be reasonable
to develop limits based on an average expected performance (although these are the correct values to
use when evaluating the economics of investments in ride through solutions).
Example of average voltage dip performance from major benchmarking projects. These values
represent voltage dip performance on medium voltage systems.
The voltage dip performance can vary dramatically for different kinds of systems (rural vs urban,
overhead vs underground). It may be important to include some of these important factors in the
specification of the power quality grades.
It will also be important to specify the performance for momentary interruptions. These events can be
a particular problem for customers and are not included in most assessments of reliability.
A previous CEA Technologies report prepared by Electrotek Concepts recommended that the SARFI
indices be calculated for the following magnitude and duration categories:
Recommended magnitude and duration categories for calculating voltage dip performance.
The reasons for these categories were explained as follows:





The 90% level provides an indication of performance for the most sensitive equipment.
The 80% level corresponds to an important break point on the ITI curve and some sensitive
equipment may be susceptible to even short sags at this level.
The 70% level corresponds to the sensitivity level of a wide group of industrial and commercial
equipment and is probably the most important performance level to specify.
The 50% level is important, especially for the semiconductor industry, since they have
adopted a standard that specifies ride through at this level.
Interruptions affect all customers so it is important to specify this level separately. These will




usually have longer durations than the voltage sags.
The first range of durations is up to 0.2 seconds (12 cycles at 60 Hz). This is the range specified
by the semiconductor industry that equipment should be able to ride through sags as long as
the minimum voltage is above 50%.
The second range is up to 0.5 seconds. This corresponds to the specification in the ITIC
standard for equipment ride through as long as the minimum voltage is above 70%. It is also
an important break point in the definition of sag durations in IEEE 1159 (instantaneous vs.
momentary).
The third duration range is up to 3 seconds. This is an important break point in IEEE 1159 and
in IEC standards (momentary to temporary).
The final duration is up to one minute. Events longer than one minute are characterized as
long duration events and are part of the system voltage regulation performance, rather than
voltage sags.
As a final note, remember that voltage sags are voltages, and therefore always occur between two
conductors - there is no such thing as a "sag on phase A" -- it must be a sag between phase A and
phase B, or a sag between phase A and Neutral.
Sources of Voltage Sags
Voltage sags are brief reductions in the voltage on ac
power systems. (The American "sag" and the British "dip"
have exactly the same meaning, and may be used
interchangeably.) How brief? Between 1/2 cycle and a few
seconds. Disturbances that last less than 1/2 cycle are
commonly called "low frequency transients"; voltage
reductions that last longer than a few seconds are
commonly called "undervoltage."
A typical voltage sag, graphed as an
RMS voltage vs. time. This sag
affected a three-phase system,
dropping the voltage to 22.5% of
nominal
for
0.236
seconds.
(Captured and displayed by a PSL
PQube)
Voltage sags have two main characteristics: depth and
duration. Do a scatter plot of depth vs. duration, and you
get a CBEMA or ITIC graph.
Power systems have non-zero impedances, so every increase in current causes a corresponding
reduction in voltage. Usually, these reductions are small enough that the voltage remains within
normal tolerances. But when there is a large increase in current, or when the system impedance is
high, the voltage can drop significantly. So conceptually, there are two sources of voltage sags:


Large increases in current
Increases in system impedance
As a practical matter, most voltage sags are caused by increases in current.
It is convenient to think of the power system as a tree, with your sensitive load connected to one
of the twigs. Any voltage sag on the trunk of the tree, or on a branch leading out to your twig, will
cause a voltage sag at your load. But a short circuit out on a distant branch can cause the trunk
voltage to diminish, so even faults in a distant part of the tree can cause a sag at your load.
Most voltage sags originate within your facility. The three
most common causes of facility-sourced voltage sags are:



Starting a large load, such as a motor or resistive
heater. Electric motors typically draw 150% to
500% of their operating current as they come up to
speed. Resisitive heaters typically draw 150% of
their rated current until they warm up.
Loose or defective wiring, such as insufficiently
tightened box screws on power conductors. This
effective increases your system impedance, and
exaggerates the effect of current increases.
Faults or short circuits elsewhere in your facility.
Although the fault will be quickly removed by a
fuse or a circuit breaker, they will drag the voltage
down until the protective device operates, which
can take anywhere from a few cycles to a few
seconds.
The most common cause of voltage
sags: starting a large load, such as a
motor or a computer room. The
motor might be a pump, a
compressor, a fan, part of an HVAC
system...
Experts can identify the specific source of a voltage sag with an advanced power quality monitor,
such as those found at PQMonitoring.com.
Voltage sags can also originate on your utility's electric
power system. The most common types of utility-sourced
voltage sags are:

