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HARMONIC SOURCES AND CONTROL

1 DEPARTEMENT OF EEE

CHAPTER-1

INTRODUCTION

1.1 Introduction

The project undertaken at jocil is to study the existing Harmonics in Power System and

to analyze its effects. It will be most useful once the harmonics levels and orders are found

so that relevant solutions for Clean power may be envisaged.

In the present project work it has been proposed to investigate the sources of

harmonics and control.

Jaya lakshmi Oil and Chemical Industries Limitedhas been taken for case study.1.2 Organization Of The Project Work

Linear loads draw current that is precisely proportional to the voltage and is also an

almost perfect sinusoid. This is because these loads do not depend on the voltage to

determine their impedance. Their response at any given frequency is completely linear.

Non-linear loads do not respond in this way. When presented with a sinusoidal voltage,

the current is not proportional to the voltage and is not sinusoidal. The non-sinusoidal

current consumed is due to the device impedance changing over a complete voltage cycle

A load is NON-LINEAR when the current it draws does not have the same waveform as

the supply voltage. The harmonic spectrum depends on the type of load i.e., switching-

mode power supplies, motors during start-up, transformers during switch-on, frequency

controlled motors. Loads that make use of semi conductor devices like transistor..

The harmonic level has a great effect on the performance of the system components

and equipments. Harmonic map for the distribution system is necessary for appreciating

system operation upgrade. During the next decade, an increase of the nonlinear loads up to

70% is expected. Understanding electrical system problems will help in implementing

appropriate solutions. Phase shifting transformers can efficiently mitigate harmonic


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distortion. They are rigid and more economically than harmonic filters. Besides, they are

secure for resonance problem that may arise in passive filter applications utilizing passiveharmonic filters require recurrent analysis, measurements and precautions from system

reconfigurations or upgrading and load changes for save system operation.

The best way to deal with harmonics problems is through prevention: choosing

equipment and installation practices that minimize the level of harmonics in any one circuit

or portion of a facility. Many power quality problems, including those resulting from

harmonics, occur when new equipment is haphazardly added to older systems. However,

even within existing facilities, the problems can often be solved with simplesolutions suchas fixing poor or nonexistent grounding on individual equipment or the facility as a whole,

moving a few loads between branch circuits, or adding additional circuits to help isolate the

sensitive equipment from what is causing the harmonic distortion.

1.3 Statement Of The Problem

In this project work an attempt has been made to analyze the actual causes for

harmonics and how we control them. For this purpose data of harmonics existing as on 01-06-2011. In the operation division, Dokiparru, Guntur has been collected.


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CHAPTER-2

HARMONICS

2.1 Introduction

Linear loads draw current that is precisely proportional to the voltage and is also an

almost perfect sinusoid. This is because these loads do not depend on the voltage to

determine their impedance. Their response at any given frequency is completely linear.

Non-linear loads do not respond in this way. When presented with a sinusoidal

voltage, the current is not proportional to the voltage and is not sinusoidal. The non-

sinusoidal current consumed is due to the device impedance changing over a complete

voltage cycle.

2.2 Definitions

Harmonics are integral of somefundamental frequency that, when added To gether,

result in a distorted waveform i.e

Distorted sine wave causes harmonics. Distorted current wave causes current harmonics. Distorted voltage wave causes voltage harmonics. Nth order harmonics is of n. Fs frequency. Generally odd harmonics are prevalent because of half wave symmetric

2.3 About The Harmonics

The power company typically supplies a reasonably smooth sinusoidal waveform:

Supply Frequency-50 Hzs ...but nonlinear devices will draw distorted waveforms, whichare comprised of harmonics of the source. Typical Harmonics are 3rd,5th, 7th & 11th

3rd = 150 Hzs.

5th = 250 Hzs.


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7th= 350 Hzs.

11th = 550 Hzs

Each harmonic order has a particular Phase sequence relationship with respectto fundamental.

By convention the fundamental is assumed to have positive phase sequence. All higher order harmonics have either positive ,negative or zero phase sequence

with respect to fundamental.

Table.2.1. Sequence and order of harmonics

Sequence Order

Zero(0) 0,3,6,9,12,15,18,21,24.3q

Positive (+) 1,4,7,10,13,16,19,22,25.3q +1

Negative (-) 2,5,8,11,14,17,20,23,26..3q- 1

Some references refer to clean or pure power as those without any harmonics.

