4
Voltage Dips and Short Supply
Interruptions – Case Study
Zbigniew Hanzelka
C4.1 HV MEASUREMENTS
Identification of the power quality in a transmission system is urgently needed in order to
formulate properly the contracts and asses the suitability of the existing documents. For this
purpose, the power quality factors in lines delivering electric power from a transmission system
have been measured at the Power Distribution Company (PDC) [46].
C4.1.1 The Measuring System
The main part of the 110 kV distribution system operated by the PDC is a ring-operated network.
The whole 110 kV distribution system is operated as a system with a solidly grounded neutral
(Figure C4.1). The PDC-operated 110 kV distribution system is connected with neighboring
operators’ distribution systems and with the 220 kV transmission system by means of three
autotransformers, 160 MVA each, installed at three substations: (a) LUB – industrial and
household customers; (b) WAN – predominantly industrial customers; (c) SKA – near to
a heat and power generating plant.1 At these points instruments have been installed to
measurer the power quality factors. The measurements were carried out over seven months,
at three measurement points. The effect of these measurements is recorded in the database
which contains, among other things, the recorded voltage dips.
1
The number of end customers connected to the distribution system exceeds 800 000.
Handbook of Power Quality Edited by Angelo Baggini
© 2008 John Wiley & Sons, Ltd
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20
Figure C4.1 Block diagram of the example distribution system connections to the transmission
system
C4.1.2 Voltage Dips and Short Supply Interruptions
Tables C4.1–C4.3 give a summary of the recorded voltage dips and short supply interruptions. Most of the disturbances occur within a short time interval.
Table C4.1 Number of voltage dips – WAN (L1/L2/L3)
Dips (%)
10–15
15–30
30–60
60–90
90–100
10–100 ms
100–500 ms
500 ms to 1 s
1–3 s
3–20 s
20 s to 1 min
1/3/0
1/0/4
0/1/3
1/1/1
1/2/3
0/0/0
2/3/3
0/0/1
0/0/0
0/0/0
0/0/0
1/1/1
0/0/1
0/0/0
0/0/0
0/0/0
0/0/0
0/0/0
0/0/0
0/0/2
0/0/0
0/0/0
0/0/0
0/0/0
2/1/3
0/0/0
0/0/0
0/0/0
0/0/0
0/1/0
Number of recorded voltage dips: 44
Table C4.2 Number of voltage dips – SKA (L1/L2/L3)
Dips (%)
10–15
15–30
30–60
60–90
90–100
10–100 ms
2/3/2
1/1/0
0/0/0
0/0/0
0/0/0
100–500 ms
500 ms to 1 s
1–3 s
3–20 s
20 s to 1 min
0/0/0
0/0/0
0/0/0
2/4/2
0/1/1
0/0/0
0/0/0
0/0/0
0/0/0
0/0/0
0/0/0
0/0/0
0/0/0
0/0/0
0/0/0
Number of recorded events: 19
0/0/0
0/0/0
0/0/0
0/0/0
0/0/0
0/0/0
0/0/0
0/0/0
0/0/0
0/0/0
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21
Table C4.3 Number of voltage dips – LUB (L1/L2/L3)
Dips (%)
10–15
15–30
30–60
60–90
90–100
10–100 ms
0/0/3
1/0/11
0/1/2
1/0/4
1/0/1
100–500 ms
500 ms to 1 s
1–3 s
3–20 s
20 s to 1 min
2/2/4
0/0/0
0/0/0
3/2/0
0/0/0
0/0/0
0/1/2
0/0/0
0/0/0
0/0/7
0/0/1
0/0/7
0/0/0
0/0/0
0/0/0
Number of recorded events: 74
0/0/0
0/0/0
0/0/0
0/0/11
0/1/0
0/0/0
0/0/0
0/0/0
0/0/5
0/0/0
Substation
Threshold value
90 %
80 %
70 %
60 %
SKA
19
6
6
6
WAN
44
35
24
21
LUB
74
59
45
42
Figure C4.2 Dependence of the number of dips on the selected threshold value, which defines the
disturbance
The dependence of the number of dips on the selected threshold value, which defines
the disturbance, is presented in Figure C4.2. For each location it can be seen that there is a
reduction in the number of disturbances with a decrease in the threshold voltage.
