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Transcript
Industry-Building
Solutions to Voltage Sags and Interruptions
Electrical Distribution
Solutions
The solution
Monitoring at the extrusion plant identified
several improvement areas for the utility.
The event depicted in figure 1 (page 1) caused
the plant engineer to think that his process may
be overly sensitive to sags. The plant was initially
designed with equipment intended
to lessen the effects of voltage sags. Controls
and sensitive electronics for the dc drives are
supplied from a motor-generator set. Other
microprocessors controlling the extrusion line are
served from a UPS system.
The plant used the information provided by
the waveform capture to strengthen the weak link
in the system: the ac drives serving cooling
rollers. Drive engineers used the waveform
capture in figure 1 as a basis to change settings
on the ac adjustable-speed drive controls. Each
drive is equipped with a “fault board,” a sensing
and control circuit that detects voltage anomalies
and turns off the drive to protect its costly power
electronic components.
The adjustment reduced the production line’s
sensitivity to voltage sags, while maintaining
adequate drive protection. These drives slow
down somewhat during sags, but they do not trip
during mild or short-duration sags. This
improvement has lessened the extrusion line
sensitivity to voltage sags.
Solutions to voltage sags are often less costly
than solutions to interruptions. In either case,
however, both utility system and customer loads
should be considered for improvement. Often,
the best solution to nuisance shutdowns due
to voltage sags is a dual approach:
c reduce the number of utility faults,
c lessen the sensitivity of customer equipment.
Customer solutions to voltage sags can be as
simple as adjusting fault board settings, as the
plant in Virginia discovered. This change did not
eliminate shutdowns, but it reduced their number
by a considerable amount. The next step is to
consider installing constant-voltage transformers
(CVTs) on sensitive control circuits. CVTs use
transformer saturation characteristics to dampen
changes in output voltage due to sudden
decreases or increases in input voltage. CVTs
are too expensive and bulky for use on largepower loads, but control circuits are typically less
than 1 kilovolt-ampere (kVA) in capacity and are
fairly inexpensive.
Solutions to voltage sags
and interruptions
The problem
In this issue,
we discuss
the solutions
to voltage
disturbances
caused
by voltage
sags and
interruptions
Conclusion
Voltage sags and interruptions can be costly
forms of power quality disturbances. These
events are caused by faults on the utility system,
or sometimes by normal operation inside a
customer facility. While both disturbances last
less than a second, voltage sags differ from
interruptions in that some voltage remains during
the disturbance. Interruptions cause a complete
loss in voltage. It is important to distinguish
between the two events because their solution
alternatives differ greatly in cost and complexity.
Waveform capture information is crucial in
determining which event is causing the
shutdowns, and in identifying the causes and
analyzing solution alternatives. PowerLogic
Circuit Monitors with high-speed waveform
capture, and some engineering expertise, can
ensure that sags and interruptions do not ruin
your business.
An extrusion plant in Virginia (USA) was being
plagued by unexpected disturbances called sags
and interruptions. Lights would blink, adjustablespeed drives would trip, and molten polyester
would begin to gum up dies and rollers. Four
hours later, workers would return the line to
normal operation and quality, only to have the
entire process happen again when the next
storm cloud appeared. During the first six
months of 1994, the plant suffered production
losses 35 times due to sags and interruptions.
Figure 1: Tree contact with an energized feeder - a common
source of a fault which causes a voltage sag.
On the utility side, solutions to voltage sags
result from reviewing common sources of faults.
One of the most common causes is incidental
tree contact with overhead lines. Overhead high
voltage lines are not insulated like the wiring in a
house. Tree contact, especially during wet or
windy weather, can permit current flow from
the feeder to the ground, using the tree as
a conductor. The high levels of current result
in depressed voltage (sag) along the entire
network until overcurrent protective devices
operate to interrupt the current flow. Utilities
attempt to keep trees out of lines by cutting limbs
that grow too close, but unusually active tree
growth, or tree-trimming budget cuts which
increase the time between trimmings, can result
in frequent tree limb contact.
ENMED199031EN
The two-pronged approach requires
a partnership, between utility and customer,
built on open communication and willingness
to share data and ideas.
