Power system harmonics

Chapter 1
Introduction
1.1 Motivation:
The primary motivation behind the study of Power System Harmonics
is the power quality in a power system has become an important issue
nowadays with significant development of power electronics technology
application of power electronic equipments and nonlinear loads in recent years
lead to harmonic interference problems in a power system. The loads are
nonlinear and harmonic currents generated by the loads will cause a voltage
drop across source impedance which causes decrease in power quality.
Power System may also contain sensitive loads such as computers or
electronic controllers which consume less power are connected in parallel with
nonlinear loads. The harmonics generated by these nonlinear loads may be
harmful to sensitive loads and could even damage the sensitive loads.
1.2 Background:
A good assumption for most utilities in the United States is that sine-wave
voltage generated in central power stations is very good. In most areas,
the voltage found on distortion system typically has much less than 1.0 percent
distortion. However, the distortion increases closer to the load. At some loads,
the current waveform barely resembles a sine wave. Electronics power
converters can chop the current into seemingly arbitrary waveforms.
While there are a few cases where the distortion is random, most
distortion is periodic, or an integer multiple of the power system fundamental
frequency.
When electronics power converters first became commonplace in the
late 1970s, many utility engineers became quite concerned about the ability of
the power system to accommodate the harmonic distortion. Many dire
predictions were made about the fate of power system if these devices were
permitted to exist. While some of these concerns were probably overstate, the
field of power quality analysis owes a great debt of gratitude to these people

because their concern over “new” problem of harmonics sparked the research
that has eventually led too much of the knowledge about all aspect of power
quality.
Presence of harmonics has been a lot since the 1990’s and has led to
deterioration in the quality of power. Moreover, there has also been an
increase in use of devices and equipments in power system also including the
nonlinear loads and electronic loads used in residential areas there by loading
the transmission and the distribution systems. This is because they operate at
very low power factors which increases the losses in line and also causes poor
regulation in voltage further leading the power plants to supply more power.
Also, some nonlinear loads and electronics equipment are such that instead of
drawing current sinusoidally they tend to draw current in short pulses thus
creating harmonics. Some of the examples of nonlinear loads would be
rectifiers, inverters, etc. Some of the examples of electronics equipments
would be computers, scanners, printers, etc.

Chapter 2
Cause of Power Quality Deterioration
2.1 Introduction:
As always, the main objective of the power system would be generation
of electrical energy to the end user. Also, associated with power system
generation is the term power quality. So much emphasis has been given to
power quality that it is considered as a separate area of power engineering.
There are many reasons for the importance given to the power quality. One of
the main reasons is, the consumers are well informed about the power quality
issues like interruptions, sagging and switching transients. Also, many power
systems are internally connected into a network. Due to this integration if a
failure exists in any one of the internal network it would result into unfavorable
consequences to the whole power system. In addition to all this, with the
microprocessor based controls, protective devices become more sensitive
towards power quality variation than were the past generation protective
devices.
Following are some of the disturbances which are common in affecting
the power system.
1.) Transients
2.) Sagging
3.) Variations in voltage
4.) Harmonics
2.2 Transients:
In terms of power system, the transients can be defined as an action or a
situation in power system with variations in power system and which is not
desirable in nature. A general understanding of transient is considered to be an
oscillatory transient who is damped due to the RLC network. A person who is
new to the power system also uses the term “surge” to define transient. A
surge may be analyzed as a transient who is resulting from the stroke of
lightening where protection is done by using a surge arrester. A person who is

