Control Engineering Online Update for May 14, 2004


Control Engineering Online Update - Technology


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Harmonic distortion often takes a double bite out of profits—from production problems that include nuisance interruptions and downtime to costly replacement of expensive equipment.

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Harmonic Distortion: An Elusive Cause of Poor Equipment Performance

As businesses retool production operations in order to stay competitive, they often allow a severe problem to creep in that can wreak havoc on profitability. This uninvited guest inexplicably overheats power distribution systems, causes power interruptions, damages vital instrumentation, and brings production to a screeching halt. The guest is harmonic distortion, and if you've got it, you may also be in store for a future hit to the bottom line in the form of penalties from your power company for non-compliance with IEEE Standard 519.

Harmonic distortion can cause incorrect meter readings, nuisance tripping of circuit breakers and fuses, failures in zero sensing circuits, motor overheating, blown fuses on power factor corrected systems and interference with telephones and other communications systems. And the potential for most such problems is undetected—until the equipment fails.

Although harmonic distortion is hard to see and measure, it is no mere gremlin. It is a very real problem that often accompanies the installation of equipment with motors controlled by VFDs (variable-frequency drives). VFDs are an integral part of today's industrial and commercial power systems because they save energy, provide added control and reduce mechanical and electrical stress during starting and stopping of loads. However, unless certain precautions are taken, VFDs can also induce harmful harmonic distortion, an often undetected condition in electric power systems that some industry reports claim to cost industry between $16-40 billion dollars in annual losses due to downtime, equipment failures, and malfunctioning systems.

"Essentially, harmonic distortion is electrical noise that occurs in power distribution networks," explains John Cherney, harmonics specialist at Saftronics, a manufacturer of advanced variable-speed drives and starters for electric motors. "This noise is created by (AC) VFDs with non-linear power loads, loads that draw current with a waveform that does not conform with the shape of the supply voltage waveform."

There are many different effects of harmonic distortion that may result in potential problems. However, the single most damaging effect of the current harmonics caused by many VFDs is the excitation of system resonance, which causes instability. IEEE Standard 519 states: "System resonant conditions are the most important factors affecting harmonic levels."

"Most industrial environments are continually in a state of flux, where new technologies are being introduced by adding new production equipment," Cherney explains. "In many cases this equipment is highly sophisticated, employing computer-controlled operations and microprocessor-controlled monitor systems that are highly dependent on clean power. Yet, the companies installing the equipment may not have the experience or personnel to understand how it affects their power system. Too often, this results in harmonic current distortion that destabilizes the power system and equipment attached to it. Because most users have no way of metering total harmonic distortion within their system, they figure the problem must belong to the power company," says Cherney.

Efforts to manage the rampant harmonic distortion that came with the influx of VFDs are what led to the writing of the standard IEEE 519. "Because users' power distribution systems that have severe harmonic distortion were causing problems for electric power suppliers, the IEEE decided to pressure those users to clean up their systems through power conditioning or the use of VFDs that create lower amounts of harmonic distortion," Cherney explains. "Otherwise, they will continue to experience the consequences of power system problems and, in the future, possibly pay increasingly stiff penalties for non-compliance."

This economic need for clean use power distribution quality is amplified by the potential for power factor penalties. Another incentive for incorporating VFD technology into the distribution environment is to lower power factor. Yet, companies with erratically performing equipment due to harmonic distortion problems are subject to higher power factor charges.

Many standard harmonic reduction solutions are available, including reactors, isolation transformers, filters, active devices and multi-pulse VFD packages. All have their strengths and weaknesses, and should be carefully considered by qualified engineers after an analysis of power system characteristics. Although applying component-level solutions provide some level of harmonic reduction, they may not meet the strict guidelines of IEEE 519.

Another method of keeping harmonic distortion within safe and IEEE 519-compliant levels is to use VFDs with a sufficiently high pulse count. Most general purpose VFDs employ a 6-diode input power section, and are referred to as 6-Pulse VFDs. Pulse "multiplication" is achieved in increments of six (12, 18, 24, etc.). The 6- and 12-pulse systems will not meet harmonic levels outlined in IEEE-519 and if either is installed, additional harmonic-reduction devices would be required. The 18-pulse VFD design is a self-contained solution that typically reduces total harmonic distortion to less than 5 percent—well within IEEE 519 compliance (see graphic for configuration).

Saftronics offers a product called the HR1018 Harmonic Reduction Technology package, which is an18-pulse AC drive designed to reduce harmonic distortion to meet IEEE 519 requirements without requiring external filters, magnetics, or active devices.

For more information, visit the Saftronics Web site .


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