CIRED
17th International Conference on Electricity Distribution
Barcelona, 12-15 May 2003
SYSTEM ORIGINATING DIPS, SHORT INTERRUPTIONS, SWELLS AND "CANADIAN POWER
QUALITY SURVEY 2000"
Francisc Zavoda
Hydro-Québec (IREQ) - Canada
zavoda.francisc@ireq.ca
Mario Tremblay
Hydro-Québec (IREQ) - Canada
tremblay.mario@ireq.ca
INTRODUCTION
In September 2001, the results of the “Canadian Power
Quality Survey 2000” [2] were published. This wide survey
involving 9 major utilities (Newfoundland Power, Nova Scotia
Power, Hydro-Quebec, Toronto Hydro, Hydro One Networks,
Manitoba Hydro, SaskPower, ATCO Electric, BC Hydro) was
led by CEA Technologies Inc. (CEATI) and its Power Quality
Interest Group (PQIG.
The survey was based on the "Canadian Power Quality
Measurement Protocol" [1], which includes guidelines to
measurements techniques and to categorize the population to
be surveyed.
CANADA PQ 2000 SURVEY
A total of 413 sites across Canada, selected randomly by the
utilities, were surveyed over a total period of 4 years. These
sites were classified in different categories, following the
guidelines of [1]. Based on voltage, load density, type of
customer etc., out of the 28 initial categories proposed in the
guide, 19 categories were acknowledged, considering they
will give an accurate representation for the population. The
classification criteria of de population are not discussed in this
paper, they are largely presented in [1] but it shall be
mentioned that the type of MV distribution power line
supplying the customer gave the site category.
The Mini-AQO, a Power Quality analyser based on the
measurement techniques described in [1], was selected for this
survey. These techniques followed standards and best
measurement practices available at the moment of the survey.
Details are available in [1].
Each site was surveyed for major power quality indices
(Frequency, Steady State Voltages, Voltage Unbalance,
Flicker, Harmonics Voltages and Currents, Transients and,
finally, Voltages Dips, Swells and Short Interruptions which
are the main object of this paper) for at least 1 complete week
and the monitoring was held at the point of common coupling
(PCC).
For each Power Quality index, a specific analysis was done to
insure the validity of the data. Moreover a statistical analysis
of data was completed to determine the confidence level of
each result corresponding to each index monitored.
With the original publication of the survey [2], the results
have permitted to establish the performance of the baseline
electricity product for Canadian utilities within the desired
IRE_Zavoda_A1
Session 2 Paper No 52
Georges Simard
Hydro-Québec - Canada
simard.georges@hydro.qc.ca
confidence level. However for indices that are not steady state
like dips, swells and short interruptions, there was still more
investigation to perform before giving final results.
Because only raw numbers were available in September 2001
for voltage dips, swells and short interruption, a new project
“Sag, Swell and Short Interruption evaluation from the
Canadian PQ Survey 2000” [3] was granted by CEATI to
review the raw data and to aggregate those numbers according
to the latest standards and recommended practices on this
matter (IEC 61000-2-8 [4], IEEE P1564 [5], IEC 61000-4-30
[6] etc.)
This paper details the theory that was used to aggregate data
on dips, swells and short interruptions from [2]. It will cover
the modification made to the analyzer's software in order to
follow the evolution of standards and to reflect a more
realistic way to measure voltage dips, swells and short
interruptions. Also it will describe the manual analysis that
was conducted according to the best practices for phase and
temporal aggregation, leading to the final aggregated results.
These discussions will hopefully contribute to the
development of voltages dips, swells and short interruption
measurements and standards.
MINI-AQO’S MEASUREMENT TECHNIQUES
FOR
DIPS,
SWELLS
AND
SHORT
INTERRUPTIONS AND THEIR LIMITATIONS
The Mini-AQO’s measurement techniques for these types of
disturbances are based on the "Canadian Power Quality
Measurement Protocol" [1]. The half cycle measurement
period, which was selected, has certain limitations. This
section discusses analyser's limitations related to dip, swell
and short interruption measurement.
Initially, the measurement technique was based on formula
(1), representing a trapezoidal integration technique. It was
used for the assessment of RMS half-cycle values during the
"Canadian PQ Survey 2000".