C4.1.3 Aggregation procedures
Analysis shows that the use of phase and time (1 and 3 min) aggregation significantly
reduces the number of disturbances. This does not concern the substation LUB, where a
particularly large number of disturbances occurred in a single phase (L3) – see Table C4.4,
Table C4.5 and Table C4.6.
Figure C4.3 shows a comprehensive comparison of the effect of various types of
aggregation on the number of voltage dips for given measurement points.
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22
Figure C4.3 Dependence of the number of dips on the selected method of aggregation
Table C4.4 Substation WAN
1 min aggregation
Dips (%)
10–15
15–30
30–60
60–90
90–100
10–100 ms
4
2
1
1
0
100–500 ms
500 ms to 1 s
0
0
2
2
0
0
0
0
1
0
Number of recorded events: 25
1–3 s
3–20 s
20 s to 1 min
0
2
1
0
0
0
0
0
0
4
0
0
1
0
4
1–3 s
3–20 s
20 s to 1 min
0
2
1
0
0
0
0
0
0
3
0
0
1
0
4
3 min aggregation
Dips (%)
10–15
15–30
30–60
60–90
90–100
10–100 ms
4
2
1
0
0
100–500 ms
500 ms to 1 s
0
0
2
2
0
0
0
0
0
0
Number of recorded events: 23
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23
Phase aggregation
Dips (%)
10–15
15–30
30–60
60–90
90–100
10–100 ms
3
5
4
3
6
100–500 ms
500 ms to 1 s
0
0
5
1
1
1
0
0
0
0
Number of recorded events: 38
1–3 s
3–20 s
20 s to 1 min
0
0
0
0
2
0
0
0
0
6
0
0
0
0
1
1–3 s
3–20 s
20 s to 1 min
2
2
0
0
0
0
0
0
0
0
0
0
0
0
0
1–3 s
3–20 s
20 s to 1 min
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Table C4.5 Substation SKA
1 and 3 min aggregation
Dips (%)
10–15
15–30
30–60
60–90
90–100
10–100 ms
3
2
0
0
0
100–500 ms
500 ms to 1 s
0
0
6
0
0
0
0
0
0
0
Number of recorded events: 15
Phase aggregation
Dips (%)
10–15
15–30
30–60
60–90
90–100
10–100 ms
5
3
0
0
0
100–500 ms
500 ms to 1 s
0
0
4
1
0
0
0
0
0
0
Total of recorded events: 13
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Table C4.6 Substation LUB
1 min aggregation
Dips (%)
10–15
15–30
30–60
60–90
90–100
10–100 ms
1
9
3
1
1
100–500 ms
500 ms to 1 s
3
0
1
0
1
2
5
1
1
0
Number of recorded events: 46
1–3 s
3–20 s
20 s to 1 min
1
1
0
1
0
2
0
0
6
1
0
0
0
5
0
1–3 s
3–20 s
20 s to 1 min
1
2
0
1
0
2
0
0
4
1
0
0
0
2
0
1–3 s
3–20 s
20 s to 1 min
0
0
0
7
0
0
0
0
11
1
0
0
0
5
0
3 min aggregation
Dips (%)
10–15
15–30
30–60
60–90
90–100
10–100 ms
0
9
3
0
1
100–500 ms
500 ms to 1 s
2
0
1
0
1
2
6
1
1
0
Number of recorded events: 44
Phase aggregation
Dips (%)
10–15
15–30
30–60
60–90
90–100
10–100 ms
3
12
3
5
2
100–500 ms
500 ms to 1 s
6
0
5
0
3
0
7
1
0
0
Number of recorded events: 71
C4.2 SELECTED CONTRACTS AND NATIONAL
REGULATIONS
The examples refer to three well-known power quality contracts.
