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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.