Information about Flicker

Flicker is a very specific problem related to human perception and incandescent light bulbs.
It is not a general term for voltage variations.
Humans can be very sensitive to light flicker that is caused by voltage fluctuations.
Human perception of light flicker is almost always the limiting criteria for controlling small voltage fluctuations. The figure illustrates the level of perception of light flicker from a 60 watt incandescent bulb for rectangular variations. The sensitivity is a function of the frequency of the fluctuations and it is also dependent on the voltage level of the lighting.

Voltage changes that will result in perceptible light flicker with a 60 watt incandescent light bulb.

Limits for flicker levels are not specified in IEEE standards. Curves similar to the one shown above have been used by individual utilities as guidelines for controlling flicker.

Flicker levels in IEC standards are characterized by two parameters:

  • Pst is a value measured over 10 minutes that characterizes the likelihood that the voltage fluctuations would result in perceptible light flicker. A value of 1.0 is designed to represent the level that 50% of people would perceive flicker in a 60 watt incandescent bulb.
  • Plt is derived from 2 hours of Pst values (12 values combined in cubic relationship).

Note that IEEE is also adopting this method of characterizing flicker (IEEE 1453).

IEC 61000-2-2 specifies flicker compatibility levels:

  • Compatibility level for short term flicker (Pst) is 1.0.
  • Compatibility level for long term flicker (Plt) is 0.8.

Recognizing that it is not always possible to maintain flicker levels within these compatibility levels, EN 50160 specifies less restrictive requirements for the supply system performance. The EN 50160 limit is that 95% of the long term flicker values (Plt) should be less than 1.0 in one week measurement period.

Note that individual step changes in the voltage, such as would be caused by motor starting or switching a capacitor bank, are often limited separately from the continuous flicker limits. IEC 61000-2-2 specifies a compatibility level of 3% for the individual voltage variations. EN 50160 specifies a limit of 5% for these variations but mentions that more significant variations (up to 10%) can occur for some switching events. Specific recommendations are not provided in IEEE but individual utilities usually have their own guidelines in the range 4-7%.

Contact Alex McEachern for more information.

Alex McEachern, 1/2004, based in part on work done with Mark McGranaghan

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