Schneider Electric
Industries SA
Centre Merlin Gerin
F - 38050 Grenoble cedex
France
Tel.: +33 (0)4 76 57 60 60
Fax: +33 (0)4 76 57 73 62
http://www.schneiderelectric.com
[email protected]
ENMED199031EN
Electrical Distribution
Solutions
Industry-Building
This paper was originally published as a part of the series PowerLogic
Solutions, copyright Square D Company 1998. Used with permission.
Figure 2: High-speed waveform, captured during a voltage sag.
This document has been
printed on ecological paper.
Published by: Schneider Electric SA
Design and layout by: Insign’
Printed by:
ENMED199031EN
ART.28263
09/1999
The plant complained to the local utility, which
agreed to help determine the causes of the
numerous disruptions, and to recommend
solutions. In partnership with the electric utility,
Schneider Electric placed a Circuit Monitor at the
plant’s service entrance in July, to measure the
voltage disturbances causing the shutdowns.
This meter is able to capture high-speed
waveforms during power system anomalies. The
monitor captured numerous voltage sag events,
several of which caused shutdowns. One of the
first events recorded (figure 2) exposed a
shortcoming in the plant’s power quality design.
Solutions to Voltage Sags and Interruptions
Key concepts and Terms
Voltage sag
A voltage sag is a brief decrease in effective
voltage lasting less than one minute. Sags are
usually caused by faults on the utility system
which occur due to lightning, tree or animal
contact with energized feeders, or equipment
failure (see figure 1). Sags can also occur when
a large motor starts or faults occur inside a
plant. Sags differ from interruptions in that
some effective voltage remains during a sag,
while interruptions cause a complete loss of
voltage (see figure 3). Since both events last
about the same brief time period (usually less
than one second), the two different types of
Figure 3: Voltage sag due to utility fault (top) and interruption
due to utility circuit breaker trip (bottom). Circuit monitors can
simultaneously capture up to 60 cycles of waveform data
on all phases.
Measuring Voltage Sags
and Interruptions
disturbances are difficult to distinguish without
high-speed monitoring equipment. This is
especially true for deep voltage sags, which
may cause the same effect on plant equipment
as interruptions. Voltage sags and interruptions
due to utility faults vary in duration and
magnitude according to their location on the
power system and the number of phases they
involve.
The high-speed capability of the CM-2350 and
higher Circuit Monitors allows them to capture
events such as the sag at the extrusion plant.
The Circuit Monitor can automatically trigger an
event based on the effective level of any of the
monitored voltage and current inputs. When preprogrammed setpoints are exceeded, the Circuit
Monitor simultaneously captures a snapshot of
the instantaneous voltages and currents on up to
seven channels. The capture plots 64 data points
per cycle for every channel, up to 60 cycles per
event.
Why is it important to distinguish
between sags and interruptions?
Customer-side solutions to voltage sags are
usually cheaper than solutions to interruptions.
Often, shutdowns due to sags can be reduced
in number with equipment costing much less
than uninterruptible power supply (UPS)
systems.
Ride-through options include constant-voltage
transformers, magnetic synthesizers,
and control modifications. Interruptions,
however, may require UPS, rotary UPS,
or expensive modifications to the utility
distribution system.
Voltage sags can completely shut down
sensitive process loads. These unexpected
disruptions can be extremely costly. Voltage
sags affect equipment used in extrusion
processes; silicon wafer fabrication; data
processing; and chemical and papermaking.
Some sags affect only one or two phases
of a three-phase circuit (figure 4). Depending
on whether plant loads are single- or threephase, and depending on transformer
connections between the load and the fault
location, only a portion of plant equipment may
shut down during sag events. Interruptions
almost always affect all phases simultaneously.
The waveform snapshot of the event includes
two to ten cycles of pre-event values, depending
on user preferences. Additionally, the user can
define a log file in which other system
information related to the event can be placed.
For instance, information about system loading,
power factor, voltage unbalance, and other
parameters (at the time of a high-speed trigger)
can be recorded in a log file.
Setpoints for the high-speed event capture are
chosen by the user. The user selects the voltage
or current at which the event capture begins
(“pickup” value) and ends (“dropout” value).
These setpoints can be set in two ways: as
absolute values or as relative values. Setup
screens from the System Manager Software
used for setting up absolute and relative
setpoints are shown in figures 5 and 6,
respectively.
