Seek the power of harmony

Any device that converts power creates electrical harmonics, but variable-frequency drives are responsible for most of the harmonic distortion in industrial factories, reports Senior Technical Editor Dan Hebert.

 By Dan Hebert, Senior Technical Editor

H

armonic distortion has been a characteristic of commercially distributed electricity since Day One, but fairly recent changes to industrial power systems have made this electrical noise a growing problem. For years, most industrial factories ran AC-powered motors operated at constant speed. Variable-speed motors/drives have grown in popularity with industrial OEMs and their customers because they save energy, provide added control and reduce mechanical and electrical stress during the starting and stopping of loads.

Any device that converts power creates electrical harmonics, but VFDs create most of the harmonic distortion in industrial factories. Those motors often range into the hundreds of horsepower and dwarf harmonic distortion generated by low-power devices such as computers and fluorescent lights in a customer’s factory.

Variable-frequency operation creates harmonic distortion because the power loads are nonlinear and draw current with a waveform that does not conform to the shape of the supply voltage.

VFDs are here to stay, so industrial plants have to confront the problem and reduce or control harmonic distortion. “Harmonic distortion can cause incorrect utility power 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,” explains John Cherney, harmonics specialist at Saftronics.

Although the effects of harmonic distortion are many and varied, the most damaging result is excitation of system resonance, which causes instability. Efforts to manage harmonic distortion that accompanied the VFDs are what led to the writing of IEEE-519, a standard which expressly links harmonic distortion with power system resonance conditions.

Power companies don’t like customer power systems with high amounts of harmonic distortion, because it can generate billing problems. From the power company’s point of view, high harmonic distortion levels usually are associated with large electrical power consumers that make extensive use of VFDs, a profile that fits many of your customers.

While many of these facilities use their scale to negotiate more favorable power rates, processors and manufacturers with high electrical distortion levels can lose negotiating power. This is due to the fact that utilities must upgrade their own power distribution systems to handle the harmonic distortion introduced by their customers and their problematic power systems. 

Since VFDs are the main source of harmonic distortion, it can be wise to deal with distortion at the source, rather than paying for its effects through higher power bills, increased maintenance expenses and plant downtime.

Don’t be surprised, however, when customers start to expect you to be part of the solution.

There are two primary methods for dealing with VFD harmonic distortion. The most simple and effective approach is to use high-pulse-count VFDs that are designed from the beginning to generate low levels of harmonic distortion. The other, more commonly used, approach is to install components that reduce the relatively high harmonic distortion generated by general purpose VFDs.

Most general-purpose VFDs have a six-diode input power section (six-pulse). Pulse “multiplication” is achieved in increments of six, so 12-pulse and 18-pulse VFDs are common. VFDs with six or 12-pulse technology, typically do not meet harmonic level guidelines in IEEE-519, but 18-pulse count VFDs can reduce total harmonic distortion to less than 5%–well within the range for IEEE-519 compliance.

An 18-pulse VFD is usually about twice the price of a six-pulse unit, so they generally are installed sparingly and applied only in specific instances where harmonic distortion problems are the most serious.

Most new—and virtually all retrofit—VFD installations use six-pulse technology fitted with reactors, isolation transformers, filters, and other distortion-reducing components.

According to Cherney, high-pulse count VFDs are a better technical solution because harmonic currents follow the path of least resistance. That path might not lead where the harmonic reduction techniques are applied. If this happens, harmonic currents can flow into sensitive equipment and cause malfunctions or failure.

When harmonic-reduction techniques are applied to existing VFDs, an electrical power system study may be needed to verify that these changes do not have adverse affects. One of the most damaging side effects can be a change in the power system’s resonance point, which can introduce instability.

Detection of harmonic distortion in existing industrial facilities usually is done as part of a comprehensive electrical power systems study. Although harmonics can be measured easily with special-purpose meters, the mere presence of harmonic distortion neither reveals potential problems, nor suggests potential solutions.

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