Power Quality,
Harmonics & Transients
By
P.K.Agarwal
DGM, NRLDC
Agenda
 Peculiarity of power
 What is quality power.
 Power quality defects.
 Who is responsible?.
 Reasons of bad power quality.
 Cost of bad power quality.
 Typical problems.
 Solutions.
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Peculiarity of Electric Power
 Electric power is the most essential raw
material.
 It is unusual commodity as it requires
continuous flow and can not be conveniently
stored.
 It is a typical example of “Just in Time”
philosophy – Components are delivered to
the production line at the point and time of
use by a trusted supplier with no requirement
of “goods in” inspection.
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Peculiarity of Electric Power (contd..)
 In reality electricity is very different from any
other product.
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It is generated far from the point of
consumption.
Is fed to the grid together with output of many
other generators.
And arrives at the point of use via several
transformers and many kilometers of overhead
or possibly underground cabling.
The network is owned and maintained by
different owners
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Peculiarity of Electric Power (contd..)
 Assuring the quality of power is not a easy
task.
 These is no way that sub-standard electricity
can be withdrawn from the supply chain or
rejected by the consumer.
 From the consumer point of view situation is
even more difficult.
 Acceptable quality level as perceived by the
supplier is different then desired by the
consumer.
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Quality of power
 The perfect power supply will be one which is
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Always available
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Always with voltage and frequency within
tolerances
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Has pure noise free sinusoidal wave shape.
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Defects in Power
 The most obvious defect in power is complete
interruption for few seconds to several hours.
 Voltage dips or sag where voltage drops to
lower value for a short duration.
 Voltage swells or surges where voltage raises
to higher value for a short duration.
 Long power interruption are problems to all
users.
 Many process are very sensitive to even a
short duration of interruption.
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Defects in Power (contd…/-)
 Continuous process industry where short
outage of power may loss synchronizing of
machinery and may result in large quantity of
semi processed product – Paper making where clean
up process is long and expensive
 Outage in multistage batch process may
destroy the value of previous process –
Semiconductor industry – Production of wafer
requires few dozen process over several days.
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Categories of Defects in Power
 Power quality defects i.e. deviations from
perfection falls in five categories:
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Under Voltage or Over Voltage
Dips (or sags) and surges (or swells).
Blackouts.
Harmonic distortion.
Transients.
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Under voltage or Over Voltage
 Over voltage or under voltages means,
supply voltage is more or less then the
allowable tolerances.
 These are the causes of unbalance of supply
and consumption of reactive energy in
network.
 If generation of reactive power is more then
the consumption supply voltage will be high.
 If generation of reactive power is less then
the consumption, voltage will be less.
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Under voltage or Over Voltage (contd..)
 Sustained over voltage or under voltage
stress the equipment.
 It weakens the insulation of system and
equipments.
 By proper reactive power management the
supply voltage can be maintained within the
tolerance.
 As far as possible reactive power
compensation should be done locally by
consumer.
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Voltage Dips (or Sags) and Surges
 Voltage dips or sags are caused by abrupt
increases in loads such as short circuits or
faults, motors starting, or electric heaters
turning on, or they are caused by abrupt
increases in source impedance, typically
caused by a loose connection.
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Voltage Dips
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Voltage Dips
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Relative Cost of Voltage Dips
Elimination
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Voltage Dips (or Sags) and Surges (contd..)
 Voltage swells are almost always caused by
an abrupt reduction in load on a circuit with a
poor or damaged voltage regulator, although
they can also be caused by a damaged or
loose neutral connection.
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Blackouts (Power Outage)
 Power outage disrupts more business that
any other factor.
 Power outage or blackout means energy is
not available for work.
 Power outages may be caused by generator
maintenance outage, failure of equipment of
any critical element.
 The quality of sustained supply of power is
known as reliability.
 Reliability indices is used to track the
performance of a utility.
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Harmonic Distortions
 Electricity generation is normally produced at
constant frequency of 50Hz and generated
voltage can be considered practically
sinusoidal.
 However, when a source of sinusoidal
voltage is applied to a nonlinear device or
load the resulting current is not perfectly
sinusoidal.
 In the presence of system impendence of this
causes a non-sinusoidal voltage drop causing
voltage distortion at the load terminals known
as Harmonic Distortion.
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Harmonic Distortions (contd…)
Circuits containing thyristor is
when supplied a sinusoidal
voltage produces the distortion.
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Harmonic Distortions (contd…)
 Harmonics generated by microwave oven
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Harmonic Distortions (contd…)
 Harmonics generated by SMPS of computer
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Harmonic Distortions (contd…)
 Harmonics generated by variable speed drive
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Transients
 Transients disturbances are high frequency
events with durations much less than one
cycle of supply.