Caution: Backup Power Needed
Absolute setpoints are applied when the user
wants to define the exact value at which the
event is triggered. Relative setpoints are
activated in cases where the user would like
to allow for large long-term fluctuations
in voltage without triggering an event, unless the
measured value quickly changes by the percent
specified as the pickup setpoint.
This prevents the measured voltage from drifting
too near a trigger threshold and capturing
spurious events.
For example, if a 5% pickup setpoint is used
to trigger a voltage sag event, the effective
voltage must change by 5% from a value based
on the measured voltage averaged over about
the last 30 seconds. For further flexibility, the
time interval for calculating the average voltage
can be reduced to about five seconds to make
the event triggering even less sensitive to voltage
variations.
How does the Circuit Monitor
determine that a sag has occurred?
Every half-cycle (8 or 10 ms), the Circuit Monitor
compares the measured value of the previous
cycle with the pickup value specified in the setup
screen. If the measured value drops below the
pickup, a high-speed capture occurs. The Circuit
Monitor records event type, pickup time, dropout
time, and magnitude of the minimum value
during the event. The event pickup and dropout
time-stamps have a one-millisecond precision.
A reliable source of control power is necessary
for the Circuit Monitor to be able operate during
a power system disturbance. DC control power
is preferred. When a reliable source of control
power is not available, an optional “Ride-Through
Module” is available to maintain operation during
most voltage disturbances.
If even greater security is desired, a small UPS
may be used for control power backup.
A Circuit Monitor’s load is 14 VA.
The Circuit Monitor should be connected in a
way that protects control power from being
inadvertently lost due to the disturbance you
wish to capture. If a circuit breaker is being
monitored, for example, connect the control
power to the source side of the circuit breaker.
Then, if the circuit breaker opens, power to the
Circuit Monitor is maintained.
Compared to other electronic devices, the Circuit
Monitor is fairly resistant to temporary voltage
loss. The monitor can ride through an
interruption lasting up to one-tenth of a second.
A typical electronic device trips during an
interruption of one-fiftieth of a second. Also, the
nominal input voltage for a Circuit Monitor can
range from 100 to 264 Vac, allowing a wide
variation of input without control power loss.
Figure 6: System Manager Software screen used when setting
relative set points. Here, a waveform capture will be triggered when
voltage drops below 5% of the nominal value.
Figure 4: Voltage sag affecting
primarily one phase of a three-phase
circuit.
Figure 5: System Manager
Software screen used when
setting absolute set points.
In this example, a waveform
capture will be triggered
when line-to-neutral voltage
drops below 250 V.
ENMED199031EN
Electrical Distribution
Solutions
Industry-Building
ENMED199031EN
Electrical Distribution
Solutions
Industry-Building
ENMED199031EN
Electrical Distribution
Solutions
Industry-Building
Solutions to Voltage Sags and Interruptions
Key concepts and Terms
Voltage sag
A voltage sag is a brief decrease in effective
voltage lasting less than one minute. Sags are
usually caused by faults on the utility system
which occur due to lightning, tree or animal
contact with energized feeders, or equipment
failure (see figure 1). Sags can also occur when
a large motor starts or faults occur inside a
plant. Sags differ from interruptions in that
some effective voltage remains during a sag,
while interruptions cause a complete loss of
voltage (see figure 3). Since both events last
about the same brief time period (usually less
than one second), the two different types of
Figure 3: Voltage sag due to utility fault (top) and interruption
due to utility circuit breaker trip (bottom). Circuit monitors can
simultaneously capture up to 60 cycles of waveform data
on all phases.
Measuring Voltage Sags
and Interruptions
disturbances are difficult to distinguish without
high-speed monitoring equipment. This is
especially true for deep voltage sags, which
may cause the same effect on plant equipment
as interruptions. Voltage sags and interruptions
due to utility faults vary in duration and
magnitude according to their location on the
power system and the number of phases they
involve.
The high-speed capability of the CM-2350 and
higher Circuit Monitors allows them to capture
events such as the sag at the extrusion plant.