more groomed in the field of power engineering would avoid using the term
“surge” unless it is specified as to what exactly.
2.3 Variations in Voltage:
There are two types of variations in the voltages.
 Short duration voltage variations
 Long duration voltage variations.
2.3.1 Short Duration Voltage Variations:
Short duration voltage variations are usually caused by faults in the
power system. Short duration voltage variations consist of sags which are
caused depending on the system conditions and faults that are caused in the
power system. It really depends on what kind of fault is caused in the power
system under what condition which may lead to voltage drops, voltage rise and
even interruptions in certain conditions. When such faults take place,
protective devices are used in order to clear the fault. But, the impact of
voltage during such faulty conditions is of short-duration variation.
 Interruptions:
When there are reductions in the voltage or current supply interruptions
take place. Interruptions may occur due to various reasons, some of them
being faults in the power system, failures in the equipment, etc.
 Sagging:
A short duration voltage variation is often referred to as sagging. When
there is a decrease between 0.1 to 0.9pu in rms voltage sagging takes place.
There are many ways to obtain the magnitude of sagging from the rms
voltages. Most of the times lowest value obtained during the event are
considered. Sagging normally has constant rms value during the deep part of
the sag. Thus, lowest value is an acceptable approximate value.
2.3.2 Long Duration Voltage Variations:
Long duration voltage variations are comprised of over voltages as well
as under voltages conditions. These under voltage and over voltage conditions
are caused by variations in the power system and not necessarily due to the

faults in the system. The long duration voltage variations refer to the steady
state condition of the rms voltage of the power system. The long duration
voltage variations are further divided into three different categories i.e.
interruptions, over voltage and under voltage.
 Under Voltage :
There are many reasons for the under voltage conditions in the power
system. When there is a decrease in the rms ac voltage to less than 90% of a
power system for some amount of time then under voltage condition exists.
Load switching on or switching off of a capacitor bank can also cause under
voltage condition. Also, when a power system is overloaded it may result into
under voltage condition.
 Over Voltage:
Compared to the under voltage condition, over voltage is an increase in
the rms ac voltage to greater than 110% of the power system for some amount
of time. Unlike under voltage condition, load switching off or capacitor bank
getting energized are main reasons for the over voltage conditions.
2.4 Harmonics:
Harmonics are one of the major concerns in a power system.
Harmonics cause distortion in current and voltage waveforms resulting into
deterioration of the power system. The first step for harmonic analysis is the
harmonics from non-linear loads. The results of such analysis are complex.
Over many years, much importance is given to the methods of analysis and
control of harmonics. Harmonics present in power system also has non-integer
multiples of the fundamental frequency and have periodic waveform. The
harmonics are generated in a power system from two distinct types of loads.
First category of loads is described as linear loads. The linear time-invariant
loads are characterized such that application of sinusoidal voltage
results in sinusoidal flow of current. A constant steady-impedance is displayed
from these loads during the applied sinusoidal voltage. As the voltage and
current are directly proportional to each other, if voltage is increased it will
also result into increase in the current. An example of such a load is
incandescent lighting. Even if the flux wave in air gap of rotating machine is

not sinusoidal, under normal loading conditions transformers and rotation
machines pretty much meet this definition. Also, in a transformer the current
contains odd and even harmonics including a dc component. More and more
use of magnetic circuits over a period of time may get saturated and result into
generation of harmonics. In power systems, synchronous generators produce
sinusoidal voltages and the loads draw sinusoidal currents. In this case, the
harmonic distortion is produced because of the linear load types for sinusoidal
voltage is small.
Non-linear loads are considered as the second category of loads. The
application of sinusoidal voltage does not result in a sinusoidal flow applied
sinusoidal voltage for non-linear devices. The non-linear loads draw a current
that may be discontinuous. Harmonic current is isolated by using harmonic
filters in order to protect the electrical equipment from getting damaged due to
harmonic voltage distortion. They can also be used to improve the power
factor. The harmful and damaging effects of harmonic distortion can be
evident in many different ways such as electronics miss-timings, increased
heating effect in electrical equipments, capacitor overloads, etc. There can be
two types of filters that are used in order to reduce the harmonic distortion i.e.
the active filters and the passive filters. Active harmonic filters are electronic
devices that eliminate the undesirable harmonics on the network by inserting
negative harmonics into the network. The active filters are normally available
for low voltage networks. The active filters consist of active components such
as IGBT-transistors and eliminate many different harmonic frequencies. The
signal types can be single phase AC, three phases AC. On the other hand,
passive harmonic filters consist of passive components such as resistors,
inductors and capacitors. Unlike the active filters which are used only for low
voltages, the passive filters are commonly used and are available for different
voltage levels.