Faults on distant circuits, which cause a
corresponding reduction in voltage on your circuit.
Typically, these faults are removed by "reclosers",
or self-resetting circuit breakers. These reclosers
typically delay 1 to 5 seconds before self-resetting. Occasionally, a voltage sag will
If the fault is still present when the recloser resets, originate on the utility grid. But inyou may see a series of voltage sags, spaced 1 to 5 plant causes are far more common.
seconds apart. Faults on utility systems may be
phase-to-phase, or phase-to-earth; depending on

the transformers between you and the fault, you
will see different levels of voltage reduction.
Voltage regulator failures are far less common.
Utilities have automated systems to adjust voltage
(typically using power factor correction capacitors,
or tap switching transformers), and these systems
do occasionally fail.
It is important to understand the source of the voltage sags before trying to eliminate them,
because the wrong solution can actually make the problem worse. For example, if you install a
ferro-resonant transformer as a voltage regulator, or a battery-operated UPS (a reasonable and
common approach), but inadvertently install it upstream from the motor that is causing your
voltage sags, the voltage sags will get worse, not better.
In most cases, the correct solution is to adjust the equipment so that it is less sensitive to voltage
sags.
Information about voltage sag sensitivity
Why does equipment fail when there are voltage sags on ac power systems? There is one obvious
way, and four not-so-obvious ways.
1. Equipment fails because there isn't
enough voltage. This is the obvious
way -- if there is not enough voltage
on the ac power system to provide the
energy that the equipment needs, it is
going to fail. Actually, the problem is
slightly more subtle. In a typical
sensitive load, the ac voltage is
rectified and coverted to pulsed dc.
With a bridge rectifier, the pulsing will
typically be either twice the power
line frequency (for single-phase loads)
or six times the power line frequency
(for three-phase loads). This pulsing
DC is stored in a filter capacitor, which
in turn supplies smooth DC as raw
material for the rest of the power
supply: regulators, etc.
Filter capacitors store voltage in power supplies (yellow
trace). If their voltage drops below a critical level (typically
several cycles after the sag begins), there will not be
enough voltage for the rest of the power supply to operate
properly.
If the DC supplied by the filter capacitor drops below some critical level, the regulators will not be
able to deliver their designed voltage, and the system will fail. Note that the filter capacitor always
stores energy, so there is always an ability to ride through some sags -- after all, the ac power
system delivers zero voltage 100 or 120 times each second! But with a deep enough sag that lasts
long enough, the filter capacitor voltage will drop below a critical level.
2. Equipment fails because an
undervoltage circuit trips. Careful
system designers may include a circuit
that monitors the ac power system for
adequate voltage. But "adequate
voltage" may not be well defined, or
understood. For example, if the
sensitive system is running at half
load, it may be able to operate at only
70% ac voltage, even though it may be
specified to operate with 90% - 110% Quick-operating relays, such as this "ice-cube" relay, can
ac voltage. So the voltage sags to 70%; inadvertently shut down sensitive systems during voltage
the equipment can operate without a sags, especially in EMO circuits.
problem; but the undervoltage
monitor may decide to shut the
system down.
3. Equipment fails because an
unbalance relay trips.On three-phase
systems, voltage sags are often
asymmetrical (they affect one or two
phases more than the remaining
phases). Three-phase motors and
transformers can be damaged by
sustained voltage unbalance; it can
cause the transformer or motor to
overheat. So it makes sense to put in
an unbalance relay, which is a device
that shuts down the system if the
voltage unbalance exceeds some
threshold, typically a few percent.
Unbalance relays, if their trip time is set too short, can shut
a system down during a hamrless voltage sag. Typically,
you can adjust both the trip delay and the re-start delay.
Some relays combine unbalance, undervoltage tripping.
But a voltage sag that causes 20-50% unbalance for a second or two is never going to cause a motor
or transformer to overheat. It just doesn't last long enough. Still, unbalance relays with inadequate
delays can cause the sensitive system to shut down, even for a brief voltage sag.
4. A quick-acting relay shuts the
system down, typically in the EMO
circuit. The EMO (emergency off)
circuit in an industrial load typically
consists of a normally-closed switch
that can disconnect power to a
latched relay coil. If the relay operates
Emergency Off circuits can inadvertently shut equipment
quickly enough, it may interpret a down during brief, harmless voltage sags.
brief voltage sag as an operator hitting
the EMO switch. The whole system
will shut down unnecessarily.
5. A reset circuit may incorrectly trip
at the end of the voltage sag. This is
the most subtle problem caused by
voltage sags. Many electronic reset
circuits are designed to operate at
"power up" -- when you first turn on
the equipment, these circuits will
ensure that the microprocessors all
start up properly, the latches are all Reset circuits deserve close attention; they can improperly
properly initialized, the displays are in reset a system, or part of a system, at the end of a voltage
their correct mode, etc. These circuits sag.
are difficult to design, because they
must operate correctly when power is
uncertain.
One common design detects a sudden increase in voltage, which always happens when you turn the
equipment on. Unfortunately, it also happens at the end of a voltage sag. If the reset circuit
misinterprets the end of a voltage sag, the equipment will operate perfectly during the voltage sag,
but will abruptly reset itself when the voltage returns to normal.
To make this problem even more difficult, it is quite common for different parts of a system to have