The Circuit Monitor can automatically trigger an
event based on the effective level of any of the
monitored voltage and current inputs. When preprogrammed setpoints are exceeded, the Circuit
Monitor simultaneously captures a snapshot of
the instantaneous voltages and currents on up to
seven channels. The capture plots 64 data points
per cycle for every channel, up to 60 cycles per
event.
Why is it important to distinguish
between sags and interruptions?
Customer-side solutions to voltage sags are
usually cheaper than solutions to interruptions.
Often, shutdowns due to sags can be reduced
in number with equipment costing much less
than uninterruptible power supply (UPS)
systems.
Ride-through options include constant-voltage
transformers, magnetic synthesizers,
and control modifications. Interruptions,
however, may require UPS, rotary UPS,
or expensive modifications to the utility
distribution system.
Voltage sags can completely shut down
sensitive process loads. These unexpected
disruptions can be extremely costly. Voltage
sags affect equipment used in extrusion
processes; silicon wafer fabrication; data
processing; and chemical and papermaking.
Some sags affect only one or two phases
of a three-phase circuit (figure 4). Depending
on whether plant loads are single- or threephase, and depending on transformer
connections between the load and the fault
location, only a portion of plant equipment may
shut down during sag events. Interruptions
almost always affect all phases simultaneously.
The waveform snapshot of the event includes
two to ten cycles of pre-event values, depending
on user preferences. Additionally, the user can
define a log file in which other system
information related to the event can be placed.
For instance, information about system loading,
power factor, voltage unbalance, and other
parameters (at the time of a high-speed trigger)
can be recorded in a log file.
Setpoints for the high-speed event capture are
chosen by the user. The user selects the voltage
or current at which the event capture begins
(“pickup” value) and ends (“dropout” value).
These setpoints can be set in two ways: as
absolute values or as relative values. Setup
screens from the System Manager Software
used for setting up absolute and relative
setpoints are shown in figures 5 and 6,
respectively.
Caution: Backup Power Needed
Absolute setpoints are applied when the user
wants to define the exact value at which the
event is triggered. Relative setpoints are
activated in cases where the user would like
to allow for large long-term fluctuations
in voltage without triggering an event, unless the
measured value quickly changes by the percent
specified as the pickup setpoint.
This prevents the measured voltage from drifting
too near a trigger threshold and capturing
spurious events.
For example, if a 5% pickup setpoint is used
to trigger a voltage sag event, the effective
voltage must change by 5% from a value based
on the measured voltage averaged over about
the last 30 seconds. For further flexibility, the
time interval for calculating the average voltage
can be reduced to about five seconds to make
the event triggering even less sensitive to voltage
variations.
How does the Circuit Monitor
determine that a sag has occurred?
Every half-cycle (8 or 10 ms), the Circuit Monitor
compares the measured value of the previous
cycle with the pickup value specified in the setup
screen. If the measured value drops below the
pickup, a high-speed capture occurs. The Circuit
Monitor records event type, pickup time, dropout
time, and magnitude of the minimum value
during the event. The event pickup and dropout
time-stamps have a one-millisecond precision.
A reliable source of control power is necessary
for the Circuit Monitor to be able operate during
a power system disturbance. DC control power
is preferred. When a reliable source of control
power is not available, an optional “Ride-Through
Module” is available to maintain operation during
most voltage disturbances.
If even greater security is desired, a small UPS
may be used for control power backup.
A Circuit Monitor’s load is 14 VA.
The Circuit Monitor should be connected in a
way that protects control power from being
inadvertently lost due to the disturbance you
wish to capture. If a circuit breaker is being
monitored, for example, connect the control
power to the source side of the circuit breaker.
Then, if the circuit breaker opens, power to the
Circuit Monitor is maintained.
Compared to other electronic devices, the Circuit
Monitor is fairly resistant to temporary voltage
loss. The monitor can ride through an
interruption lasting up to one-tenth of a second.
A typical electronic device trips during an
interruption of one-fiftieth of a second. Also, the
nominal input voltage for a Circuit Monitor can
range from 100 to 264 Vac, allowing a wide
variation of input without control power loss.
Figure 6: System Manager Software screen used when setting
relative set points. Here, a waveform capture will be triggered when
voltage drops below 5% of the nominal value.
Figure 4: Voltage sag affecting
primarily one phase of a three-phase
circuit.
Figure 5: System Manager
Software screen used when
setting absolute set points.