Chapter 3
Fundamentals Of Harmonics
3.1 General:
When we talk about ac we are talking about alternating current. The
voltage pushing that current through the load circuit is described in terms of
frequency and amplitude. The frequency of the current will be identical to the
frequency of the voltage as long as the load resistance/impedance does not
change. In a linear load, like a resistor, capacitor or inductor, current and
voltage will have the same frequency. As long as the characteristics of the load
components do not change, the frequency component of the current will not
change. When we deal with non-linear loads such as switching power supplies,
transformers which saturate, capacitors which charge to the peak of the supply
voltage, and converters used in drives, the characteristics of the load are
dynamic. As the amplitude of the voltage changes and the load impedance
changes, the frequency of the current will change. That changing current and
resulting complex waveform is a result of: these load changes. The complex
current waveform can be described by defining each component of the
waveform. The component of any waveform can be defined in terms of dc, and
all frequencies from 0 to infinity. The frequencies that are normally dealt with
using drives are 50 and 60 Hertz. By definition, these frequencies are termed
fundamental in their respective distribution systems.
3.2 Definition:
The Fourier theorem states that all non-sinusoidal periodic functions
can be represented as the sum of terms (i.e. a series) made up of:
1. Sinusoidal term at the fundamental frequency
2. Sinusoidal terms (harmonics) whose frequencies are whole multiples of the
fundamental frequency
3. DC component, where applicable

The nth order harmonic (commonly referred to as simply the nth
harmonic) in a signal is the sinusoidal component with a frequency that is n
times the fundamental frequency.
The equation for the harmonic expansion of a periodic function is presented
below:
∞
y(t)=Y0+ Σ Yn√2 sin (nωt-φn)
n=1
Where:
Yo: value of the DC component, generally zero and considered as such here in
after
Yn: rms value of the nth harmonic
ω: angular frequency of the fundamental frequency
φn: displacement of the harmonic component at t = 0.
 Example of signals (current and voltage waves) on the French electrical
distribution System:
The value of the fundamental frequency (or first order harmonic) is 60 Hz
The third harmonic has a frequency of 180 Hz
The fifth harmonic has a frequency of 300 Hz
The seventh harmonic has a frequency of 420 Hz
Etc.
A distorted signal is the sum of a number of superimposed harmonics

Fig 3.2 Voltage waveform showing the effect of harmonics
3.3 Representation of harmonics: the frequency spectrum
The frequency spectrum is a practical graphical means of representing
the harmonics contained in a periodic signal.
 The graph indicates the amplitude of each harmonic order.
 This type of representation is also referred to as spectral analysis.
 The frequency spectrum indicates which harmonics are present and their
relative importance.
Fig 3.3 Graph of Harmonics Spectrum

3.4 Triplen Harmonics:
The triplen harmonics are defined as the odd multiples of the 3rd
harmonic (ex. 3rd, 9th, 15th, 21st etc.).They deserve special consideration
because the system response is often considerarably different for triplens
than for rest of harmonics. The normal mode for tripplen harmonics is to be
zero sequence. During imbalances, triplen harmonics may have positive or
negative sequence components.
3.5 Interharmonics:
Inter-harmonics are those spectra components whose frequencies are
not integer multiple of the fundamental frequency. In recent years,
interharmonics have gained some attention because they can cause voltage
flicker. A voltage flicker can be viewed as the modulation of the peak (or
RMS) value of a voltage waveform. The inter-harmonics can beat with the
fundamental frequency component to cause modulated peak voltage.

