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CHAPTER 1

INTRODUCTION

1.1 QUALITY OF ELECTRICAL POWER

Electrical power is one of the essential commodities used in

industry and commerce today. Electrical power is generated, transmitted,

distributed and utilised mostly in the form of Alternating Current (AC) at

present. It is an unusual energy since it is required as a continuous flow

with quality. It cannot be conveniently stored in bulk quantity. The

reliability of the supply must be known before hand and the resilience of the

process to supply variations must be understood. In reality electricity is very

different from any other product: it is generated far from the point of use and

is fed to the grid synchronising with the output of many generators. Finally

it is connected to the customers as per their requirements.

Though electricity is generated within the specified nominal

values, due to various network and load conditions at various locations, it is

subjected to quality degradation. Quality power supply is the one which is

made available, within voltage and frequency tolerances with a pure

sinusoidal waveform. Power quality is a set of electrical boundaries that

allows an equipment to function in its intended manner without significant

loss of its performance or life.

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1.1.1 Power Quality Issues

The problems in the power supply could be due to variations in

voltage or frequency and distortion in the waveform. Arrillaga et al (2000) list

the broad categories of power quality issues as under:

• Disturbances: Disturbance is a temporary deviation from the

steady state waveform caused by faults of brief duration or by

sudden changes in the power system. The disturbances

considered by the International Electrotechnical Commission

(IEC) include voltage dips, voltage increases, brief

interruptions and impulsive and oscillatory transients. The

Institute of Electrical and Electronics Engineers (IEEE) has

termed voltage dips as sags and voltage increases as swells

(IEEE 1995).

• Unbalance: Unbalance in a three-phase system is a situation in

which either the three-phase voltages are not equal in

magnitude or the phase differences between them are not 120°

or both.

• Distortion: Distortion in the voltage or current waveform

occurs when nonlinear loads are connected to the electrical

system. Nonlinear loads draw currents in a nonsinusoidal

manner though the supply voltage is sinusoidal.

• Voltage fluctuations: Continuous changes in the nominal value

of the supply voltage are called voltage fluctuations.

• Flicker: Flicker is described as the impression of unsteadiness

of visual sensation induced by a light stimulus whose

luminescence or spectral distribution fluctuates with time

(IEEE 1995).

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Sankaran (2002) lists the following as the factors contributing to

the quality of a typical electrical power system:

1. Power frequency disturbance

2. Electro Magnetic Interference (EMI)

3. Power system transients

4. Electro Static Discharge (ESD)

5. Power system harmonics

6. Power Factor (PF)

7. Grounding and bonding.

1.1.2 Causes and Effects of Power Quality Issues

PF deviation is mainly due to the inductive loads in the system. PF

is defined as

current in a load. In a resistive circuit, since the phase angle between voltage

and current is 0º, the PF is 1. In an inductive load, current lags the voltage and

the PF is considered as lagging PF and it has a value between 0 and 1. A non-

unity PF causes reactive power losses in the system.

Voltage swells are caused by lightning strikes. Voltage sags are

caused by faults in the electrical network. Interruptions are caused by

generator abnormalities. Flicker is caused by large fluctuating loads.

Transients are caused by switching in or out heavy loads. Harmonics are

generated by nonlinear loads.

Voltage swells, voltage sags, interruptions and transients cause

malfunction and premature failure of electrical and associated equipment.

Flicker causes irritation and strain to the user. Harmonics produce many ill

effects which affect the entire electrical system and cause energy losses and

failure of equipment.

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In a power system, voltage swells, voltage sags, interruptions,

transients and flicker occur and persist for a short duration only. PF and

harmonics are steady state phenomena (IEEE 1995). The losses due to PF and

harmonics are quite significant compared to the other issues.

Improvement of PF to the ideal value has already been achieved by

many methods. Identification and mitigation of harmonics is under active

research in recent times.

1.2 HARMONICS IN ELECTRICAL SYSTEMS

IEEE (1992) defines harmonics as sinusoidal voltages and currents

having frequencies that are integer multiples of the supply frequency.

Harmonics are produced by nonlinear loads in the electrical system.

