A different kind of poison
Every time I see the lights on a Christmas tree blinking on and off, I am reminded of the old adage, “One person’s medicine is another person’s poison.” Flickering lights in an office, factory or residential home can be more than an annoyance. The voltage fluctuations that result in the lights flickering generally do not interrupt a process or cause equipment failures, though they can cause different colors (light shades) of woven fabric or vary the product diameter in plastic pipe or fiber optic extrusion processes. However, as businesses realize that their employees are their most valuable assets, keeping those “knowledge assets” running at full productivity is increasingly important. The flicker phenomena can be subtle enough as to not be consciously detected by those effected, but can induce discomfort, nausea and in extreme cases, epileptic seizures. Some people are reconsidering the “sick building syndrome” as not always being related to the air quality and the HVAC system, but the “silent nauseator.”
Many of the latest generation of PQ monitors provide the ability to monitor flicker, though not all comply with the harmonized standards, IEC 61000-4-15 and IEEE P1453. The perceived light flicker is the result of the relatively small voltage modulations or variations in amplitude at subharmonic frequencies (below 30Hz). Different types of light sources and different wattage levels respond differently to these voltage variations, and the light output (measured in lumens) also varies. For 120V, 60W incandescent light bulbs, as little as a quarter of a percent voltage fluctuation at 9Hz can be perceived by most people. Other factors, such as the ambient light level and the activity of the person, can also affect perception.
Though more subtle than sags and swells, the voltage fluctuations or modulations of the RMS envelope follows the same basic rules as the RMS variations that result in sags (dips) or swells. They usually are the result of a change in load current, which causes a change in the voltage drop across the source impedance, which then results in a change in the voltage supply to the lighting. Having a higher source impedance will exaggerate the effects of the same current variations, the same as it would result in a more severe sag.
The new standards measure the perception of the light flickering by two parameters, Pst and Plt. Pst is the short-term perception index, usually computed over a 10-minute interval; Plt is the long-term perception index, usually over a two-hour interval. A value of 1 is assigned to the lower bound of the observable flicker perception curve for 60W incandescent light bulbs. The larger the number, the more perceptible it is.
Numerous field tests have shown that the value of 1 for Pst is a very good indicator of when most people will observe the light flicker. It doesn’t indicate what the effect on the people will be, nor on the quality of a product, but it does serve a good basis for needing further investigation to determine the source.
However, in some other countries that have adopted EN50160 Quality of Supply standard, especially in Europe, the Pst limit of 1 must be met 95 percent of the time. Common sources of flicker include: arc furnaces, arc welders, resistance welding machines, lamp dimmers, rolling mills, large electric motors with variable loads, heat-pumps or air conditioning, medical imaging machines (X-ray, MRI, CT scan), large-capacity copy machines, even appliances such as refrigerators. Arc furnaces and welders are perhaps the most notorious in this list. The electric arc used in both processes is basically a short circuit, drawing very large currents at a plethora of frequencies in a varying pattern over time. The voltage drop in the source impedance can be measured all the way back to the transmission substations, affecting thousands of customers over a wide geographic area. Customers in a 10 to 20 mile radius can experience the flicker, though the Pst value seems to decay in an exponential curve relative to the distance.
The solutions to eliminating the flicker can range for relatively inexpensive solutions (change the type of lighting), to very expensive solutions (feeding the offending facility with a separate feeder circuit from a transmission substation.) Specially designed filters placed in the feeder to the source can reduce the rapid changes in current flow. Inductors don’t like the current to change instantly and capacitors don’t like the voltage to change instantly.
Lowering the impedance between the source and the supply can also lessen the effects, since Ohm’s and Kirchoff’s Laws apply to flicker, just like for sags, swells, transients and transients. But sometimes, changing from a 60W to 75W light bulb, or mixing in natural light sources, is all it takes. Lastly, you may experience the flicker sensation if your computer monitor has a 72 Hz scan rate, and 60 Hz fluorescent light sources reflect off the screen. This produces a 12 Hz frequency that is in the frequency range to make some people nauseous. EC
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.