Chapter 4
Harmonic indices
The two most commonly used indices for measuring the harmonic
content ofa waveform are the total harmonics distortion(THD) and the total
demand distortion. Both are measures of the effective value of waveform and
may be applied to eitger voltage or current.
4.1 Total Harmonic Distortion
Total harmonics distortion is the ratio between the RMS value of sum
of harmonics to the RMS value of the fundamental.
THD can be used to describe voltage or current distortion and is calculated as
follows:
푆푢푚 표푓 푠푞푢푎푟푒푠 표푓 푎푚푝푙푖푡푢푑푒푠 표푓 푎푙푙 ℎ푎푟푚표푛푖푐푠
푇퐻퐷 = √
푠푞푢푎푟푒 표푓 푎푚푝푙푖푡푢푑푒 표푓 푓푢푛푑푎푚푒푛푡푎푙
× 100
The THD is very useful quantity for many applications, but its
limitation must be realized. It can provide a good idea of how much extra heat
will be realized when distorted voltage is applies across a resistive load. It can
give an idea about the additional losses caused by the current flowing through a
conductor.
4.2 Total Demand Distortion
Current distortion levels can be characterized by a THD value, as has been
described, but this can often be misleading. A small current have a high THD
but not be a significant threat to the system.
Some analysts have attempted to avoid this difficulty by referring THD
to the fundamental of peak demand load current rather than to the fundamental
of the peak demand load current rather than the fundamental of present sample.
This is called total demand distortion and serves as the basis for the guidelines
in IEEE standard 519-1192.

TDD =
2 + I3
√I2
2 + I4
2 + I5
2 + ⋯
IL
IL is the peak, or maximum, demand load current at the funamental
frequency component measured at the point of common coupliing
The two definitions are very much alike. The only difference is the
denominator. The THD calculation compares the measured harmonics with the
measured fundamental current. The TDD calculation compares the measured
harmonics with the maximum demand current.
Similarly, the individual harmonic current limits are not given in terms of
percent of fundamental (as is typical of most harmonic measurements) at a
given point of time. The current limits are given in terms of, “Maximum
Harmonic Current Distortion in Percent of IL.”

Chapter 5
Sourecs of Harmonics
Devices causing harmonics are present in all industrial, commercial and
residential installations. Non-linear equipment or components in the power
system cause distortion of the current and to a lesser extent of the voltage.
5.1 Non-linear loads
5.1.1 Definition:
A load is said to be non-linear when the current it draws does not have the
same wave form as the supply voltage.
Fig 5.1.1 Current waveform in non-linear load
5.1.2 Harmonics in non-linear load:
DC Bus will only charge when the AC sine wave voltage is greater than
the DC capacitor voltage, this results current draw only at the peaks of the sine
waves instead of the whole sine wave
Fig 5.1.2 Common Single Phase Bridge Rectifier Circuit and waveform

5.2 Types of equipment that generate harmonics
Harmonic load currents are generated by all non-linear loads. These include:
Single phase loads, e.g.
 Switched mode power supplies (SMPS)
 Electronic fluorescent lighting ballasts
 Small uninterruptible power supplies (UPS) units
Three phase loads, e.g.
 Variable speed drives
 Large UPS units
5.2.1 Single phase loads
 Switched mode power supplies (SMPS):
The majority of modern electronic units use switched mode power
supplies (SMPS). These differ from older units in that the traditional step-down
transformer and rectifier is replaced by direct controlled rectification of the
supply to charge a reservoir capacitor from which the direct current for the load
is derived by a method appropriate to the output voltage and current required.
The advantage to the equipment manufacturer is that the size, cost and weight
is significantly reduced and the power unit can be made in almost any required
form factor. The disadvantage to everyone else is that, rather than drawing
continuous current from the supply, the power supply unit draws pulses of
current which contain large amounts of third and higher harmonics and
significant high frequency components. A simple filter is fitted at the supply
input to bypass the high frequency components from line and neutral to ground
but it has no effect on the harmonic currents that flow back to the supply.
Single phase UPS units exhibit very similar characteristics to SMPS.For high
power units there has been a recent trend towards so-called power factor
corrected inputs. The aim is to make the power supply load look like a resistive
load so that the input current appears sinusoidal and in phase with the applied
voltage. It is achieved by drawing input current as a high frequency triangular
waveform that is averaged by the input filter to a sinusoid.

