ENMED199031EN

Schneider ElectricIndustries SA

ART.28263

Centre Merlin GerinF - 38050 Grenoble cedexFranceTel.: +33 (0)4 76 57 60 60Fax: +33 (0)4 76 57 73 62http://www.schneiderelectric.comservice-commc4@mail.schneider.fr

This paper was originally published as a part of the series PowerLogicSolutions, copyright Square D Company 1998. Used with permission.

Published by: Schneider Electric SADesign and layout by: InsignPrinted by:

09/1999

This document has beenprinted on ecological paper.

ENM

ED19

9031

EN

Solutions to Voltage Sags and Interruptions

ENMED199031EN Electrical DistributionSolutionsIndustry-Building

An extrusion plant in Virginia (USA) was beingplagued by unexpected disturbances called sagsand interruptions. Lights would blink, adjustable-speed drives would trip, and molten polyesterwould begin to gum up dies and rollers. Fourhours later, workers would return the line tonormal operation and quality, only to have theentire process happen again when the nextstorm cloud appeared. During the first sixmonths of 1994, the plant suffered productionlosses 35 times due to sags and interruptions.

The problem

Figure 2: High-speed waveform, captured during a voltage sag.

In this issue,we discussthe solutionsto voltagedisturbancescausedby voltagesags andinterruptions

Figure 1: Tree contact with an energized feeder - a commonsource of a fault which causes a voltage sag.

The plant complained to the local utility, whichagreed to help determine the causes of thenumerous disruptions, and to recommendsolutions. In partnership with the electric utility,Schneider Electric placed a Circuit Monitor at theplants service entrance in July, to measure thevoltage disturbances causing the shutdowns.This meter is able to capture high-speedwaveforms during power system anomalies. Themonitor captured numerous voltage sag events,several of which caused shutdowns. One of thefirst events recorded (figure 2) exposed ashortcoming in the plants power quality design.

Electrical Distribution

Solutions to voltage sagsand interruptions

Solutions

Industry-Building

The solution

Monitoring at the extrusion plant identifiedseveral improvement areas for the utility.The event depicted in figure 1 (page 1) causedthe plant engineer to think that his process maybe overly sensitive to sags. The plant was initiallydesigned with equipment intendedto lessen the effects of voltage sags. Controlsand sensitive electronics for the dc drives aresupplied from a motor-generator set. Othermicroprocessors controlling the extrusion line areserved from a UPS system.

The plant used the information provided bythe waveform capture to strengthen the weak linkin the system: the ac drives serving coolingrollers. Drive engineers used the waveformcapture in figure 1 as a basis to change settingson the ac adjustable-speed drive controls. Eachdrive is equipped with a fault board, a sensingand control circuit that detects voltage anomaliesand turns off the drive to protect its costly powerelectronic components.The adjustment reduced the production linessensitivity to voltage sags, while maintainingadequate drive protection. These drives slowdown somewhat during sags, but they do not tripduring mild or short-duration sags. Thisimprovement has lessened the extrusion linesensitivity to voltage sags.

Solutions to voltage sags are often less costlythan solutions to interruptions. In either case,however, both utility system and customer loadsshould be considered for improvement. Often,the best solution to nuisance shutdowns dueto voltage sags is a dual approach:c reduce the number of utility faults,c lessen the sensitivity of customer equipment.

The two-pronged approach requiresa partnership, between utility and customer,built on open communication and willingnessto share data and ideas.

On the utility side, solutions to voltage sagsresult from reviewing common sources of faults.One of the most common causes is incidentaltree contact with overhead lines. Overhead highvoltage lines are not insulated like the wiring in ahouse. Tree contact, especially during wet orwindy weather, can permit current flow fromthe feeder to the ground, using the tree asa conductor. The high levels of current resultin depressed voltage (sag) along the entirenetwork until overcurrent protective devicesoperate to interrupt the current flow. Utilitiesattempt to keep trees out of lines by cutting limbsthat grow too close, but unusually active treegrowth, or tree-trimming budget cuts whichincrease the time between trimmings, can resultin frequent tree limb contact.

