Don't Let Sags and Interruptions Disturb You

Aug 1, 1998 12:00 PM, By Larry Ray, Square D

Voltage sags and interruptions can be costly forms of power quality disturbance. But using waveform capture information can help you find which event is causing the shutdowns.

An extrusion plant in Tennessee was plagued by unexpected voltage disturbances. Lights blinked, adjustable-speed drives tripped, and molten polyester began to gum up dies and rollers. Four hours later, workers returned to the line, only to have the entire process happen again, when the next storm cloud appeared.

During the first six months of the year, this plant suffers production losses around 35 times because of these voltage sags and interruptions. What are the solutions? Let's get some basic understanding before learning how the plant solved these problems.

Why is it important to distinguish between sags and interruptions? Solutions to voltage sags usually are cheaper than solutions to interruptions. Often, you can use equipment costing much less than uninterruptable power supply (UPS) systems to reduce the number of shutdowns due to sags. Ride-through options include constant-voltage transformers, magnetic synthesizers, and control modifications. However, interruptions may require a static or rotary UPS, or expensive modifications to the utility distribution system.

Voltage sags can shut down sensitive process loads completely. These unexpected disruptions can be extremely costly. Voltage sags affect equipment used in extrusion processes, silicon wafer fabrication, data processing, and chemical and paper making. Some sags effect only one or two phases of a three-phase circuit. Depending on whether plant loads are single- or three-phase, 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, on the other hand, nearly always affect all phases simultaneously.

Circuit monitors can capture events. These monitors can automatically trigger an event based on the effective level of any of the monitored voltage and current inputs. When the event exceeds pre-programmed set points, the circuit monitor simultaneously captures a snapshot of the instantaneous voltages and currents up to seven channels. The capture plots 64 data points per cycle for every channel, up to 60 cycles per event.

The waveform snapshot includes two to ten cycles of pre-event values, depending on user performance. You can also define a log file to place other related system information. For instance, you can record information about system loading, power factor, voltage imbalance, and other parameters (at the time of a high-speed trigger) in a log file.

You can choose the set points for the high-speed event capture. This involves selecting the voltage or current at which the event capture begins (pickup value) and ends (dropout value). These set points are set in absolute values or in relative values.

You apply absolute set points when you want to define the exact value at which the event is triggered. You use relative set points in cases where you want 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 set point). This prevents the measured voltage from drifting too near a trigger threshold and capturing spurious events.

For example, if you use a 5% pickup set point to trigger a voltage sag event, the effective voltage must change by 5% from a value based on the measured voltage averaged during the past 30 sec.

For further flexibility, you can reduce the time interval for calculating the average voltage to about five sec to make the event triggering even less sensitive to voltage variations.

How does the circuit monitor determine a sag has occurred? Every half cycle (0.008 sec), it 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 then records event type, pickup time, dropout time, and magnitude of the minimum value during the event.

Caution: You need a backup power supply for the circuit monitor to operate during a power system disturbance.

Equipping the current monitor with an optional ride-through module can provide about 8 sec of backup capability. This is usually enough to ride through these disturbances, allowing you to capture the beginning and end of duration events.

Back to the extrusion plant. After complaining, the local utility agreed to help determine the causes of the numerous disruptions, and to recommend solutions. In partnership with the utility, an electrical distribution equipment manufacturer placed a circuit monitor at the plant's service entrance to measure voltage disturbances. It captured numerous voltage sag events, several of which caused shutdowns.

The plant strengthened the weak link in the system (the AC drives serving cooling rollers) with the information provided by the waveform capture. Drive engineers use the waveform capture as a basis to change settings on the AC adjustable-speed drive controls. Each drive is equipped with a "fault board." This is a sensing and control circuit that detects voltage anomalies and turns off the drive to protect its costly power electronic components. The adjustment made reduces the production line's sensitivity to voltage sags, while maintaining adequate drive protection. The drives do slow down somewhat during sags, but they don't trip during mild or short-duration sags.

Sidebar: What is the Difference Between a Voltage Sag and an Interruption?

Voltage sags are a brief decrease in effective voltage that lasts less than 1 min. Faults on the utility system resulting from lightning, tree or animal contact with energized feeders, or equipment failure usually cause sags. They also occur when a fault occurs inside a plant or when a large motor st arts.

Interruptions, however, differ from sags. While some effective voltage remains during a sag, interruptions cause a complete loss of voltage. Since both usually last less than 1 sec, both types of 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 involved.

Sidebar: What's the Best Approach to a Solution?

Solutions to voltage sags are often less costly than solutions to interruptions. But in either case, you should consider the utility's system and your loads for improvement. Often the best solution to nuisance shutdowns due to voltage sags is a dual approach:

Reduce the number of utility faults.
Lessen the sensitivity of your equipment.

This two-pronged approach requires a partnership between you and your utility. This partnership should involve open communication and a willingness to share data and ideas.

Utility solutions to voltage sags result from reviewing common sources of faults, such as animal contacts, storms, and overhead lines coming in contact with trees. Overhead high voltage lines are not insulated like the wiring in a house. A tree can conduct a current flow from the utility line to ground, especially during wet or windy weather. The high levels of current result in depressed voltage (sag) along the entire network until overcurrent protective devices operate to interrupt the current flow.

Customer solutions to voltage sags can be as simple as adjusting fault board settings, as the plant in Tennessee discovered. This did not eliminate shutdowns, but it reduced the number considerably.

The next step is to consider installing constant-voltage transformers (CVTs) on sensitive control circuits, due to sudden decreases or increases in input voltage. CVTs use transformer saturation characteristics to dampen changes in output voltage. They are too expensive and bulky for use on large power loads, but control circuits typically are less than 1kVA in capacity. CVTs in this range cost less than $500.