IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 20, NO. 1, JANUARY 2005
375
Sensitivity of Personal Computers to
Voltage Sags and Short Interruptions
S. Ž. Djokić, J. Desmet, Member, IEEE, G. Vanalme, J. V. Milanović, Senior Member, IEEE, and
K. Stockman, Student Member, IEEE
Abstract—This paper discusses the sensitivity of personal computers (PCs) to voltage sags and short interruptions on the basis of
the extensive test results. Existing standards and previously published works are reviewed, and a description of a used testing procedure is presented. The following tests were performed: sensitivity
to rectangular voltage sags with ideal and nonideal supply characteristics, and sensitivity to voltage sags caused by the starting of
large motors. The results obtained emphasize the importance of
clear definition of the malfunction criteria for this equipment.
Index Terms—Malfunction criterion, power quality, short interruption, voltage sag, voltage-tolerance curve.
I. INTRODUCTION
P
ERSONAL computer (PC) is a general-purpose, rather
complex electronic computing device designed to be
operated by one person at a time. Most often, and especially
when used as an abbreviation, this term denotes an IBM-compatible (desktop) computer. The continually increasing ratio of
computing power to cost, coupled with decreasing weight, size,
and power consumption, as well as the introduction of more
efficient and convenient (user-friendly) software influenced
both the rapid development of PCs and extraordinary diversity
of their usage. Today, PCs are standard, if not a basic part of
scientific, engineering, business, industrial, commercial, educational, and other environments, affecting almost all aspects
of life.
PCs can be implemented as “real-time” systems (i.e., for
real-time control of various external devices), for online control
of communication between two or more locations, or as a part
of continuous process-control applications. Such computers
are often called microcontrollers, or programmable logic controllers (PLCs), or programmable electronic devices (PEDs).
On the other hand, PCs can be used as an off-line tool in some
action/time noncritical applications (e.g., offline computing,
word-processing, computer-aided-design (CAD) applications,
etc.). This categorization allows easier assessment of possible
losses and costs related to power-quality disturbances that
yield to malfunction of PCs. Malfunction of PCs incorporated
Manuscript received April 8, 2003; revised October 13, 2003. This work was
supported by the U.K.’s Engineering and Physical Sciences Research Council
(EPSRC) under Grant GR/R40265/01, the Copper Development Association
(U.K.), and Electrotek Concepts Inc. Paper no. TPWRD-00159-2003.
S. Ž. Djokić and J. V. Milanović are with the School of Electrical and Electronic Engineering, The University of Manchester, Manchester M60 1QD, U.K.
J. Desmet, G. Vanalme, and K. Stockman are with the Department Provinciale
Industriële Hogeschool, Hogeschool West-Vlaanderen, Kortrijk, Belgium.
Digital Object Identifier 10.1109/TPWRD.2004.837828
Fig. 1. Rectangular voltage-tolerance curve.
in a real-time system is related to potentially bigger consequences, because there are losses associated to the controlled
system/process too. Malfunction of the offline used computer is
usually associated with substantially lesser consequences (i.e.,
the loss of those pieces of information gained between the last
save/backup and the moment of malfunction).
The sensitivity of PCs to voltage sags is usually expressed
only in terms of the magnitude and duration of the voltage sag.
For this purpose, “rectangular voltage-tolerance curve” shown
in Fig. 1 is used. This curve indicates that voltage sag longer than
the specified duration and deeper than the specified voltage magnitude will lead to a malfunction (i.e., to a restarting/rebooting
of the PC). The reported threshold values for voltage magnitude vary from as low as 30–40% to as high as 80–90% of rated
in Fig. 1) and from as short as 1–2 cycles to as long
voltage (
as ten cycles for the duration (
in Fig. 1) [1].
It may be possible, however, that the voltage sag (or short
interruption) causes lockup of the operations performed by the
computer and/or blockage of its operating system without the
restarting/rebooting of the computer. This would mean that the
voltage-tolerance curves obtained using restarting/rebooting
criterion only are not conservative. Therefore, if only such
voltage-tolerance curve is available, it may be misleading,
especially in the case when the process controlled by the
computer is of particular importance and interest. PCs are
already identified as the equipment very sensitive to various
power-quality disturbances and, therefore, their behavior during
voltage sags and short interruptions should be assessed with
respect to the additional malfunction criteria.