Harmonics levels are increasing in the electrical power systems due to

proliferation of nonlinear loads like power electronic converter circuits which

include Switched Mode Power Supplies (SMPS), Uninterruptible Power

Supplies (UPS), Personal Computers (PC), Adjustable Speed Drives (ASD),

arc and induction furnaces, fluorescent lights with electronic ballasts, etc.

(Arrillaga et al 1985). ASD is also known as Variable Speed Drive (VSD).

1.2.1 Types of Electrical Loads

The loads connected in the electrical systems can be broadly

classified into two categories based on the relationship between the supply

voltage and the current drawn by them. They are: linear loads and nonlinear

loads. In linear loads, the current drawn by the load has a linear relationship

with the supply voltage. In nonlinear loads, the current drawn by the load has

a nonlinear relationship with the supply voltage.

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1.2.2 Linear Loads

Loads like resistor, inductor, capacitor or a combination of any of

these are called as linear loads. When a supply voltage with sinusoidal shape

is applied to the linear loads, the current flowing through them is also

sinusoidal in shape. Typical voltage and current waveforms in linear loads are

shown in Figure 1.1.

S. No. Type of load Circuit Voltage and current

waveformsPF

1 Resistive (R)

e.g.Incandescent lamps, Resistive Heaters, etc.

1

2 Inductive (L) e.g.Motors,Transformers, etc. (with inherent resistance)

Lag,< 1

3 Capacitive (C) e.g. PF correction capacitors (with inherent resistance)

Lead,< 1

Figure 1.1 Voltage and current waveforms in linear loads

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Examples of linear loads are:

• Incandescent lamps (resistive)• Resistive Heaters (resistive) • Motors and Transformers (inductive) • PF correction capacitors (capacitive) • Transmission lines (combination of resistance, inductance and

capacitance).

1.2.3 Nonlinear Loads

Nonlinear loads do not exhibit linear relationship between supply voltage and load current (Mohan et al 1995). Examples for nonlinear loads are

power electronic converter circuits like diode rectifiers, SMPS, UPS, VSDetc. Arc and induction furnaces also have nonlinear characteristics. Loads like transformers, which are normally linear, also behave nonlinearly when they are saturated due to over load or faulty circuit conditions. When nonlinear loads are connected to sinusoidal AC supply voltage, the current will be flowing in a nonsinusoidal manner. The typical voltage and current waveforms in a nonlinear load is shown in Figure 1.2.

-400

-300

-200

-100

0

100

200

300

400

1 25 49 73 97

sampling instance

V

-3

-2

-1

0

1

2

3

4

A

Voltage

Current

Figure 1.2 Voltage and current waveforms in a nonlinear load

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A given periodic nonsinusoidal (distorted) waveform can be

resolved into a sum of many sine waves using Fourier series (Bracewell

2000). The resolved components will have a sine wave which has the same

frequency as that of the given waveform (called the fundamental or 1st

harmonic) and many other sine waves whose frequencies are integer multiples

of the fundamental. The components other than the fundamental are called

harmonics. Thus in an AC power system with a supply frequency of 50Hz,

the fundamental (f1) has a frequency of 50 Hz; the 2nd harmonic (f2) has a

frequency of 100 Hz; 3rd harmonic (f3) has a frequency of 150 Hz and so on.

The multiplying integers are called the orders of the harmonics, denoted by h.

Many a times the order of harmonics is also denoted by n. Since in practical

situations the higher order harmonics are insignificant in magnitude, h is

limited to 50 in IEEE Std 519-1992 (IEEE 1992). Subsequent definition by

IEC 61000-4-7 (IEC 2002) standard specifies that harmonics need to be

resolved up to 40th order. The order of harmonics and their magnitude depend

on the shape of the waveform. The harmonics in the nonsinusoidal load

current, flowing through the electrical network cause harmonics in the supply

voltage. An example of one complete cycle of a distorted waveform and its

resolved components is shown in Figure 1.3.

An electrical network is basically designed for the fundamental

frequency. When the high frequency components flow through the network,

more heating and losses occur. Additionally, the current harmonics, flowing

through the finite impedance of the electrical network, cause distortion in

supply voltage waveform. The Position Paper (United States National

Committee 1999) explains that the increase in current and voltage harmonics

has degraded the quality of power that is transmitted and distributed.

