Download POWER QUALITY

Power Quality
M.Jegadeesan
ASP/EEE
KLN College of Engineering
Pottapalayam
Electricity
• Electricity provides the power you need to run
your home electronic equipment.
• Sometimes, interference in the supply of
electricity affects your equipment runs.
• Many older appliances can tolerate short power
disturbances.
• Many newer appliances, such as personal
computers, Microwave ovens and sophisticated
stereo systems, have sensitive electronics that
can be disrupted or damaged.
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Electricity
Utilities transmit electricity over power lines and into
your home as an alternating current (AC) wave, which
looks something like an electrocardiogram.
 This is how power travels through your wiring and
passes into your appliances.
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Power quality
Our Power Systems are
designed for
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Now the power system
serves
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What is Power Quality?
• ‘Power quality’ broadly refers to the
delivery of a sufficiently high grade of
electric service.
• In general, it involves maintaining a
sinusoidal load bus voltage at
stipulated magnitude and frequency.
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Why is Power Quality Important?
• It affects both utilities as
suppliers and customers as
users
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Impact on Customer Side
• Computers and communication equipment
are
susceptible
to
power
system
disturbances which can lead to loss of data
and erratic operation.
• Automated manufacturing processes such
as paper-making machinery, chip-making
assembly lines, etc. can shutdown in case of
even short voltage sags.
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Impact on Customer Side (cont.)
• Induction and synchronous motors can have
excessive losses and heating.
• Home electronic equipment are vulnerable to
power quality problems - e.g., blinking VCR
machines and digital clocks.
• Equipment and process control malfunction
translates to dollars of expense for
replacement parts and for down time,
impacting adversely on profitability and
product quality.
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Impact on Utility Side
• Failure of power-factor correction
capacitors
due
to
resonance
conditions.
• Increased
losses
in
cables,
transformers
and
conductors,
especially neutral wires.
• Errors in energy meters, which are
calibrated to operate under sinusoidal
conditions.
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Impact on Utility Side (cont.)
• Incorrect operation of
protective relays,
particularly
in
solid-state
and
microprocessor-controlled systems.
• Interference with ripple control and power
line carrier systems used for remote
switching, load control, etc.
• Unhappy customers as well as malfunction
and failure of system components and
control systems, impacting adversely on
profitability.
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Sources of Power Quality Problems
•
•
•
•
•
•
•
•
Power electronic devices
IT and office equipments
Arching devices
Load switching
Large motor starting
Embedded generation
Sensitive equipment
Storm and environmental related damage
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Common Manifestations of Power quality
Reactive power - Low power factor
Harmonics - current & voltage distortions
Frequency limits - under & over frequencies
Steady state voltage limits - under & over voltages
Transients
Sags & Swells
Unbalance
Sequence components
Black outs & Brown outs
 Flicker
Neutral shifts
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Several typical PQ disturbances
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POWER QUALITY
What do we need ?
What do we have ?
Culprits
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Victims
POWER QUALITY
?
Power Quality issue is defined as "any occurrence manifested in
voltage, current or frequency deviation that results in damage, upset,
failure or malfunction of end use equipment".
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Why is it a Concern?
Power quality problems can
cause:
 Equipment malfunctions
 Excessive wear or premature
 failure of equipment
 Increased costs from downtime
 Increased maintenance, repair
time and expense
 Outside consultant expense
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POWER QUALITY
• Power quality is the combination of
Voltage Quality and Current Quality.
Voltage Quality
• Voltage Quality is concerned with
deviation of the actual voltage from the
ideal voltage.
• The ideal voltage is the single frequency
sine of constant frequency and amplitude.
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POWER QUALITY
Current Quality
• Current Quality is concerned with
deviation of the actual Current from the
ideal Current.
• The ideal current is the single frequency
sine of constant frequency and amplitude.
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Linear vs. Non-linear loads
Linear
 Pure resistance, inductance, and
capacitance are all linear.
 a specific value of ohms, the
relationship of volts and amperes is a
straight line.
