JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV Year POWER QUALITY AND FACTS Previous Question Paper (September - 2022 - R18)

A voltage sag is a decrease in RMS voltage at the power frequency for durations from 0.5 cycles to 1 minute, typically caused by fault conditions, motor starting, or large load switching. It is characterized by a magnitude between 0.1 and 0.9 per unit.

A voltage swell is an increase in RMS voltage at the power frequency for durations from 0.5 cycles to 1 minute. Swells are often associated with system fault conditions, such as a single-line-to-ground fault on an unfaulted phase, or the sudden switching off of a large load.

(i) DC Offsets: These occur during transient conditions, such as fault inception or switching operations. The DC component represents the non-periodic part of the fault current that decays exponentially based on the circuit's time constant (L/R ratio). It causes the waveform to be asymmetrical about the zero axis.

(ii) Fluctuations: These are systematic variations in the voltage envelope or a series of random voltage changes, the magnitude of which does not normally exceed the specified voltage ranges. They are typically caused by loads that exhibit rapidly changing current requirements, such as arc furnaces or large industrial drives.

(iii) Flicker: This is the subjective impression of unsteadiness of visual sensation induced by a light stimulus whose luminance or spectral distribution fluctuates with time. It is a direct consequence of voltage fluctuations and is measured using the short-term (Pst) and long-term (Plt) flicker severity indices, which quantify the impact on human perception of light intensity variations.