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Figure 1.3 Resolving a nonsinusoidal current waveform

1.3 EFFECTS AND SOURCES OF HARMONICS

Wakileh (2001) divides the effects of harmonics into three general

categories:

1. Thermal stress,

2. Insulation stress and

3. Load disruption.

All the elements of the electrical system, namely generators,

transmission lines / cables, distribution transformers, switch gears and

protection relays, meters, motors and various loads are affected by the

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harmonics. Though the harmonics are generated at the load end, they travel

upstream up to the generator. During the travel, they cause damages to

equipment connected in the same bus along their path.

1.3.1 Effects of Harmonics

Specifically, harmonics produce the following unwanted effects in

the electrical systems and the connected equipment:

• Very high current in neutral conductor of a ‘3P-4W’ system.

(In many applications a 3 ½ core cable is used for distribution

in which the size of the neutral conductor is only half that of a

phase conductor.) The higher neutral current causes over

heating of the neutral conductor.

• Increase in voltage between neutral and ground

• Reduction in current carrying capacity of conductors

• Overloading and overheating of transformers and electrical

apparatus

• Increase in hysteresis losses

• Reduction in PF & failure of PF correction capacitors.

• Inaccurate meter reading

• Nuisance breaker tripping

• Malfunctioning of protective relays

• Inductive interference into telephone lines

• Energy loss and higher electricity payment

• Inefficient distribution system

• Increased maintenance cost, etc.

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Certain specific orders of harmonics cause some distinctive effects. The triple n harmonics (i.e. Harmonics of order 3, 9, 15, 21, etc.) cause high

current in neutral. The 5th harmonic produces a negative torque in rotating machines. The 7th harmonics creates a crawling effect in the motors.

1.3.2 Sources of Harmonics

Harmonic currents are generated to a small extent by generation,transmission and distribution equipment. A large amount of harmonics is

produced by the nonlinear industrial and domestic loads (Arrillaga et al 2000).Nonlinear loads are the origin of nonsinusoidal load currents. Most of the

electronic equipment that draw power from AC distribution have a rectifier

and filter capacitor in the first stage of their power supply circuits. As far as

the distribution is concerned the rectifier is the load for the AC mains. The

rectifier-capacitor combination draws current from the mains in a nonsinusoidal manner. The following types of electrical equipment generate

considerable amount of harmonics:

• VSD consisting of :

Direct Current (DC) drives and

AC drives known as Variable Frequency Drives (VFD)

• UPS • SMPS

• PC • Rectifiers

• Arc and induction furnaces

• Fluorescent lamps with electronic ballasts • Television receivers with SMPS

• Electronic fan regulators and light dimmers

• Computer Numerically Controlled (CNC) machines

• Appliances with SMPS • Transformers under saturated condition, etc.

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1.4 INDICES AND LIMITS FOR HARMONICS DISTORTION

IEEE Std 519, “IEEE Recommended Practices and Requirements

for Harmonic Control in Electrical Power Systems” was published in 1981.

This standard established limits for voltage distortion levels acceptable to the

distribution system. This standard was widely used in the industry. However,

with the increased usage of nonlinear load and considering the effects of

harmonic currents and voltage, a rewrite of IEEE Std 519 was necessitated.

The revised IEEE Std 519-1992, (IEEE 1992), sets the limits for both

harmonic voltages on the utility transmission and distribution system and

harmonic currents within the industrial distribution systems. Supply voltage

distortion is caused by the harmonic currents flowing in the network which

has finite impedance. Thus it can be inferred that by reducing either the

current harmonics or the network impedance or both, harmonics in voltage

can be reduced. IEEE Std 519-1992 specifies the allowable limits for voltage

and current distortion at various bus and system voltages. These are given in

Tables 1.1 to 1.4. 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. PCC can be either the primary or secondary of a

utility transformer or at the service entrance of the facility. In

some cases, harmonics can be measured or calculated between

the nonlinear loads and other loads of an industrial plant.

