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CHAPTER 1

INTRODUCTION

1.1 Power Inverter

An Inverter is basically a converter that converts DC-AC power. Inverter

circuits can be very complex so the objective of this method is to present some of the

inner workings of inverters without getting lost in some of the fine details. The word

„inverter‟ in the context of power electronics denotes a class of power conversion

circuits that operates from a dc voltage source or a dc current source and converts it

into ac voltage or current. Even though input to an inverter circuit is a dc source, it not

uncommon to have this dc derived from an ac source such as utility ac supply. Thus,

for example, the primary source of input power may be utility ac voltage supply that is

„converted, to dc by an ac to dc converter and then „inverted‟ back to ac using an

inverter. Here, the final output may be of a different frequency and magnitude than the

input ac of the utility supply [1]. Typical Applications such as Un-interruptible Power

Supply (UPS), Industrial (induction motor) drives, Traction, HVDC.

1.2 Multi Level Inverters (MLI)

Numerous industrial applications have begun to require higher power apparatus

in recent years. Some medium voltage motor drives and utility applications require

medium voltage and megawatt power level [26]. For a medium voltage grid, it is

troublesome to connect only one power semiconductor switch directly. As a result, a

multilevel power converter structure has been introduced as an alternative in high

power and medium voltage situations [65]. A multilevel converter not only achieves

high power ratings, but also enables the use of renewable energy sources. Renewable

energy sources such as photovoltaic, wind, and fuel cells can be easily interfaced to a

multilevel converter system for a high power application [3].

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The concept of multilevel converters has been introduced since 1975. The term

multilevel began with the three-level converter. Subsequently, several multilevel

converter topologies have been developed. However, the elementary concept of a

multilevel converter to achieve higher power is to use a series of power semiconductor

switches with several lower voltage dc sources to perform the power conversion by

synthesizing a staircase voltage waveform. Capacitors, batteries, and renewable energy

voltage sources can be used as the multiple dc voltage sources. The commutation of the

power switches aggregate these multiple dc sources in order to achieve high voltage at

the output; however, the rated voltage of the power semiconductor switches depends

only upon the rating of the dc voltage sources to which they are connected [6].

Although multilevel inverters were basically developed to reach higher

voltage operation, before being restricted by semiconductor limitations, the extra

switches and dc sources (supplied by dc-link capacitors) could be used to generate

different voltage levels, enabling the generation of stepped waveform with less

harmonic distortion, reducing dv/dt and common-mode voltages. These

characteristics have made them popular for high-power medium-voltage applications

but the large number of semiconductor switches in these inverters, result in a reduction

both of the reliability and efficiency of the drive. Therefore, many power electronic

researchers have made great effort in developing multilevel inverters with the same

benefits and less number of semiconductor devices [7].

1.2.1Importance of multilevel inverter

The importance of multilevel inverters has been increased since last few

decades. These new types of inverters are suitable for high voltage and high power

application due to their ability to synthesize waveforms with better harmonic spectrum

and with less Total Harmonic Distortion (THD). Numerous topologies have been

introduced and widely studied for utility of non-conventional sources and also for drive

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applications. Amongst these topologies, the multilevel cascaded inverter was

introduced in Static VAR compensation and in drive systems [18].

The Multi-Level Inverter [MLI] is a promising inverter topology for high voltage and

high power applications. This inverter synthesizes several different levels of DC

voltages to produce a stepped AC output that approaches the pure sine waveform. It has

the advantages like high power quality waveforms, lower voltage ratings of devices,

lower harmonic distortion, lower switching frequency and switching losses, higher

efficiency, reduction of dv/dt stresses etc. It gives the possibility of working with low

speed semiconductors in comparison with the two-level inverters [19].

1.2.2 Main feature of Multi-Level Inverter (MLI)

1. Ability to reduce the voltage stress on each power device due to the utilization of

multiple levels on the DC bus.