Customer solutions to voltage sags can be assimple as adjusting fault board settings, as theplant in Virginia discovered. This change did noteliminate shutdowns, but it reduced their numberby a considerable amount. The next step is toconsider installing constant-voltage transformers(CVTs) on sensitive control circuits. CVTs usetransformer saturation characteristics to dampenchanges in output voltage due to suddendecreases or increases in input voltage. CVTsare too expensive and bulky for use on large-power loads, but control circuits are typically lessthan 1 kilovolt-ampere (kVA) in capacity and arefairly inexpensive.

Voltage sags and interruptions can be costlyforms of power quality disturbances. Theseevents are caused by faults on the utility system,or sometimes by normal operation inside acustomer facility. While both disturbances lastless than a second, voltage sags differ frominterruptions in that some voltage remains duringthe disturbance. Interruptions cause a completeloss in voltage. It is important to distinguishbetween the two events because their solutionalternatives differ greatly in cost and complexity.Waveform capture information is crucial indetermining which event is causing theshutdowns, and in identifying the causes andanalyzing solution alternatives. PowerLogicCircuit Monitors with high-speed waveformcapture, and some engineering expertise, canensure that sags and interruptions do not ruinyour business.

Conclusion

ENMED199031EN Electrical DistributionSolutionsIndustry-Building

Solutions to Voltage Sags and Interruptions

ENMED199031EN Electrical DistributionSolutionsIndustry-Building

ENMED199031EN Electrical DistributionSolutionsIndustry-Building

Voltage sag

A voltage sag is a brief decrease in effectivevoltage lasting less than one minute. Sags areusually caused by faults on the utility systemwhich occur due to lightning, tree or animalcontact with energized feeders, or equipmentfailure (see figure 1). Sags can also occur whena large motor starts or faults occur inside aplant. Sags differ from interruptions in thatsome effective voltage remains during a sag,while interruptions cause a complete loss ofvoltage (see figure 3). Since both events lastabout the same brief time period (usually lessthan one second), the two different types of

Key concepts and Terms

Figure 3: Voltage sag due to utility fault (top) and interruptiondue to utility circuit breaker trip (bottom). Circuit monitors cansimultaneously capture up to 60 cycles of waveform dataon all phases.

disturbances are difficult to distinguish withouthigh-speed monitoring equipment. This isespecially true for deep voltage sags, whichmay cause the same effect on plant equipmentas interruptions. Voltage sags and interruptionsdue to utility faults vary in duration andmagnitude according to their location on thepower system and the number of phases theyinvolve.

Why is it important to distinguishbetween sags and interruptions?

Customer-side solutions to voltage sags areusually cheaper than solutions to interruptions.Often, shutdowns due to sags can be reducedin number with equipment costing much lessthan uninterruptible power supply (UPS)systems.Ride-through options include constant-voltagetransformers, magnetic synthesizers,and control modifications. Interruptions,however, may require UPS, rotary UPS,or expensive modifications to the utilitydistribution system.Voltage sags can completely shut downsensitive process loads. These unexpecteddisruptions can be extremely costly. Voltagesags affect equipment used in extrusionprocesses; silicon wafer fabrication; dataprocessing; and chemical and papermaking.Some sags affect only one or two phasesof a three-phase circuit (figure 4). Dependingon whether plant loads are single- or three-phase, and depending on transformerconnections between the load and the faultlocation, only a portion of plant equipment mayshut down during sag events. Interruptionsalmost always affect all phases simultaneously.

Figure 4: Voltage sag affectingprimarily one phase of a three-phasecircuit.

Figure 5: System ManagerSoftware screen used whensetting absolute set points.In this example, a waveformcapture will be triggeredwhen line-to-neutral voltagedrops below 250 V.

Measuring Voltage Sagsand Interruptions

The high-speed capability of the CM-2350 andhigher Circuit Monitors allows them to captureevents such as the sag at the extrusion plant.The Circuit Monitor can automatically trigger anevent based on the effective level of any of themonitored voltage and current inputs. When pre-programmed setpoints are exceeded, the CircuitMonitor simultaneously captures a snapshot ofthe instantaneous voltages and currents on up toseven channels. The capture plots 64 data pointsper cycle for every channel, up to 60 cycles perevent.