Fig 5.2.1 a. Harmonic spectrum of a typical PC
 Fluorescent lighting ballasts
Electronic lighting ballasts have become popular in recent years
following claims for improved efficiency. Overall they are only a little
more efficient than the best magnetic ballasts and in fact, most of the gain
is attributable to the lamp being more efficient when driven at high
frequency rather than to the electronic ballast itself. Their chief advantage
is that the light level can be maintained over an extended lifetime by
feedback control of the running current a practice that reduces the overall
lifetime efficiency. Their great disadvantage is that they generate
harmonics in the supply current. So called power-factor corrected types are
available at higher ratings that reduce the harmonic problems, but at a cost
penalty. Smaller units are usually uncorrected. Compact fluorescent lamps
(CFL) are now being sold as replacements for tungsten filament bulbs.
Miniature electronic ballast, housed in the connector casing, controls a
folded 8mm diameter fluorescent tube. CFLs rated at 11 watt are sold as
replacements for a 60 watt filament lamp and have a life expectancy of
8000 hours. The harmonic current spectrum is shown in Figure. These
lamps are being widely used to replace filament bulbs in domestic
properties and especially in hotels where serious harmonic problems are
suddenly becoming common.

Fig 5.2.1 a. Harmonic spectrum of a typical CFL
5.2.2 Three Phase load
Variable speed controllers, UPS units and DC converters in general are
usually based on the three-phase bridge, also known as the six-pulse bridge
because there are six pulses per cycle (one per half cycle per phase) on the DC
output. The six pulse bridge produces harmonics at 6n +/- 1, i.e. at one more
and one less than each multiple of six. In theory, the magnitude of each
harmonic is the reciprocal of the harmonic number, so there would be 20 %
fifth harmonic and 9 % eleventh harmonic, etc.
Fig 5.2.2 Harmonic spectrum of a typical 6-pulse bridge

Chapter 6
Effect of Harmonics
The effects of three-phase harmonics on circuits are similar to the
effects of stress and high blood pressure on the human body. High levels of
stress or harmonic distortion can lead to problems for the utility's distribution
system, plant distribution system and any other equipment serviced by that
distribution system. Effects can range from spurious operation of equipment to
a shutdown of important plant equipment, such as machines or assembly lines.
Harmonics can lead to power system inefficiency. Some of the negative ways
that harmonics may affect plant equipment are listed below:
6.1 Current Harmonics
These important non-linear circuits produce current harmonics. Current
Harmonics do have an effect on the electrical equipment supplying harmonic
current to the device (transformers, conductors). Current Harmonics can cause
issues with distribution equipment with has to handle the current from the
utility transformer all the way down to the device, but generally don’t affect
other equipment connected to the electrical system. Harmonic currents can
cause excessive heating to transformers. For electrical systems feeding single
phase loads the third harmonic has gained attention in design consideration and
transformer selection for causing the neutral conductor to draw excessive
current.
6.2 Voltage Harmonics
Voltage Harmonics can affects sensitive equipment throughout your
facility. Voltage Harmonics arise when Current Harmonics are able to create
sags in the voltage supply. When any device draws current it creates a voltage
dip which is required for current to flow. This voltage dip is visible with larger
loads when turning on a hair dryer or a table saw and seeing the lights dim
down. The amount of sag depends on many factors like transformer impedance
wire size. Current Harmonics create Voltage Harmonics, but the magnitude of