This paper summarizes the results of a comprehensive study
of the behavior of PCs during voltage sags and short interruptions. After reviewing the existing standards and previously
published work, the paper presents extensive experimental
results that demonstrate that the sensitivity of PCs to voltage
sags should not be assessed only regarding the hardware malfunction criterion (i.e., restarting/rebooting of the computer).
Two additional software malfunction criteria that may result in
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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 20, NO. 1, JANUARY 2005
different voltage-tolerance curves for PCs were considered in
the tests: a) lockup of a read/write operation, and b) blockage
of the operating system (OS). Tests were performed with
rectangular voltage sags and an ideal voltage supply, as well
as with nonrectangular sags occurring during the start of the
large motors. The influence of nonideal supply characteristics
was also investigated. The paper presents and discusses the
results of testing of PCs only. The conclusions regarding their
behavior, however, are to some extent also applicable to microcontrollers, PLCs, PEDs, and other microprocessor/CPU-based
devices that utilize the switch-mode power supply.
II. EXISTING STANDARDS AND PREVIOUS RESEARCH
A. Overview of the Existing Standards
1) General Power Quality Standards: Existing power
quality standards [2]–[7] define voltage sags and short interruptions as a short duration variation of voltage of any or
of all phase voltages of a single-phase or a polyphase power
supply at a point in the electrical system. Although there are
slight differences between the various standards, voltage sag
is always expressed and referred to as a root-mean-square
(rms) voltage event. It is defined as: “ sudden or temporary
reduction/decrease of the supply voltage to a value between 1%
(e.g., [7]) or 10% (e.g., [4]) and 90% of the nominal or declared
voltage, at the power frequency, which is followed by voltage
recovery after a short period of time” [2]–[7]. This “short period
of time” is the duration of the voltage sag. It is usually defined
as the time measured from the moment the rms voltage of any
phase drops below 90% of the declared voltage to the moment
when the voltages in all phases rise above 90% of the declared
voltage. In standards, it varies from a 0.5 cycle (e.g., [4], [5]) or
one cycle (e.g., [6]) to a few seconds (e.g., [2], [5], [6]) or even
a minute (e.g., [4]). In other words, voltage sags are generally
defined and described by only two parameters—magnitude and
duration. It is assumed that the voltage magnitude is constant
during the sag (i.e., the sag has a “rectangular” shape). For
example, in [8], a voltage sag is described as “ a two-dimensional (2-D) electromagnetic disturbance, the level of which is
determined by both voltage and time (duration).”
Similarly to voltage sag, standards [2]–[7] define a short interruption as: “ disappearance, or complete loss or reduction
of the supply voltage to a value less than 1% (or 10%) of the
nominal (or declared) voltage, for a time interval whose duration is between a few tenths of a second (or 0.5 cycle) and few
seconds (or minutes).”
2) Standards Related to Computers: Historically, the first
and still one of the most involved community in defining and
solving various power-quality problems was the computer-related community. This community proposed several guidelines
for the equipment manufacturers and for the end users, which
were subsequently incorporated in related standards ([9]–[11],
see Fig. 2). Recently, their interest in this subject even increased,
mainly due to the working strategies associated with the Internet
” and “aland the Web-based computer applications (e.g., “
ways on-line” concepts).
The first standard proposed was the Computer Business
Equipment Manufacturers Association (CBEMA) power
Fig. 2. Power acceptability curves—trends in recommended power
acceptability.
acceptability curve [9]. The curve defines the tolerance level of
“automatic data processing” equipment to voltage sags, swells,
and short interruptions. Although it was originally developed for
the equipment connected to the 120-V/60-Hz ac supply only, the
curve has been widely used to evaluate the performance of various electrical and electronic equipment operating at different
nominal voltages and/or frequencies, simply because there were
no other appropriate standards (curves).
After the extensive research in computer power supplies in
1995, the CBEMA curve was revised and “renamed” to ITI
(CBEMA) or the Information Technology Industry Council
(ITIC) curve [10]. The new curve was constructed in steps,
which simplifies its realization and reproduction for testing and
validating purposes. It should be strictly applied to single-phase
“information technology” equipment with 120-V/60–Hz-rated
conditions. Other voltages and frequencies, as well as usage in
the case of three-phase equipment were not specified.