2. Important when a high DC side voltage is imposed by an application (e.g. traction

systems).

3. Even at low switching frequencies, smaller distortion in the multilevel inverter AC

side waveform can be achieved (with stepped modulation technique).

1.2.3 Advantages of Multi-Level Inverter (MLI)

A multilevel converter has several advantages over a conventional two-level

converter that uses high switching frequency pulse width modulation (PWM). The

attractive features of a multilevel converter can be briefly summarized as follows.

1. Staircase waveform quality: Multilevel converters not only can generate the output

voltages with very low distortion, but also can reduce the dv/dt stresses; therefore

Electro Magnetic Compatibility (EMC) problems can be reduced.

2. Common-Mode (CM) voltage: Multilevel converters produce smaller CM voltage;

therefore, the stress in the bearings of a motor connected to a multilevel motor drive

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can be reduced. Furthermore, CM voltage can be eliminated by using advanced

modulation strategies

3. Input current: Multilevel converters can draw input current with low distortion.

4. Switching frequency: Multilevel converters can operate at both fundamental

switching frequency and high switching frequency PWM. It should be noted that lower

switching frequency usually means lower switching loss and higher efficiency.

Unfortunately, the multilevel converters do have some disadvantages. One

particular disadvantage is the greater number of power semiconductor switches needed.

Although lower voltage rated switches can be utilized in a multilevel converter, each

switch requires a related gate drive circuit [40]. This may cause the overall system to be

more expensive and complex. Plentiful multilevel converter topologies have been

proposed during the last two decades. Contemporary research has engaged novel

converter topologies and unique modulation schemes.

1.3 Types of Multi-Level Inverter

Currently, in terms of topology, multilevel inverters can be mainly divided into

three major groups [44]:

“Cascaded multilevel inverters”: These inverters include several H-bridge

cells (Full-bridge inverters) connected in series. One leg of a cascaded multilevel

inverter is shown in Fig1.1. In the same figure, it is possible to observe the structure of

one individual cell [48].

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Fig. 1.1 Cascaded multilevel inverter.

“Diode-clamped multilevel inverters”: These inverters use clamped diodes and dc

capacitors in order to generate ac voltage. This inverter is manufactured in 3, 4 and 5-

level structures. The 3-level structure is known as “neutral-point clamped (NPC)”

and is widely used in medium voltage, high power drives [23]. One leg of an NPC

inverter can be seen in Fig .1.2.

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Fig. 1.2 NPC inverter.

“Flying-capacitor multilevel inverter [79][50]”: In this topology, semiconductor

devices are in series and their connecting points are clamped by extra capacitors, as it

can shown in Fig. 1.3.

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Fig. 1.3 Flying-capacitor inverter.

1.3.1 Other Multi Level Inverter

1. Generalized Multilevel Topology.

2. Mixed-Level Hybrid.

3. Multilevel Converter.

4. Soft-Switched Multilevel Converter.

5. Back-to-Back Diode-Clamped Converter.

1.3.2 Hybrid and Asymmetric Multi Level Inverters

The topologies mentioned before are typically called “symmetric multilevel

inverters”, because the dc link capacitors have the same voltages [66]. Asymmetric

multilevel inverters have the same topology as symmetric ones; the only

difference is in the dc link voltages. However, the asymmetric multilevel inverters

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can generate higher number of output voltage levels with the same number of

semiconductor switchers in symmetric ones. Therefore, in these inverters the

efficiency is improved by using less semiconductor devices [32] and more complicated

switching algorithms; while, output filters are very small or even removed. One leg of

an asymmetric multilevel inverter is shown in Fig.1.4.

Since the different cells of asymmetric inverter work with different dc link

voltages and different switching frequencies, it is more efficient to use

appropriate semiconductor devices in different cells. For example, using IGCT

integrated (Gate-Commutated Thyristor) switches which are suitable for high voltages

low frequency applications, in higher voltage cells decreases the power losses.