Total Harmonic Distortion (THD) is the distortion in voltage

waveform calculated or measured at PCC.

Total Demand Distortion (TDD) is the distortion in the current

waveform calculated or measured at PCC.

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1.4.1 THD and TDD

THD and TDD are defined as below:

The THD is used to define the effects of the harmonics on the power

system voltage. It is used in low-voltage, medium-voltage and high-voltage

systems. It is expressed as a percent of the fundamental and is defined as,

sum of all squares of amplitude of all harmonic voltagesTHD 100%square of the amplitude of the fundamental voltage

(1.1)

502h

h 2

1

VTHD 100%

V (1.2)

where ‘V1’ is the RMS value of the fundamental voltage and ‘Vh’ is the RMS

value of the harmonic voltage of order h at the PCC.

TDD expressed in percentage, is given by,

sum of all squares of amplitude of all harmonic currentsTDD 100%square of the max imum demand load current

(1.3)

502h

h 2

L

ITDD 100%

I (1.4)

where ‘IL’ is maximum RMS demand load current at the fundamental frequency

and ‘Ih’ is the RMS harmonic current of order h at the PCC. In literature, Total

Harmonic Distortion in voltage (THDv) and Total Harmonic Distortion in

current (THDi) are interchangeably used for THD and TDD respectively.

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1.4.2 Limits for Voltage and Current Harmonics

IEEE Std 519-1992 specifies the allowable limits for voltage and

current distortion at various bus voltages. These are given in Tables 1.1 to 1.4.

Table 1.1 IEEE Std 519 – 1992 voltage distortion limits

Bus Voltage at PCC Individual voltage

Distortion (%) Total Harmonic voltage

Distortion THD (%)

69 kV and below 3.0 5.0

69.0001 kV - 161 kV 1.5 2.5

> 161.001 kV 1.0 1.5

Table 1.2 IEEE Std 519 – 1992 current distortion limits for general

distribution systems (120 V to 69 000 V)

Isc / I L

Maximum Harmonic Current Distortion in % of I L

Individual Harmonic order (Odd Harmonics )

TDD

< 20 4.0 2.0 1.5 0.6 0.3 5.0

20 - 50 7.0 3.5 2.5 1.0 0.5 8.0

50 - 100 10.0 4.5 4.0 1.5 7 12.0

100 -1000 12.0 5.5 5.0 2.0 1.0 15.0

>1000 15.0 7.0 6.0 2.5 1.4 20.0

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Table 1.3 IEEE Std 519- 1992 current distortion limits for general sub

transmission systems (69001 V to 161 000 V)

Isc / I L

Maximum Harmonic Current Distortion in % of I L

Individual Harmonic order (Odd Harmonics )

TDD

< 20 2.0 1.0 0.75 0.3 0.15 2.5

20 - 50 3.5 1.75 1.25 0.5 0.25 4.0

50 - 100 5.0 2.25 2.0 0.75 0.35 6.0

100 - 1000 6.0 2.75 2.5 1.0 0.5 7.5

>1000 7.5 3.5 3.0 1.25 0.7 10.0

Table 1.4 IEEE 519 – 1992 current distortion limits for general

transmission systems (>161 kV)

Isc / I L

Individual Harmonic order (Odd Harmonics )

TDD

< 50 2.0 1.0 0.75 0.3 0.15 2.5

50 3.0 1.5 1.15 0.45 0.22 3.75

where ISC = maximum short circuit current at PCC and IL = maximum demand load current at fundamental frequency

at PCC. (Even harmonics are limited to 25% of the odd harmonic

limits above.)

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1.5 PRESENCE OF HARMONICS IN INDUSTRIES

The author was granted a project by the state utility Tamilnadu

Electricity Board (TNEB) to conduct a survey and to assess the orders and

their magnitudes of harmonics present at the PCC of various electrical

installations of its consumers. The voltage and current waveforms were

recorded and THDv and THDi were measured at the PCC with the help of a

Power Quality Analyser (PQA) model C.A 8332 (Chauvin-Arnoux 2003).

The measured THDv and THDi are given in Figures 1.4 to 1.7.

