A Better Approach to Harmonics Considered Too High?

I read with interest the Specifier's Notebook, "Moving Harmonics Out Of Harm's Way," in the October issue of Consulting-Specifying Engineer. Having written an article on harmonics ("Waving Goodbye to Harmonics," (February 1996), I always take an interest in other viewpoints.

11/01/2000


I read with interest the Specifier's Notebook, "Moving Harmonics Out Of Harm's Way," in the October issue of Consulting-Specifying Engineer . Having written an article on harmonics ("Waving Goodbye to Harmonics," (February 1996), I always take an interest in other viewpoints. However, the solutions to fighting harmonics suggested in the commentary have very limited utility at best and, in some cases, just simply do not work.

Starting with "the first line of defense," the transient-voltage surge suppressor (TVSS) has two functions: to reduce the surge-voltage peak to a selected level by some type of clamping circuitry, and to reduce the magnitude of high-frequency noise on the circuit to which the TVSS is attached. Neither of these functions have any potential to reduce harmonics in any manner whatsoever.

Harmonics are low multiples of a 60-Hertz sine wave, so even if one considers harmonic content up to the 50th harmonic, that is only 3 kilohertz. The components in a typical TVSS are not designed to filter frequencies this low, but attenuation is typically achieved in the 50- to 100-kilohertz range, 16 or more times the maximum frequency that is considered to be associated with harmonic problems. There is one manufacturer that claims to reduce harmonics with their TVSS, but that claim needs substantial technical back-up before it may be considered. And even their claim is for a minimal reduction of harmonics, so it certainly would not be "the first line of defense."

The use of additional copper in the neutral of panels, feeders and other current carrying components may make better accommodation of the triplen harmonic currents, but it does no good in attenuating the harmonics within the system.

The commentary does say that the K-factor transformer does nothing to alleviate the voltage-wave harmonics, but it implies that it may help on other harmonic cases. In reality, the K-factor transformer has no more capability in attenuating system harmonics than adding copper in the neutral conductors. For a well-designed electrical distribution system with well-placed harmonic filters, K-factor transformers and their premium price have no place in this system. This is an admitted marketing ploy on the part of some of the electrical equipment manufacturers to improve their bottom line at the expense of inexperienced owners and engineers.

Installing low-total-harmonic-distortion ballasts in fluorescent lighting fixtures is the one item that I believe has real merit in reducing harmonics in the electrical systems of buildings. However, a cautionary note should be added to this suggestion clarifying that the total harmonics in a building are only minimally affected by the presence or absence of low-THD ballasts in the fluorescent fixtures. It should be remembered that the standard electromagnetic ballasts have medium to high harmonics relative to their total load current and have always been a concern as far as how to mitigate their harmonics. The harmonics issue is not new to ballasts just since the introduction of electronic ballasts!

For many years, I believed that isolation transformers would reduce harmonics traveling between variable-frequency drives (VFDs) and the electrical distribution system.

After some investigation on one project, however, I found that the level of attenuation provided by the isolation transformers did not merit their consideration for any real reduction in harmonic levels. If the distribution system does not have any harmonic issues before the addition of isolation transformers, it will not have any after their installation. And, likewise, if the distribution system does have significant harmonic issues that are affecting the operation of other devices in the system, the addition of isolation transformers will not significantly mitigate these problems.

The only available solutions are to use harmonic-reduction techniques on the devices that produce the majority of harmonics in the facility. The chief generators are VFD's, uninterruptible power supplies and major SCR-type power supplies. The triplen harmonics may be accommodated by the increased neutral-conductor size, but this will not help at all for the other odd harmonics. Only active or passive filters or multipulse rectifier input sections with twelve or more pulses per cycle may achieve a reduction in harmonic sources.

KENNETH L. LOVORN, P.E.

Manager, Electrical Department

Eichleay Engineers & Construction Inc.





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