different reset circuits, so it is possible for one part of the system to be reset even when the rest of
the system is not. Without a sag generator with a good data acquisition system, this problem is very
difficult to detect and solve.
Information about Transient Overvoltages
Transient overvoltages are brief, high-frequency increases in voltage on
AC mains.
Broadly speaking, there are two different types of transient overvoltages:
low frequency transients with frequency components in the few-hundredhertz region typically caused by capacitor switching, and high-frequency
transients with frequency components in the few-hundred-kilohertz
region typically caused by lighting and inductive loads.
Low frequency transients are often called "capacitor switching transients".
High frequency transients are often called "impulses", "spikes", or
"surges".
Surge suppressors are devices that conduct across the power line when
some voltage threshold is exceeded. Typically, they are used to absorb the
energy in high frequency transients. However, the resulting high
frequency current pulses (often in the hundreds of amps) can still create
problems for sensitive
instrumentation.
electronic
systems,
especially
delicate
Low frequency transients are caused when a discharged power-factorcorrection capacitor is switched on across the line. The capacitor then
resonates with the inductance of the distribution system, typically at 400 600 Hz, and produce and exponentially damped decaying waveform. The
peak of this waveform, in theory, cannot exceed twice the peak voltage of
the sine wave, and is more typically 120% - 140% of the sine peak.
However, in some specific cicumstances, there can be "multiplication" of
this transient by resonance with other power factor correction capacitors.
High frequency transients are caused by lightning, and by inductive loads
turning off. Typical rise times are on the order of a microsecond; typical
decay times are on the order of a tens to hundreds of microseconds.
Often, the decay will be an exponential damped ringing waveform, with a
frequency of approximately 100 kHz, which corresponds to the frequency
of equivalent inductor/capacitor model of low voltage power lines. Typical
peak voltages for end-use applications are hundreds of volts to a few
thousand volts; several thousand amps of current may be available.
(Extremely fast transients, or EFT's, have rise and fall times in the
nanosecond region. They are caused by arcing faults, such as bad brushes
in motors, and are rapidly damped out by even a few meters of
distribution wiring. Standard line filters, included on almost all electronic
equipment, remove EFT's.)
About Harmonics
The electric power distribution system is designed to operate with sinusoidal voltages and
currents.
But not all waveforms are sine waves. Electronic loads, for example, often draw current only
at the peak of the voltage waveform, which always means that the current is distorted, and
may distort the voltage as well. One convenient way to describe these waveforms is to make
a list of sine waves that, when added together, reproduce the distorted waveform. The sine
waves in this list are always multiples, or harmonics, of the fundamental frequency (50 Hz or
60 Hz).
A typical input circuit of a single-phase supply.
All of the graphs below are automatically produced by the Industrial Power Corruptor's
Power Flow Option.
A typical distorted current waveform, drawn by the supply above. It only draws current at the
peak of the voltage waveform, because the diodes in BR1 only conduct when the AC voltage is
higher than the voltage on C1.
This is the same waveform, expressed as a frequency spectrum. Note that the frequency
content of the waveform consists of odd multiples (3,5,7,9, etc.) of the fundamental. This is
typical for electronic loads.
Again, the same waveform, expressed as a frequency spectrum. This time the values are
listed. Sometimes, the phase angles of the harmonics can be important, too, but they are not
shown
here.
THD, or Total Harmonic Distortion, is one measure of the total distortion. It is the RMS sum of
the harmonics, divided by one of two values: either the fundamental value, or the RMS value
of the total waveform. Both are legitimate definitions of THD. For small values of distortion,
they both produce roughly the same number. For the waveform above, using the
fundamental as the reference produces a THD value of 93.2%, and using the RMS as the
reference produces a THD value of 67.8%. Both values are correct.
For this and other reasons, most experts in power system harmonics frown on using THD as a
measure of harmonics. Other measures such as TDD (IEEE 519) or volts and amps make more
sense. For example, the waveform above consists of 32.4 amps at 60 Hz, plus 25.4 amps at
180 Hz, plus 14.8 amps at 300 Hz, etc.
Many devices on the power system respond poorly to non-sinusoidal waveforms.
Transformers, for example, become less efficient. Many revenue meters become less
accurate. Protection devices such as circuit breakers may trip too soon, or too late.
Balanced harmonics at multiples-of-3-of-the-fundamental, or triplen harmonics (3rd, 9th,
15th, etc.), fail to rotate on three-phase systems. As a result, neutral conductors may
overheat, and transformers and motors become less efficient.
Information about Flicker
Flicker is a very specific problem related to human
perception
and
incandescent
light
bulbs.
It is not a general term for voltage variations.
Humans can be very sensitive to light flicker that is caused by voltage fluctuations.
Human perception of light flicker is almost always the limiting criteria for controlling small
voltage fluctuations. The figure illustrates the level of perception of light flicker from a 60 watt
incandescent bulb for rectangular variations. The sensitivity is a function of the frequency of
the fluctuations and it is also dependent on the voltage level of the lighting.
Voltage changes that will result in perceptible light flicker with a 60 watt incandescent light
bulb.
Limits for flicker levels are not specified in IEEE standards. Curves similar to the one shown
above have been used by individual utilities as guidelines for controlling flicker.
Flicker levels in IEC standards are characterized by two parameters:


Pst is a value measured over 10 minutes that characterizes the likelihood that the
voltage fluctuations would result in perceptible light flicker. A value of 1.0 is designed
to represent the level that 50% of people would perceive flicker in a 60 watt
incandescent bulb.
Plt is derived from 2 hours of Pst values (12 values combined in cubic relationship).
Note that IEEE is also adopting this method of characterizing flicker (IEEE 1453).
IEC 61000-2-2 specifies flicker compatibility levels:


Compatibility level for short term flicker (Pst) is 1.0.
Compatibility level for long term flicker (Plt) is 0.8.
Recognizing that it is not always possible to maintain flicker levels within these compatibility
levels, EN 50160 specifies less restrictive requirements for the supply system performance. The
EN 50160 limit is that 95% of the long term flicker values (Plt) should be less than 1.0 in one
week measurement period.
Note that individual step changes in the voltage, such as would be caused by motor starting or
switching a capacitor bank, are often limited separately from the continuous flicker limits. IEC
61000-2-2 specifies a compatibility level of 3% for the individual voltage variations. EN 50160
specifies a limit of 5% for these variations but mentions that more significant variations (up to
10%) can occur for some switching events. Specific recommendations are not provided in IEEE
but individual utilities usually have their own guidelines in the range 4-7%.
Information about Voltage Regulation
The term "voltage regulation" is used to discuss long-term variations in voltage. It does not
include short term variations, which are generally called sags, dips, or swells.
The ability of equipment to handle steady state voltage variations varies from equipment to
equipment. The steady state voltage variation limits for equipment is usually part of the
equipment specifications. The Information Technology Industry Council (ITIC) specifies
equipment withstand recommendations for IT equipment according to the ITI Curve (formerly
the CBEMA curve). The 1996 ITI Curve specifies that equipment should be able to withstand
voltage variations within ± 10% (variations that last longer than 10 seconds).
Voltage regulation standards in North America vary from state to state and utility to utility. The
national standard in the U.S.A. is ANSI C84.1. Voltage regulation requirements are defined in
two categories:


Range A is for normal conditions and the required regulation is ± 5% on a 120 volt base
at the service entrance (for services above 600 volts, the required regulation is -2.5% to
+5%).
Range B is for short durations or unusual conditions. The allowable range for these
conditions is -8.3% to +5.8%. A specific definition of these conditions is not provided.
Voltage regulation requirements from ANSI C84.1. This is not a universal standard; it is only used
in North America.
Other countries have different standards. For example, IEC 61000-2-2 mentions that the normal
operational tolerances are ± 10% of the declared voltage. This is the basis of requirements for
voltage regulation in EN 50160 for the European Community. EN 50160 requires that voltage
regulation be within ± 10% for 95% of the 10 minute samples in a one week period, and that all
10 minute samples be within -15% to +10%, excluding voltage dips.
Other Disturbances
The most common disturbances on AC power systems are voltage sags or dips. Other problems,
such as transient overvoltages and brief interruptions, occur almost everywhere. Problems with
harmonics, voltage regulation, and flicker occur at a wide range of sites.
Some other disturbances that occur at specific locations include:





Frequency variations. On utility grids, these are rare events, usually associated with
catastrophic collapses on the grid. However, at sites with back-up diesel generators, they
are common.
High frequency noise. This can be caused by anything from arcing brushes on a motor, to
local radio transmitters.
Mains signalling Some utilities intentionally place small signals on the mains voltage to act
as control signals (for example, they may control a capacitor switch, or they may instruct
revenue meters to go to a different rate structure).
EFT Extremely Fast Transients are nano-second range transient overvoltages. Due to their
high frequency content, they do not travel well over the mains circuits, getting damped out
within a few meters. However, they can be caused by nearby contact arcing.
Unbalance On three-phase systems, the voltages and currents on each phase should, in
theory, match the voltages and currents on the other phases. Sometimes they don't.