the Voltage Harmonics depends on the “Stiffness” of your electrical
distribution’s “System Impedance”.
6.3 Effect of Harmonics
 Conductor overheating: A function of the square rms current per unit volume
of the conductor. Harmonic currents on undersized conductors or cables can
cause a “skin effect”, which increases with frequency and is similar to a
centrifugal force.
 Capacitors: Can be affected by heat rise increases due to power loss and
reduced life on the capacitors. If a capacitor is tuned to one of the characteristic
harmonics such as the 5th or 7th, overvoltage and resonance can cause
dielectric failure or rupture the capacitor.
 Fuses and Circuit Breakers: Harmonics can cause false or spurious
operations and trips, damaging or blowing components for no apparent reason.
 Transformers: have increased iron and copper losses or eddy currents due to
stray flux losses. This causes excessive overheating in the transformer
windings. Typically, the use of appropriate “K factor” rated units is
recommended for non-linear loads.
 Generators: Have similar problems to transformers. Sizing and coordination
is critical to the operation of the voltage regulator and controls. Excessive
harmonic voltage distortion will cause multiple zero crossings of the current
waveform. Multiple zero crossings affect the timing of the voltage regulator,
causing interference and operation instability.
 Utility Meters: May record measurements incorrectly, result in higher
billings to consumers.
 Drives/Power Supplies: Can be affected by misoperation due to multiple
zero crossings. Harmonics can cause failure of the commutation circuits, found
in DC drives and AC drives with silicon controlled rectifiers (SCRs).

Chapter 7
Power and Harmonics
7.1 Active Power
The active power P of a signal distorted by harmonics is the sum of the
active powers corresponding to the voltages and currents in the same frequency
order. The expansion of the voltage and current into their harmonic
components may be written as:
∞
P = Σ Vh Ih cos φh
h=1
Where φh is the displacement between voltage and current of harmonic order
h.
Note:
 it is assumed that the signal does not contain a DC component, i.e. V0= I0 = 0
 When the signal is not distorted by harmonics, the equation P = V1 I1 cos φ1
again applies, indicating the power of a sinusoidal signal, where cos φ1 is
equal to "cos φ").
7.2 Reactive Power
Reactive power applies exclusively to the fundamental and is defined
by the equation:
Q = V1I1 sin φ1
7.3 Distortion Power
Consider the apparent power S:
S=VrmsIrms

In the presence of harmonics, the equation becomes:
∞
S2 = (Σ Vh
h=1
∞
)(Σ Ih
h=1
)
Consequently, in the presence of harmonics, the equation S2=P2+Q2 is no
longer valid. The distortion power D is defined as S2=P2+Q2+D2, i.e.:
퐷 = √푆 2 − 푃2 − 푄2
7.4 Power factor in presence of harmonics
There are two different types of power factor that must be considered
when voltage and current waveforms are not perfectly sinusoidal.
 Input Displacement Factor (IDF) =which refers to the cosine of the angle
between the 50/60 Hz voltage and current waveforms.
 Distortion Factor (DF) is defined as follows:
DF =
1
√1 + THD2
Total Power Factor (PF) = Product of the Input Displacement Factor and
the Distortion Factor as follows:
PF = IDF × DF

Chapter 8
Harmonics Reduction Techniques
8.1 Genral Tecniques
Following techiques are use to reduce the effect of harmonics
 Grouping the disturbing loads
 Separating the sources
 Using transformers with special connections
 Installing inductors
8.1.1 Grouping the disturbing loads:
When preparing the single-line diagram, separate where possible the
disturbing equipment from the other loads. Practically speaking, the different
types of loads should be supplied by different bus bars.
By grouping the disturbing loads, the possibilities of angular
recomposition are increased. The reason is that the vector sum of the harmonic
currents is lower than their algebraic sum.
An effort should also be made to avoid the flow of harmonic currents in
the cables, thus limiting voltage drops and temperature rise in the cables.
Fig 8.1.1 Grouping of non-linear loads and supply as far upstream as possible
8.1.2 Separating the sources
In efforts to attenuate harmonics, an additional improvement may be
obtained by supplying the different loads via different transformers, as
indicated in the simplified diagram below

Fig 8.1.2 Supply of the disturbing loads via a separate transformer
This disadvantage of this solution is the increase in the cost of the installation.
8.1.3 Using transformers with special connections
Special types of connection may be used in transformers to eliminate
certain harmonic orders.
The harmonic orders eliminated depend on the type of connection
implemented:
 A delta-star-delta connection eliminates harmonic orders 5 and 7
 A delta-star connection eliminates harmonic order 3 (the harmonics
flow in each of the phases and loop back via the transformer neutral)
 A delta-zigzag connection eliminates harmonic order 5 (loop back via
the magnetic circuit).
Fig 8.1.3 A delta-star-delta transformer prevents propagation of harmonic orders 5 and
7 upstream in the distribution system.