 Causes are switching, lightening strikes on
the network, switching of reactive loads on
consumer sites.
 Transients can have magnitudes of several
thousands volts and so can cause serious
damage to both the installation and the
equipment connected to it.
 A good earthing greatly reduces the
transients.
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Transients (contd…/-)
 Majority of power system transients are due to
switching action.
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Transients (contd…/-)
Voltage across the
capacitor while
Switching a RC circuit
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Transients (contd…/-)
 Transients are triggered from
capacitor switching.
 Capacitor transients normally
causes the highest peak
magnitudes. If the capacitor is
switches when the system
voltage near its peak, the
capacitor pulls the system
voltage down (as the current
ruches to capacitor to charges
it). The system rebounds, the
voltage
overshoots,
and
oscillates about the system
fundamental frequency wave
Capacitor Switching Transients
forms. This transients normally
decays quickly.
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Switching Transients
A restrike can occur when
switching a capacitor off:
the switch opens at current
zero, trapping the peak
voltage on capacitor. As the
system voltage decreases
from the peak, the voltage
across the switch rapidly
increases. By the time
system voltage reaches
opposite peak, the voltage
across the switch is double.
Switch may restrikes.
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Restrike of a capacitor Bank
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Who is Responsible for Quality
 Suppliers argue – Critical users must bear the
cost of ensuring quality of supply.
 Rather than expecting the supply industry to
provide very high reliability to every customer
everywhere on the network.
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Who is Responsible for Quality (Cond..)
 Some quality problems are the result of
shared infrastructure.
 Fault on a one part of network may cause dip
that will affect some customers.
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Who is Responsible for Quality (Cond..)
 Higher the level of the fault, the greater will
be the number of customers affected.
 Problem on one customer’s site may cause a
transient that affects all the customers on the
same subsystem.
 Harmonics, arise within the customer’s won
installation may propagate onto the network
and affect other customers.
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Reasons
 Such a guaranteed quality supply would
require a very substantial investment for the
benefits of relatively few customers (in count,
not in consumption) and would be
uneconomic.
 It may not be technical feasible with in the
current social and legal frame work.
 Any customer is normally entitled to be
connected to the supply.
 Utility providers has the right to excavate
roadways with the risk of cable damage.
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Reasons (contd…)
 Weather conditions, such as high winds and
rains, snow fall, frequently cause damages
overhead lines.
 Repair of lines under the same conditions,
are difficult and time consuming.
 It is therefore the consumers responsibility to
take steps to ensure that the quality of power
delivered to his process is good enough.
 This quality level may well be higher than that
delivered to the plant by the supplier.
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Cost of Power Quality defects
 It is difficult to attribute a business loss to a
particular event.
 The electricity supply industry tends to value
an interruption in terms of the cost of the
electricity that was not supplied as a result.
 Consumer values it in terms of the revenue
lost as a consequence of the break in
production.
 Electricity is relatively cheap and the supply
interruption is relatively short.
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How much good is good enough
 This question is impossible to answer.
 While it is relatively simple to quantify the
behavior of a particular piece of equipment to
voltage dips.
 Determining the likely incidence of voltage
dips at a particular on supply system is
difficult.
 It will change over time as new consumers
are added and assets replaced.
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How much good is good enough
 There are some international standards
available that set limit of voltage variation and
harmonic voltage distortion below which the
equipment should function without error.
 Similarly, there are standard limits for voltage
deviation and harmonic voltage distortion of
the supply.
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Typical Problems
 A study was performed by European Copper
Institute in 2001, covering 1400 sites in 8
countries.
 Any given site in Europe has 5-20%
probability of suffering from one or more of
the problem listed.
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Computer Lockups
Flickering screens
Flickering lights.
Overheating of transformers at moderate load.
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Typical Problems (contd…)
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Induction Motors
Heating of conductors due to skin effects.
Correct functioning of process control
equipments
Data network congestion
Problem with power factor correction
equipment.
Problem with specific (long) lines or when
switching heavy loads.
Overloaded neutrals.
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Typical Problems (contd…)
 Nuisance tripping of protective devices.
 Utility claims resulting from harmonics
affecting supply.
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Typical Problems (contd…)
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Solutions
 The list of possible solutions for power quality
problems is long and non-complete. Solutions
listed below are adopted according to a the
survey:
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Surge Protection
UPS
Back-up generator
True RMS measurement
Transformer derating
Motor derating
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Solutions (contd…)
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Dedicated circuit
Multiple cables for harmonic loads.
Complete rewiring of the installation.
Zoning of electrical loads.
Meshed Earth.
Passive filters
Active conditioners
TN-S wiring
Natural Upsizing
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Typical Problems (contd…)
 Computer Lockups:
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Solutions (contd…)
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Thank You
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