But such clean waveforms typically only exist in a laboratory. Harmonics have been around

for a long time and will continue to do so. In fact, musicians have been aware of such since

the invention of the first string or woodwind instrument. Harmonics (called overtones in

music) are responsible for what makes a trumpet sound like a trumpet, and a clarinet like a

clarinet.


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In European countries and other parts of the world, this frequency is usually 50 Hz.

Aircraft often uses 400 Hz as the fundamental frequency. At 60 Hz, this means that sixtytimes a second, the voltage waveform increases to a maximum positive value, then

decreases to zero, further decreasing to a maximum negative value, and then back to zero.

The rate at which these changes occur is the trigometric function called a sine wave, as

shown in figure 1. This function occurs in many natural phenomena, such as the speed of a

pendulum as it swings back and forth, or the way a string on a violin vibrates when

plucked.

Fig 2.1. Sine wave

Figure2.2 Fundamental with two harmonics


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In order to be able to analyze complex signals that have many different frequencies

present, a number of mathematical methods were developed. One of the more popular iscalled the Fourier Transform. However, duplicating the mathematical steps required in a

microprocessor or computer-based instrument is quite difficult. So more compatible

processes, called the FFT for Fast Fourier transform, or DFT for Discrete Fourier

Transform, are used.

These methods only work properly if the signal is composed of only the

fundamental and harmonic frequencies in a certain frequency range (called the Nyquist

frequency, which is one-half of the sampling frequency). The frequency values must not

change during the measurement period. Failure of these rules to be maintained can result in

mis-information. For example, if a voltage waveform is comprised of 60 Hz and 200 Hz

signals, the FFT cannot directly see the 200 Hz. It only knows 60, 120, 180, 240,..., which

are often called bins. The result would be that the energy of the 200 Hz signal would

appear partially in the 180Hz bin, and partially in the 240 Hz bin. An FFT-based processer

could show a voltage value of 115V at 60 Hz, 18 V at the 3rd harmonic, and 12 V at the 4th

harmonic, when it really should have been 30 V at 200 Hz.

These in-between frequencies are called inter harmonics. There is also a special

category of inter harmonics, which are frequency values less than the fundamental

frequency value, called sub-harmonics. For example, the process of melting metal in an

electric arc furnace can result large currents that are comprised of the fundamental , inter

harmonic, and sub harmonic frequencies being drawn from the electric power grid. These

levels can be quite high during the melt-down phase, and usually effect the voltage

waveform.

2.4 Problem Of Harmonics

Most electrical loads (except half-wave rectifiers) produce symmetrical current

waveforms, which mean that the positive half of the waveform looks like a mirror image of

the negative half. This results in only odd harmonic values being present. Even harmonics


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will disrupt this half-wave symmetry. The presence of these even harmonics should cause

the investigator to suspect there is a half-wave rectifier on the circuit. This also results froma full wave rectifier when one side of the rectifier has blown or damaged components. Early

detection of this condition in a UPS system can prevent a complete failure when the load is

switched onto back-up power.

To determine what is normal or acceptable levels, a number of standards have been

developed by various organizations. ANSI/IEEE C57.110 Recommended Practice for

Establishing Transformer Compatibility When Supplying No sinusoidal Load Currents is a

useful document for determining how much a transformer should be derated from its

nameplate rating when operating in the presence of harmonics. There are two parameters

typically used, called K-factor and TDF (transformer de reading factor). Some power

quality harmonic monitors will automatically calculate these values.

IEEE 519-1992 Recommended Practices and Requirements for Harmonic Control in

Electrical Power Systems provides guidelines from determining what acceptable limits are.

The harmonic limits for current depend on the ratio of Short Circuit Current (SCC) at PCC

(or how stiff it is) to average Load Current of maximum demand over 1 year, as illustrated

in Table 5. Note how the limit decreases at the higher harmonic values, and increases with

larger ratios.

2.5 Sources Of Harmonics

How this electricity is used by the different type of loads can have an effect on

purity of the voltage waveform. Some loads cause the voltage and current waveforms to

lose this pure sine wave appearance and become distorted. This distortion may consist ofpredominately harmonics, depending on the type of load and system impedances. Since this

article is about harmonics, we will concentrate on those types of sources.

The main sources of harmonic current are at present the phase angle controlled

rectifiers and inverters. These are often called static power converters. These devices take


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AC power and convert it to another form, sometimes back to AC power at the same or

different frequency, based on the firing scheme. The firing scheme refers to the controllingmechanism that determines how and when current is conducted. One major variation is the

phase angle at which conduction begins and ends.

