**2.2. Voltage flickers**

Voltage flickers are defined as a continuous rapid variation of input supply voltage sustained for an appropriate period to enable visual recognition of a variation in electric light intensity. Flicker is a power quality problem in which the magnitude of the voltage or frequency changes at a rate that is to be noticeable to the human eye [6]. The main causes of the voltage flicker are the loads that draw large starting currents during initial energization such as elevators, arc furnaces, and arc welders. If load starting cases are rapidly repeated, then light flicker effects can be quite noticeable. The severity of voltage flickers is measured using short-term and long-term flicker severity terms, where an expected flicker severity over a short duration (typically 10 minutes) is known as *Pst*, and that evaluated over a long duration (typically 2 hours) is known as *Plt*. Thus, *Plt* is a combination of 12 *Pst* values. (0.0314 <sup>×</sup> *Pa*) <sup>+</sup> (0.0525 <sup>×</sup> *Pb*) <sup>+</sup> (0.0657 <sup>×</sup> *Pc*) <sup>+</sup> (0.28 <sup>×</sup> *Pd*) <sup>+</sup> (0.08 <sup>×</sup> *Pe*) (1)

$$P\_{st} = \sqrt{(0.0314 \times P\_s) + (0.0525 \times P\_b) + (0.0657 \times P\_c) + (0.28 \times P\_d) + (0.08 \times P\_e)}\tag{1}$$

where *Pa , Pb , Pc , Pd ,* and *Pe* are the surpassed flicker levels during 0.1, 1, 3, 10, and 50% of the surveillance period. By definition, a value of one for *Pst* expresses a visible disturbance, a level of optical severity at which 50% of persons might sense a flicker in a 60-W incandescent lamp. Excessive light flicker can cause a severe headache and can lead to the so-called 'sick building syndrome.'