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25
C4.2.1 USA (based on [22])
In this contract the concept of qualifying sag has been defined. A voltage dip is referred to
as the qualifying sag when the r.m.s. value of any of the three phase voltages is lower than
75 % of the nominal voltage.2 The minimum duration of a voltage dip has not been defined.
Thus the provisions of the contract comprise all cases of disturbances, with the exception
of voltage dips associated with protection systems operation, and voltage dips which occur
on unloaded lines and therefore are causing no effects.3
The deepest voltage dip occurring within a 15-minute time interval is regarded as the
qualifying sag. The observation interval starts at the instant of occurrence of the first dip and
ends at the instant of the last voltage dip ending, or after a time of 15 minutes has elapsed.
Voltage dips which occur after this time are treated as belonging to the next observation
interval. The further procedure consists of the aggregation of this way of obtaining the
results of measurement. For example, according to the contract, for a single customer when
measurements are taken on several busbars, a program seeks for the worst case (the deepest
dip). This kind of data processing, which may be referred to as time (15 minutes) and spatial
(according to location) aggregation, significantly reduces the number of dips with respect
to the number of non-aggregated qualified dips.
If the residual voltage of one out of the three phase voltages drops below 75 % of the
nominal value then, for contractual purposes, the so-called sag score4 comes into effect:
1 − UA + UB + UC /3
It is equal to the average voltage dip on the three phases. For instance, for UA = UB =
UC = 0722 (pu) the sag score = 0278; for UA = 0818 UB = 0574 UC = 0823 the sag
score = 0262.5 If instead of a voltage dip, a voltage swell above 1 pu occurs on one or
two phases as a result of, say, the neutral point potential shift, then the value 1 is assumed
for calculation of the sag score. Thus all values of the sag score belong to the interval:
0.0833–1. The value 0.0833 applies to the case where UA = 075 UB = UC = 1. The greater
the value of the sag score index, the more severe the disturbance.
Also the concept of the so-called sag score target was introduced. This index is
determined for a customer who has many points of recording voltage dips. It is the sum
of sag scores for all measurement points, which is compared to the value set forth in the
contract. Mutual financial commitments are determined at the end of each year if the sag
score targets of a given customer exceed the value agreed upon by the parties to the contract.
If, for example, the sum of the sag scores is 3.28 and the limit value in the contract is 3,
the difference of 0.28 is multiplied by the agreed compensation rate.
2
This value has been chosen based on the new ITIC curve. Also customers have adopted this value as critical for
their equipment.
3
Whether the supplying line is loaded or not is decided from the current measurement at the instant of a voltage dip
occurrence. If the current value is lower than a certain level, the line is considered a hot reserve. The measurement
of current often helps to settle who caused the dip: the customer or the supplier?
4
The r.m.s. voltage values are assumed constant during the considered voltage dip. The sag score can also be a
function of time if phase voltage r.m.s. values are assumed to vary with time.
5
In the case of supply interruption, contractual provisions relevant to this disturbance are applicable. There are
two separate contracts, pertaining to voltage dips and supply interruptions.
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C4.2.2 France [44],[57]
In the Emerald Contract (EdF),6 changes in r.m.s. voltage value with amplitude contained
in the range 10–99 % and with a duration of 10 ms to 3 min are regarded as voltage dips.
The threshold values adopted in the contract are: dip duration longer than 600 ms, depth
greater than 30 %, respectively. Voltage dips which successively occur in times shorter than
100 ms are considered a single disturbance.
According to the contract, a short interruption occurs when the residual voltages of the
three line-to-line voltages are simultaneously lower than 10 % for a duration greater than
1 s and shorter than 3 min.
The number of interruptions that occur within one year is a subject of the contract.
Table C4.7 gives numbers of short supply interruptions, which have occurred on systems
with a voltage higher than 50 kV. The supplier guarantees that these values will not be
increased in the future.
Under the Emerald Contract EdF agrees not to exceed the number of seven short
interruptions per year for customers with a declared voltage above 50 kV. There is an option
of defining the limit value for the sum of short and long supply interruptions.
At the customer’s request, where technically practicable and financially feasible, the
supplier may guarantee better supply conditions on individually negotiated terms.