(1)
Prior to the calculation, each point Ui is calibrated for
correcting the internal voltage offset of the analyser. N, the
number of points depends on 12 cycles window used for the
steady state analysis. 2048 points are sampled during that
interval, which correspond to almost 85 or points for each
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17th International Conference on Electricity Distribution
half-cycle period.
A Phase-Locked Loop (PLL) is used for the adjustment of that
sampling rate. Sometimes PLL synchronization is lost and the
sampling rate is affected for more than a second, until the PLL
resynchronisation. During that period, the RMS value
calculated over half-cycle is distorted because of sampling
rate variation and signal amplitude distortion. The variation
magnitude is related to the upper and lower limits of the
sampling frequency, which were set for the PLL. For the
Mini-AQO, the sampling frequency range goes from 40Hz to
70Hz. In spite of allowing a good control over the 50Hz or
60Hz grid frequency, those limits are the main cause for ripple
producing false dip and swell triggering.
Barcelona, 12-15 May 2003
the real amplitude is lost as seen in Figure 1. That graph
shows an important three-phase short interruption followed by
voltage oscillations during the PLL recovery period when the
voltage amplitude returns to normal. The use of half-cycle
calculation, in that case, causes significant voltage amplitude
variations seen as a saw tooth waveform. This problem is also
visible in instruments with fixed sampling rate monitoring
grids with fundamental frequency fluctuation.
The graph in Figure 2 is another example of the saw tooth
waveform problem caused by half cycle measurement period
technique.
Figure 2 Half-Cycle DC offset Ripple
Figure 1 PLL recovery following a short interruption
Most of PLL desynchronizations occur during short
interruption. Also in fewer cases, they are generated during
dips and swells because of the phase shift coming with. Figure
1 shows that kind of analyser problem related to disturbance
detection and recording.
However, the phase shift doesn’t cause as much sampling rate
variation as a voltage drop, because the PLL doesn't go up to
his frequency limit. In 95% cases of dip and swell occurrence,
there is no visible PLL perturbation. Most of the remaining
5% cases are coming from false triggering during voltage
recovery periods following short interruptions. Those cases
are eliminated by a mere concatenation of dip events over
one-minute period, namely the temporal aggregation
recommended by [4].
Time distortions are actually corrected in the analyser with
simple functions. Those functions give real period versus
elapsed time since PLL lost synchronisation. A correlation
between the dip’s time coordinate versus dip’s real time
coordinate was experimentally obtained, allowing to
determinate the time correction formulas (2):
(2)
The in-time reconstruction of the RMS vector is possible, but
IRE_Zavoda_A1
Session 2 Paper No 52
The correction of steady state ripple is possible with a DC
voltage measurement, but it is not very accurate. DC
measurement can be done with precision over long periods.
When those measurements go under a cycle, transient voltages
created by dips, swells and short interruptions cause an invalid
DC offset measurement, which can't be used for correction of
half-cycle RMS values. In fact, EXCEL simulations show an
erroneous measurement when DC corrections are applied on
half cycle measurement, especially when dip duration is
between a half-cycle and a cycle.
Upgrade to IEC 61000-4-30
Since the end of “Canadian PQ Survey 2000”, the analyser
was modified for using a moving window as recommended by
[6]. This upgrade to [6] solves most of the problems stated
above, in particular for synchronized signals with DC or
transient DC. RMS values are calculated over one-cycle
periods and refreshed each half-cycle. Definitely the graphic
of Figure 3 shows a smoother 3 phases signal, which was
recorded after the upgrading.
The use of moving window was helpful in some cases but it
has some limitations. Often, the event duration is longer than
in reality because moving window acts as a first order low
pass filter. Sometimes the event duration is lower and could
be undetectable because dip and swell detection thresholds are
not exceeded. It could be concluded that the duration variation
depends on the signal dv/dt and on the dip or swell depth or
severity.
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17th International Conference on Electricity Distribution
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eliminated, but half cycle measurement is very sensitive, as it
is shown in the graphic of Figure 5. The ratio between DC
offset (% of crest) and ripple (% of RMS) is in the range from
2 to 5. In that context, 10% DC offset creates a 25% ripple on
RMS signal.
Figure 6 illustrates the error for a lost PLL or fundamental
frequency deviation for both measurement techniques i.e.
simple half-cycle and moving window cycle.