In this example, a waveform
capture will be triggered
when line-to-neutral voltage
drops below 250 V.
ENMED199031EN
Electrical Distribution
Solutions
Industry-Building
ENMED199031EN
Electrical Distribution
Solutions
Industry-Building
ENMED199031EN
Electrical Distribution
Solutions
Industry-Building
Solutions to Voltage Sags and Interruptions
Key concepts and Terms
Voltage sag
A voltage sag is a brief decrease in effective
voltage lasting less than one minute. Sags are
usually caused by faults on the utility system
which occur due to lightning, tree or animal
contact with energized feeders, or equipment
failure (see figure 1). Sags can also occur when
a large motor starts or faults occur inside a
plant. Sags differ from interruptions in that
some effective voltage remains during a sag,
while interruptions cause a complete loss of
voltage (see figure 3). Since both events last
about the same brief time period (usually less
than one second), the two different types of
Figure 3: Voltage sag due to utility fault (top) and interruption
due to utility circuit breaker trip (bottom). Circuit monitors can
simultaneously capture up to 60 cycles of waveform data
on all phases.
Measuring Voltage Sags
and Interruptions
disturbances are difficult to distinguish without
high-speed monitoring equipment. This is
especially true for deep voltage sags, which
may cause the same effect on plant equipment
as interruptions. Voltage sags and interruptions
due to utility faults vary in duration and
magnitude according to their location on the
power system and the number of phases they
involve.
The high-speed capability of the CM-2350 and
higher Circuit Monitors allows them to capture
events such as the sag at the extrusion plant.
The Circuit Monitor can automatically trigger an
event based on the effective level of any of the
monitored voltage and current inputs. When preprogrammed setpoints are exceeded, the Circuit
Monitor simultaneously captures a snapshot of
the instantaneous voltages and currents on up to
seven channels. The capture plots 64 data points
per cycle for every channel, up to 60 cycles per
event.
Why is it important to distinguish
between sags and interruptions?
Customer-side solutions to voltage sags are
usually cheaper than solutions to interruptions.
Often, shutdowns due to sags can be reduced
in number with equipment costing much less
than uninterruptible power supply (UPS)
systems.
Ride-through options include constant-voltage
transformers, magnetic synthesizers,
and control modifications. Interruptions,
however, may require UPS, rotary UPS,
or expensive modifications to the utility
distribution system.
Voltage sags can completely shut down
sensitive process loads. These unexpected
disruptions can be extremely costly. Voltage
sags affect equipment used in extrusion
processes; silicon wafer fabrication; data
processing; and chemical and papermaking.
Some sags affect only one or two phases
of a three-phase circuit (figure 4). Depending
on whether plant loads are single- or threephase, and depending on transformer
connections between the load and the fault
location, only a portion of plant equipment may
shut down during sag events. Interruptions
almost always affect all phases simultaneously.
The waveform snapshot of the event includes
two to ten cycles of pre-event values, depending
on user preferences. Additionally, the user can
define a log file in which other system
information related to the event can be placed.
For instance, information about system loading,
power factor, voltage unbalance, and other
parameters (at the time of a high-speed trigger)
can be recorded in a log file.
Setpoints for the high-speed event capture are
chosen by the user. The user selects the voltage
or current at which the event capture begins
(“pickup” value) and ends (“dropout” value).
These setpoints can be set in two ways: as
absolute values or as relative values. Setup
screens from the System Manager Software
used for setting up absolute and relative
setpoints are shown in figures 5 and 6,
respectively.
Caution: Backup Power Needed
Absolute setpoints are applied when the user
wants to define the exact value at which the
event is triggered. Relative setpoints are
activated in cases where the user would like
to allow for large long-term fluctuations
in voltage without triggering an event, unless the
measured value quickly changes by the percent
specified as the pickup setpoint.
This prevents the measured voltage from drifting
too near a trigger threshold and capturing
spurious events.
For example, if a 5% pickup setpoint is used
to trigger a voltage sag event, the effective
voltage must change by 5% from a value based
on the measured voltage averaged over about
the last 30 seconds. For further flexibility, the
time interval for calculating the average voltage
can be reduced to about five seconds to make
the event triggering even less sensitive to voltage
variations.
