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

Power quality in distribution systems refers to the ability of the electrical network to provide a clean, stable, and sinusoidal voltage supply to sensitive loads. Problems arise due to the increasing use of non-linear loads, power electronic converters, and intermittent renewable energy sources. Key issues include:

• Harmonic Distortion: Caused by non-linear loads like VFDs and rectifiers, leading to overheating of transformers and motors.
• Voltage Fluctuations: Rapid variations in voltage magnitude caused by arc furnaces or large motor starting, resulting in light flicker.
• Transients: Short-duration high-energy impulses caused by lightning strikes or switching operations, potentially damaging insulation.
• Voltage Unbalance: Unequal phase voltages or phase angles, often due to unbalanced single-phase loading on a three-phase system.
• Poor Power Factor: Excessive reactive power demand from inductive loads, increasing distribution losses and reducing capacity.

These power quality phenomena are defined by their duration and magnitude relative to nominal voltage:

• Unbalance: A condition where the three-phase voltages or currents differ in magnitude or phase displacement from the standard 120-degree separation. It is typically caused by uneven distribution of single-phase loads across the three phases.
• Sags (Dips): A decrease in RMS voltage to between 0.1 and 0.9 per unit at the power frequency for durations from 0.5 cycles to 1 minute. They are usually caused by faults on the transmission or distribution system or the starting of large induction motors.
• Swells: An increase in RMS voltage to between 1.1 and 1.8 per unit for durations from 0.5 cycles to 1 minute. These often occur during single-line-to-ground faults on the unfaulted phases or due to the sudden switching off of large capacitor banks.
• Interruptions: The complete loss of supply voltage (less than 0.1 per unit) for a specific duration. A momentary interruption lasts up to 3 seconds, while a sustained interruption exceeds 1 minute, typically resulting from protective device operations like circuit breakers or fuses clearing a fault.