R M S E r r o r V e r su s F r e q u e n c y D e v i a ti o n
Figure 3 Smoothing with RMS on a cycle moving on Half-Cycle
In spite of certain DC offset ripple correction brought by the
moving window, the fundamental frequency variation always
produces ripple. In fact, depending on the signal phase angle
for the window sample used for the calculation and the period
covered by that window, the RMS value will fluctuate.
However, the ripple amplitude for a moving window is lower
than for half-cycle measurement. This fact is interesting
because the use of moving window can attenuate the false diptriggering occurrence. Figure 4 shows the difference between
methods involving ripple and fundamental frequency
fluctuation.
R i p p l e V e r su s F r e q u e n c y D e v i a ti o n
% Deviation on RMS Measurement
25
A b s Er r H C
20
A b s Er r F C
15
10
5
0
0 ,6
0 ,7
0 ,8
0 ,9
1
1 ,1
1 ,2
Ne tw o r k Fr e q u e n c y (p u )
Figure 6 RMS error at fundamental frequency deviation
A 1% deviation near the fundamental frequency, create 0,6%
error in the RMS measurement no matter what technique was
used. This aspect should be considered when accurate
measurements are necessary.
% Ripple on RMS Measurement
40
R ip p le H C
35
30
R ip p le F C
25
20
15
10
5
0
0 ,6
0 ,7
0 ,8
0 ,9
1
1 ,1
1 ,2
Ne tw o r k Fr e q u e n c y (p u )
Figure 4 Ripple at fundamental frequency deviation
Another interesting comparison between the moving window
(FC – full cycle) and the half-cycle (HC) measurements is
related to their effect on DC offset voltage ripple.
R i p p l e V e r su s O ffse t D C
% Ripple on RMS Measurement
80
70
R ip p le H C
60
R ip p le F C
50
By integrating the recommendations of [6] in the analyser
software, the number of errors caused by the DC offset and
also the ripple caused by the lost of PLL synchronisation
diminished. In consequence by using moving window
technique, the number of false dip or swell triggering was
lowered but not completely eliminated. The use of hardware
PLL instead of software, in order to comply with synchronous
sampling recommended by [6] can cause important deviation
in the RMS measurement technique used for the dips, swells
and short interruptions analysis. Users and future developers
should be aware of it.
DIP AND SHORT INTERRUPTIONS
The measurement of PQ indices characterizing short duration
undervoltages and overvoltages like dips, swells and short
interruptions are based on voltage RMS measurement. These
disturbances are caused by different electrical events occurred
either on the power distribution network side or on the
customer side. They affect one, two or all of three-phase
electrical distribution systems.
40
30
20
10
0
0
5
10
15
20
25
30
O ffs e t DC (% c r e s t)
Figure 5 % of ripple versus DC offset signal
As mentioned earlier, full cycle moving window DC offset is
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Session 2 Paper No 52
A pair of data characterizes a voltage dip: retained voltage and
duration [5]. The retained voltage quantifies the severity of
the dip, namely the smallest Urms(1/2) voltage value measured
during the dip. The duration represents the time quantification
of the phenomenon.
Usually PQ analysers detect and record dips and swells on
each phase separately. Afterwards they aggregate them (phase
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CIRED
17th International Conference on Electricity Distribution
Barcelona, 12-15 May 2003
aggregation) in order to determinate disturbances
characteristics (severity and duration) at a three-phase level.
Phase aggregation - Simultaneous events (sag and swell on
different phases)
Because the power distribution system is a three-phase
system, from the utilities viewpoint it is more interesting to
have a statistical occurrence evaluation of phase aggregated
dips, swells and short interruptions.
The phase aggregation was possible by reprocessing the
survey results in [3].
Due to their one, two or three phase equipment power
supplies, the customers for their part are interested in
statistical results for phase and phase-aggregated events.
The classification criterion of disturbances was based on dip’s
depth and duration and the choice of the magnitude and
duration followed the recommendation of UNIPEDE DISDIP
working group [4]. Table 1 contains dip occurrence global
values from [3], which were calculated by summation method
and classified respecting UNIPEDE’s recommendation.
IEC 61000-4-30 specifies for three-phase systems that a dip
begins when the Urms(1/2) voltage of one or more phases falls
below the threshold and ends when the Urms(1/2) voltage on all
phases is equal or above the threshold.