Figure C4.4 shows the annual cumulative numbers of long and short supply interruptions for 3000 feed points on French HV and LV systems. A global trend toward better
supply quality, which results from company decisions taken in the early 1980s, continued in
the years 1988–1998. When the period of 1990–1995 is considered, the effectiveness of these
actions might be questionable. This situation has resulted from a period of an exceptional
number of thunderstorm days during this time. Figure C4.4 illustrates two theses:
1. An abrupt improvement of power quality factors should not be expected; such a situation
never happens in practice. The time constant of changes in the quality of power supply
is extremely long.
2. The indices characterizing voltage dips are very sensitive to atmospheric phenomena.
Table C4.7 The number of short supply interruptions on
HV system, according to French data [44]
Short interruptions
6
1996
5
1997
5
1998
5
As stated in [45], the number of long and short supply interruptions was the most difficult obstacle to reaching
agreement between the parties. Many customers have had no interruptions over several years. Therefore, the
provision that allows for the possibility of a certain number of interruptions is regarded as a deterioration of the
quality of supply. This is one of the reasons for introducing customized contracts. In such a case, a provision
allowing for one interruption over three years has been introduced. Usually the number of disturbances experienced
by the customer over the last 4–5 years has been set as the permissible value.
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27
Figure C4.4 Cumulative numbers of long and short supply interruptions on HV and LV
systems [45]
C4.2.3 South Africa [50],[51],[66]
The duration of a voltage dip is the time measured between the instant at which the r.m.s
voltage value falls below 90 % of the declared voltage and the instant at which it rises above
90 % of the declared value. The amplitude of a voltage dip equals the maximum voltage
change during the disturbance, and its duration is the maximum voltage dip duration for the
most disturbed phase.
A South African user specification [50] [51] contains an interesting classification table
for voltage dips that may be considered when doing an expert evaluation (Table C4.8).
The Y-type area reflects dips that are expected to occur frequently in typical HV and
MV systems, and against which customers should protect their plant. The X-type areas (X1
and X2) reflect normal HV protection clearance times and hence a significant number of
events in this area. Customers should attempt to protect against at least X1-type dips. The
T-type area reflects close-up faults, which are not expected to happen too regularly – and
Table C4.8 Characterization of depth and duration of voltage dips [51]
Range of dip depth
U (expressed as
a % of Ud )
Range of residual
voltage Ur (expressed
as a % of Ud )
10 < U ≤ 15
90 > U ≥ 85
15 < U ≤ 20
85 > U ≥ 80
20 < U ≤ 30
80 > U ≥ 70
Duration t
20 < t ≤ 150 ms
150 < t ≤ 600 ms
06 < t ≤ 3 s
Y
Z1
30 < U ≤ 40
70 > U ≥ 60
X1
40 < U ≤ 60
60 > U ≥ 40
X2
60 < U ≤ 100
40 > U ≥ 0
S
Z2
T
Note: In the case of measurements on LV systems it is acceptable to set the dip threshold at 0.85 pu.
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28
which a utility should specifically address if excessive. S-type dips are not as common
as X- and Y-type events, but may occur where impedance protection schemes are used,
or where voltage recovery is delayed. Z-type dips are very uncommon in HV systems
(particularly Z2-type events), as this generally reflects problematic protection operation
[25],[26]. Voltage dips with a longer duration than 3 s are considered in [50] as undervoltage
events. This standard contains, however, no classification of undervoltage events. Based
on the classification in [50] voltage dips may be presented as total numbers that occur
differentiated on the seven categories Y, X1, X2, T, S, Z1 and Z2.
The South African standard gives limits for voltage dips in the form of a maximum
number of voltage dips per year for defined ranges of voltage dip duration and retained
voltage, designated as dip window categories (Table C4.8). All voltage dips caused by force
majeure or caused on the customer’s side (short circuits, large drive starts, etc.) are excluded
from this number. According to the document it is expected, for most of the time and most
customers, that the number of dips will be considerably less than the numbers set as the
minimum standards.
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