Most recently [11], a similar curve was proposed for “semiconductor processing” equipment (SEMI F47). It is related only
to sags and undervoltages that last between 50 ms and 1 s (three
and 60 cycles in a 60-Hz frequency supply). This new curve
incorporates a single change with respect to the original ITIC
curve introduced to 50 ms–0.2 s time region. Overvoltages and
exswells are not considered in this standard due to the “
tremely low number of semiconductor equipment interruptions”
caused by these events [11].
It is interesting to note that the region encircled in Fig. 2
and initially identified at the CBEMA curve as a “problematic”
cycle and 10–100 cycles), in two successive
(sags between
curves, still remains as the region of the “most interest.” This is
the region where all three curves (illustrated in Fig. 2) overlap.
3) Standards Related to Testing: Current standards related
to the testing of the equipment sensitivity to voltage sags
and short interruptions suggest that the tests should be performed preferably at 0 point on wave of the voltage waveform
[12]–[14]. Standard [14] concludes that testing of the equipcan start and stop at any phase angle” (preferably
ment “
at 0 ), and suggests testing for additional angles only if they
are “ considered critical by product committees or individual
product specifications.” If so, a range from 0 to 360 (in steps
of 45 ) is recommended for such additional testing. The testing
is always related to simple rectangular voltage sags. There are
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DJOKIĆ et al.: SENSITIVITY OF PCs TO VOLTAGE SAGS AND SHORT INTERRUPTIONS
no suggestions related to testing of the equipment sensitivity
to nonrectangular voltage sags (for example, two-stage voltage
sags), or voltage sags caused by the starting of large motors.
The voltage waveforms used in the test should be the ideal sine
waves at rated frequency. The allowed deviations in magnitude,
frequency, and total harmonic distortion from the ideal voltage
supply conditions (e.g., given in [7]) are not considered.
None of the existing power-quality standards considers the
influence of several disturbances occurring simultaneously (for
example, cumulative effects of both voltage magnitude and harmonic content variations).
B. Overview of the Previous Research
The sensitivity of PCs to voltage sags is addressed in several
references [15]–[19]. Some of these present results of testing
only against short interruptions, some have results for both short
interruptions and voltage sags, but none of them reported different sensitivities with respect to different software/hardware
malfunction criteria.
The results of testing presented in [15] show that the PC may
malfunction (reboot) for voltage sag with the magnitude as high
as 75% of the rated voltage and the duration as short as four
cycles. However, only one table is presented in the paper without
any further discussion.
In [16], effects of various power-quality disturbances on PCs
connected to a local-area network (LAN) were investigated.
Several malfunction criteria were used in testing: lockup of the
PC, slowdown of the network traffic, file corruption, etc. The
behavior of the PCs, however, was assessed only with respect to
short interruptions, but not with respect to voltage sags. It was
concluded that: “ some PC’s can not ride through outages
lasting more than 5–6 cycles ” and that both, the client and
the server PCs (always) need to be rebooted if they cannot
ride-through.
Lack of the response to commands from the keyboard and
restarting of the computer were used as the malfunction criteria
in tests with PCs reported in [17]. There were no observed differences, however, in computer sensitivity for these two criteria.
Reported voltage magnitude and duration thresholds for tested
computers varied between 30–65% of rated voltage and 80–450
ms, respectively. The voltage-tolerance curves developed following the tests were rectangular, having the flat vertical and
flat horizontal part with the sharp knee between them.
Reference [18] reported results of testing of the variety of
equipment used in the semiconductor industry (33 different
types in total). The PCs and PLCs were identified as some of
the most sensitive components to voltage sags.
The results of testing of electrical equipment (including several different PCs) are also presented in [19]. The developed
voltage-tolerance curves are of the same shape as those reported
in [17]. The malfunction criterion for PCs was the automatic reboot occurring due to the voltage sag. Software malfunction criteria, again, were not considered.
Sensitivity of PCs to nonrectangular voltage sags and the influence of nonideal voltage supply conditions on computer behavior during the voltage sags and short interruptions have not
yet been reported in the literature.