Although phase aggregation was conducted with respect to
[6], the standard doesn’t specify whether dip and swell type
disturbances should be aggregated separately or together.
Example 1:
Simultaneous disturbances, namely sags and swells occurring
on different phases, are very often the result of a one-phase
short circuit to ground (see Figure 7).
Table 1: Dips occurrence (summation method)
Amplitude
16-100ms
100-500ms
500ms-1s
1-3s
3-20s
20-60s
DIP15
Factors
85-90%
641
138
98
92
404
56
DIP30
70-85%
329
145
42
23
16
21
DIP60
40-70%
95
62
7
7
14
3
DIP90
10-40%
23
29
5
3
1
1
DSI
< 10%
4
11
11
22
69
6
DIPS
AND
SHORT
AGGREGATIONS
INTERRUPTIONS
The fundamental concepts of aggregation processes like
measurement, temporal and spatial aggregation are discussed
in [5]
The statistical evaluation of dips swells and short interruptions
(DSI) recorded during the Canada 2000 Survey was made
according to the following categorization methods:
• Summation (dips and swells on each phase),
• Phase aggregation - Simultaneous events (dips and
swells on different phases),
• Temporal aggregation - Consecutive events (dips
and swells on the same phase and on different
phases).
Figure 7: Simultaneous sags and swells
Such phenomena could be approached from their impact on:
1. 3-phase loads,
2. 1-phase loads.
One phase loads connected to the phase affected by the sag
may react differently than loads connected to the other phases
affected by the swell. For this reason and for statistical
purposes, sags and swells are accounted separately as
different types of disturbances in spite of their simultaneous
occurrence and their common originating cause.
Summation
The analyser software was developed before standards begun
recommending disturbances aggregations and therefore it
recorded dips on each phase separately.
For this reason, the statistical analysis of dips in the first
edition of [2] was restricted to the summation method.
By the summation method, the numbers of events calculated
separately for each phase were accounted for the total number
of events recorded.
The global results for 19 categories of customers are
illustrated in Table 1 and in Figure 11.
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Figure 8 - DIP30 following DIP90
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Example 2:
Figure 8 illustrates six significant dips classified as DIP90 and
DIP30, two on each phase. The summation method classifies
these disturbances as six dips while the phase aggregation
method classifies the same event as two resultant dips.
Barcelona, 12-15 May 2003
Example 4:
From the viewpoint of the impact produced by consecutive
dips occurred on the same phase and separated by only few
cycles (see Figure 10), where each dip probably has the same
effect on customer’s equipment, the temporal aggregation is
appropriate.
Temporal Aggregation - Consecutive events (sags or
swells on the same phase)
IEC 61000-2-8 mentions that reclosing operations can result
in multiple voltage dips and short interruptions (see Figure 9)
from the same primary causative event, which are unlikely to
affect equipments and processes multiple times and therefore
these disturbances shouldn’t be accounted separately.
Figure 10: Consecutive sags on the same phase
Because the suggestion made in [4], how to evaluate the
duration of time-aggregated dips (i.e. to consider only the
duration of most severe event from a string of events within 1
minute period) was again questionable, a different evaluation
of resultant event duration was adopted.
Figure 9: Dips due to reclosing operations
Moreover the standard recommends classification of all
events within a one-minute interval as a single event whose
amplitude and duration are those of the most severe sag or
swell observed during the interval.
Also this type of aggregation agrees with the definition of the
minimum duration of a sustained interruption given by IEEE
Std 1159-1995 [7].
So the accounting of the duration starts with the beginning of
the first event of the string, within a one- minute interval, and
stops at the end of last event of the string.
COMMENT ON RESULTS
From 413 databases recorded during the survey only 403 were
valid. 19 tables containing results for each of 19 categories of
sites selected for the survey and a table for global results were
generated.
The one-minute time aggregation of results from Canada 2000
survey was conducted partially with respect to [4].
Example 3:
Very short sags or swells (1/2 cycle or 1 cycle duration) occur
during the transient period of the voltage recovery following
short interruptions or important dips as shown in Figure 1.
The phase aggregation method takes into consideration 3
events (one DSI followed by two DIP30) that occurred during
the period. In the time aggregation process, only one of them
will be accounted for because all of them happened within a
one-minute interval. Its severity will be that of the most severe
sag observed during the interval.