How does the Circuit Monitor
determine that a sag has occurred?
Every half-cycle (8 or 10 ms), the Circuit Monitor
compares the measured value of the previous
cycle with the pickup value specified in the setup
screen. If the measured value drops below the
pickup, a high-speed capture occurs. The Circuit
Monitor records event type, pickup time, dropout
time, and magnitude of the minimum value
during the event. The event pickup and dropout
time-stamps have a one-millisecond precision.
A reliable source of control power is necessary
for the Circuit Monitor to be able operate during
a power system disturbance. DC control power
is preferred. When a reliable source of control
power is not available, an optional “Ride-Through
Module” is available to maintain operation during
most voltage disturbances.
If even greater security is desired, a small UPS
may be used for control power backup.
A Circuit Monitor’s load is 14 VA.
The Circuit Monitor should be connected in a
way that protects control power from being
inadvertently lost due to the disturbance you
wish to capture. If a circuit breaker is being
monitored, for example, connect the control
power to the source side of the circuit breaker.
Then, if the circuit breaker opens, power to the
Circuit Monitor is maintained.
Compared to other electronic devices, the Circuit
Monitor is fairly resistant to temporary voltage
loss. The monitor can ride through an
interruption lasting up to one-tenth of a second.
A typical electronic device trips during an
interruption of one-fiftieth of a second. Also, the
nominal input voltage for a Circuit Monitor can
range from 100 to 264 Vac, allowing a wide
variation of input without control power loss.
Figure 6: System Manager Software screen used when setting
relative set points. Here, a waveform capture will be triggered when
voltage drops below 5% of the nominal value.
Figure 4: Voltage sag affecting
primarily one phase of a three-phase
circuit.
Figure 5: System Manager
Software screen used when
setting absolute set points.
In this example, a waveform
capture will be triggered
when line-to-neutral voltage
drops below 250 V.
ENMED199031EN
Electrical Distribution
Solutions
Industry-Building
ENMED199031EN
Electrical Distribution
Solutions
Industry-Building
ENMED199031EN
Electrical Distribution
Solutions
Industry-Building
Industry-Building
Solutions to Voltage Sags and Interruptions
Electrical Distribution
Solutions
The solution
Monitoring at the extrusion plant identified
several improvement areas for the utility.
The event depicted in figure 1 (page 1) caused
the plant engineer to think that his process may
be overly sensitive to sags. The plant was initially
designed with equipment intended
to lessen the effects of voltage sags. Controls
and sensitive electronics for the dc drives are
supplied from a motor-generator set. Other
microprocessors controlling the extrusion line are
served from a UPS system.
The plant used the information provided by
the waveform capture to strengthen the weak link
in the system: the ac drives serving cooling
rollers. Drive engineers used the waveform
capture in figure 1 as a basis to change settings
on the ac adjustable-speed drive controls. Each
drive is equipped with a “fault board,” a sensing
and control circuit that detects voltage anomalies
and turns off the drive to protect its costly power
electronic components.
The adjustment reduced the production line’s
sensitivity to voltage sags, while maintaining
adequate drive protection. These drives slow
down somewhat during sags, but they do not trip
during mild or short-duration sags. This
improvement has lessened the extrusion line
sensitivity to voltage sags.
Solutions to voltage sags are often less costly
than solutions to interruptions. In either case,
however, both utility system and customer loads
should be considered for improvement. Often,
the best solution to nuisance shutdowns due
to voltage sags is a dual approach:
c reduce the number of utility faults,
c lessen the sensitivity of customer equipment.
Customer solutions to voltage sags can be as
simple as adjusting fault board settings, as the
plant in Virginia discovered. This change did not
eliminate shutdowns, but it reduced their number
by a considerable amount. The next step is to
consider installing constant-voltage transformers
(CVTs) on sensitive control circuits. CVTs use
transformer saturation characteristics to dampen
changes in output voltage due to sudden
decreases or increases in input voltage. CVTs
are too expensive and bulky for use on largepower loads, but control circuits are typically less
than 1 kilovolt-ampere (kVA) in capacity and are
fairly inexpensive.