A typical such converter is the switching-type power supplies found in most

personal computers and peripheral equipment, such as printers. While they offer many

benefits in size, weight and cost, the large increase of this type of equipment over the past

fifteen years is largely responsible for the increased attention to harmonics.

Figure shows below how a switching-type power supply works. The AC voltage is

converted into a DC voltage, which is further converted into other voltages that the

equipment needs to run. The rectifier consists of semi-conductor devices (such as diodes)

that only conduct current in one direction. In order to do so, the voltage on the one end must

be greater than the other end. These devices feed current into a capacitor, where the voltage

value on the cap at any time depends on how much energy is being taken out by the rest of

the power supply.

When the input voltage value is higher than voltage on the capacitor, the diode will

conduct current through it. This results in a current waveform as shown in Figure 5, and

harmonic spectrum in Figure 6. Obviously, this is not a pure sinusoidal waveform with only

a 60 Hz frequency component.

2.5.1 Harmonics Generating loads

A load is NON-LINEAR when the current it draws does not have the same

waveform as the supply voltage. The harmonic spectrum depends on the type of load i.e.,switching-mode power supplies, motors during start-up, transformers during switch-on,

frequency controlled motors. Loads that make use of semi conductor devices like transistor.

i.e.static rectifies. (AC/DC conversation using SCRS),


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1. static frequency converters, static inverters like

a. Static power converters (AC-DC conversion using SCRS)

b. Static rectifiers

c. Static frequency converter

d. Static uninterruptible power supplies e. Static induction regulators.

2. Variable impedance loads, using electric arcs, arc furnaces, welding units,

fluorescent tubes, discharge lamps, Light control brightness etc.

3. Loads using strong magnetizing currents saturated transformer, inductance

furnaces, reactors etc.

2.5.2 Harmonic Analysis

There are some simple methods for estimating the harmonic voltage due to an

installation and whether a capacitor may cause an un wanted resonance.The steps in simple

harmonic analysis are

1. Obtain information on supply system. This is usually given in the form of the short

circuit current or fault level, from which an equipment impedance can be

calculated.

2 Estimate the major harmonic sources in an installation.

3. For each harmonic order ,model the power system and installation. It is assumed

that inductive reactance will increase with frequency,capactive reactive reactance

will decrease while reactance remain unchanged.

4. Determine the voltage at the point of common coupling from the distorting current

injected and calculated harmonic impedance


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2.6 Types Of Harmonics

Generally voltage harmonics depend on grid / source stability, current harmonics

and network impedance. Current harmonics mainly depends upon the loads. These high

voltages and currents cause additional system losses, breakdown of equipment insulation,

capacitor fuse melting, errors in metering and protective relays and interference with other

consumers which may result in fall in efficiency.

1. Voltage Harmonics

2. Current Harmonics

2.6.1.Voltage Harmonics

Voltage harmonics can effects 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 the current to flow.

The voltage dip is visible with larger loads when turning on 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 distributions system impedance.

2.6.2.Current Harmonics

Current harmonics do have an effect on the electrical equipment supplying

harmonic current to the devices (transformers, conductors). Current harmonics can cause

issues with distribution equipment which has to handle the current from the utility

transformer all the way down to the device, but generally dont affect other equipmentconnected to electrical system. Harmonic currents can cause excessive heating to

transformers. For electrical systems feeding single phase loads the third harmonics has gain

attention in design consideration and transformer selection for causing the neutral

conductor to draw excessive current.


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Figure 2.3. Current Waveform

Figure 2.3. Harmonic Spectrum of Current Waveform Shown in Figure 2.2


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2.7. Measuring Of Harmonics

Hand-held harmonic meters can be useful tools for making spot checks for known

harmonic problems. However, harmonic values will often change during the day, as

different loads are turned on and off within the facility or in other facilities on the same

electric utility distribution system. This requires the use of a harmonic monitor or power

quality monitor with harmonic capabilities (such as shown in Figure 8), which can record

the harmonic values over a period of time.

Figure 2.4. Power Quality Monitor with Harmonic Analysis

Typically, monitoring will last for one business cycle. A business cycle is how long

it takes for the normal operation of the plant to repeat itself. For example, if a plant runs

three identical shifts, seven days a week, then a business cycle would be eight hours. More

typically, a business cycle is one week, as different operations take place on a Monday,

when the plant equipment is restarted after being off over the weekend, then on a

Wednesday, or a Saturday, when only a Skelton crew may be working.