These inverters are called “hybrid multilevel inverters [31]”. A hybrid inverter which

uses several types of semiconductors has many advantages Active power is

transferred by semiconductors with low losses and high reliability and the output

harmonic spectrum is improved by other semiconductors.

Fig. 1.4 Asymmetric multilevel inverter.

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1.3.3 Symmetric Multi Level Inverters

symmetric multilevel inverters are characterized by the fact that the voltages

across the different dc link capacitors are equal[59], importance to mention that the

switches applied in the symmetric inverter have the same off-state voltage.

1.4 Different Switching Methods to Reduce Harmonic Distortion

Together with the converter topology, great effort has been addressed from the

research community in investigating different switching methods for these inverters.

This is mainly due to the fact that the adopted switching strategy impacts the

harmonic spectrum of output waveforms as well as the switching and the conduction

power losses. In case of multilevel converters, three switching methods are usually

used.

1. “Selective Harmonic Elimination”. In this method, each switch is turned on

and turned off once in a switching cycle and switching angles are usually chosen

based on specific harmonics elimination or minimization of output voltage Total

Harmonic Distortion (THD)[57].

2. “Carrier-Based PWM”. In this method, drive signals of switches are derived from

comparison of reference signal with carrier signals.

3. “Space-Vector PWM”. The space vector modulation technique is based on

reconstruction of sampled reference voltage with help of switching space vectors of a

voltage source inverter in a sampling period [52].

1.5 Cascade Multi Level Inverter

Cascade Multilevel Inverter (CMLI) is more recent and popular type of power

electronic converter [62] that synthesizes a desired output voltage from several levels

of dc voltages as inputs [24]. If sufficient number of dc sources is used, a nearly

sinusoidal voltage waveform can be synthesized. CMLI offers several advantages such

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as, its capabilities to operate at high voltage with lower dv/dt per switching, high

efficiency and low electromagnetic interference [EMI]. CMLI is one of the most

important topology in the family of multilevel and multi pulse inverters. It requires

least number of Components with compare to diode-clamped and flying capacitors type

multilevel inverters and no specially designed transformer is needed as compared to

multi pulse inverter. It has modular structure with simple switching strategy and

occupies less space.

A cascaded multilevel inverter is discussed to eliminate the excessively large number

of component.

1. Bulky transformers required by conventional multi pulse inverters,

2. Clamping diodes required by multilevel diode-clamped inverters, and

3. Flying capacitors required by multilevel flying-capacitor inverters.

1.5.1 Features of CMLI

1. It is much more suitable to high-voltage, high-power applications than the

conventional inverters.

2. It switches each device only once per line cycle and generates a multistep

staircase voltage waveform approaching a pure sinusoidal output voltage by increasing

the number of levels.

3. Since the inverter structure itself consists of a cascade connection of many single-

phase, Full-Bridge Inverter (FBI) units and each bridge is fed with a separate DC

source, it does not require voltage balance (sharing) circuits or voltage matching of the

switching devices.

4. Packaging layout is much easier because of the simplicity of structure and lower

component count.

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5. Soft-switching can be used in this structure to avoid bulky and lossy resistor -

capacitor-diode snubbers.

6. For real power conversions, (ac to dc and dc to ac), the cascaded-inverter needs

separate dc sources. The structure of separate dc sources is well suited for

various renewable energy sources such as fuel cell, photovoltaic, and biomass, etc.

7. Connecting separated dc sources between two converters in a back-to-back

fashion is not possible because a short circuit will be introduced when two back-

to-back converters are not switching synchronously.

1.5.2 Advantages of CMLI

1. The regulation of the DC buses is simple.

2. Modularity of control can be achieved. Unlike the diode clamped and capacitor

clamped inverter where the individual phase legs must be modulated by a

central controller, the full-bridge inverters of a cascaded structure can be

modulated separately.

