Voltage Sags: What causes them?
Voltage sags are the most common cause of poor power quality. They are also the most costly. The more complex a plant is, the more voltage sags it is susceptible to, such as in-process controllers, PLCs, variable speed drives, and robots. A sag in voltage can cause relays and contactors in motor starters to fail, resulting in a shutdown.
Power quality is mainly affected by voltage sags. In addition to being the most expensive, they also have the longest lead times.
Complex industrial equipment, such as process controllers, PLCs, adjustable speed drives, and robots, is more sensitive to voltage sags as their complexity increases. As a result of voltage sags, motor starter relays and contactors break down, causing downtime.
What are voltage sags?
According to IEEE, voltage sag refers to a short-term reduction in voltage. At 60 Hz, the magnitude of the voltage reduction is between 10% and 90% of the normal root mean square voltage (RMS). By definition, a voltage sag event lasts less than 1 minute and more than 8 milliseconds, or a half cycle of 60-Hz electrical power. Undervoltage, transients, voltage unbalance among phases, voltage fluctuations, and electrical noise are also power quality issues affecting voltage amplitude and duration.
What causes voltage sags?
In both directions of an electric meter, power quality issues exist. It is typically the local utility that is blamed for voltage sags. The equipment within a plant is often to blame for voltage sag events. Switching operations are the most common human-created events. Human-created events, like switching operations, also constitute the majority of natural events. Even if the problem is caused by sags in power meters or other power quality issues, it’s difficult to determine the source of the issue.
Trees falling onto power lines
Construction workers digging into buried cables
Squirrels and rodents
During voltage sag, there can be a difference in individual phase voltages and an accompanying phase-angle shift. The most common cause of sags is a single-line-to-ground fault (SLGF). Double and three-phase symmetrical faults are rare, occurring less than 20% of the time. Users beyond a 100-mile radius of the causing event can be affected by voltage sag on the grid. In some cases, a large motor can also cause voltage sags within the plant.
Detecting voltage sags
Monitoring power quality can detect voltage fluctuations, surges, interruptions, and other issues related to power quality. Power is measured as it enters a facility and compared with accepted standards. Systems that provide web-enabled power monitoring provide information on total harmonic distortion (THD), voltage, power factor, current demand, voltage unbalance, voltage sag, voltage swell, and alarms for multiple locations. Using one browser, a company in central Tennessee can monitor its facilities in Mexico, Canada, and across the country.
Typically, the utility is responsible for sag, Undervoltage, and interruption events that occur outside the plant. Contacting the utility should be made when power quality problems occur, but when they occur within the plant, plant personnel should determine the cause. This problem may be resolved by providing more power, lowering the voltage drop, and using a soft-starting method. Several industrial devices can be easily tested for sensitivity using the sag generation apparatus.
Voltage sag reduction
Uninterruptible power supplies (UPS), constant voltage transformers (CVT), and solid-state sag correction devices are common power line conditioning technologies. The location of a machine affects its voltage sag sensitivity. Increasing wire size can reduce voltage drop when the wiring is to blame. The plant service entrance can often be used as an entry point for sag correction devices, such as on the control panel, at the machine level, or at the bus level. During voltage drops, momentary power blackouts, and extended power outages, UPS devices provide protection.
Power quality: A relationship-based challenge
Power quality and the ability to reduce the need for reliability and dependability must be understood in order to solve problems. Assuring PQ’s impact on operating facilities, and minimizing the costs of solving problems, requires an understanding of its relationships. Power quality problems are typically caused by the addition of new, more sensitive devices to an existing plant, not by a change in the power coming from the utility. A manufacturer knows the relationship between raw materials and the finished product. Why would a manufacturer losing $100,000 per year to scrap, misdirected labor, and lost production not spend half as much to stop these losses?
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