A friend reminded me of the extensive flicker monitoring program that the Colombian utility commission required of electric utilities beginning in 2005. Based on preliminary measurements, it was obvious that the perceptibility short term (Pst) value used to quantify the complex process of the eye-brain response to voltage fluctuations below 30 hertz was greater than the typically accepted annoyance level of 1.0 in many measurements, as shown in the figure below. Yet the number of complaints about the lights flickering wasn’t that large.
Likewise, studies in Australia and elsewhere have found the magic 1.0 number has been exceeded with minimal complaints. An October 2011 U.K. study found that the “shadow flicker” effect (modulation of the voltage waveform as a blade of a wind turbine passes in line with the tower) that was a big concern with increased deployment of wind turbines has been insignificant there. Does this mean that, after years of effort to develop and adopt standards for accurately measuring flicker (IEC 61000-4-15 and IEEE 1453), it’s much ado about nothing?
The voltage fluctuations that result in the perception of light flicker clearly aren’t going away. Anyone who lives within 10 miles of an electric arc furnace is probably aware of the effects, as is anyone working in a plant with large horsepower reciprocating motors or even arc welder neighbors. It goes back to Ohm’s and Kirchoff’s Laws for sags, swells, harmonics and even transients. Changes in current will result in changes in voltage. Large increases in current will result in measurable decreases in voltage, especially as one gets farther away from the generator or source of the voltage.
The unique part about flicker is the human aspect, where the ability of the eye to detect the variation in light and the brain to interpret such determines if it is considered annoying (or worse for those susceptible to light-triggered epileptic seizures). Multiple factors come into play, including the person, the type of light source, the amount of natural light available, and the person’s activity. The light source, though, is changing.
The Australian government passed legislation on efficiency standards for lamps in 2007 that resulted in the virtual elimination of certain lamps by the time the Pst study was conducted. This legislation was similar to the U.S. Energy Independence and Security Act of 2007, which established the phase-out of incandescents (and other types of lighting) that can’t pass minimum efficiency and lifetime requirements. The higher frequency of the electronic ballasts in compact-fluorescent lamps (CFLs) makes them more immune to voltage fluctuations than standard fluorescents and far better than incandescents. The filaments of the incandescent lamps in North and South America, which have 115–120V as the standard for single-phase, point-of-utilization circuits, have four times the “photon-ride-thru” of the 230-volt equivalents in Europe, Africa, the Middle East and Asia. That alone reduces the effect.
The voltage fluctuations that result in the light flicker aren’t just a human factor, though. Some processes, such as fiber extrusion and textile weaving, can be susceptible to slight variations that don’t trip the process offline but change the fiber diameter or weave tightness. It also can be a precursor to larger problems brewing. Contact impedance can cause a small voltage variation today; as it gets worse over time, so will the resulting voltage drop.
In the international standards community, another power quality parameter is gaining acceptance—rapid voltage changes (RVC). RVCs are defined as a “single rapid variation of the rms value of a voltage between two consecutive levels which are sustained for definite but unspecified durations,” where the variation is less than the +/–10 percent from the nominal value normally classified as a swell or sag. The magnitude and duration factors are different than the rms variations of sags and swells, and there is no frequency component as a weighting factor that is found in the flicker calculations.
But, to paraphrase Mark Twain, rumors of flicker’s demise have been greatly exaggerated. There are and will be millions of lamps around for years to come that will flicker when the voltage fluctuates. And though CFLs and other newer lighting technologies may be much less susceptible to voltage fluctuations and also much lower in energy consumption, the overall impact of the current harmonics from these nonlinear loads is still a subject of study and debate.
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.