24-09-2016, 09:23 AM
Abstract - This paper summarizes the results from a
number of different voltage sag investigations. These
investigations involve characterizing the voltage sag
performance at a customer facility and evaluating
equipment sensitivity to different voltage sag magnitudes
and durations. Possible solutions to voltage sag sensitivity
problems are also described.
INTRODUCTION
Voltage sags and momentary power interruptions are
probably the most important power quality problems
affecting industrial and large commercial customers.
These events are usually associated with a fault somewhere
on the supplying power system. Actual interruptions occur
when the fault is on the circuit supplying the customer.
Voltage sags are much more common since they can be
associated with faults remote from the customer. Even
voltage sags lasting only 4-5 cycles can cause a wide range
of sensitive customer equipment to drop out.
Analysis of voltage sag concerns requires a knowledge of
the voltage sag characteristics, statistical information
describing the likelihood of a voltage sag occurring, and
information describing the sensitivity of important loads
within the facility. Developing this knowledge base
requires close cooperation between the utility, the
customer, and equipment manufacturers.
In order to develop a better understanding in all of these
areas, the Electric Power Research Institute (EPRI) and a
number of individual electric utilities have been sponsoring
case studies to investigate voltage sag concerns and
available solutions. This paper summarizes some of the
important results from these case studies.
CHARACTERISTICS OF VOLTAGE SAGS
Voltage sags which can cause equipment impacts are
usually caused by faults on the power system. Motor
starting also results in voltage sags but the magnitudes are
usually not severe enough to cause equipment
misoperation. The simplified one line diagram in Figure 1
can be used to explain how a fault results in a voltage sag
at a customer facility.
Figure 1. Example Power System
Consider a customer that is supplied from the feeder
designated with breaker 1 on the diagram. If there is a
fault on this feeder, the customer will experience a voltage
sag during the fault and then an interruption when the
breaker opens to clear the fault. If the fault is temporary in
nature, a reclosing operation on the breaker may be
successful and the interruption will only be temporary.
Regardless, sensitive equipment will almost surely trip
during this interruption.
A much more common event would be a fault on one of the
other feeders from the substation or a fault somewhere on
the transmission system (see fault locations shown on the
figure). In either of these cases, the customer will
experience a voltage sag during the period that the fault is
2
actually on the system. As soon as breakers open to clear
the fault, normal voltage will be restored at the customer.
Figure 2 is a plot of the rms voltage vs. time and the
waveform characteristic at the customer location for one of
these fault conditions.
p
Phase C-A Voltage
RMS Variation
Trigger
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
80
85
90
95
100
105
110
115
Time (Seconds)
Voltage (%)
0 25 50 75 100 125 150 175
-150
-100
-50
0
50
100
150
Time (mSeconds)
Voltage
Duration
0.150 Sec
Min
81.38
Ave
96.77
Max
101.4
BMI/Electrotek
Figure 2. Example Voltage Sag Characteristic During a
Fault on a Parallel Feeder Circuit
The waveform given in Figure 2 is typical of the customer
voltage during a fault on a parallel feeder circuit that is
cleared quickly by the substation breaker. The total
duration of the fault is 150 msec, or about nine cycles.
The voltage during a fault on a parallel feeder will depend
on the distance from the substation to the fault location. A
fault close to the substation will result in a much more
significant sag than a fault near the end of the feeder.
Figure 3 shows the voltage sag magnitude at the plant bus
as a function of fault location for an example system. Note
that a single line-to-ground fault condition results in a
much less severe voltage sag than a three phase fault
condition due to a delta-wye transformer connection at the
plant.
Plant Service Entrance Bus Voltage vs. Fault Location
Distance From Substation to Fault (Feet)
0
10
20
30
40
50
60
70
80
90
100
0 2500 5000 7500 10000 12500 15000
Normal Voltage (100%)
Single-Line-To-Ground Fault
3 Phase Fault
Figure 3. Plant Phase-to-Phase Bus Voltage (%) as a
Function of Fault Location on a Parallel Feeder
Transmission related voltage sags are normally much more
consistent in duration than distribution voltage sags.
Because of the large amounts of energy associated with
transmission faults, they are cleared as soon as possible.
This normally corresponds to 3-6 cycles, which is the total
time for fault detection and breaker operation.