The waveform snapshot of the event includestwo to ten cycles of pre-event values, dependingon user preferences. Additionally, the user candefine a log file in which other systeminformation related to the event can be placed.For instance, information about system loading,power factor, voltage unbalance, and otherparameters (at the time of a high-speed trigger)can be recorded in a log file.Setpoints for the high-speed event capture arechosen by the user. The user selects the voltageor current at which the event capture begins(pickup value) and ends (dropout value).These setpoints can be set in two ways: asabsolute values or as relative values. Setupscreens from the System Manager Softwareused for setting up absolute and relativesetpoints are shown in figures 5 and 6,respectively.

Absolute setpoints are applied when the userwants to define the exact value at which theevent is triggered. Relative setpoints areactivated in cases where the user would liketo allow for large long-term fluctuationsin voltage without triggering an event, unless themeasured value quickly changes by the percentspecified as the pickup setpoint.This prevents the measured voltage from driftingtoo near a trigger threshold and capturingspurious events.

For example, if a 5% pickup setpoint is usedto trigger a voltage sag event, the effectivevoltage must change by 5% from a value basedon the measured voltage averaged over aboutthe last 30 seconds. For further flexibility, thetime interval for calculating the average voltagecan be reduced to about five seconds to makethe event triggering even less sensitive to voltagevariations.

How does the Circuit Monitordetermine that a sag has occurred?

Every half-cycle (8 or 10 ms), the Circuit Monitorcompares the measured value of the previouscycle with the pickup value specified in the setupscreen. If the measured value drops below thepickup, a high-speed capture occurs. The CircuitMonitor records event type, pickup time, dropouttime, and magnitude of the minimum valueduring the event. The event pickup and dropouttime-stamps have a one-millisecond precision.

Caution: Backup Power Needed

A reliable source of control power is necessaryfor the Circuit Monitor to be able operate duringa power system disturbance. DC control poweris preferred. When a reliable source of controlpower is not available, an optional Ride-ThroughModule is available to maintain operation duringmost voltage disturbances.If even greater security is desired, a small UPSmay be used for control power backup.A Circuit Monitors load is 14 VA.

The Circuit Monitor should be connected in away that protects control power from beinginadvertently lost due to the disturbance youwish to capture. If a circuit breaker is beingmonitored, for example, connect the controlpower to the source side of the circuit breaker.Then, if the circuit breaker opens, power to theCircuit Monitor is maintained.Compared to other electronic devices, the CircuitMonitor is fairly resistant to temporary voltageloss. The monitor can ride through aninterruption lasting up to one-tenth of a second.A typical electronic device trips during aninterruption of one-fiftieth of a second. Also, thenominal input voltage for a Circuit Monitor canrange from 100 to 264 Vac, allowing a widevariation of input without control power loss.

Figure 6: System Manager Software screen used when settingrelative set points. Here, a waveform capture will be triggered whenvoltage drops below 5% of the nominal value.

ENMED199031EN Electrical DistributionSolutionsIndustry-Building

Solutions to Voltage Sags and Interruptions

ENMED199031EN Electrical DistributionSolutionsIndustry-Building

ENMED199031EN Electrical DistributionSolutionsIndustry-Building

Voltage sag

A voltage sag is a brief decrease in effectivevoltage lasting less than one minute. Sags areusually caused by faults on the utility systemwhich occur due to lightning, tree or animalcontact with energized feeders, or equipmentfailure (see figure 1). Sags can also occur whena large motor starts or faults occur inside aplant. Sags differ from interruptions in thatsome effective voltage remains during a sag,while interruptions cause a complete loss ofvoltage (see figure 3). Since both events lastabout the same brief time period (usually lessthan one second), the two different types of

Key concepts and Terms

Figure 3: Voltage sag due to utility fault (top) and interruptiondue to utility circuit breaker trip (bottom). Circuit monitors cansimultaneously capture up to 60 cycles of waveform dataon all phases.

disturbances are difficult to distinguish withouthigh-speed monitoring equipment. This isespecially true for deep voltage sags, whichmay cause the same effect on plant equipmentas interruptions. Voltage sags and interruptionsdue to utility faults vary in duration andmagnitude according to their location on thepower system and the number of phases theyinvolve.

Why is it important to distinguishbetween sags and interruptions?