377
TABLE I
TEST CONDITIONS
III. TESTING OF PCs
A. List of Tests
Initially, sensitivity of PCs to voltage sags was assessed using
simple rectangular voltage sags. The voltage sags were initiated at one particular point on the voltage sine wave, with fixed
presag, during-sag and postsag voltage magnitudes, and with a
constant phase shift during the sag. Two different presag and
postsag voltage waveforms were used in this part: a) supply
from an ideal voltage sine wave at 50 Hz, and b) supply from a
% variation in voltage
nonideal voltage source with up to
magnitude, up to
% variation in frequency, and with a different harmonic content with total harmonic distortion (THD)
up to 20%. In all cases, the during-sag voltage was an ideal sine
wave with rated frequency (50 Hz). The second set of tests involved nonrectangular voltage sags similar to those caused by
the starting of large motors. Table I summarizes the tests that
were conducted.
B. Testing Procedure
To ensure a high degree of repeatability for of all the tests performed, the testing was conducted according to a well-defined
procedure. The following procedure was used in tests with rectangular voltage sags.
1) The computer (with all input/output (I/O) and pointing
devices connected) was switched on and allowed to boot
and load the operating system.
2) Read/write operation (copying of different files from
CD-ROM to the computer’s hard drive) was initiated.
3) Voltage sags were applied in steps of 1% of rated voltage,
starting from 0 V. The point on wave of the sag initiation
and the phase shift during the sag were both kept constant.
For each voltage sag magnitude, the duration of the sag
was progressively increased until lockup of copying operation was obtained, or the operating system was blocked,
or the computer was forced to restart, or if none of the
former happened up to a few seconds. The critical sag
duration for each of these hardware/software criteria was
ascertained by up to ten repeated measurements for each
value of the sag magnitude. In cases when the tested computer had different sensitivities to voltage sags with respect to different software/hardware criteria, it would first
lock up the read/write operation, then it would block the
OS, and, finally, it would restart. A recovery time of 5–10
s was allocated between the consecutive voltage sags.
4) The point on wave of voltage sag initiation was adjusted
in steps of 15 (from 0 to 360 ) and the measurements
described in Step 3 were repeated.
5) The phase shift during the sag was changed in steps of 15
(from 0 to
) and the measurements described in Step
3 were repeated.
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TABLE II
BASIC DATA OF TESTED PCS
Fig. 3. Examples of the VSG outputs: (a) Programmed voltage sag (50%
magnitude, 100-ms duration, and 100 point on wave). (b) The same voltage
sag measured at the VSG output; voltage (dotted line) and current (solid line)
waveforms of two voltage sags with the same magnitude and point on wave
and different duration c) 50 ms d) 55 ms.
C. Multiplicity of the Voltage-Tolerance Curves
The tests described above may not therefore result in a single
voltage-tolerance curve, but in a set of different curves corresponding to the one hardware and various software malfunction
criteria. Each voltage-tolerance curve obtained in tests consisted
of 101 magnitude-duration pairs, because the voltage is adjusted
in 1% steps of rated voltage. The software criteria used were
lockup of the read/write operation and blockage of the operating
system. The first software criterion (the lockup or corruption
of copying operation from CD-ROM to hard drive) was chosen
because it was shown previously in [20] that the power/current
consumption of the PC is the highest for this operation. It was assumed that the sensitivity of the PC to voltage sags for all other
operations (e.g., copying from network/floppy/hard drive to a
computer’s hard drive, formatting of the floppy disk, opening
the file, etc.) would be between the two chosen software criteria.
D. Voltage Sag Generator
The characteristics of the commercially available voltage sag
generator (Schaffner “Profline 2100”) used in the tests are given
in [21]. Illustrative examples of the output of the voltage sag
generator (VSG) for different sag characteristics are shown in
Fig. 3. These waveforms were measured recorded by the independent data-acquisition system (Dranetz “Power quality analyzer,” Model 658) described in [22]. Preliminary measurements
with this data-acquisition system showed that the VSG can reproduce desired voltage sag characteristics with an error of less
than a few percent for all controlled parameters [see Figs. 3(a)
and (b)]. Keeping in mind that this is the top of the range VSG,
this small error was accepted as the inevitable and unavoidable
at the present time.
E. Testing Conditions and Tested Computers
In order to assess and quantify the effects of the voltage sags
and short interruptions on PCs in the most general way, dif-
ferent computers, made by different manufacturers, with different switch-mode power-supply characteristics, different operating systems installed, and manufactured between 1996 and
2002, were tested. The list of the PCs used in tests is given in
Table II.
All tested computers were in normal exploitation before the
tests were performed. During the testing, a rated voltage (having
ideal or nonideal supply characteristics) was applied to the PCs
before and after the voltage sag.