Solutions to voltage sags
and interruptions
The problem
In this issue,
we discuss
the solutions
to voltage
disturbances
caused
by voltage
sags and
interruptions
Conclusion
Voltage sags and interruptions can be costly
forms of power quality disturbances. These
events are caused by faults on the utility system,
or sometimes by normal operation inside a
customer facility. While both disturbances last
less than a second, voltage sags differ from
interruptions in that some voltage remains during
the disturbance. Interruptions cause a complete
loss in voltage. It is important to distinguish
between the two events because their solution
alternatives differ greatly in cost and complexity.
Waveform capture information is crucial in
determining which event is causing the
shutdowns, and in identifying the causes and
analyzing solution alternatives. PowerLogic
Circuit Monitors with high-speed waveform
capture, and some engineering expertise, can
ensure that sags and interruptions do not ruin
your business.
An extrusion plant in Virginia (USA) was being
plagued by unexpected disturbances called sags
and interruptions. Lights would blink, adjustablespeed drives would trip, and molten polyester
would begin to gum up dies and rollers. Four
hours later, workers would return the line to
normal operation and quality, only to have the
entire process happen again when the next
storm cloud appeared. During the first six
months of 1994, the plant suffered production
losses 35 times due to sags and interruptions.
Figure 1: Tree contact with an energized feeder - a common
source of a fault which causes a voltage sag.
On the utility side, solutions to voltage sags
result from reviewing common sources of faults.
One of the most common causes is incidental
tree contact with overhead lines. Overhead high
voltage lines are not insulated like the wiring in a
house. Tree contact, especially during wet or
windy weather, can permit current flow from
the feeder to the ground, using the tree as
a conductor. The high levels of current result
in depressed voltage (sag) along the entire
network until overcurrent protective devices
operate to interrupt the current flow. Utilities
attempt to keep trees out of lines by cutting limbs
that grow too close, but unusually active tree
growth, or tree-trimming budget cuts which
increase the time between trimmings, can result
in frequent tree limb contact.
ENMED199031EN
The two-pronged approach requires
a partnership, between utility and customer,
built on open communication and willingness
to share data and ideas.
Schneider Electric
Industries SA
Centre Merlin Gerin
F - 38050 Grenoble cedex
France
Tel.: +33 (0)4 76 57 60 60
Fax: +33 (0)4 76 57 73 62
http://www.schneiderelectric.com
[email protected]
ENMED199031EN
Electrical Distribution
Solutions
Industry-Building
This paper was originally published as a part of the series PowerLogic
Solutions, copyright Square D Company 1998. Used with permission.
Figure 2: High-speed waveform, captured during a voltage sag.
This document has been
printed on ecological paper.
Published by: Schneider Electric SA
Design and layout by: Insign’
Printed by:
ENMED199031EN
ART.28263
09/1999
The plant complained to the local utility, which
agreed to help determine the causes of the
numerous disruptions, and to recommend
solutions. In partnership with the electric utility,
Schneider Electric placed a Circuit Monitor at the
plant’s service entrance in July, to measure the
voltage disturbances causing the shutdowns.
This meter is able to capture high-speed
waveforms during power system anomalies. The
monitor captured numerous voltage sag events,
several of which caused shutdowns. One of the
first events recorded (figure 2) exposed a
shortcoming in the plant’s power quality design.
Industry-Building
Solutions to Voltage Sags and Interruptions
Electrical Distribution
Solutions
The solution
Monitoring at the extrusion plant identified
several improvement areas for the utility.
The event depicted in figure 1 (page 1) caused
the plant engineer to think that his process may
be overly sensitive to sags. The plant was initially
designed with equipment intended
to lessen the effects of voltage sags. Controls
and sensitive electronics for the dc drives are
supplied from a motor-generator set. Other
microprocessors controlling the extrusion line are
served from a UPS system.
The plant used the information provided by
the waveform capture to strengthen the weak link
in the system: the ac drives serving cooling
rollers. Drive engineers used the waveform
capture in figure 1 as a basis to change settings
on the ac adjustable-speed drive controls. Each
drive is equipped with a “fault board,” a sensing
and control circuit that detects voltage anomalies
and turns off the drive to protect its costly power
electronic components.
The adjustment reduced the production line’s
sensitivity to voltage sags, while maintaining
adequate drive protection. These drives slow
down somewhat during sags, but they do not trip
during mild or short-duration sags. This
improvement has lessened the extrusion line
sensitivity to voltage sags.