Normally, customers do not experience an interruption for
a transmission system fault. Transmission systems are
looped or networked, as opposed to distribution systems
which are radial. This means that if a single line trips, or
is out of service, the remaining system supplies the load. If
a fault occurs as shown on the 115 kV system, the
protective relaying will sense the faults and breakers A and
B will open to clear the fault. While the fault is on the
transmission system, the entire power system, including
the distribution system, will experience a voltage sag.
Figure 4 shows the magnitude of measured voltage sags at
an industrial plant supplied from a 115 kV system. Most
of the voltage sags were 10-30% below nominal voltage,
and no momentary interruptions were measured at the
plant during the monitoring period (almost one year).
Magnitude of Voltage Sags at Industrial Plant
0-10%
Below Normal Voltage
44%
40-50%
Below Normal Voltage
2%
30-40%
Below Normal Voltage
10%
20-30%
Below Normal Voltage
10%
10-20%
Below Normal Voltage
34%
>50%
Below Normal Voltage
0%
Figure 4. Industrial Plant Sag Magnitude Data
(magnitudes are in % below nominal voltage)
Figure 5 gives a three dimensional plot illustrating the
number of sags experienced as a function of both the
voltage sag magnitude and the duration. This is a
convenient way to completely characterize the actual or
expected voltage sag conditions at a site. Evaluating the
impact of voltage sags at a customer plant involves
estimating the number of voltage sags that can be expected
as a function of the voltage sag magnitude and then
comparing this with the equipment sensitivity.
The estimates of voltage sag performance are developed by
performing short circuit simulations to determine the plant
voltage as a function of fault location throughout the power
system. Total circuit miles of line exposure that can affect
3
the plant (area of vulnerability) are determined for a
particular voltage sag level. Historical fault performance
(faults per year per 100 miles) can then be used to estimate
the number of sags per year that can be expected below that
magnitude. Finally, a chart such as the one in Figure 6 can
be constructed breaking down the expected voltage sags by
magnitude and cause (voltage level of faulted line). This
information can be used directly by the customer to
determine the need for power conditioning equipment at
sensitive loads in the plant.
number of different voltage sag investigations. These
investigations involve characterizing the voltage sag
performance at a customer facility and evaluating
equipment sensitivity to different voltage sag magnitudes
and durations. Possible solutions to voltage sag sensitivity
problems are also described.
INTRODUCTION
Voltage sags and momentary power interruptions are
probably the most important power quality problems
affecting industrial and large commercial customers.
These events are usually associated with a fault somewhere
on the supplying power system. Actual interruptions occur
when the fault is on the circuit supplying the customer.
Voltage sags are much more common since they can be
associated with faults remote from the customer. Even
voltage sags lasting only 4-5 cycles can cause a wide range
of sensitive customer equipment to drop out.
Analysis of voltage sag concerns requires a knowledge of
the voltage sag characteristics, statistical information
describing the likelihood of a voltage sag occurring, and
information describing the sensitivity of important loads
within the facility. Developing this knowledge base
requires close cooperation between the utility, the
customer, and equipment manufacturers.
In order to develop a better understanding in all of these
areas, the Electric Power Research Institute (EPRI) and a
number of individual electric utilities have been sponsoring
case studies to investigate voltage sag concerns and
available solutions. This paper summarizes some of the
important results from these case studies.
CHARACTERISTICS OF VOLTAGE SAGS
Voltage sags which can cause equipment impacts are
usually caused by faults on the power system. Motor
starting also results in voltage sags but the magnitudes are
usually not severe enough to cause equipment
misoperation. The simplified one line diagram in Figure 1
can be used to explain how a fault results in a voltage sag
at a customer facility.
Figure 1. Example Power System
Consider a customer that is supplied from the feeder
designated with breaker 1 on the diagram. If there is a
fault on this feeder, the customer will experience a voltage
sag during the fault and then an interruption when the
breaker opens to clear the fault. If the fault is temporary in
nature, a reclosing operation on the breaker may be
successful and the interruption will only be temporary.
Regardless, sensitive equipment will almost surely trip
during this interruption.