 A linear element in a power system is a
component in which the current is
proportional to the voltage. In general,
this means that the current wave shape
will be the same as the voltage
Example: Incandescent lighting,
heating loads, and motors
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Non-Linear
The current wave shape on a non-linear
load is not the same as the voltage.
 These loads do not exhibit a constant
impedance during the entire cycle of
applied sinusoidal voltage.
Examples of non-linear loads
 In single phase
Computers,
Fax
Machines,
Photocopiers, UPS’s, TV’s, VCR’s,
Lighting dimmers & Electronic ballasts
for high efficiency lighting Single-phase
AC & DC drives, Ultra-violet disinfection
systems.
 Three Phases
Variable speed AC & DC drives,
UPS systems, Arc furnaces & SCR
temperature
controllers,
Battery
chargers, etc.
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POWER QUALITY PROBLEMS
Overloading,
 under voltage
 sustained interruption
Waveform Distortion
Harmonics, Noise & Interference
Voltage Fluctuations
Voltage Sags & Swells
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Steady state Disturbance
Long Duration Disturbances
 Overvoltage
 Under voltage
 Sustained Interruptions
Short Duration Disturbances
 Sag
 Swells
 Interruptions
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Long Duration Disturbances
• Long-duration variations encompass root-meansquare (rms) deviations at power frequencies for
longer than 1 min.
• Long-duration variations can be either over
voltages or under voltages.
• Overvoltages and undervoltages generally are
not the result of system faults, but are caused by
load variations on the system and system
switching operations.
• Such variations are typically displayed as plots
of rms voltage versus time.
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Overvoltage
An overvoltage is an increase in the
rms ac voltage greater than 110percent at
the power frequency for a duration longer
than 1 min.
• load switching (e.g., switching off a large
load or energizing a capacitor bank).
• Incorrect tap settings on transformers can
also result in system overvoltages.
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A
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Under voltage
An undervoltage is a decrease in the rms ac voltage
to less than 90 percent at the power frequency for a
duration longer than 1 min.
 opposite of the events that cause overvoltages.
 load switching on or a capacitor bank switching off can
cause an under voltage
Brownout
The term brownout is often used to describe sustained
periods of undervoltage initiated as a specific utility
dispatch strategy to reduce power demand.
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RMS Measurement of under voltage during one day
Sustained Interruptions
• When the supply voltage has been zero for a period of
time in excess of 1 min, the long-duration voltage
variation is considered a sustained interruption.
• Voltage interruptions longer than 1 min are often
permanent and require human intervention to repair the
system for restoration.
Outage
• Utilities use outage or interruption to describe
phenomena of similar nature for reliability reporting
purposes. However, this causes confusion for end users
who think of an outage as any interruption of power that
shuts down a process.
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Short Duration Disturbances
Voltage sag
Voltage sag is a reduction in RMS voltage at the power
frequency for duration of 0.5 cycles to 300 cycles.
Typical end-use equipment sensitive to voltage sags are:
computers, programmable logic controllers, controller power supplies,
motor starter contactors, control relays and adjustable speed drives.
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Voltage Sag
Sags can cause computers and other sensitive equipment to malfunction
or simply shut off. Undervoltage conditions can damage certain types of
electrical equipment.
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Voltage swell
Voltage swell is an increase in RMS voltage at the power
frequency for duration of 0.5 cycles to 300 cycles.
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Voltage Swell
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Short interruption
Short interruption is the complete loss of the supply voltage with in
a time period of 0.5 cycles up to 150 cycles.
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Voltage Variation Sources
 Supply side variations
 Short circuits
 Capacitor switching
 Load switching
 Regulator malfunction
 Load side variations
 Motor starting
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Voltage Flicker
• A waveform may exhibit voltage flicker if its
waveform amplitude is modulated at frequencies less
than 25 Hz, which the human eye can detect as a
variation in the lamp intensity of a standard bulb.
• Voltage flicker is caused by an arcing condition on
the power system.