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Certain types of loads also generate typical harmonic spectrum signatures that can

point the investigator towards the source. This is related to the number of pulses, or paths ofconduction. The general equation is h = ( n * p ) +/- 1, where h is the harmonic number, n is

any integer (1,2,3,..) and p is the number of pulses in the circuit, and the magnitude

decreases as the ration of 1/h (1/3, 1/5, 1/7, 1/9,...). Table 4 shows examples of such.

2.8. Limits of Harmonics

As per IS 13021, following are limits for various Harmonics.

Table 2.2 limits for various Harmonics.

Harmonics Number % of harmonic value

9th 3%

7th

4%

5th 7%

2nd 5%

3rd 30%

11th 2%

THD 8%

2.9. Case Studies

2.9.1 Case Study 1

oxygen plant

line voltage(v) line currents(A) %THD harmonic dominant

L1 L2 L3 L1 L2 L3 L1 L2 L3

428.2 432.1 427.4 77.3 86 78.4 13.8% 14.8% 11.2% 5,7,11,13


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2.9.2 Case Study2

line voltage(v) line currents(A) %THD harmonic dominants

L1 L2 L3 L1 L2 L3 L1 L2 L3

436.2 435.4 434.3 394 410 400 2.7% 2.7% 2% 5,7


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2.9.3 Case Study3

line voltage(v) line currents(A) %THD harmonic dominants

L1 L2 L3 L1 L2 L3 L1 L2 L3

432.9 428.9 426.6 61.8 76.6 66.3 10.2 10.8 9.4 5,7,11


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CHAPTER-3

IDENTIFYING THE HARMONICS

Without obvious symptoms such as nuisance breaker trips or overheated transformers,

how do you determine whether harmonic current or voltages are a cause for concern? Here

are several suggestions for simple, inexpensive measurements that a facility manager or

staff electrician could take, starting at the outlet and moving upstream

Measure the peak and root mean square (RMS) voltage at a sample of receptacles .The crest factor is the ratio of peak to RMS voltage. For a perfectly sinusoidal

voltage, the crest factor will be 1.4. Low crest factor is a clear indicator of the

presence of harmonics. Note that these measurements must be performed with a

true RMS meterone that doesnt assume a perfectly sinusoidal waveform.

Inspect distribution panels. Remove panel covers and visually inspect componentsfor signs of overheating, including discolored or receded insulation or discoloration

of terminal screws. If you see any of these symptoms, check that connections are

tight (since loose connections could also cause overheating), and compare currents

in all conductors to their ratings.

Measure phase and neutral currents at the transformer secondary with clamp-oncurrent probes. If no harmonics are being generated, the neutral current of a three-

phase distribution system carries only the imbalance of the phase currents. In a well-

balanced three-phase distribution system, phase currents will be very similar, and

current in the neutral conductor should be much lower than phase current and far

below its rated current capacity. If phase currents are similar and neutral current

exceeds their imbalance by a wide margin, harmonics are present. If neutral current

is above 70percent of the conductors rated capacity, you need to mitigate the

problem.


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Compare transformer temperature and loading with nameplate temperaturerise and capacity ratings. Even lightly loaded transformers can overheat if harmoniccurrent is high. A transformer that is near or over it srated temperature rise but is

loaded well below it srated capacity is a clear sign that harmonics are at work.

(Many transformers have built-in temperature gauges. If yours does not, infrared

thermo graphy can be used to detect overheating.)

In addition to these simple measurements, many power-monitoring devices are now

commercially available from a variety of manufacturers to measure and record harmonic

levels. These instruments provide detailed information on THD, as well as on the intensity

of individual harmonic frequencies. After taking the appropriate measurements to

determine whether you have high levels of harmonics and, if so, to find the source, you will

be well-positioned to choose the best solution.

3.1 Effects Of Harmonics

3.1.1Potential effects of harmonics

Power system problems related to harmonics are rare but it is possible for a number

of undesirable effects to occur. High levels of harmonic distortion can cause such effects as

increased transformer, capacitor, motor or generator heating, mis operation of electronic

equipment (which relies on voltage zero crossing detection or is sensitive to wave shape),

incorrect readings on meters, mis operation of protective relays, interference with telephone

circuits, etc. The likelihood of such ill effects occurring is greatly increased if a resonant

condition occurs. Resonance occurs when a harmonic frequency produced by a non-linear

load closely coincides with a power system natural frequency.

3.1.2 Effects of Harmonics on Transformers

1. Increase in No load Loss.

2. Increase in Load Loss.


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3. Increase in Energy loss.

4. Over heating of transformer which leads to

faster Insulation deterioration Lower Transformer life Premature failure of transformer, etc

1. Harmonic current Effect on IR loss

The RMS value of the load current is increased due to harmonic component, IR

loss increases accordingly.