Customer-side solutions to voltage sags areusually cheaper than solutions to interruptions.Often, shutdowns due to sags can be reducedin number with equipment costing much lessthan uninterruptible power supply (UPS)systems.Ride-through options include constant-voltagetransformers, magnetic synthesizers,and control modifications. Interruptions,however, may require UPS, rotary UPS,or expensive modifications to the utilitydistribution system.Voltage sags can completely shut downsensitive process loads. These unexpecteddisruptions can be extremely costly. Voltagesags affect equipment used in extrusionprocesses; silicon wafer fabrication; dataprocessing; and chemical and papermaking.Some sags affect only one or two phasesof a three-phase circuit (figure 4). Dependingon whether plant loads are single- or three-phase, and depending on transformerconnections between the load and the faultlocation, only a portion of plant equipment mayshut down during sag events. Interruptionsalmost always affect all phases simultaneously.

Figure 4: Voltage sag affectingprimarily one phase of a three-phasecircuit.

Figure 5: System ManagerSoftware screen used whensetting absolute set points.In this example, a waveformcapture will be triggeredwhen line-to-neutral voltagedrops below 250 V.

Measuring Voltage Sagsand Interruptions

The high-speed capability of the CM-2350 andhigher Circuit Monitors allows them to captureevents such as the sag at the extrusion plant.The Circuit Monitor can automatically trigger anevent based on the effective level of any of themonitored voltage and current inputs. When pre-programmed setpoints are exceeded, the CircuitMonitor simultaneously captures a snapshot ofthe instantaneous voltages and currents on up toseven channels. The capture plots 64 data pointsper cycle for every channel, up to 60 cycles perevent.

The waveform snapshot of the event includestwo to ten cycles of pre-event values, dependingon user preferences. Additionally, the user candefine a log file in which other systeminformation related to the event can be placed.For instance, information about system loading,power factor, voltage unbalance, and otherparameters (at the time of a high-speed trigger)can be recorded in a log file.Setpoints for the high-speed event capture arechosen by the user. The user selects the voltageor current at which the event capture begins(pickup value) and ends (dropout value).These setpoints can be set in two ways: asabsolute values or as relative values. Setupscreens from the System Manager Softwareused for setting up absolute and relativesetpoints are shown in figures 5 and 6,respectively.

Absolute setpoints are applied when the userwants to define the exact value at which theevent is triggered. Relative setpoints areactivated in cases where the user would liketo allow for large long-term fluctuationsin voltage without triggering an event, unless themeasured value quickly changes by the percentspecified as the pickup setpoint.This prevents the measured voltage from driftingtoo near a trigger threshold and capturingspurious events.

For example, if a 5% pickup setpoint is usedto trigger a voltage sag event, the effectivevoltage must change by 5% from a value basedon the measured voltage averaged over aboutthe last 30 seconds. For further flexibility, thetime interval for calculating the average voltagecan be reduced to about five seconds to makethe event triggering even less sensitive to voltagevariations.

How does the Circuit Monitordetermine that a sag has occurred?

Every half-cycle (8 or 10 ms), the Circuit Monitorcompares the measured value of the previouscycle with the pickup value specified in the setupscreen. If the measured value drops below thepickup, a high-speed capture occurs. The CircuitMonitor records event type, pickup time, dropouttime, and magnitude of the minimum valueduring the event. The event pickup and dropouttime-stamps have a one-millisecond precision.

Caution: Backup Power Needed

A reliable source of control power is necessaryfor the Circuit Monitor to be able operate duringa power system disturbance. DC control poweris preferred. When a reliable source of controlpower is not available, an optional Ride-ThroughModule is available to maintain operation duringmost voltage disturbances.If even greater security is desired, a small UPSmay be used for control power backup.A Circuit Monitors load is 14 VA.

The Circuit Monitor should be connected in away that protects control power from beinginadvertently lost due to the disturbance youwish to capture. If a circuit breaker is beingmonitored, for example, connect the controlpower to the source side of the circuit breaker.Then, if the circuit breaker opens, power to theCircuit Monitor is maintained.Compared to other electronic devices, the CircuitMonitor is fairly resistant to temporary voltageloss. The monitor can ride through aninterruption lasting up to one-tenth of a second.A typical electronic device trips during aninterruption of one-fiftieth of a second. Also, thenominal input voltage for a Circuit Monitor canrange from 100 to 264 Vac, allowing a widevariation of input without control power loss.