• Flicker problems can be corrected with the
installation of filters, static VAR systems, or
distribution static compensators
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Example voltage waveforms showing flicker created
by an arc furnace
POWER QUALITY PROBLEMS
Harmonic distortion
Harmonic distortion is the periodic deviation of the voltage or current
from the ideal sinusoidal waveform, which have frequencies of
multiple integral of the fundamental frequency.
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What Are Harmonics?
• “A component frequency of a harmonic
motion of an electromagnetic wave that is
an integral multiple of the fundamental
frequency”
• US fundamental frequency is 60 Hertz
– 3rd Harmonic is 3 x 60Hz or 180Hz
– 5th Harmonic is 5 x 60Hz or 300Hz, etc.
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What Causes Harmonics?
Non-Linear Loads
Current is not proportional to the applied voltage
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Harmonics
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Harmonics
• The base frequency of the power supply is said
to be the fundamental frequency or first
harmonic.
• The fundamental frequency or first harmonic of a
60 Hz power supply is 60 Hz.
• Additional harmonics can appear on the power
supply. These harmonics are usually whole
number multiplies of the first harmonic.
• The third harmonic of a 60 Hz power supply, for
example, is 180 Hz (60 x 3).
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Harmonics
• When
a
harmonic
waveform
is
superimposed on the fundamental sine
wave a distinctive waveform is produced.
• In this example, the third harmonic is seen
superimposed
on
the
fundamental
frequency. The problem of waveform
distortion becomes more complex when
additional harmonics are present.
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Resultant wave form
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A.S.S.Murugan,SL/EEE,KLNCE,P
ottapalayam
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Harmonic Distortion
Harmonics are multiples of the fundamental
frequency
=
+
f(x) = sin(x)
Sin(5 X )
f (X ) 
5
Sin(5 X )
f ( X )  Sin( X ) 
5
When added together
result in a distorted waveform
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Distorted Waveform Composed of Fundamental and 3rd
Harmonic. THD approximately 30%
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Effect of harmonics on
waveform
180 Out
of Phase
In
Phase
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What do harmonics do?
• Harmonics are carried through the system from
the source and can nearly double the amount of
current on the neutral conductor in three phase
four wire distribution systems.
• Distorted currents from harmonic-producing
loads also distort the voltage as they pass
through the system impedence. Therefore, a
distorted voltage can be presented to other end
users on the system.
• Overall electrical system and power quality is
affected by the introduction of harmonics.
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Sources of Harmonics
• Solid State Electronic Devices which contain a poor
power supply
– Computers (PCs/CPUs)
– Laser Printers
– Copy Machines
•
•
•
•
•
Solid State UPS Units
Solid State Devices (Fluorescent lighting ballasts)
Rectifiers (AC-DC Converters  VFDs)
Welding Units
Arc Furnaces
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Interharmonics
Interharmonics are defined as frequency
components of voltages or currents that are not an
integer multiple of the normal system frequency (e.g., 60
or 50 Hz).
The main sources of interharmonics are static
frequency converters, cycloconverters, induction motors,
and arcing devices. Power line carrier signals can be
considered as interharmonics. The effects of
interharmonics are not well known but have been shown
to affect power line carrier signaling and induce visual
flicker in display devices such as cathode ray tubes
(CRTs).
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Common Power System Harmonics In Bold
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Square Wave Harmonic Content
Fund
3
3,5
3,5,7
3,5,7,9
3,5,7,9,11
3,5,7,9,11,13
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Practical Motor Drive Circuits
Almost all motor drive circuits consist of three parts:
A input converter to change the AC to DC;
A DC link to store and filter the DC;
An output inverter to change the DC into AC.
Both output voltage and frequency must be controlled together for motor load.
DC Link
AC-DC
Conversion
AC Input;
fixed Frequency,
fixed Voltage
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Capacitor
or
Inductor
DC-AC
Conversion
Motor
AC Output;
variable Frequency,
variable Voltage
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Point of Common Coupling
• The point of common coupling is the
location in the power distribution system
where harmonic distortion is to be
measured, usually where harmonic
currents flow into a bus which feeds other
equipment. Its location must be specified!
• In the absence of a specified location, the
POCC for current harmonics is the plantutility interface
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How can Harmonics be Reduced?