The first software malfunction criterion (lockup, interruption, or corruption of read/write operation—copying files
from CD-ROM to the computer’s hard drive) was identified
in tests by obtaining the “blue screen,” or “fatal error,” or
“source/task/file cannot be identified” messages. As a consequence of the malfunction, some of the files copied before
the disturbance could not be deleted from the hard drive.
Otherwise, after the termination of the unsuccessful read/write
operation, the computer remained in normal operating state,
with the operating system running.
The second software malfunction criterion (blocking of the
operating system) was identified as a “frozen screen” and a lack
of response to any command issued from the keyboard, pointing
device, or other I/O device.
Hardware malfunction criterion was identified by automatic
restarting/rebooting of the computer, or as a permanent “black
screen” after which manual restarting was necessary. After
restarting the computer (sometimes in the “safe mode”), the
following happened: surface scan disk was performed, screen
resolution, and other settings were changed, and system registry
errors were reported.
IV. TEST RESULTS
A. Testing of Computers With Rectangular Voltage Sags
The results of computer testing are presented in the form of
voltage-tolerance curves.
1) Supply From the Ideal Voltage Source: First, the PCs
were tested using simple rectangular voltage sags. It was found
that voltage-tolerance curves (of all tested PCs) have two
distinctive parts: a flat vertical part, and a flat horizontal part,
with a very sharp knee between them. It was also found that
different points on wave of voltage sag initiation and different
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DJOKIĆ et al.: SENSITIVITY OF PCs TO VOLTAGE SAGS AND SHORT INTERRUPTIONS
379
Fig. 4. Test results for computer PC1.
Fig. 5.
phase shifts during the sag do not have any noticeable influence
on the PC’s behavior during the voltage sag. The detailed test
results for computer PC1 are shown in Fig. 4. It can be seen
that there are significant differences between voltage-tolerance
curves developed using different malfunction criteria.
In Fig. 4, two software criteria used (lockup of read/write operation and blockage of operating system) resulted in a wider
vertical part (a family of vertical lines whose boundaries are
shown in Fig. 4) than the restart/reboot criterion. This is due to
different power consumption related to different stages of operation and different OS execution states. The hardware criterion,
however, resulted in a single vertical line because it is related to
the limit of the switch-mode power supply. There are also significant differences in the horizontal parts of these voltage-tolerance curves. The curve corresponding to the read/write operation lockup criterion has the highest (the least tolerant), and the
curve corresponding to the restart/reboot criterion has the lowest
voltage magnitude (the most tolerant) threshold.
The internal dc voltage at the computer power-supply output
(5-V dc output) was also monitored during the tests. It was found
that for the PC1, it starts to decay at about 200 ms after the initiation of the voltage sag. The voltage sags and interruptions
shorter than 200 ms, therefore, will not influence the behavior
of the PC1 at all. This happens because there is enough energy
stored in a dc bus capacitance of the power supply to maintain correct operation of dc voltage regulator (i.e., correct operation and functioning of the PC1). If the voltage sag lasts between 200–250 ms, and has a magnitude lower than 46% of the
rated voltage, the dc voltage starts to decay, initiating the corruption or interruption of the ongoing process. There is, however,
still enough energy in a dc bus capacitance to maintain functioning of the operating system. If the voltage drops below 36%
of the rated voltage and such condition lasts between 260–380
ms, PC1 becomes useless as its operating system gets blocked.
There is still no shutdown/restart of the computer. Only if the sag
magnitude drops below 21% of the rated voltage and sag lasts
longer than 380 ms, the PC1 will restart. The difference in sensitivity thresholds between curves obtained using the first software criterion (lockup of read/write operation) and the hardware
criterion (restarting/rebooting) is almost 100% for the voltage
sag duration and more than a 100% for the voltage sag magnitude. This difference indicates the range of possible error if
ride-through capabilities of the PC1 were assessed only on the
basis of the hardware malfunction criterion.
Similar sets of voltage-tolerance curves were obtained for all
six tested PCs (see Fig. 5). As with computer PC1, the general
shape of the voltage-tolerance curves is the same: flat vertical
and flat horizontal parts with almost instantaneous transition between them. The voltage sag magnitude threshold for the tested
computers varied between 20–65% of rated voltage, and the duration threshold varied between is 40-400 ms.
