March 2007 Electrical Apparatus

March 2007 Electrical Apparatus

This is a summary of the Electrical Apparatus March 2007 featured technical article,  by Richard L. Nailen, P.E.    

From the earliest use of electric distribution systems to serve incandescent lighting, utility customers have been bothered by “flicker”–the periodic dimming and brightening of electric lamps caused by fluctuations in system voltage. Typical causes of such variations are the switching on and off of other loads. Arcing devices such as welders and furnaces are particularly troublesome.

Both the perception of flicker and the annoyance it may cause are determined largely by the disturbance frequency. A higher repetition rate tends to increase observer reaction. Many experimental studies during the 20th century resulted in publication of graphs (flicker curves) relating the voltage disturbance rate to the threshold of observer perception–and objection to–the consequent light variation. Those reactions are subject to wide variation among individuals.

Most studies involved the simplest form of voltage disturbance: a rectangular dip in voltage, applied to incandescent lamps. Since then, the advent of electronic dimmers, fluorescent lighting with electronic ballasts, and newer technologies such as the metal halide lamp, have greatly changed the relationship between voltage fluctuation (which often involves non-rectangular wave shapes) and the visible effects.

That has led to development of more sophisticated standards to predict, measure, and evaluate flicker, such as IEC 61000-3. The relationship between voltage and light variations in those standards was developed for specific incandescent lamps. Also, only rectangular voltage dips were considered. To account for other waveforms, and for the duration of individual disturbances, the IEC has developed shape factor curves to modify the measurement standards.

To govern the measurement process itself, the IEC (in standard 61000-4-15) calls for use of an electronic flickermeter incorporating a mathematical model of the typical human eye-to-brain response to light variation. Based on that, the meter produces a numerical index of flicker severity. The IEC document has now been incorporated into an IEEE standard (No. 1453). Some commercially available power quality analyzers now provide that meter capability.

Although use of the flickermeter is considered a much more scientific approach to evaluation of expected human response to light fluctuations, the fact remains that no two individuals will exhibit identical reactions.

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