Figure 6: System Manager Software screen used when settingrelative set points. Here, a waveform capture will be triggered whenvoltage drops below 5% of the nominal value.

ENMED199031EN Electrical DistributionSolutionsIndustry-Building

Solutions to Voltage Sags and Interruptions

ENMED199031EN Electrical DistributionSolutionsIndustry-Building

ENMED199031EN Electrical DistributionSolutionsIndustry-Building

Voltage sag

A voltage sag is a brief decrease in effectivevoltage lasting less than one minute. Sags areusually caused by faults on the utility systemwhich occur due to lightning, tree or animalcontact with energized feeders, or equipmentfailure (see figure 1). Sags can also occur whena large motor starts or faults occur inside aplant. Sags differ from interruptions in thatsome effective voltage remains during a sag,while interruptions cause a complete loss ofvoltage (see figure 3). Since both events lastabout the same brief time period (usually lessthan one second), the two different types of

Key concepts and Terms

Figure 3: Voltage sag due to utility fault (top) and interruptiondue to utility circuit breaker trip (bottom). Circuit monitors cansimultaneously capture up to 60 cycles of waveform dataon all phases.

disturbances are difficult to distinguish withouthigh-speed monitoring equipment. This isespecially true for deep voltage sags, whichmay cause the same effect on plant equipmentas interruptions. Voltage sags and interruptionsdue to utility faults vary in duration andmagnitude according to their location on thepower system and the number of phases theyinvolve.

Why is it important to distinguishbetween sags and interruptions?

Customer-side solutions to voltage sags areusually cheaper than solutions to interruptions.Often, shutdowns due to sags can be reducedin number with equipment costing much lessthan uninterruptible power supply (UPS)systems.Ride-through options include constant-voltagetransformers, magnetic synthesizers,and control modifications. Interruptions,however, may require UPS, rotary UPS,or expensive modifications to the utilitydistribution system.Voltage sags can completely shut downsensitive process loads. These unexpecteddisruptions can be extremely costly. Voltagesags affect equipment used in extrusionprocesses; silicon wafer fabrication; dataprocessing; and chemical and papermaking.Some sags affect only one or two phasesof a three-phase circuit (figure 4). Dependingon whether plant loads are single- or three-phase, and depending on transformerconnections between the load and the faultlocation, only a portion of plant equipment mayshut down during sag events. Interruptionsalmost always affect all phases simultaneously.

Figure 4: Voltage sag affectingprimarily one phase of a three-phasecircuit.

Figure 5: System ManagerSoftware screen used whensetting absolute set points.In this example, a waveformcapture will be triggeredwhen line-to-neutral voltagedrops below 250 V.

Measuring Voltage Sagsand Interruptions

The high-speed capability of the CM-2350 andhigher Circuit Monitors allows them to captureevents such as the sag at the extrusion plant.The Circuit Monitor can automatically trigger anevent based on the effective level of any of themonitored voltage and current inputs. When pre-programmed setpoints are exceeded, the CircuitMonitor simultaneously captures a snapshot ofthe instantaneous voltages and currents on up toseven channels. The capture plots 64 data pointsper cycle for every channel, up to 60 cycles perevent.

The waveform snapshot of the event includestwo to ten cycles of pre-event values, dependingon user preferences. Additionally, the user candefine a log file in which other systeminformation related to the event can be placed.For instance, information about system loading,power factor, voltage unbalance, and otherparameters (at the time of a high-speed trigger)can be recorded in a log file.Setpoints for the high-speed event capture arechosen by the user. The user selects the voltageor current at which the event capture begins(pickup value) and ends (dropout value).These setpoints can be set in two ways: asabsolute values or as relative values. Setupscreens from the System Manager Softwareused for setting up absolute and relativesetpoints are shown in figures 5 and 6,respectively.

Absolute setpoints are applied when the userwants to define the exact value at which theevent is triggered. Relative setpoints areactivated in cases where the user would liketo allow for large long-term fluctuationsin voltage without triggering an event, unless themeasured value quickly changes by the percentspecified as the pickup setpoint.This prevents the measured voltage from driftingtoo near a trigger threshold and capturingspurious events.