The ITIC and SEMI F47 power acceptability curves are also
plotted in Fig. 5 (with thick solid and thick dashed line, respectively). It can be seen from Fig. 5 that all tested PCs satisfy the ITIC power acceptability curve. On the other hand, all
of them except one, violate the most recent SEMI F47 standard. Regarding the SEMI F47 standard, results for computer
PC6 are of a particular interest. If only the hardware criterion
is used for assessing the ride-through capability of this PC, the
relevant voltage-tolerance curve would be practically at the borderline of the SEMI F47 curve. However, the voltage-tolerance
curve related to the software malfunction criterion violates the
SEMI F47 standard and emphasizes the ambiguities associated
with PC6 ride-through capabilities. (Note: The ride-through capability of the computer is usually specified by the manufacturer only for hardware criterion and with regards to some of
the above mentioned power acceptability curves. The inclusion
of the software malfunction criteria in the manufacturer’s specification and the provision of the exact information about the
magnitude and duration thresholds would make purchasing a PC
with appropriate ride-through capability straightforward, even
for the users with sensitive processes).
In addition to the tests described above, two different computers (PC2 and PC3) were tested with the same power supply.
From Fig. 5, it can be seen that they have almost completely
identical voltage-tolerance curves. Although PC2 and PC3 have
very similar specifications, they were assembled using different
hardware components and different I/O and peripheral devices.
This suggests that the behavior of the PC during the voltage sag
and short interruption is influenced more by the characteristics
of its power supply than by its hardware configuration.
One of the tested computers (PC1) had a built-in switch
in a power-supply case that permits connection to either
230-V/50-Hz or 110-V/60-Hz power supply. Different sensitivities obtained for these two specified rated conditions are
illustrated in Fig. 6. Higher sensitivity (about 10–15% for
the vertical part and about 3–5% for the horizontal part) was
obtained in the case of the 110-V/60-Hz power supply for all
Voltage tolerance curves for computers PC1 to PC6.
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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 20, NO. 1, JANUARY 2005
Fig. 6. Computer PC1, different rated supply conditions: 230 V/50 Hz and 110
V/60 Hz.
tested software/hardware criteria. (This can be explained by the
fact that due to the reduced supply voltage, the current limits of
the components were reached faster).
[Note: Very high peak-inrush current was observed (measured) for all tested computers during the transient from
during-sag to postsag voltage waveform. If both the PC and
the monitor/video display unit (VDU) are connected to the
same supply circuit, minimum 10-A C class automatic fuse
is necessary for normal operation (4-A and 6-A fuses would
blow). If only a PC is connected, a 4-A fuse would suffice, but
the inrush current may be up to 20–25 times greater than the
rated current [24]. It was also found that the sensitivity of the
individual PCs to voltage disturbances is not affected if two
computers are connected to the same supply circuit. Finally,
it should be mentioned that one monitor and one computer’s
power supply were permanently damaged during the tests].
2) Supply From the Nonideal Voltage Source: During the
second stage of testing, the PCs were supplied from the nonideal
voltage source. Deviations from the ideal supply characteristics
were within the following limits: voltage magnitude variation up
% of the rated voltage, frequency variations up to
%
to
of the rated frequency, and different harmonics superimposed
to the fundamental frequency waveform with the THD not exceeding 20%. The above disturbances were applied to both, separately and simultaneously, in order to assess their individual
and cumulative effect on the PC’s sensitivity.
The influence of voltage magnitude variation on the PC1 sensitivity to voltage sags is illustrated in Fig. 7.
Results of the tests performed show that the sensitivity thresholds change for all criteria. The change, however, is only in the
vertical part of the related voltage-tolerance curves. If the supply
voltage magnitude is 10% lower than the rated voltage, the sensitivity of the computers to voltage sags increases. If the supply
voltage magnitude is 10% higher than the rated voltage, sensitivity of the computers to voltage sags decreases. (Generally, the
shifting of the vertical line of the sensitivity curve depends on
the amount of energy stored in the dc bus capacitance (
CV ), as it is the only available source of energy during the interruptions and deep sags. With a higher presag supply voltage,
there is more energy stored in the dc bus capacitance than with
a lower presag voltage. The horizontal part, on the other hand,
depends on the amount of the power that can be handled by
the switch-mode power supply. This power depends on the current rating of the switch-mode supply. It is determined by the
Fig. 7.