8.1.4 Installing inductors
In installations comprising variable-speed drives, the current can be
smoothed by installing line inductors. By increasing the impedance of the
supply circuit, the harmonic current is limited.
Use of harmonic inductors on capacitor banks is a means of increasing
the impedance of the inductor and capacitor assembly, for harmonics with high
frequencies.
8.2 Solutions when limit values are exceeded
8.2.1 Passive Filter
Operating principle: an LC circuit, tuned to each of the harmonic
frequencies requiring filtering, is installed in parallel with the device causing
the harmonic distortion this bypass circuit draws the harmonics, thus avoiding
the flow of harmonics to the power source.
Fig 8.2.1 Operating principle of a passive filter.
Generally speaking, the passive filter is tuned to a harmonic order near
the one to be eliminated. A number of parallel-connected filters may be used
when a significant reduction in distortion over a range of orders is required.

Typical applications:
 Industrial installations comprising a set of devices causing harmonics
with a total power rating greater than approximately 200 kVA.
(variable-speed drives UPSs, rectifiers, etc.)
 installations where power factor correction is required.
 situations where voltage distortion must be reduced to avoid disturbing
sensitive loads.
 situations where current distortion must be reduced to avoid overloads.
8.2.2 Active filters (active harmonic conditioners)
Operating principle: active filters are systems employing power
electronics, installed in series or in parallel with the non-linear load, to provide
the harmonic currents required by non-linear loads and thereby avoid distortion
on the power system.
.
Fig 8.2.2 Operating principle of an active filter.
The active filter injects, in opposite phase, the harmonics drawn by the
load, such that the line current is remains sinusoidal.
 Commercial installations comprising a set of devices causing harmonics with a
total power rating less than 200 kVA (variable-speed drives, UPSs, office
equipment, etc.)
 situations where current distortion must be reduced to avoid overloads.

8.2.3 Hybrid filters
Operating principle: The two types of filters presented above can be combined
in a single device, thus constituting a hybrid filter. This new filtering solution
combines the advantages of the existing systems and provides a high-performance
solution covering a wide power range.
Fig 8.2.3 Operating principle of a hybrid filter.
 Industrial installations comprising a set of devices causing harmonics with a
total power rating greater than 200 kVA approximately (variable-speed drives,
UPSs, rectifiers, etc.)
 Installations where power factor correction is required.
 Situations where voltage distortion must be reduced to avoid disturbing
sensitive loads.
 Situations where current distortion must be reduced to avoid overloads.
 Situations where conformity with strict harmonic-emission limits is required.
8.2.4 12-pulse Rectifiers
This is effectively two six-pulse bridges, fed from a star and a delta
transformer winding, providing a 30 degrees phase shift between them. A
twelve-pulse system can also be constructed from two six-pulse rectifiers
connected in series. In this configuration, two six-pulse rectifiers, each
generating one half of the DC link voltage, are series connected.

Fig 8.2.4 12-Pulse Rectifier
Very effective in the elimination of 5th and 7th harmonics. Stops
harmonics at the source. Insensitive to future system changes.

Chapter 9
IEEE Standard 519-1992
IEEE standard 519-1992 is a useful document for understanding harmonics and
applying harmonic limits in power systems. It is therefore useful to measure and limit
harmonics in electric power systems. IEEE Std 519-1992, IEEE Recommended
Practices and Requirements for Harmonic Control in Electric Power Systems (IEEE
519), provides a basis for limiting harmonics.
9.1 IEEE STD 519 Voltage Harmonics Distortion
According to IEEE 519, harmonic voltage distortion on power systems 69 kV
and below is limited to 5.0% total harmonic distortion (THD) with each individual
harmonic limited to 3%. The current harmonic limits vary based on the short circuit
strength of the system they are being injected into. Essentially, the more the system is
able to handle harmonic currents, the more the customer is allowed to inject.
Table 9.1 IEEE Std 519-1992 Harmonic Voltage Limits
Bus Voltage at PCC Individual Voltage
Distortion (%)
Total Voltage
Distortion THD (%)
69 kV and below 3.0 5.0
69.001 kV through 161 kV 1.5 2.5
161.001 kV and above 1.0 1.5
NOTE: High-voltage systems can have up to 2.0% THD where the cause is an HVDC
terminal that will attenuate by the time it is tapped for a user.
9.2 IEEE STD 519 Current Harmonics Distortion
The level of harmonic voltage distortion on a system that can be attributed to
an electricity consumer will be the function of the harmonic current drawn by that
consumer and the impedance of the system at the various harmonic frequencies. A
system’s impedance can be represented by the short circuit capacity of that system
since the impedance will limit current that will be fed into a short circuit. Therefore,
the short circuit capacity can be used to define the size and influence of a particular

