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+∑n=1

∞

Yn√2sin (nωt-φn)

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:

THD=√∑ of squares of amplitudes of all harmonicssquare of amplitude of fundamental

× 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=√I 2

2+ I32+ I 4

2+ I 52+…

I L

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=∑h=1

∞

V h I h cosφh

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=V 1 I 1sin φ 1

7.3 Distortion Power

Consider the apparent power S:

S=VrmsIrms

In the presence of harmonics, the equation becomes:

S2=(∑h=1

∞

V h)(∑h=1

∞

I h)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.:

D=√S2−P2−Q2

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 exceeded8.2.1 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.

Typical applications:

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.

Typical applications:

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 VoltageDistortion (%)

Total VoltageDistortion 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