For example, if a 5% pickup setpoint is usedto trigger a voltage sag event, the effectivevoltage must change by 5% from a value basedon the measured voltage averaged over aboutthe last 30 seconds. For further flexibility, thetime interval for calculating the average voltagecan be reduced to about five seconds to makethe event triggering even less sensitive to voltagevariations.

How does the Circuit Monitordetermine that a sag has occurred?

Every half-cycle (8 or 10 ms), the Circuit Monitorcompares the measured value of the previouscycle with the pickup value specified in the setupscreen. If the measured value drops below thepickup, a high-speed capture occurs. The CircuitMonitor records event type, pickup time, dropouttime, and magnitude of the minimum valueduring the event. The event pickup and dropouttime-stamps have a one-millisecond precision.

Caution: Backup Power Needed

A reliable source of control power is necessaryfor the Circuit Monitor to be able operate duringa power system disturbance. DC control poweris preferred. When a reliable source of controlpower is not available, an optional Ride-ThroughModule is available to maintain operation duringmost voltage disturbances.If even greater security is desired, a small UPSmay be used for control power backup.A Circuit Monitors load is 14 VA.

The Circuit Monitor should be connected in away that protects control power from beinginadvertently lost due to the disturbance youwish to capture. If a circuit breaker is beingmonitored, for example, connect the controlpower to the source side of the circuit breaker.Then, if the circuit breaker opens, power to theCircuit Monitor is maintained.Compared to other electronic devices, the CircuitMonitor is fairly resistant to temporary voltageloss. The monitor can ride through aninterruption lasting up to one-tenth of a second.A typical electronic device trips during aninterruption of one-fiftieth of a second. Also, thenominal input voltage for a Circuit Monitor canrange from 100 to 264 Vac, allowing a widevariation of input without control power loss.

Figure 6: System Manager Software screen used when settingrelative set points. Here, a waveform capture will be triggered whenvoltage drops below 5% of the nominal value.

ENMED199031EN

Schneider ElectricIndustries SA

ART.28263

Centre Merlin GerinF - 38050 Grenoble cedexFranceTel.: +33 (0)4 76 57 60 60Fax: +33 (0)4 76 57 73 62http://www.schneiderelectric.comservice-commc4@mail.schneider.fr

This paper was originally published as a part of the series PowerLogicSolutions, copyright Square D Company 1998. Used with permission.

Published by: Schneider Electric SADesign and layout by: InsignPrinted by:

09/1999

This document has beenprinted on ecological paper.

ENM

ED19

9031

EN

Solutions to Voltage Sags and Interruptions

ENMED199031EN Electrical DistributionSolutionsIndustry-Building

An extrusion plant in Virginia (USA) was beingplagued by unexpected disturbances called sagsand interruptions. Lights would blink, adjustable-speed drives would trip, and molten polyesterwould begin to gum up dies and rollers. Fourhours later, workers would return the line tonormal operation and quality, only to have theentire process happen again when the nextstorm cloud appeared. During the first sixmonths of 1994, the plant suffered productionlosses 35 times due to sags and interruptions.

The problem

Figure 2: High-speed waveform, captured during a voltage sag.

In this issue,we discussthe solutionsto voltagedisturbancescausedby voltagesags andinterruptions

Figure 1: Tree contact with an energized feeder - a commonsource of a fault which causes a voltage sag.

The plant complained to the local utility, whichagreed to help determine the causes of thenumerous disruptions, and to recommendsolutions. In partnership with the electric utility,Schneider Electric placed a Circuit Monitor at theplants service entrance in July, to measure thevoltage disturbances causing the shutdowns.This meter is able to capture high-speedwaveforms during power system anomalies. Themonitor captured numerous voltage sag events,several of which caused shutdowns. One of thefirst events recorded (figure 2) exposed ashortcoming in the plants power quality design.

Electrical Distribution

Solutions to voltage sagsand interruptions

Solutions

Industry-Building

The solution

Monitoring at the extrusion plant identifiedseveral improvement areas for the utility.The event depicted in figure 1 (page 1) causedthe plant engineer to think that his process maybe overly sensitive to sags. The plant was initiallydesigned with equipment intendedto lessen the effects of voltage sags. Controlsand sensitive electronics for the dc drives aresupplied from a motor-generator set. Othermicroprocessors controlling the extrusion line areserved from a UPS system.