consumer on a power system. It can be used to reflect the level of voltage distortion
that current harmonics produced by that consumer would contribute to the overall
distortion of the power system to which it is connected.
To define current distortion limits, IEEE Std 519 uses a short circuit ratio to
establish a customer’s size and potential influence on the voltage distortion of the
system. The short circuit ratio (ISC/IL) is the ratio of short circuit current (ISC) at the
point of common coupling with the utility, to the customer’s maximum load or
demand current (IL). Lower ratios or higher impedance systems have lower current.
Table 9.1 IEEE Std 519-1992 Harmonic current Limits
Maximum Harmonic Current Distortion in Percent of IL
Individual Harmonic Order (Odd Harmonics)
ISC/IL <11 11≤h<17 17≤h<23 23≤h<35 35≤h TDD
<20* 4 2 1.5 0.6 0.3 5
20<50 7 3.5 2.5 1 0.5 8
50<100 10 4.5 4 1.5 0.7 12
100<1000 12 5.5 5 2 1 15
>1000 15 7 6 2.5 1.4 20
Even harmonics are limited to 25% of the odd harmonic limits above.
Current distortions that result in a dc offset, e.g. half-wave converters, are not allowed.
* All power generation equipment is limited to these values of current distortion,
regardless of actual Isc/IL.
Where
Isc = maximum short-circuit current at PCC.
IL = maximum demand load current (fundamental frequency component) at PCC.
TDD = Total demand distortion (RSS), harmonic current distortion in % of maximum
demand load current (15 or 30 min demand).
PCC = Point of common coupling.

Chapter 10
Conclusion
10.1 Conclusion
Virtually all modern electrical and electronic equipment contains a SMPS or
involves some form of power control and so is a non-linear load. Linear loads are
comparatively rare, the fundamental voltage, applied on a non-linear load, cause
harmonic currents (called characteristic harmonics). The main distortion consists of
odd multiples of the fundamental component (50 or 60 Hz). Single phase non-linear
loads have a current distortion, THD, around 120 %. All odd harmonics exist in the
current spectrum. Three phase non-linear loads have a current distortion, THD, up to
200 %. All odd harmonics exist in the spectrum, except triplen harmonics. As the
quantity of installed equipment rises and without very strong standards backed up by
rigid enforcement measures, it is likely that harmonic pollution will continue to
increase. Harmonic pollution is a major contributor to power quality degradation and
must be kept under IEEE 519 standard

References
1. IEEE Std 519-1992, “IEEE Recommended Practices and Requirements for Harmonic
Control in Electric Power Systems,” © Institute of Electrical and Electronics
Engineers, Inc. 1993.
2. Boger C.Dugan, Mark F.McGRANAGHN, Electrical Power System Quality, Tata
McGRAW-Hill
3. en.wikipedia.org/wiki/Harmonics_(electrical power)
4. literature.rockwellautomation.com/idc/groups/.../mvb-wp011_-en-p.pdf
5. Enrique Acha, Manuel Madrigal, Power System Harmonics, John Wiley & Sons

Power system harmonics

More Related Content

What's hot (20)

Viewers also liked (20)

Similar to Power system harmonics (20)

Recently uploaded (20)

Power system harmonics