S. No. Type of Load

Supply voltageand measured

harmonic valuesVoltage and current waveforms

1 PC

1 phase, 230 V

THDv=4.2 % THDi=152.6%

2Fan with electronic regulator

1 phase, 230 V

THDv = 1.8%THDi = 58.1%

3 Television with SMPS

1 phase, 230 V

THDv= 3.4% THDi=153.7%

Figure 1.4 Voltage and current in loads connected to 230V supply

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S. No.Type of

Load

Supply voltageand measured

harmonic valuesVoltage and current waveforms

1 VFD

3 phase, 415 V

Figure shows the line voltage and

current in 1 phase only;

THDv= 2.8% THDi=59.4%

2Welding

transformer

3 phase, 415 V

Figure shows only the current

waveforms in the three phases,

R,Y,B.

THDiR=37.7%THDiY=26.2%THDiB=46.7%

Figure 1.5 Voltage and current in loads connected to 415V supply

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S. No.Type of

Load

Supply voltageand measured

harmonic valuesVoltage and current waveforms

1Induction furnace

3 phase,22 kV

Measurement done only in 2

phases of current measuring circuit.

THDiR=21.0%THDiY=20.2%

Figure 1.6 Voltage and current in a load connected to 22kV supply

S. No. Type of Load

Supply voltageand measured

harmonic valuesVoltage and current waveforms

1Railway traction feeder

1 phase,110 kV

THDv=0.7 % THDi=18.1 %

2Steel

Melting Shop

3 phase,110 kV

THDiR=3.8%THDiY=6.9%

Figure 1.7 Voltage and current in loads connected to 110kV supply

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1.6 PROBLEM FORMULATION: MEASUREMENT OF

HARMONICS USING FAST FOURIER TRANSFORM

Considering the ill effects produced by harmonics, many utilities

have started imposing penalty to the consumers for generating harmonics and

dumping them into the supply lines. To effectively monitor the order and

magnitude of harmonics present in the system, it is imperative to have an

affordable cost, accurate and fast on-line harmonics measurement method.

The utilities would like to quantify the current harmonics generated by the

user so as to fix the penalty along with the consumption charges. The user

would like to know the current harmonics generated in their installation, so

that suitable remedial measures can be taken up. Since, harmonies affect both

the utility and the user, measuring the harmonics helps both of them.

Harmonics in electrical power systems are complex phenomena to

detect, observe, capture and analyse. A large number of data samples of

voltage and current is acquired over many cycles of power frequency and

resolved into fundamental and harmonics using Fourier Transform (FT). The

Fast Fourier Transform (FFT) algorithm developed by Cooley and Tukey

(1965) laid the way for FT to be executed on digital computers and

subsequently on microprocessors.

A methodology for measurement of harmonics is given by IEC in

the standard IEC-61000-4-7 Edition 2 (IEC 2002). This standard specifies that

1000 data samples are to be acquired from 10 cycles of voltage or current

waveforms and to be applied to the FFT. This method necessitates more

computational power and complexity. The data acquisition time for using this

method is 200ms for a 50Hz power system. The computational time, for the

-

depending on the type of Digital Signal Processor (DSP) and the clock speed.

The total time for measuring the harmonics is the sum of data acquisition time

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and the computational time. Thus the minimum measurement time in the

present methods using FFT algorithm is 200.08 ms.

Both supply voltage and load current waveforms contain

harmonics. Measurement of current harmonics is crucial for the following

reasons:

1. Current harmonics generated by the nonlinear loads are the

cause for distortion in the voltage waveform,

2. Utilities are fixing the penalty for the quantum of current

harmonics generated by the user and dumped into the supply

grid (network) and

3. Design and implementation of harmonic filters take

magnitude of the current harmonics as their control input.

1.7 PROPOSED METHOD FOR MEASUREMENT OF

HARMONICS USING ADALINE ALGORITHM

A technique for measurement of current harmonics is required for

stand alone metering as well as filtering purposes. The requirements for

harmonics measurement are faster calculation, less computational complexity,

less computational power and ease of implementation on a stand alone

hardware system. In view of the above requirements, a faster method for

measurement of harmonics using an ADAptive LINear Element (ADALINE)

which is a class of Artificial Neural Network (ANN) is proposed. In this

thesis, a methodology for implementation of ADALINE in a DSP and in a

Field Programmable Gate Array (FPGA) is developed.

