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Dealing with non-linear loads and harmonic distortion.

Do you have a site that has been working reliably until now, and for some strange reason you are suddenly having problems? It couldn't be electrical power because it is all on a UPS (uninterruptible power supply), right?

"Wrong," according to Ed Hodgson, ADCU Standards chairman. He says there are two issues here that can be the underlying cause of hard-to-detect problems in this area which he categorizes as non-linear loads and harmonic distortion.

Hodgson explains that when most of your equipment used linear power supplies, UPSs functioned properly. But now, with most of the newer equipment utilizing switched-mode power supplies, problems are occurring. Even if you do not have a UPS, non-linear loads and harmonic distortion can create problems.

The switched-mode power supplies are non-linear loads which feed back harmonics into the source power which, coupled with the output harmonics of your solid-state UPS, create severe power distortions.

He says one problem can be with equipment utilizing the older linear power supplies and the pass-through of the harmonics as noise; i.e., communications problems (even with new modems).

Another problem is excessive neutral current. The new requirements for the sizing of the neutral wiring is that it be 200% of the phase current. In most facilities, this is not the case. The result can be fires, insulation meltdown, transformer and UPS failures, etc.

Hodgson says you can measure the neutral current to see if you have a problem if you use a true RMS (root mean square) current meter. Your regular ammeter with the snap-around current transformer will not work.

He cautions against using an oscilloscope to look at the waveform. This is not recommended because of the voltages involved. Incorrect connections or contact can be fatal.

Hodgson says it is difficult to determine the THD (total harmonic distortion) at which a problem occurs. It depends on a lot of things and no one really knows for certain. If your clocks are running backwards, that is a sure sign you have surpassed the threshold.

While it is difficult to measure the THD, the solution to this is under examination although there is no ready answer right now. Meanwhile, there are several measures you can take, namely:

* Use rotary filters and rotary UPSs. These are motor generators with a true 60 Hz sine wave output and a low impedance that will absorb the harmonics. Low-end units start at 1.5 kVA (1500 VA = 120 volts X 12.5 amps).

* Besure your neutral wiring is sized properly.

* Derate your power conditioners, transformers, and solid-state UPS by 25%.

ADCU is fortunate to have sources (like Hodgson) for advice on both present and future technical problems of this type. You all realize you must devote a significant portion of your managerial time to long-range planning; but if the present-day systems don't work, you may never make it into the future.
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Title Annotation:Datacomm User
Author:Blegen, August
Publication:Communications News
Article Type:column
Date:Jul 1, 1991
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