The way in which the duration of the resultant sag or short
interruption is assessed with respect to [4] for this particular
event raised some questions. Should the disturbance duration
include the time span corresponding to the transient period
following the main disturbance?
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Session 2 Paper No 52
Figure 11: Sag distribution (summation)
The charts in Figure 11, 12, and 13 represent the graphical
presentation of dip data for three cases:
(1) Summation without any aggregation,
(2) Phase aggregation (according to [6]),
(3) Time aggregation one-minute (according to [4], [7]).
They facilitate the comparison between statistical values
corresponding to those cases.
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17th International Conference on Electricity Distribution
Barcelona, 12-15 May 2003
the percentage variation of dips occurrence numbers after
phase aggregation and time aggregation processes.
For example the occurrence number for DIP15 (16-100ms)
calculated by summation method decreased by 31% after
phase aggregation and by other 38% after time aggregation.
Contrary the statistical number for DIP90 (20-60ms)
increased by 100% after phase aggregation, then by 50% after
time aggregation one-minute.
TABLE 3- Percentage variation of dips occurrence numbers after time
aggregation one minute
Factors
Figure 12: Sag distribution (phase aggregation)
It is normal that after the phase aggregation, the resultant
numbers of disturbances to be diminished with respect to
summation numbers. The temporal aggregation one-minute
was conducted on numbers already phase-aggregated.
In comparing the numbers in sag distribution charts for phase
aggregation and aggregation one-minute, the reader will
notice that some of the numbers in the temporal aggregation
one-minute tables increased although they should be lower
according to what would be expected.
Amplitude
16-100ms
100-500ms
500ms-1s
1-3s
3-20s
DIP15
85-90%
38%
51%
57%
27%
9%
20-60s
-60%
DIP30
70-85%
29%
18%
39%
-69%
-56%
-36%
DIP60
40-70%
29%
37%
0%
-67%
-120%
-900%
DIP90
10-40%
39%
60%
0%
-100%
0%
-50%
DSI
< 10%
0%
33%
100%
45%
4%
-50%
The survey at any site lasted one week though [6] suggests a
one-year minimum assessment period. Because the
monitoring interval was too short, a prediction over one-year
period based on extrapolation of the numbers of dips swells
and short interruptions could be erroneous.
REFERENCES
[1] Bergeron, R., 1996, “Power Quality Measurement
Protocol - CEA Guide to Performing Power Quality
Surveys (CEA - 220 D 711)”.
[2] Gaétan Ethier, revision 1 2003, “Canadian Power Quality
Survey 2000 – CEATI Project No. T984700-5103”.
[3] Francisc Zavoda, 2003, “Sag, Swell and Short
Interruption evaluation from the Canadian PQ Survey
2000 - CEATI Project No. T014700-5113”.
Figure 13: Sag distribution (temporal aggregation one-min)
[4] IEC 61000-2-8 (2002) Electromagnetic Compatibility
(EMC):Part 2-8: Environment – Voltage dips and short
interruptions on public electric power supply systems
with statistical measurement results.
In fact, several shorter duration sags or swells are combined
together during the aggregation one-minute process and the
duration of the resultant disturbance is practically longer. So
generally the numbers of short duration sags or swells
decrease and those of long duration increase exceeding
corresponding numbers from phase aggregation tables.
[5] Voltage Sag Indices – Draft 2 : Working document for
IEEE P1564, November 2001.
Table 2 -- Percentage variation of dips occurrence numbers after phase
aggregation
[7] IEEE 1159 (1995) Recommended
Monitoring Electric Power Quality.
Amplitude
16-100ms
100-500ms
500ms-1s
1-3s
3-20s
20-60s
DIP15
Factors
85-90%
31%
28%
64%
52%
54%
25%
DIP30
70-85%
32%
30%
45%
43%
44%
48%
DIP60
40-70%
20%
34%
29%
57%
64%
67%
DIP90
10-40%
22%
48%
20%
33%
100%
-100%
DSI
< 10%
50%
73%
64%
50%
61%
33%
[6] IEC 61000-4-30 (2001) Electromagnetic Compatibility
(EMC) Part 4-30: Testing and measurement techniques –
Power quality measurement methods.
Practice
Table 2 and Table 3 respectively contain values representing
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for