2. Harmonic current effect on the winding eddy current los

Winding eddy current loss in power frequency spectrum tends to proportional to the

square of the load current and square of the frequency. It is the characteristic that can

cause excessive winding loss and hence higher temperature.

3. Harmonic current effect on other stray loss

These loss also increase with square of the load current and by a harmonic exponent

factor of 0.8 or less.

4. DC component of load current

A DC component of load current increase the magnetizing current and audible sound

level.

3.1.3 Effects Of Harmonics On The Equipments

Equipment responds to harmonics differently depending on there method of

operation. for example incandescent lights and most types of household electric heaters and

stoves are not effected adversely at all.


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On the other hand, induction motor windings are over heated by the harmonics,

causing accelerated degradation of insulation and losses of life .harmonic voltages can givecorrespondingly higher currents than do 50hz voltages and one can easily under estimated

the degree of additional heating in the motor. the operation of some equipment depends on

an accurate voltage wave shape and they can malfunction when harmonics are present.

Examples of this equipment containing scrs (thyristor) such as light dimmers and seem

welders.

Harmonics due to many single phase distorting loads spread across three phases,

such as occurs in commercial office buildings, can give neutral currents exceeding the

active line currents. when harmonics are absent ,the neutral conductor carries a very small

current and it has been the practice to rate the neutral for all of or maybe for half of the

active line current .with excessive levels of harmonics due to single phase loads, there is a

risk of over loading the neutral with two possible consequences.

overheating the neutral conductor with losses of conductor life and possible risk offire

there have been some claims that high neutral earth voltages can affect digitalequipment and local area networks(LANS) if the earthing system is poor.

In the supply system, substation transformers and power factor correction capacitors

are most affected. transformers are affected by a distorted current waveform which can

cause extra heating leading to reduction in their service life. capacitors can affected by the

applied voltage waveform which can cause overheating of the di-electric with a risk of

explosion.Many plant engineers are aware only of power supply problems which lead to

immediate malfunctioning or equipment trips. We have seen that harmonic effects can lead

to equipment overheating and reduction in service life by a factor of up to half with

consequent economic losses. Unlike most other supply problems, harmonics can go un


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noticed for many years unless equipment temperature or the voltage waveform is routinely

monitored.

3.1.4 Effects on Network

overvoltage and harmonic distortion in own network. overvoltage and harmonic distortion that can disturb other subscribers in the same

network.

Resonance phenomena between inductive and capacitive parts of the network.3.2 Problems Caused By Harmonics

1. Overheating of transformers (K- Factor), and rotating equipment

2. Increased hysteresis losses

3. Neutral overloading / unacceptable neutral-to-ground voltage Distorted voltage and

current waveforms

4. Failed capacitor banks

5. Breakers and fuses tripping, Unreliable operation of electronic equipment, and

Generators

6. Erroneous register of electric meters

7. Wasted energy / higher electric bills -KWD & KWH.

8. Wasted capacity Inefficient distribution of power

9. Increased maintenance cost of equipment and machinery

10. Cogging & Crawling of Induction Motor.

11. Blinking of Incandescent Lights - Transformer Saturation

12. Flickering of Fluorescent Lights Transformer Saturation

13. Neutral Conductor and Terminal Failures Additive Triple Currents


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14. Overheating of Metal Enclosures - Inductive Heating

15. Power Interference on Voice Communication Harmonic Noise

3.3 Solution To Harmonic Problems

The best way to deal with harmonics problems is through prevention: choosing

equipment and installation practices that minimize the level of harmonics in any one circuit

or portion of a facility. Many power quality problems, including those resulting from

harmonics, occur when new equipment is haphazardly added to older systems. However,

even within existing facilities, the problems can often be solved with simplesolutions such

as fixing poor or nonexistent grounding on individual equipment or the facility as a whole,

moving a few loads between branch circuits, or adding additional circuits to help isolate the

sensitive equipment from what is causing the harmonic distortion.

If the problems cannot be solved by these simple measures, there are two basic

choices: to reinforce the distribution system to withstand the harmonics or to install devices

to attenuate or remove the harmonics. Reinforcing the distribution system means installing

double-size neutral wires or installing separate neutral wires for each phase, and/or

installing oversized or K rated transformers, which allow for more heat dissipation. There

are also harmonic-rated circuit breakers and panels, which are designed to prevent

overheating due to harmonics. This option is generally more suited to new facilities,

because the costs of retrofitting an existing facility in this way could be significant.