The plant used the information provided bythe waveform capture to strengthen the weak linkin the system: the ac drives serving coolingrollers. Drive engineers used the waveformcapture in figure 1 as a basis to change settingson the ac adjustable-speed drive controls. Eachdrive is equipped with a fault board, a sensingand control circuit that detects voltage anomaliesand turns off the drive to protect its costly powerelectronic components.The adjustment reduced the production linessensitivity to voltage sags, while maintainingadequate drive protection. These drives slowdown somewhat during sags, but they do not tripduring mild or short-duration sags. Thisimprovement has lessened the extrusion linesensitivity to voltage sags.

Solutions to voltage sags are often less costlythan solutions to interruptions. In either case,however, both utility system and customer loadsshould be considered for improvement. Often,the best solution to nuisance shutdowns dueto voltage sags is a dual approach:c reduce the number of utility faults,c lessen the sensitivity of customer equipment.

The two-pronged approach requiresa partnership, between utility and customer,built on open communication and willingnessto share data and ideas.

On the utility side, solutions to voltage sagsresult from reviewing common sources of faults.One of the most common causes is incidentaltree contact with overhead lines. Overhead highvoltage lines are not insulated like the wiring in ahouse. Tree contact, especially during wet orwindy weather, can permit current flow fromthe feeder to the ground, using the tree asa conductor. The high levels of current resultin depressed voltage (sag) along the entirenetwork until overcurrent protective devicesoperate to interrupt the current flow. Utilitiesattempt to keep trees out of lines by cutting limbsthat grow too close, but unusually active treegrowth, or tree-trimming budget cuts whichincrease the time between trimmings, can resultin frequent tree limb contact.

Customer solutions to voltage sags can be assimple as adjusting fault board settings, as theplant in Virginia discovered. This change did noteliminate shutdowns, but it reduced their numberby a considerable amount. The next step is toconsider installing constant-voltage transformers(CVTs) on sensitive control circuits. CVTs usetransformer saturation characteristics to dampenchanges in output voltage due to suddendecreases or increases in input voltage. CVTsare too expensive and bulky for use on large-power loads, but control circuits are typically lessthan 1 kilovolt-ampere (kVA) in capacity and arefairly inexpensive.

Voltage sags and interruptions can be costlyforms of power quality disturbances. Theseevents are caused by faults on the utility system,or sometimes by normal operation inside acustomer facility. While both disturbances lastless than a second, voltage sags differ frominterruptions in that some voltage remains duringthe disturbance. Interruptions cause a completeloss in voltage. It is important to distinguishbetween the two events because their solutionalternatives differ greatly in cost and complexity.Waveform capture information is crucial indetermining which event is causing theshutdowns, and in identifying the causes andanalyzing solution alternatives. PowerLogicCircuit Monitors with high-speed waveformcapture, and some engineering expertise, canensure that sags and interruptions do not ruinyour business.

Conclusion

ENMED199031EN

Schneider ElectricIndustries SA

ART.28263

Centre Merlin GerinF - 38050 Grenoble cedexFranceTel.: +33 (0)4 76 57 60 60Fax: +33 (0)4 76 57 73 62http://www.schneiderelectric.comservice-commc4@mail.schneider.fr

This paper was originally published as a part of the series PowerLogicSolutions, copyright Square D Company 1998. Used with permission.

Published by: Schneider Electric SADesign and layout by: InsignPrinted by:

09/1999

This document has beenprinted on ecological paper.

ENM

ED19

9031

EN

Solutions to Voltage Sags and Interruptions

ENMED199031EN Electrical DistributionSolutionsIndustry-Building

An extrusion plant in Virginia (USA) was beingplagued by unexpected disturbances called sagsand interruptions. Lights would blink, adjustable-speed drives would trip, and molten polyesterwould begin to gum up dies and rollers. Fourhours later, workers would return the line tonormal operation and quality, only to have theentire process happen again when the nextstorm cloud appeared. During the first sixmonths of 1994, the plant suffered productionlosses 35 times due to sags and interruptions.

