Power System

Power Quality: An Introduction

Power Quality-

According to definition of power quality given in IEEE standard, “Power quality is the concept of powering and grounding sensitive equipment in a matter that is suitable to the operation of that equipment.”

According to Institute of Electrical and Electronic Engineers (IEEE) standard IEEE 1100 defines power quality as “the concept of powering and grounding sensitive electronic equipment in a manner suitable for the equipment”.

Problems in Regarding to Power Quality-

Power quality problems encompass a wide range of disturbances such as voltage sags, swells, flickers, harmonic distortion, impulse transients, and interruptions.

  • Voltage Sag (Or Dip)-  Voltage sag is the decreasing in voltage between 10% and 90% of nominal voltage for half cycle to one minute. The Sags comes from the utility have a variety of cause like lightning, animal and human activity, and normal and abnormal utility equipment operation.


  • Voltage Swell-  Swell is the opposite of sag. An increasing in voltage above 110% of nominal for half cycle to one minute. Voltage swell is caused due to line faults,badly dimensioned power sources and incorrect tap settings in tap changers in sub-stations.


  • Over voltage – An over voltage is an increase in the rms ac voltage greater than 110 percent at the power frequency for duration longer than 1 min. Over voltages are usually due to load switching or incorrect tap settings on transformers.


  • Under voltage –   Under voltage is a decrease in voltage below 90% of its nominal value for more than one minute.


  • Voltage Fluctuations –  Voltage fluctuations are systematic variation of voltage or a series of random changes in voltage magnitude which lies in the range of 0.9 to 1.1p.u. Voltage fluctuations are caused due to frequent start or stop of electric motors, oscillating loads and arc furnaces.


  • Harmonics Distortion –  Harmonics are periodic sinusoidal distortions of the supply voltage or load current caused due to non-linear loads. Harmonics are measured in integer multiples of the fundamental supply frequency. In commercial facilities, computers, lighting, and electronic office equipment generate harmonic distortion.


  • Transients- 
    • Impulsive Transient –  An impulsive transient is a unidirectional variation in voltage, current, or both on a power line. Lightning strikes, switching of inductive loads, or switching in the power distribution system are the most common causes of impulsive transients.06
    • Oscillatory Transient –  An oscillatory transient is a bidirectional variation in voltage, current, or both on a power line. These are caused due to power factor correction capacitors, switching of inductive loads and transformer Ferro-resonance.07.JPG
  • Interruption –  When a voltage drops below 10% of its nominal value it is called an interruption or a blackout.
    • Very Short Interruption – Very short interruption is defined as total interruption of electrical supply for duration from few milliseconds to one or two seconds.
    • 10.JPG
    • Long Interruptions – Long interruptions are defined as loss of utility power lasting more than 2 minutes due to major local area or regional electrical events.11.JPG

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