Strategies for attenuating harmonics, from cheap to more expensive, include passive

harmonic filters, isolation transformers, harmonic mitigating transformers (HMTs), the

Harmonic Suppression System (HSS) from Harmonics Ltd., and active filters.

3.4 Impact Of Harmonics

3.4.1 Impact of harmonics on power factor

PF is a ratio of useful power to perform real work to the apparent power supplied

by a utility, which measures the efficiency of utilization of a power distribution system. The


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PF for a system delivering only linear loads is called the displacement power factor. Today

many electrical systems have harmonic currents on their lines. Harmonics are caused bynon linear loads and their current causes the apparent power to exceed the active and

reactive powers by a substantial amount. The apparent power for non linear loads can be

calculated using the equations.

KVA = (P+Q+D)

Where, D = distortion volt amperes

P = active power at all frequency

Q = reactive power at all frequency

Q = VkIksink

The presence of harmonics increases the apparent power that must be delivered,

therefore lowering the PF. In these situations the form of power factor is called

DISTORTION POWER FACTOR. In a system consisting of both linear and nonlinear

loads. The true power factor is a sum of cosine of both displacement and distortion angles.

If harmonic currents are introduced in to their system the true PF will always be lower thanthe displacement PF.

3.4.2 Impact of harmonics on capacitors

Harmonic component affects the performance of a capacitor unit significantly due to

diminishing reactance at higher frequencies, which adds to its loading substantially and can

be analyzed as follows.

This means that the capacitor will offer low reactance to the higher harmonics and

will tend to magnify the harmonic effect due to higher harmonic current. In fact, harmonic

currents have a greater heating effects compared to fundamental current. The effective

current caused by all the harmonics present in the system can be expressed as


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Where, Over current resulting to an over voltage across the capacitor units, which

would flick greater dielectric stress on capacitor elements. Since the harmonic disordersoccur at higher frequencies then the fundamental, they cause higher dielectric losses.

Fig 3.1. capacitors

The rating of the capacitor unit will does vary in a square proportion of the

effective harmonic voltage and in direct proportion to the harmonic frequency. This rise in

the KVAR, however will not contribute to the improvement of system PF, but only to the

overloading of the capacitor themselves.

3.4.3 Impact of harmonics on motor

For frequencies higher than the fundamental, 3phase induction motors can be

approximated by positive / negative shunt impedances. Where Rw is the motor winding

resistance, k is the harmonic order; X is the fundamental frequency reactance. The

harmonic voltage distortion is translated to harmonic fluxes with in the motor. Harmonic

fluxes do not contribute significantly to motor path, but induces additional losses by several

percent

Fig 3.2. Motor


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.CHAPTER-4

DECREASING OF HARMONICS

Care should be undertaken to make sure that the corrective action taken to minimize

the harmonic problems dont actually make the system worse. This can be the result of

resonance between harmonic filters, PF correcting capacitors and the system impedance.

Isolating harmonic pollution devices on separate circuits with or without the use of

harmonic filters are typical ways of mitigating the effects of such. Loads can be relocated to

try to balance the system better. Neutral conductors should be properly sized according to

the latest NEC-1996 requirements covering such. Whereas the neutral may have been

undersized in the past, it may now be necessary to run a second neutral wire that is the same

size as the phase conductors. This is particularly important with some modular office

partition-type walls, which can exhibit high impedance values. The operating limits of

transformers and motors should be derated, in accordance with industry standards from

IEEE, ANSI and NEMA on such. Use of higher pulse converters, such as 24-pulse

rectifiers, can eliminate lower harmonic values, but at the expense of creating higher

harmonic values.

4.1 Harmonic Mitigation Requirement

Need and cause analysis Root cause identification Solutions analysis. Optimize solution based on effectiveness, speed of implementation and viability Propose solutions along with cost benefits and return on investment Undertake, and implement proposal.Harmonic mitigation transformer actually do relieve problematic harmonics. HMTs can

be quite cost-effective in the right application, because they can both improve reliability

and reduce energy costs. The right application includes transformers that are heavily or


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moderately loaded and where high levels of harmonic currents are present. In addition,

HMTs are very effective in supporting critical loads that are backed up by a UPS. UPSs andbackup generators tend to have high impedance, which results in high voltage distortion

under nonlinear loading. Because of this, equipment that operates flawlessly when supplied

by utility power may malfunction when the backup system engages during a utility outage.

Note that some of these power systems have output filters (either passive or active) to

control harmonic levels. The pre senceor absence of such filters should be determined

before adding an HMT.