3. Requires the least number of components among all multilevel converters to achieve

the same number of voltage levels.

4. Soft-switching can be used in this structure to avoid bulky and lossy resistor-

capacitor-diode snubbers. These advantages are our motivation to work on the

harmonic analysis of the cascaded Three-level, Five-level, Seven-level, Nine level

&Twenty seven level inverters.

1.5.3 Cascaded H-Bridges MLI (Multi Level Inverter)

Cascaded H-Bridge (CHB) configuration has recently become very popular

in high-power AC supplies and adjustable-speed drive applications. A cascade

multilevel inverter consists of a series of H-bridge (single-phase full bridge)

inverter units in each of its three phases. Each H-bridge unit has its own dc

source, which for an induction motor would be a battery unit, fuel cell or solar cell.

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Each SDC (separate D.C. source) is associated with a single-phase full-bridge inverter.

The ac terminal voltages of different level inverters are connected in series. Through

different combinations of the four switches, S1-S4, each converter level can

generate three different voltage outputs, + , - and zero.

The AC outputs of different full-bridge converters in the same phase are

connected in series such that the synthesized voltage waveform is the sum of the

individual converter outputs. Note that the number of output -phase voltage levels is

defined in a different way from those of the two previous converters (i.e. diode

clamped and flying capacitor). In this topology, the number of output-phase

voltage levels is defined by m= 2N+1, where N is the number of DC sources. A

seven-level cascaded converter, for example, consists of three DC sources and three

full bridge converters. Minimum harmonic distortion can be obtained by controlling the

conducting angles at different converter levels. Each H- bridge unit generates a

quasi-square waveform by phase shifting its positive and negative phase switching

timings. Each switching device always conducts for 180° (or half cycle) regardless of

the pulse width of the quasi-square wave.

This switching method makes all of the switching devices current stress equal.

In the motoring mode, power flows from the batteries through the cascade

inverters to the motor. In the charging mode, the cascade converters act as rectifiers,

and power flows from the charger (ac source) to the batteries. The cascade converters

can also act as rectifiers to help recover the kinetic energy of the vehicle if regenerative

braking is used. The cascade inverter can also be used in parallel HEV

configurations. This new converter can avoid extra clamping diodes or voltage

balancing capacitors.

The combination of the 180° conducting method and the pattern-swapping

scheme make the cascade inverter‟s voltage and current stresses the same and

battery voltage balanced. Identical H-bridge inverter units can be utilized, thus

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improving modularity and manufacturability and greatly reducing production costs.

Battery-fed cascade inverter prototype driving an induction motor at 50% and 80%

rated speed both the voltage and current are almost sinusoidal. Electromagnetic

interference (EMI) and common mode voltage are also much less than what would

result from a PWM inverter because of the inherently low dv/dt and sinusoidal voltage

output.

The main advantages of multilevel cascaded H-bridge converters are as follows.

1. The number of possible output voltage levels is more than twice the number of dc

sources (m = 2s + 1).

2. The series of H-bridges makes for modularized layout and packaging. This will

enable the manufacturing process to be done more quickly and cheaply.

1.6 Total Harmonics Distortion (THD)

Harmonic currents, generated by non-linear electronic loads, increase power

system heat losses and power bills of end-users. These harmonic-related losses reduce

system efficiency, cause apparatus overheating, and increase power and air

conditioning costs. As the number of harmonics-producing loads has increased over the

years, it has become increasingly necessary to address their influence when making any

additions or changes to an installation. Harmonic currents can have a significant impact

on electrical distribution systems and the facilities they feed. It is important to consider

their impact when planning additions or changes to a system. In addition, identifying

the size and location of non-linear loads should be an important part of any

maintenance, troubleshooting and repair program [2].

