The document discusses power quality disturbances, defined as changes in voltage, current, or frequency that disrupt electrical equipment operation. It categorizes these disturbances into various types including sags, swells, overvoltage, and interruptions, explaining their causes and characteristics. The increasing reliance on automated devices has made power quality an important concern in electrical engineering.

1. Power Quality Disturbances
By
Dr. Sunil Singh
Associate Professor
Department of EE
GB Pant University of Ag and Tech Pantnagar

2. Introduction
 Increased use of automated control devices, computers,
PLC, and other safety devices quality of power become
very important issue.
 Power quality, or more specifically, a power quality
disturbance, is generally defined as any change in voltage,
current, or frequency that interferes with the normal
operation of electrical equipment.
 Cause of poor power quality: nonlinear loads, adjustable-
speed drives, traction drives, computers, UPS, battery
chargers, variable frequency drives and other power
electronics or electronic based equipments, load
unbalancing etc.

3. Classification of PQ Disturbances
 Sag (dip)
 Swell
 Overvoltage and Under voltage
 Voltage Imbalance
 Voltage fluctuations
 Frequency variations
 Waveform distortion
 Transients
 Interruptions

4. Power Quality Disturbances
Sag (dip)
 When normal voltage (rms) level decreases from .9 pu to
.1pu at the power frequency, for durations of 0.5 cycle to 1
minute.
 It may be further divided: Instantaneous (.5 to 30 cycle),
Momentary (30 cycle to 3 s), temporary(3s to 1 min).
Cause
 system faults
 Sudden connection of heavy loads
 Starting of large motors.

5. Swells
 A swell is defined as an increase in voltage to between 1.1
and 1.8 pu of the nominal voltage or current at the power
frequency for durations from 0.5 cycle to 1 min.
 It may be further divided: Instantaneous (.5 to 30 cycle),
Momentary (30 cycle to 3 s), temporary(3s to 1 min).
Causes:
 Fault
 switching off a large load or energizing a large capacitor
bank.

6. Overvoltage
 An overvoltage is an increase in the rms ac voltage
greater than 110 percent at the power frequency for a
duration longer than 1 min.
Cause
 load switching (e.g., switching off a large load or
energizing a capacitor bank).
 Incorrect tap settings on transformers

7. Under voltage
 An under voltage is a decrease in the rms ac voltage to
less than .9 pu at the power frequency for a duration
longer than 1 min.
Cause
 Switching on loads
 Switching off capacitor bank

8. Voltage Imbalance/ unbalance
 If magnitude of all phases are not equal
 Phase angle differences is deviates from 120
Causes:
 Large single-phase loads (induction furnaces, traction
loads), incorrect distribution of single-phase loads on
three phase supply

9. Voltage fluctuations
 A voltage fluctuation is a systematic variation of the
voltage waveform or a series of random voltage changes,
of small dimensions, namely 95 to 105% of nominal at a
low frequency, generally below 25 Hz.
Cause
 Arc furnaces

10. Frequency variations
 Variation in frequency of voltage or current from 50 Hz
 Very rare
 Due to generator is heavily loaded.

11. Waveform Distortion
 Waveform distortion is defined as a steady-state
deviation from an ideal sine wave of power frequency
principally characterized by the spectral content of the
deviation
 There are five primary types of waveform distortion:
■ Harmonics
■ DC offset
■ Inter-harmonics
■ Notching
■ Noise

12. Harmonics
 A non sinusoidal periodic waveform can be represented
as a sum of different sine-waves with different magnitude
and phase, having frequencies that are multiples of
power-system frequency. These components are known
as harmonics
Causes
 Classic sources: electric machines working above the
knee of the magnetization curve (magnetic saturation),
arc furnaces, welding machines, rectifiers, and DC brush
motors.
 Modern sources: all non-linear loads, such as power
electronics equipment including ASDs.

13. Continue.....
 THD (total harmonic distortion) is a measure of
effective value of harmonic distortion.

14. Voltage fluctuations
 A voltage fluctuation is a systematic variation of the
voltage waveform or a series of random voltage changes,
of small dimensions, namely 95 to 105% of nominal at a
low frequency, generally below 25 Hz.
Cause
 Arc furnaces

15. Transients
 Very fast variation of the voltage value for durations
from a several microseconds to few milliseconds. These
variations may reach thousands of volts, even in low
voltage.
 Impulsive
 Oscillatory

16. Interruptions
 An interruption is a condition in which voltage at supply
terminal is close to zero for less then one minuets
(less then 10% of nominal voltage, as per IEEE Std 1159).
 If interruption more then one minuets then it is called
sustained interruption

17. Thank You
For your kind attention