Overcome potential problems with power quality

Don’t let harmonic distortion, voltage transients, or grounding issues affect your production.

By William Atkinson, Contributing Editor

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Power is a critical resource to an industrial plant, so power problems require power solutions. Many industrial facilities have their own on-site power-generation capabilities to reduce energy costs and to compensate for the potential of short-term utility outages.  Power quality can have different kinds of impact on operations and production. There are just as many causes of poor power quality as there are options to solve them, and many of them are extremely affordable.

Harmonic distortion

"Voltage imbalance is a huge issue with motors," says Dan Chamberlain, electrical PDG lead CCMNA for Cargill in Dayton, Ohio. "Allied Reliability Group has been working with all of Cargill's corn milling facilities for a number of years and always helped us investigate the sources of this imbalance. For example, they would conduct on-line motor testing and look for total harmonic distortion, both from current and voltage. One solution was to install harmonic filters."

Harmonic distortion from poor power quality in industrial facilities can cause overheating, says Dave Sirmans, CMRP, electrical subject matter expert with Allied Reliability Group. "When variable frequency drives are applied, they are a non-linear load, and they distort the current sine wave of the incoming power that is reflected back to the service, such that transformers feeding that service will often overheat," he says. "Transformers are not equipped to deal with harmonic overheating." Negative sequence harmonics also cause electric motors to overheat. "Negative sequence harmonics will actually impose torque in an electric motor," says Sirmans.

One solution is to install isolation transformers on certain loads that have an impact, such as a group of motor loads that are run by variable frequency drives (VFDs), or non-linear loads in one part of the facility, says Sirmans. "A transformer that is rated for the kind of harmonics that you get can isolate those loads from the rest of the system," he says. "This usually takes care of the majority of problems."

Another solution is to make sure that you have the appropriate electrical system grounding. "You might have a plant that was designed to package food products 40 years ago but was sold and is now making Styrofoam cups," says Sirmans. "However, the electrical infrastructure might have remained unchanged the entire time." An electrical design from 40 years ago wasn't necessarily designed to handle the kind of distortion that you get from modern equipment, and a lot of facilities neglect testing the electrical integrity of their systems. "You need to test the grounds and make sure the whole system is electrically sound," he says.

Voltage transients

Figure 1. Insulation tests can be performed on electric motors to ensure they can withstand high voltage power transients.
(Source: SKF)

"It is important to monitor power quality," says Randy Keener, president of Torq Engineering. "It doesn't need to be done constantly but should be done periodically. During the monitoring, it is important to measure three things — voltage levels, voltage unbalance, and harmonics." If you have these problems, it can cause electrical equipment, especially motors, to run hot (Figure 1). "This shortens the life of the insulation, such that the equipment fails sooner than it otherwise would," he says.

While most professionals are aware of the importance of monitoring voltage levels, voltage unbalance, and harmonics, fewer pay as much attention to the problems associated with voltage transients, which are voltage spikes of very short duration that occur primarily internally but can also result from outside sources, such as lighting. Voltage transients can sometimes be two to five times higher than normal line voltage, explains Keener, and they can cause insulation systems to fail or at least be permanently degraded and fail very rapidly from there. "A lot of people don't even realize that transients exist, and they are not detected during normal power quality monitoring with typical lower-quality power meters," says Keener. "You need specialized equipment for this."

One solution to address voltage transients is to install low-voltage transient suppression devices. Another is to use motor soft starts to avoid instantaneous high changes in current that cause voltage transients. However, this won't eliminate the problem of transients when you stop the motor. One alternate solution, says Keener, is a variable speed device, which is useful in reducing voltage transients because it will soft-start and soft-stop motors. "Some of these variable speed devices generate a little bit of transients themselves, but not nearly as much as the motors themselves," he says.

Grounding systems

Figure 2. Many power disturbances result from degradation of, or damage to, power wiring systems in an industrial facility, especially to the grounding systems.
(Source: Schneider Electric)

At best, poor power quality equals higher production costs in an industrial facility, says Larry Ray, director of engineering for Schneider Electric Engineering Services. "Severe power quality issues can result in damage to sensitive equipment and even harm to workers," he says. In those cases, the costs are more than just lost productivity. Electronic devices associated with computers and communications networks contain sensitive components that can be affected by small variations in supply voltage, whether line-line, line-neutral, neutral-ground, or even ground-ground.

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