6
Computer PC1, nonideal supply ( 10% magnitude variation).
Fig. 8. Computer PC1, nonideal supply (third harmonic only, 20% THD).
Fig. 9. Computer PC1, nonideal supply (fifth harmonic only, 20% THD).
minimum dc link voltage that allows normal operation of the
power-supply voltage regulator. As can be seen from Fig. 7, the
horizontal part of the characteristic is independent on the presag
supply voltage).
% of the rated frequency do not
Frequency variations of
have any noticeable influence on the sensitivity of tested PCs.
The influence of different harmonic content (third, fifth, and
seventh harmonic with 20%THD and different phase angles) for
computer PC1 is illustrated in Figs. 8–10.
Different harmonic content and different phase angles of the
harmonics superimposed to the fundamental have significant influence (up to about 30%) on the vertical part of the voltage-tolerance curves. This influence is strongly related to the voltage
crest factor (CF) and the shape of the distorted waveform. The
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DJOKIĆ et al.: SENSITIVITY OF PCs TO VOLTAGE SAGS AND SHORT INTERRUPTIONS
381
Fig. 10. Computer PC1, nonideal supply (seventh harmonic only, 20% THD).
Fig. 12.
Representation of voltage sags due to the startup of large motors.
TABLE III
TIME FOR THE PC1 TO RESTART/REBOOT (IN ms) AS A FUNCTION OF THE
INITIAL VOLTAGE DROP AND THE VOLTAGE GRADIENT
6
Fig. 11. Cumulative influence of nonideal supply ( 10% voltage magnitude
and third harmonic with 20% THD).
CF and the shape of the distorted waveform, on the other hand,
will depend on the magnitude and phase angle of the harmonics
presented in the waveform, so the general conclusion about the
influence of the individual harmonics cannot be drawn. It can
be said though that if the harmonics reduce the CF (e.g., third
with 0 ), the vertical part of the sensitivity curve will move to
the left (increased sensitivity). Otherwise (e.g., 3rd with 180 ),
it will move to the right (decreased sensitivity).
Finally, the cumulative effect of both voltage magnitude
and harmonic content variations (both in magnitude and phase
angle) is illustrated in Fig. 11. Only the most and the least sensitive voltage-tolerance curves are presented (boundary cases)
in this figure. It can be seen that the simultaneous variation of
the presag voltage magnitude and the harmonic content has a
tremendous effect on computer sensitivity to voltage sags. (The
observed variation in the “set” of “critical” sag characteristics
is between 110 V/110 ms and 48 V/560 ms for different malfunction criteria). Similar behavior of all tested computers was
identified when nonideal supply conditions were used.
B. Testing of Computers to Nonrectangular Voltage Sags
The last set of tests was performed with the nonrectangular
voltage sags. Here, the influence of voltage sags, similar to those
caused by the startup of large motors, was investigated.
Fig. 12 illustrates the general shape of this type of sag. After an
initial instantaneous drop down to 30% of the rated voltage, the
voltage recovers progressively to the rated value. Five different
voltage recovery gradients for various initial drops were applied
to PCs. Results related to time needed for the restart/reboot of the
PC1 are shown in Table III. The critical sag duration causing the
restarting of the computer is almost constant (differences are inside 5 ms). This suggests that the vertical part of the voltage tolerance curve is flat and not affected by the shape of the voltage
sag. Similar responses were identified for all tested computers.
V. CONCLUSION
The paper presented the results of the investigation of the behavior of PCs during the voltage sags and short interruptions.
Extensive laboratory tests on different computers were carried out, including testing with rectangular voltage sags, nonideal voltage supply characteristics, and nonrectangular voltage
sags. The results of the tests clearly show that the response of
PCs to power-quality disturbances can be rather complex.
Beside the standard restart/reboot (hardware) malfunction
criterion, two additional software criteria (lockup of read/write
operation and blockage of operating system) were introduced.
It was found that they may result in a substantially higher
sensitivity to voltage sag characteristics than the hardware
criterion.
The following specific conclusions can be drawn from the
tests performed.
• Although the computers tested covered a wide range of
model, type, and hardware configurations, the tests highlighted a few fundamental differences in general behavior
of the computers.
• The voltage-tolerance curves for different computers have
the same rectangular shape with two clearly distinctive
parts: vertical and horizontal, with a very sharp “knee”
between them.