• Isolate harmonic loads on separate circuits
(with or without harmonic filters)
• Harmonic mitigating transformers
• Phase shifting (zig-zag) transformers
– Used to cancel out specific harmonics by
making one voltage circuit 180 degrees outof-phase
• Filter capacitor backs
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NOISE
Noise refers to unwanted electrical signals (with broadband
spectral content lower than 200 kHz) that produce undesirable effects in
the circuits of control systems in which they Noise in power systems can
be caused by power electronic devices, control circuits, arcing
equipment, loads with solid-state rectifiers, and switching power
supplies. Noise problems are mainly caused by improper grounding.
There are two types of noise voltages:
Common-mode noise voltage:
A noise voltage that appears between current carrying conductors
and ground. That is, this noise voltage appears equally and in phase
from each current-carrying conductor to the ground.
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Normal-mode noise voltage:
A noise voltage that appears between or among active circuit
conductors, but not between the grounding conductor and the active
circuit conductors.
Noise disturbs electronic devices such as microcomputer and
programmable controllers.
The problem can be mitigated by using filters, isolation
transformers, and some line conditioners.
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Others
Voltage
Fluctuation
PQ
Frequency
Variation
Notching
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Voltage Unbalance
• Deviation of magnitude and/or phase in 3phase system
• Can result in heating of induction motors
Notching
– Caused by commutation in semiconductor
converters
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What is a Transient or Surge?
• Transients, commonly called as surges are sub
cycle disturbances of very short duration that
vary greatly in magnitude.
• When transient occurs, thousands of voltage can
be generated into the electrical system
• A Transient can be classified into two
categories, impulsive and oscillatory Duration
< 50 ns to 50 ms
• .00000005 seconds to .002 seconds
• .000005 seconds to .050 seconds
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Oscillatory & Impulsive Transient
 Oscillatory Transient is a sudden,
non – power frequency
change, include both positive & negative polarity values.
 Impulsive transient is a sudden, non – power frequency
change, unidirectional in polarity ( primarily either positive or
negative)
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Sources of Transients
Lightning
• Static
• Arc Welding
Switching
• Contactor
• Relays
• SCR’s
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Harmonic Indices
• Two important indices, THD and TDD, are
used to describe the effects of harmonics
on power system components and
communication systems.
• These indices are used to measure the
deviation of a periodic waveform
containing harmonics from a perfect sine
wave.
• For a perfect sine wave, the deviation (or
the distortion) is zero.
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IEEE 519-1992
IEEE standard 519 - 1992 specifies the allowable
limits for voltage and current distortion at various bus
and system voltages. These are given in Tables 1 and 2.
The important terminology used in this standard are
explained below:
• The Point of Common Coupling (PCC) is the location of
the harmonic voltage and current distortion to be
calculated or measured.
• Total Harmonic Distortion (THD) is the total harmonic
voltage distortion calculated or measured at PCC.
• Total Demand Distortion (TDD) is the percentage of total
harmonic current distortion calculated or measured at
PCC.
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Total Demand Distortion (TDD)
The ratio of the root-mean- square of
the harmonic current to the root-meansquare value of the rated or maximum
demand fundamental current, expressed
as a percent.
Total Harmonic distortion (THD)
Total harmonic distortion (THD) is a ratio of
harmonic distortion to the fundamental frequency.
The greater the THD the more distortion there is of
the 60 Hz sine wave.
Harmonic distortion occurs in voltage and current
waveforms. Typically, voltage THD should not
exceed 5% and current THD should not exceed 20%.
Some of the power meters offered by Siemens are
capable of reading THD.
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Total Harmonic distortion (THD)
Sum of squares of amplitudes of all harmonics
THD 
Square of amplitude of fundamental
x 100
Mathematically, THD of a voltage wave form can be defined as,
h 

THDV =
h2
Vh2
X 100
2
V1
THD of a current wave form can be defined as,
h 
THDI =

h2
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I h2
x 100
2
I1
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CBEMA: Computer Business Equipment Manufacturers Association
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THANK YOU