1.6.1 Sources of Harmonic Distortion

Non-linear equipment or components in the power system cause distortion of the

current and to a lesser extent of the voltage. These sources of distortion can be divided

in three groups:

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1. Loads

2. The power system itself (HVDC, SVC, transformers, etc)

3. The generation stage (synchronous generators)

Subdivision can also be made regarding the connection at different voltage

levels. In general, loads can be considered connected at lower voltage levels, the power

system exists at all voltage levels and the generation stage at low and medium voltage

levels. The dominating distortion-producing group, globally, are the loads. At some

locations HVDC-links, SVC‟s, arc furnaces and wind turbines contributes more than

the other sources. The generation stage can, during some special conditions, contribute

to some voltage distortion at high voltage transmission level.

The characteristic behavior of non-linear loads is that they draw a distorted

current waveform even though the supply voltage is sinusoidal. Most equipment only

produces odd harmonics but some devices have a fluctuating power consumption, from

half cycle to half cycle or shorter, which then generates odd, even and inter harmonic

currents. The current distortion, for each device, changes due to the consumption of

active power, background voltage distortion and changes in the source impedance.

1.6.2 Harmonics in Electrical Systems

One of the biggest problems in power quality aspects is the harmonic contents in

the electrical system. Generally, harmonics may be divided into two types: 1) voltage

harmonics, and 2) current harmonics. Current harmonics is usually generated by

harmonics contained in voltage supply and depends on the type of load such as resistive

load, capacitive load, and inductive load. Harmonic currents can produce a number of

problems, namely: Equipment heating, Equipment malfunction, Equipment failure,

Communications interference, Fuse and breaker disoperation Process problems,

Conductor heating. Both harmonics can be generated by either the source or the load

side.

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Harmonics generated by load are caused by nonlinear operation of devices,

including power converters, arc-furnaces, gas discharge lighting devices, etc. Load

harmonics can cause the overheating of the magnetic cores of transformer and motors

[54]. On the other hand, source harmonics are mainly generated by power supply with

non-sinusoidal voltage waveform. Voltage and current source harmonics imply power

losses, Electro Magnetic Interference (EMI) and pulsating torque in AC motor drives.

Any periodic waveform can be shown to be the superposition of a fundamental and a

set of harmonic components. By applying Fourier transformation, these components

can be extracted. The frequency of each harmonic component is an integral multiple of

its fundamental.

1.6.3 Current distortion

On three-phase star systems, current distortion causes higher than expected

currents in shared neutrals. A shared neutral is one that provides the return path for two

or three-phases. Currents as high as 200% of the phase conductors have been seen in

the field. This large level of current can easily burn up the neutral creating an open

neutral environment. This open neutral creates voltage swells and overvoltage. These

voltage conditions easily destroy equipment, particularly power supplies.

Another indirect problem introduced by current distortion is called resonance.

Certain current harmonics may excite resonant frequencies in the system. This

resonance can cause extremely high harmonic voltages, possibly damaging

equipment. There is one additional comment about current distortion. When the current

is non-sinusoidal, our conventional ammeters and voltmeters will not respond

accurately. To accurately measure currents that are harmonically distorted, use a True-

RMS meter. This applies equally to distorted voltages.

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1.6.4 Voltage distortion

Voltage distortion, on the other hand, directly affects loads. Distorted voltage

can cause motors to overheat and vibrate excessively. It can also cause damage to the

motor shaft. Even non-linear loads are prey to voltage distortion. Equipment ranging

from computers to electronically-ballasted fluorescent lights may be damaged by

voltage distortion. If damage occurs due to current distortion, except for high neutral

current, then one solution is to reduce the distortion. There are three methods for this.

First, a passive filter can be used to reduce the current from the one or two specific

harmonics.

In the second method, an active filter reduces all the harmonic currents. It is

more costly and complex to use, but it works better than passive filters. The third

method involves the use of transformers. Delta-Star transformers reduce certain

harmonics, particularly what are called zero sequence harmonics. Zigzag transformers

can also be used to reduce zero sequence harmonics, but without changing the system

type between delta and star. In addition, they can help reduce high neutral currents.