The problem

Figure 2: High-speed waveform, captured during a voltage sag.

In this issue,we discussthe solutionsto voltagedisturbancescausedby voltagesags andinterruptions

Figure 1: Tree contact with an energized feeder - a commonsource of a fault which causes a voltage sag.

The plant complained to the local utility, whichagreed to help determine the causes of thenumerous disruptions, and to recommendsolutions. In partnership with the electric utility,Schneider Electric placed a Circuit Monitor at theplants service entrance in July, to measure thevoltage disturbances causing the shutdowns.This meter is able to capture high-speedwaveforms during power system anomalies. Themonitor captured numerous voltage sag events,several of which caused shutdowns. One of thefirst events recorded (figure 2) exposed ashortcoming in the plants power quality design.

Electrical Distribution

Solutions to voltage sagsand interruptions

Solutions

Industry-Building

The solution

Monitoring at the extrusion plant identifiedseveral improvement areas for the utility.The event depicted in figure 1 (page 1) causedthe plant engineer to think that his process maybe overly sensitive to sags. The plant was initiallydesigned with equipment intendedto lessen the effects of voltage sags. Controlsand sensitive electronics for the dc drives aresupplied from a motor-generator set. Othermicroprocessors controlling the extrusion line areserved from a UPS system.

The plant used the information provided bythe waveform capture to strengthen the weak linkin the system: the ac drives serving coolingrollers. Drive engineers used the waveformcapture in figure 1 as a basis to change settingson the ac adjustable-speed drive controls. Eachdrive is equipped with a fault board, a sensingand control circuit that detects voltage anomaliesand turns off the drive to protect its costly powerelectronic components.The adjustment reduced the production linessensitivity to voltage sags, while maintainingadequate drive protection. These drives slowdown somewhat during sags, but they do not tripduring mild or short-duration sags. Thisimprovement has lessened the extrusion linesensitivity to voltage sags.

Solutions to voltage sags are often less costlythan solutions to interruptions. In either case,however, both utility system and customer loadsshould be considered for improvement. Often,the best solution to nuisance shutdowns dueto voltage sags is a dual approach:c reduce the number of utility faults,c lessen the sensitivity of customer equipment.

The two-pronged approach requiresa partnership, between utility and customer,built on open communication and willingnessto share data and ideas.

On the utility side, solutions to voltage sagsresult from reviewing common sources of faults.One of the most common causes is incidentaltree contact with overhead lines. Overhead highvoltage lines are not insulated like the wiring in ahouse. Tree contact, especially during wet orwindy weather, can permit current flow fromthe feeder to the ground, using the tree asa conductor. The high levels of current resultin depressed voltage (sag) along the entirenetwork until overcurrent protective devicesoperate to interrupt the current flow. Utilitiesattempt to keep trees out of lines by cutting limbsthat grow too close, but unusually active treegrowth, or tree-trimming budget cuts whichincrease the time between trimmings, can resultin frequent tree limb contact.

Customer solutions to voltage sags can be assimple as adjusting fault board settings, as theplant in Virginia discovered. This change did noteliminate shutdowns, but it reduced their numberby a considerable amount. The next step is toconsider installing constant-voltage transformers(CVTs) on sensitive control circuits. CVTs usetransformer saturation characteristics to dampenchanges in output voltage due to suddendecreases or increases in input voltage. CVTsare too expensive and bulky for use on large-power loads, but control circuits are typically lessthan 1 kilovolt-ampere (kVA) in capacity and arefairly inexpensive.

Voltage sags and interruptions can be costlyforms of power quality disturbances. Theseevents are caused by faults on the utility system,or sometimes by normal operation inside acustomer facility. While both disturbances lastless than a second, voltage sags differ frominterruptions in that some voltage remains duringthe disturbance. Interruptions cause a completeloss in voltage. It is important to distinguishbetween the two events because their solutionalternatives differ greatly in cost and complexity.Waveform capture information is crucial indetermining which event is causing theshutdowns, and in identifying the causes andanalyzing solution alternatives. PowerLogicCircuit Monitors with high-speed waveformcapture, and some engineering expertise, canensure that sags and interruptions do not ruinyour business.

Conclusion