The block diagram for measurement of harmonics in the load

current in an electrical system consisting of a nonlinear load supplied from a

sinusoidal AC source is shown in Figure 1.8. A PC is taken as a nonlinear

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Nonlinear ….load

Current sensor

AC Source 230V, 50Hz Signal conditioning circuit

Measurement circuit -ADALINE algorithm on .DSP / FPGA hardware

load. The proposed method is implemented and tested on three different

platforms:

1. MATLAB running on a desktop PC

2. DSP

3. FPGA

Figure 1.8 Block diagram for the measurement of harmonics

All the three implementations have produced results with accuracy

comparable with that of the FFT based PQA. The proposed method needs

data samples from a single cycle of load current waveform. Thus the data

acquisition time, in the proposed ADALINE method, is reduced to 20ms from

200ms required for the FFT method. It is also observed that in the DSP

implementation, the computation time for ADALINE is almost the same as

that of the FFT. In the FPGA implementation, the computation time for

ADALINE is significantly less than that of the FFT. Thus, there is a reduction

in the overall measurement time required for measuring harmonics using the

proposed methodology without significantly affecting the accuracy.

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The DSPs have on-chip analog-to-digital converters (ADC) with

their associated sample-and-hold amplifiers (SHA). This feature helps in

reducing the number of hardware components and consequently reducing the

size of the measurement system. FPGA needs external SHA and ADC and

hence this increases the hardware size. The FPGA has parallel processing

circuit elements, which inherently help to implement the ANN with more ease

and achieve faster execution.

1.8 CONTRIBUTIONS OUT OF THIS RESEARCH

This research work, has contributed an understanding of the issues

concerned with harmonics in electrical power systems and various

measurement techniques. Specific outcomes of this research are listed below:

• A methodology for implementation of ADALINE algorithm

has been proposed. The method was implemented on three

different platforms.

• The ADALINE algorithm has been coded in MATLAB.

• The usage of PQA for reading the instantaneous values of load

current in real time has been explored.

• Convergence of ADALINE algorithm, with real time data

samples, has been proved with MATLAB.

• A current measuring circuit has been fabricated with a current

sensor, amplifier and a DC level shifter and interfaced with

the DSP TMS320F2812.

• The load current has been measured in real-time using the

Code Composer Studio (CCS) and the ADALINE algorithm

was executed on TMS320F2812.

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• The convergence of ADALINE algorithm on DSP has been

verified.

• A circuit has been fabricated with a current sensor, amplifier

and ADC that is interfaced with a FPGA, Xilinx Virtex-II.

• Real time data has been acquired by the FPGA and the

convergence of ADALINE has been verified

• During the research period, the author has also carried out

several consultancy projects for measurement of harmonics at

various industries connected to state electrical network. The

observations and the report were presented to the utility

engineers and to the Tamilnadu Electricity Regulatory

Commission (TNERC) to help them during the policy

planning.

• Further the author has also measured the level of harmonics,

identified the sources of harmonics and quantified the losses

due to harmonics during several energy audits conducted at

user industries. In some industries, it was also identified that

the root cause for certain failures in electrical machines

were due to excessive harmonics present in their electrical

system.

1.9 ORGANISATION OF THE CHAPTERS

Chapter 2 deals with the Literature Review. Chapter 3 briefly

explains about the FFT, its applications, advantages and limitations. It

explains the measurement of harmonics using ANN especially ADALINE.

Chapter 4 develops the implementation methodology of ADALINE algorithm

for measurement of harmonics in MATLAB environment and presents the

simulation results. Chapter 5 describes the implementation of ADALINE on a

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DSP and the experimental results. Chapter 6 explains the implementation of

ADALINE on an FPGA and the analysis of results. Finally in chapter 7 the

results of the above proposed methods are discussed and conclusions are

brought out. Further scope in this area is also indicated for future research

work.