Quality Control

In the August issue, I reviewed some common power-quality related issues, including insulation failures on a motor’s outer windings, unexplained offline trips of adjustable speed drives, blown fuses on surge suppression strips, damaged electronic equipment power supplies, premature lamp and ballast burnout, and noisy motor bearings. This month, I highlight more common issues.

Lightning-induced damage

The typical lightning bolt delivers 100 million volts and 10,000 amperes in just 10 microseconds, or 100,000 joules. Considering that a good surge-protector strip has a 1,000 joule rating, anything struck directly by a bolt would be obliterated. However, indirect or induced strikes can still do plenty of damage.

In one case, lightning struck a tree in the back of a property, traveled 25 feet through the earth into the exterior electrical circuit that fed a pump for a fish pond, another 35 feet into the house before destroying the GFCI receptacle, then went into the house wiring and “removed” chunks of sheetrock around other receptacles as they were damaged, coupled over from the electrical circuits to the telephone circuits in an alarm panel, blew up DSL filters, electronic equipment, and more. Adequate surge protection on all circuits, electrical or communication, that enter or exit a facility with low impedance paths to earth ground are a minimum to protect against such an event.

Nausea without illness

The fluctuation of the amplitude of the voltage at rates less than 30 hertz can result in the phenomena called light flicker. The intensity variation of the light change is measured in photons and varies depending on the type of lamp. The effect on humans depends on several factors: other lighting sources (such as sunlight), what the person is doing, the time of day, and the person’s eye-brain response. The parameter perceptibility short term (Pst) is the standard way of measuring this. A Pst greater than 1 will result in most people experiencing the flicker. For some, this can induce nausea. Though the standards were written around incandescent lamps, the phenomena can occur with fluorescents and LED lighting, especially when powered through dimmers or with high harmonic levels.

Not-so-neutral wire

The neutral (grounded) conductor in a three-phase wye circuit should not have any significant current flow in it. Originally, the National Electrical Code allowed this conductor to be undersized compared to the phase conductors. With today’s nonlinear dominated loads, the conductor should be nearly twice as large, because some of the harmonic currents that result from the nonlinear loads will add in the neutral conductor instead of cancelling out with linear loads. 

In particular, the triplen harmonics (3, 6, 9, 12, 15 …) are the offenders; the harmonic currents in Phase A, B and C are in sync with each other, rather than at different phase angles that would yield the cancellation effect. There have been instances of neutral conductors getting so overloaded that the insulation melts and further damage results, including fires. Harmonics also can cause overheating in electromagnetic devices, such as motors and transformers, and require derating.

Blinky lights

Early in my field experience, the troubleshooter for a utility in the upper Midwest showed me his trouble sheet, in which there was a category of “problem found” entitled “blinky lights.” This was a popular category, which more formally would be described as “voltage sag resulting from sudden significant increase in current.”

There should have been a secondary check box where the increase in current came from the load or the source. If the latter, it would be the troubleshooter’s problem to find the source. This often would be a call to the dispatch operator to see if there was a fault on the distribution system at the time that the customer was reporting the problem(s). If not, then it was deemed likely that the problem was load side and was the customer’s responsibility to correct. In residential dwellings, it is often caused by heat pumps or HVAC units cycling on or other significant current drawing equipment, such as microwaves, saws or even washing machines and electrical dryers. Industrial/commercial facilities have similar sources, though there are often circuits with many more and larger loads on them to investigate.

The list can go on, but these issues and the ones in the prior article are among the most common. Some problems can be solved with a simple investigation and a walk-through the circuits. Others require a PQ monitor to be installed for at least a day or preferably a week. Some require calling in the experts. Fortunately, in most facilities, those cases are few and far between.

About the Author

Richard P. Bingham

Power Quality Columnist
Richard P. Bingham, a contributing editor for power quality, can be reached at 732.287.3680.

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