The blinks and flickers referred to here can best be described as “voltage fluctuations on electric power systems [that] sometimes give rise to noticeable illumination changes from lighting equipment.” The Institute of Electrical and Electronics Engineers (IEEE) standard that covers this is about to be published with major revisions in content and direction.
As one of the first IEEE power quality documents to harmonize with the International Electrotechnical Commission (IEC) standards, IEEE Std 1453 2004 adopted the IEC flickermeter standard and provided recommended levels, using the parameters of perceptibility short term (Pst) and perceptibility long term (Plt). If the frequency and amplitude of the voltage modulation produced a Pst of 1 or more, then most people would perceive flickering or blinking by a 60-watt (W) incandescent lamp.
IEEE Std 1453 2011 adopted the 2010 edition of the IEC 61000-4-15 standard, moving the recommended acceptable flicker levels to its annex as a precursor to the adoption of the IEC/TR 61000-3-7 in IEEE 1453.1 2012. This revision eliminates the common text of the IEC standards within them, relying on the user to know about (and purchase) those IEC documents. Instead, it focuses on practical applications of this and a better understanding of the methodology limitations.
For example, different types of lamps or luminaires have varying light output resulting from the voltage’s same modulation. Compact fluorescent and light-emitting diode lamps react differently and are generally less susceptible than a 60W incandescent, which can produce a Pst of 1 with just a 0.25 percent modulation in the amplitude at 8.8 hertz. The new revision also examines the effect of interharmonic voltages, as it has been found that IEC 61000-4-15-compliant flickermeters are capable of detecting light flicker only due to low-frequency interharmonics (below twice the fundamental frequency). Therefore, the flickermeters are unable to properly measure light flicker produced by lamps technologies different from incandescent, which is problematic as incandescent is being phased out.
A significant portion of the latest revision of IEEE 1453 covers the limit values that should be allowable on different voltage utility systems, not something with which the typical electrical contractor may be involved. However, methods for determining individual customer emission limits, evaluating compliance with emission limits, and estimating flicker contribution of a single customer is useful information for one with a power quality meter with flickermeter capability.
Along these lines, three case studies in the annex section provide valuable insight in how to apply such meters to real world situations. As in the harmonics standard, IEEE 519 2014, much of the focus is on measurements at the point of common coupling, which is also one of the areas of misuse of IEEE 519, where people use the recommended limits of 519 for individual pieces of equipment.
Like most power quality phenomena, we tend to monitor the voltage and characterize it based on how it varies from a pure sine wave. If we measure the rms of the voltage waveform and see that it decreases below a predetermined limit, it’s a sag (or a rapid voltage change). If there are periodic signals superimposed with the fundamental frequency waveform, we measure the resulting distorted waveform based on its harmonic spectrum. But like most power quality phenomena, the variations in the current result in the variations in the voltage that we measure. Ohm’s and Kirchhoff’s Laws apply, even to flicker.
Loads that draw large currents in a somewhat random manner are typical flicker sources. Electric arc furnaces (EAF) and electric welders do exactly this. The electric arc is basically a short circuit between two or more phases, with the source impedance limiting the current. Since the arc varies as the metal melts, the frequency components of the resulting signals cover a wide spectrum, falling into the range that causes visible light flicker from the voltage fluctuations. The effects from an EAF can be seen 10–20 miles or more away from the source, depending on the lighting type, time of day, individual person susceptibilities and activity. For example, reading a book at night using a candelabra-type lamp doesn’t require much voltage fluctuation to make the flicker visible and annoying.
Some have proposed that directly measuring the lamp’s light output with a luminescence device would be a better flickermeter. However, the Pst-based ones, even with their limitations for the newer lighting technologies, are still an effective way to track down customer complaints of “blinks.” The latest IEEE Std 1453, Recommended Practice for Analysis of Fluctuating Loads on Power Systems, can help you do just that.
About The Author
BINGHAM, a contributing editor for power quality, can be reached at 908.499.5321.