• A wide ranges for both voltage sag magnitude and duration thresholds were identified for tested computers.
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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 20, NO. 1, JANUARY 2005
• No correlation between the age/price/operating system
installed and the sensitivity to voltage sags has been
found/established for tested computers.
• When the computer is designed to operate connected to
either the 230-V/50-Hz or 110-V/60-Hz power supply,
somewhat higher sensitivity is to be expected in the latter
case.
• All tested computers satisfy the ITIC standard voltagetolerance curve. However, all of them except one, violate
the most recent SEMI F47 standard.
• The point on wave of voltage sag initiation and the phase
shift during the voltage sag do not have any significant
influence on the computer sensitivity to voltage sags.
• Nonideal supply characteristics (variation in the supply
voltage magnitude and different harmonic content) influence only the vertical part of the related voltage-tolerance curves (i.e., duration threshold). The identified differences may be significant, especially when variations in
supply-voltage magnitude and harmonic content are introduced simultaneously.
• When two computers are connected in parallel and
supplied from the same power point, they do not influence each other’s sensitivity to voltage sags and short
interruptions.
• For some computers, three different voltage-tolerance
curves were obtained for lockup of read/write operations,
blockage of the operating system, and restart/reboot
malfunction criteria. The shortest duration threshold was
determined for lockup of read/write operations, and the
lowest voltage magnitude threshold for restart/reboot
malfunction criteria.
ACKNOWLEDGMENT
The authors wish to thank the European accredited laboratory
“LEMCKO,” Belgium, for the access and use of their equipment. The authors gratefully acknowledge the contributions
that M. T. Aung, Dr. C. P. Gupta, Prof. D. Kirschen, and Prof.
G. Strbac made to this research project.
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Saša Ž. Djokić received the Dipl.Ing. and M.Sc. degrees in electrical engineering from the University of Niš, Niš, Yugoslavia, and the Ph.D. degree from
the University of Manchester Institute of Science and Technology, Manchester,
U.K.
Currently, he is a Research Associate with the School of Electrical and Electronic Engineering, The University of Manchester, Manchester, U.K., where he
has been since 2001. From 1993 to 2001, he was with the Faculty of Electronics
Engineering at the University of Niš.
Jan J. M. Desmet (M’01) received the polytechnical engineer degree from the
Polytechnic, Kortrijk, Belgium, in 1983, and the M.Sc. degree in electrical engineering from the Vurije Universiteit Brussels, Brussels, Belgium, in 1993.
Currently, he is a Professor with the Department “Provinciale Industriële
Hogeschool,” Hogeschool West-Vlaanderen, Kortrijk, Belgium, where he has
been since 1984.
Mr. Desmet is an IASTED member and a member of SC77A (IEC) and
TC210 (CENELEC).
Greet Vanalme received the M.Sc. degree in electrical engineering and the
Ph.D. degree in sciences from the University of Ghent, Ghent, Belgium, in 1994
and 2000, respectively.
Currently, she is a Researcher with Department “Provinciale Industriële
Hogeschool,” Hogeschool West-Vlaanderen, Kortrijk, Belgium.
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DJOKIĆ et al.: SENSITIVITY OF PCs TO VOLTAGE SAGS AND SHORT INTERRUPTIONS
Jovica V. Milanović (M’95–SM’98) received the Dipl.Ing. and M.Sc. degrees
from the University of Belgrade, Belgrade, Yugoslavia, and the Ph.D. degree
from the University of Newcastle, Newcastle, Australia.
Currently, he is a Reader with the School of Electrical and Electronic Engineering, The University of Manchester, Manchester, U.K., where he has been
since 1998. He was also with “Energoproject-MDD,” Belgrade, Yugoslavia,
consulting and engineering company, as well as the Universities of Belgrade
in Yugoslavia and Newcastle and Tasmania in Australia. His research interests
include power quality and power system dynamics.
383
Kurt Stockman (M’02) was born in Kortrijk, Belgium, on September 24,
1972. He received the industrial engineer degree in electrical engineering from
“Provinciale Industriële Hogeschool,” Kortrijk, Belgium, in 1994, and the
Ph.D. degree from Katholieke Universiteit Leuven, Leuven, Belgium, in 2003.
Currently, he is with the Department of Electrical Engineering of the
Hogeschool West-Vlaanderen, Kortrijk, Belgium.
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