4.1.1 Harmonic mitigation solutions

4.2 Types Of Filters


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4.2.1 Passive Filters

Passive filters (also called traps) include devices that provide low-impedance paths to

divert harmonics to ground and devices that create a higher-impedance path to discourage

the flow of harmonics. Both of these devices, by necessity, change the impedance

characteristics of the circuits into which they are inserted.

Another weakness of passive harmonic technologies is that, as their name implies, they

cannot adapt to changes in the electrical systems in which they operate. This means that

changes to the electrical system (for example, the addition or removal of power factor

correction capacitors or the addition of more nonlinear loads) could cause them to be

overloaded or to create resonances that could actually amplify, rather than diminish,

harmonics.

Passive filters use resistors, capacitors, and inductors (RLC networks). To minimize distortion in the filter characteristic, it is desirable to use inductors

with high quality factors (remember the model of a practical inductor includes a

series resistance), however these are difficult to implement at frequencies below 1

kHz.

Drawbacks They are particularly non-ideal (lossy) They are bulky and expensive

Fig 4.1 Passive filters


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4.2.2 Active Filters

Active filters overcome these drawbacks and are realized using resistors, capacitors,and active devices (usually op-amps) which can all be integrated:

Active filters replace inductors using op-amp based equivalent circuits.Active filters in contrast, continuously adjust their behavior in response to the harmonic

current content of the monitored circuit, and they will not cause resonance. Like an

automatic transmission in a car, active filters are designed to accommodate a full range of

expected operating conditions upon installation, without requiring further adjustments by

the operator.

SOLID-STATE control of ac power using thyristors and other semiconductor switches

is widely employed to feed controlled electric power to electrical loads, such as adjustable

speed drives (ASD's), furnaces, computer power supplies, etc. Such controllers are also

used in HV dc systems and renewable electrical power generation. As nonlinear loads,

these solid-state converters draw harmonic and reactive power components of current from

ac mains. In three-phase systems, they could also cause unbalance and draw excessive

neutral currents. The injected harmonics, reactive power burden, unbalance, and excessiveneutral currents cause low system efficiency and poor power factor. They also cause

disturbance to other consumers and interference in nearby communication networks.

Conventionally passiveL-Cfilters were used to reduce harmonics and capacitors were

employed to improve the power factor of the ac loads. However, passive filters have the

demerits of fixed compensation, large size, and resonance. The increased severity of

harmonic pollution in power networks has attracted the attention of power electronics and

power system engineers to develop dynamic and adjustable solutions to the power quality

problems. Such equipment, generally known as active filters (AF's) are also called active

power line conditioners (APLC's), instantaneous reactive power compensators (IRPC's),

active power filters (APF's), and active power quality conditioners(APQC's).


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Fig. 4.2 Active filters

Active filter-features

Nominal outputs of 50A,100A & 300A High dynamic response. Up to 5afhi can be operated in parallel. Very compact design.

Easy to integrate into existing installation. Suppression of all harmonic currents up to 49th order Comprehensive communication facilities. Harmonic attenuation factor up to 97%. Avoidance of risk for resonance and takes care for unbalanced loads.

4.2.4 De-tuned filters

Tuned away from the predominant harmonics.

Blocks harmonics above tuned frequency. Acts as a controlled filter safeguard capacitors. Safeguard capacitors. Percentage tuning factor is defined as Standard detuned filters are available for 7%

tuning factor


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4.2.5 Tuned filters

Tuned closer to predominant harmonics. Combination of tuned & detuned nature. Acts as uncontrolled filter. Susceptible for failure on load profile change / modification.

4.3 Filter design highlights

Selection of size

Selection of configuration. Selecting of tuning factor. Location Sizing of components Verification of parallel & series resonance conditions. Detailed computer simulation to verify resonance & transient condition Verification of optimality through computer software.

4.4 Economic Problem

Evaluating the life-cycle costs and effectiveness of harmonics mitigation

technologies can be very challengingbeyond the expertise of most industrial facility

managers. After performing the proper measurement and analysis of the harmonics

problem, this type of evaluation requires an analysis of the costs of the harmonics problem

(downtime of sensitive equipment, reduced power factor, energy losses or potential energy

savings) and the costs of the solutions. A good place to start in performing this type of

analysis is to ask your local utility or electricity provider for assistance. Many utilities offer

their own power quality mitigation services or can refer you to outside power quality

service providers


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CHAPTER-5

REGULATION FOR INSTALLATION

These are imposed by the electricity supply authority to protect other electricity

consumers from the effects of excessive harmonics. They are usually based on agreed level

of voltage distortion which can be tolerated by correctly designed equipment. This is

specified in terms of total harmonic distortion (THD). The internationally accepted

maximum THD compatibility level in a low voltage level is 8%, and to achieve this with

a high degree of confidence it is usual to aim for a rather low level as the planning level,

typically 5%. Individual harmonics are subjected to limits.

