Power Quality
Disturbance
Effects in Relation to
60-Hz Sine Wave
Common Symptoms
Typical Causes
Typical Solutions
Harmonics
Currents reflected back
into system and distort
fundamental sine wave:
3rd harmonic = 180 Hz
5th harmonic = 300 Hz
7th harmonic = 420 Hz
• Overheating neutral
conductors, transformers,
motors
• Excessive total harmonic
distortion (THD) values
• Nuisance tripping of
breakers
• Misoperation of electronic
loads
Created by nonlinear
loads.
Examples:
• VFDs
• UPS systems
• Single-phase
loads, such as copy
machines, laser
printers, lighting with
electronic ballasts
• Add filters to those
nonlinear loads
creating excessive
THD.
• Oversize neutral
conductors
• Isolate harmonic
sensitive loads
• Replace transformers
with “K-factor” rated
transformers
Transients
Very short-duration,
high-amplitude
increases along the
voltage sine wave
• Destroy equipment
• Breakdown insulation
• Trip VFD on overvoltage
• Lightning strike
• Switching of large
loads
• Switching of
capacitor banks
• Proper grounding
• Surge protection
Sags
A decrease to between
10% and 90% of the
normal voltage for a
duration of 0.5 cycles to
1 min.
• Equipment may trip on
under voltage
• Inductive loads may
overheat
Cycling “ON” of large
loads
Changing transformer
tap settings, soft starts
on large loads, and
isolating equipment
Swells
An increase to between Damage insulation over
110% and 180% of
time
normal voltage for a
duration of 0.5 cycles to
1 min.
Cycling “OFF” of large
loads
Changing transformer
tap settings and
isolating equipment.
Long-duration
under voltage
A decrease of the
supply voltage to
less than 90% of the
nominal voltage for a
duration greater than
1 min.
• Motors may run hot
• Electronic equipment,
such as computers or
microprocessor-based
devices, might function
erratically
• Utility system
experiencing heavy
load conditions
• Excessive plant
electrical system
loading
• Shut down and
disconnect voltage
sensitive equipment
• Provide standby
power
Sustained voltage
interruptions
The supply voltage
drops to less than 10%
on one or more phases
for a period greater
than 1 min.
Loss of power for extended
period of time
• Utility system faults
• Downed power lines
• Protective relaying
operating due to faults
Provide standby power
Single phasing
Complete loss of a
phase
• Motor damage
• Loss of certain singlephase loads
Utility or distribution
system fault
• Proper selection of
motor overloads
• Protective relaying to
sense fault
Unbalanced voltage Unequal voltage values
on 3-phase circuits
Overheat motors,
transformers, generators
and associated wiring
Unbalanced loading
across the three
phases
Redistribute loads
across all three phases
Noise
• Data loss or corruption
• Hum in telephone
systems
• Malfunction of electronic
equipment
• “Snow” in video systems
• Computer lockup
Can be introduced
from various sources.
• Electromagnetic
interference (EMI)
is interference
electromagnetically
coupled into a wiring
system
• Capacitively coupled
interference is radio
frequency interference
(RFI)
• Improper bonding at
service equipment
Proper grounding and
bonding
Undesirable electrical
signals in an electrical
or electronic circuit.
Common-mode noise
is seen equally and
in phase between
any phase conductor
or neutral and the
grounding conductor.
Transverse-mode noise
occurs in phase on all
3-phase conductors
and the neutral.
Interharmonics
Harmonic currents that
occur at less than the
fundamental 60-Hz
frequency
• Lights flickering
• CRT flicker
• Overload of outlet strip
filters
• Communications circuit
interference
•
•
•
•
Cyclo-converters
Induction motors
Arc furnaces
Arc welders
Flicker
The change in
light output from a
lamp caused by the
fluctuation of the supply
voltage
Produces human
discomfort typically in the
form of headaches and
possible nausea
Equipment that
causes rapid voltage
fluctuations:
• Lamp dimmers
• Welding machines
• Rolling mills
• Large electric motors
with variable loads
• Arc furnaces and arc
welders
• Medical imaging
machines
• Large-capacity copy
machines
• Electric motor starts
• Household
appliances
Filtering, isolation of
sensitive equipment
• Isolate lighting
circuits
• Eliminate
incandescent lighting