Solutions to voltage sags are often less costly
than solutions to interruptions. In either case,
however, both utility system and customer loads
should be considered for improvement. Often,
the best solution to nuisance shutdowns due
to voltage sags is a dual approach:
c reduce the number of utility faults,
c lessen the sensitivity of customer equipment.
Customer solutions to voltage sags can be as
simple as adjusting fault board settings, as the
plant in Virginia discovered. This change did not
eliminate shutdowns, but it reduced their number
by a considerable amount. The next step is to
consider installing constant-voltage transformers
(CVTs) on sensitive control circuits. CVTs use
transformer saturation characteristics to dampen
changes in output voltage due to sudden
decreases or increases in input voltage. CVTs
are too expensive and bulky for use on largepower loads, but control circuits are typically less
than 1 kilovolt-ampere (kVA) in capacity and are
fairly inexpensive.
Solutions to voltage sags
and interruptions
The problem
In this issue,
we discuss
the solutions
to voltage
disturbances
caused
by voltage
sags and
interruptions
Conclusion
Voltage sags and interruptions can be costly
forms of power quality disturbances. These
events are caused by faults on the utility system,
or sometimes by normal operation inside a
customer facility. While both disturbances last
less than a second, voltage sags differ from
interruptions in that some voltage remains during
the disturbance. Interruptions cause a complete
loss in voltage. It is important to distinguish
between the two events because their solution
alternatives differ greatly in cost and complexity.
Waveform capture information is crucial in
determining which event is causing the
shutdowns, and in identifying the causes and
analyzing solution alternatives. PowerLogic
Circuit Monitors with high-speed waveform
capture, and some engineering expertise, can
ensure that sags and interruptions do not ruin
your business.
An extrusion plant in Virginia (USA) was being
plagued by unexpected disturbances called sags
and interruptions. Lights would blink, adjustablespeed drives would trip, and molten polyester
would begin to gum up dies and rollers. Four
hours later, workers would return the line to
normal operation and quality, only to have the
entire process happen again when the next
storm cloud appeared. During the first six
months of 1994, the plant suffered production
losses 35 times due to sags and interruptions.
Figure 1: Tree contact with an energized feeder - a common
source of a fault which causes a voltage sag.
On the utility side, solutions to voltage sags
result from reviewing common sources of faults.
One of the most common causes is incidental
tree contact with overhead lines. Overhead high
voltage lines are not insulated like the wiring in a
house. Tree contact, especially during wet or
windy weather, can permit current flow from
the feeder to the ground, using the tree as
a conductor. The high levels of current result
in depressed voltage (sag) along the entire
network until overcurrent protective devices
operate to interrupt the current flow. Utilities
attempt to keep trees out of lines by cutting limbs
that grow too close, but unusually active tree
growth, or tree-trimming budget cuts which
increase the time between trimmings, can result
in frequent tree limb contact.
ENMED199031EN
The two-pronged approach requires
a partnership, between utility and customer,
built on open communication and willingness
to share data and ideas.
Schneider Electric
Industries SA
Centre Merlin Gerin
F - 38050 Grenoble cedex
France
Tel.: +33 (0)4 76 57 60 60
Fax: +33 (0)4 76 57 73 62
http://www.schneiderelectric.com
[email protected]
ENMED199031EN
Electrical Distribution
Solutions
Industry-Building
This paper was originally published as a part of the series PowerLogic
Solutions, copyright Square D Company 1998. Used with permission.
Figure 2: High-speed waveform, captured during a voltage sag.
This document has been
printed on ecological paper.
Published by: Schneider Electric SA
Design and layout by: Insign’
Printed by:
ENMED199031EN
ART.28263
09/1999
The plant complained to the local utility, which
agreed to help determine the causes of the
numerous disruptions, and to recommend
solutions. In partnership with the electric utility,
Schneider Electric placed a Circuit Monitor at the
plant’s service entrance in July, to measure the
voltage disturbances causing the shutdowns.
This meter is able to capture high-speed
waveforms during power system anomalies. The
monitor captured numerous voltage sag events,
several of which caused shutdowns. One of the
first events recorded (figure 2) exposed a
shortcoming in the plant’s power quality design.