From the point of view of the supply authority, the relevant harmonic voltage is at

the point of common coupling (PCC) with other power consumers. The harmonic levels

with in the consumers premises may be higher because of the impedance of cables and

transformers. In large installations measures may be necessary to prevent harmonic

problems with in a site. Since there are no stationary requirements, a relaxed version of the

authority limits can be applied internally. It is not advisable to allow the 8% THD limit to

be exceeded, because the majority of equipment will have been designed to be immune

only up to this level.

Calculating the voltage distortion of a proposed installation can be an expensive

matter, because it requires existing harmonics to be measured over a period of time, the

system parameters such as source impedance to be derived, and the effect of planned new

load to be estimated.

5.1 IEEE-519 Regulation

IEEE-519 is a recommended guideline for designing electrical systems in buildings,

NOT a mandatory standard.

The IEEE-519 recommended practice defines dedicated, general, andspecial classifications. Hospitals and airports fit into the special category


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while most others fit in the general classification. Systems with only VSD

loads are defined by IEEE as dedicated and allow higher distortion

Limits for general installations are 5% Total Harmonic Voltage Distortion(THVD) and 12% Total Harmonic Current Distortion (THID). Limits for

special applications are 3% THVD and 8% THID. Dedicated systems allow

10%THVD and up to 22% THID.

IEEE States that the estimated load current should be an average runningcurrent for a 1 year period. If not known 80% of Full Load Amps is a good

approximation.

IEEE-519 should not be blindly specified. Owners and engineers must beeducated on applying IEEE-519. Raising costs for customers without rational

clarification of the guidelines is not the optimum engineering solution - its like

specifying 100,000 CFM when only 20,000CFM is required.

IEE-519 is a system issue more than a particular equipment issue.IEE-519 setslimits on the voltage and current harmonics distortion at the point of common

coupling (PCC, usually the secondary of the supply transformer) .

The total harmonic distortion is dependant on the percent of harmonicsdistortion from each non-linear device with respect to the total capacity of the

transformer and the relative load of the system.

Table 5.1 Input impedance of harmonics


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Reactors are by far, the most economical way of reducing harmonic distortion on a

drive system.Actual harmonic distortion is determined for linear and non-linear loads both

on the system. Harmonics distortion depends on the percentage of non-linear loads on the

system.

Table 5.2 Percentage of non-linear loads


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CONCLUSION

The harmonic level has a great effect on the performance of the system components

and equipments. Harmonic map for the distribution system is necessary for appreciating

system operation upgrade. During the next decade, an increase of the nonlinear loads up to

70% is expected. Understanding electrical system problems will help in implementing

appropriate solutions. Phase shifting transformers can efficiently mitigate harmonic

distortion. They are rigid and more economically than harmonic filters. Besides, they are

secure for resonance problem that may arise in passive filter applications utilizing passive

harmonic filters require recurrent analysis, measurements and precautions from system

reconfigurations or upgrading and load changes for save system operation.

Solving harmonic problem is not just for satisfying standard regulations, it is an

economical business. It decreases the overall power losses on the system, includes voltage

profile and improves power factor. It also saves a deferred capacity for both transformers

and lines and improves the life time of the system components and equipments. Finally,

care full considerations are necessary when studying harmonic problems in many power

systems and on instrumentation requirements for measurements. Important issues must be

included as types of loads, power factor characteristics, harmonic generating characteristics,

frequency response characteristics of the supply system, power factor correction in the

customer facility and harmonic filters in the customer facility. It is recommended to precede

a no sinusoidal tariff schemes towards a regulatory tariff system for the non linear loads.


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BIBILIOGRAPHY

TEXTBOOKS

As 2279.2 -1991,disturbances in mains supply networks,part:2 Limitations ofharmonics caused by industrial equipment, standards Australia,1991

Arrillaga,J., Bradley,D.A., and Bodger ,P.S..Power system harmonics,JohnWiely,1985.

IEEE:bibiliography of powersystem harmonics ,part:1,IEEE Trans.,1984,PAS-103,PP.2460-2469.

IEEE:Bibilography of power system harmonics ,part:1,IEEE Trans.,1984,PAS-103,PP.2470-2479

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