A much more common event would be a fault on one of the
other feeders from the substation or a fault somewhere on
the transmission system (see fault locations shown on the
figure). In either of these cases, the customer will
experience a voltage sag during the period that the fault is
2
actually on the system. As soon as breakers open to clear
the fault, normal voltage will be restored at the customer.
Figure 2 is a plot of the rms voltage vs. time and the
waveform characteristic at the customer location for one of
these fault conditions.
p
Phase C-A Voltage
RMS Variation
Trigger
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
80
85
90
95
100
105
110
115
Time (Seconds)
Voltage (%)
0 25 50 75 100 125 150 175
-150
-100
-50
0
50
100
150
Time (mSeconds)
Voltage
Duration
0.150 Sec
Min
81.38
Ave
96.77
Max
101.4
BMI/Electrotek
Figure 2. Example Voltage Sag Characteristic During a
Fault on a Parallel Feeder Circuit
The waveform given in Figure 2 is typical of the customer
voltage during a fault on a parallel feeder circuit that is
cleared quickly by the substation breaker. The total
duration of the fault is 150 msec, or about nine cycles.
The voltage during a fault on a parallel feeder will depend
on the distance from the substation to the fault location. A
fault close to the substation will result in a much more
significant sag than a fault near the end of the feeder.
Figure 3 shows the voltage sag magnitude at the plant bus
as a function of fault location for an example system. Note
that a single line-to-ground fault condition results in a
much less severe voltage sag than a three phase fault
condition due to a delta-wye transformer connection at the
plant.
Plant Service Entrance Bus Voltage vs. Fault Location
Distance From Substation to Fault (Feet)
0
10
20
30
40
50
60
70
80
90
100
0 2500 5000 7500 10000 12500 15000
Normal Voltage (100%)
Single-Line-To-Ground Fault
3 Phase Fault
Figure 3. Plant Phase-to-Phase Bus Voltage (%) as a
Function of Fault Location on a Parallel Feeder
Transmission related voltage sags are normally much more
consistent in duration than distribution voltage sags.
Because of the large amounts of energy associated with
transmission faults, they are cleared as soon as possible.
This normally corresponds to 3-6 cycles, which is the total
time for fault detection and breaker operation.
Normally, customers do not experience an interruption for
a transmission system fault. Transmission systems are
looped or networked, as opposed to distribution systems
which are radial. This means that if a single line trips, or
is out of service, the remaining system supplies the load. If
a fault occurs as shown on the 115 kV system, the
protective relaying will sense the faults and breakers A and
B will open to clear the fault. While the fault is on the
transmission system, the entire power system, including
the distribution system, will experience a voltage sag.
Figure 4 shows the magnitude of measured voltage sags at
an industrial plant supplied from a 115 kV system. Most
of the voltage sags were 10-30% below nominal voltage,
and no momentary interruptions were measured at the
plant during the monitoring period (almost one year).
Magnitude of Voltage Sags at Industrial Plant
0-10%
Below Normal Voltage
44%
40-50%
Below Normal Voltage
2%
30-40%
Below Normal Voltage
10%
20-30%
Below Normal Voltage
10%
10-20%
Below Normal Voltage
34%
>50%
Below Normal Voltage
0%
Figure 4. Industrial Plant Sag Magnitude Data
(magnitudes are in % below nominal voltage)
Figure 5 gives a three dimensional plot illustrating the
number of sags experienced as a function of both the
voltage sag magnitude and the duration. This is a
convenient way to completely characterize the actual or
expected voltage sag conditions at a site. Evaluating the
impact of voltage sags at a customer plant involves
estimating the number of voltage sags that can be expected
as a function of the voltage sag magnitude and then
comparing this with the equipment sensitivity.
The estimates of voltage sag performance are developed by
performing short circuit simulations to determine the plant
voltage as a function of fault location throughout the power
system. Total circuit miles of line exposure that can affect
3
the plant (area of vulnerability) are determined for a
particular voltage sag level. Historical fault performance
(faults per year per 100 miles) can then be used to estimate
the number of sags per year that can be expected below that
magnitude. Finally, a chart such as the one in Figure 6 can
be constructed breaking down the expected voltage sags by
magnitude and cause (voltage level of faulted line). This
information can be used directly by the customer to
determine the need for power conditioning equipment at
sensitive loads in the plant.