If there is concern that these special transformers or the regular distribution

transformers may overheat, then transformer de-rating, or the use of K-rated

transformers, is recommended. If high neutral currents are the culprit, then the first step

is to eliminate shared neutrals wherever possible. Where this cannot be done, try over

sizing the neutral wire so it won't overheat. If this doesn't work, then the distortion must

be reduced as described above.

There are two ways to reduce voltage distortion. Remember that internal voltage

distortion is the result of the business's non-linear loads interacting with the wiring. The

first way to reduce the distortion is to reduce the harmonic current. The second way is

to reduce the impedance of the wiring. This is done by increasing the size of the

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conductors. Where the total voltage distortion is the sum of internal and external

distortion, these techniques reduce the internal contribution.

1.6.5 Most common causes

While motor drives and commercial power supplies are most often blamed for

harmonics, the most likely culprits in the typical commercial power system is

"switched-mode-power-supplies [33]" such as those seen in personal computers and

other electronically driven devices. The typical office can have as much as 50% of its

load being determined by devices of this type. There are several methods to indicate of

the quantity of harmonics contents. The most widely used measure in North America is

the total harmonics distortion (THD) which is defined in terms of the amplitudes of the

harmonics, at frequency n , where is frequency of the fundamental component

whose amplitude of H1 and n is integer. The THD is mathematically given by

1.6.6 The Trouble with Harmonics in Modern Power Systems

Harmonics are a distortion of the normal electrical current waveform, generally

transmitted by nonlinear loads. Switch-Mode Power Supplies (SMPS), variable speed

motors and drives, photocopiers, personal computers, laser printers, fax machines,

battery chargers and UPSs [43] are examples of nonlinear loads. Single-phase non-

linear loads are prevalent in modern office buildings, while three-phase, non-linear

loads are widespread in factories and industrial plants.

A large portion of the non-linear electrical load on most electrical distribution

systems comes from SMPS equipment. For example, all computer systems use SMPS

that convert utility AC voltage to regulated low-voltage DC for internal electronics.

These non-linear power supplies draw current in high-amplitude short pulses that create

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significant distortion in the electrical current and voltage wave shape harmonic

distortion, measured as Total Harmonic Distortion (THD). The distortion travels back

into the power source and can affect other equipment connected to the same source.

Most power systems can accommodate a certain level of harmonic currents but will

experience problems when harmonics become a significant component of the overall

load. As these higher frequency harmonic currents flow through the power system, they

can cause communication errors, overheating and hardware damage, such as:

1. Overheating of electrical distribution equipment, cables, transformers, standby

generators, etc.

2. High voltages and circulating currents caused by harmonic resonance

3. Equipment malfunctions due to excessive voltage distortion

4. Increased internal energy losses in connected equipment, causing component failure

and shortened life span

5. False tripping of branch circuit breakers

6. Metering errors

7. Fires in wiring and distribution systems

8. Generator failures

9. Crest factors and related problems

10. Lower system power factor, resulting in penalties on monthly utility bills.

1.6.7 A Technical View of Harmonics

Harmonics are currents or voltages with frequencies that are integer multiples of

the fundamental power frequency. If the fundamental power frequency is 60 Hz, then

the 2nd

harmonic is 120 Hz, the 3rd

is 180 Hz, etc. When harmonic frequencies are

prevalent, electrical power panels and transformers become mechanically resonant to

the magnetic fields generated by higher frequency harmonics. When this happens, the

power panel or transformer vibrates and emits a buzzing sound for the different

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harmonic frequencies. Harmonic frequencies from the 3rd

to the 25th

are the most

common range of frequencies measured in electrical distribution systems.

All periodic waves can be generated with sine waves of various frequencies. The

Fourier theorem breaks down a periodic wave into its component frequencies.

1.6.8 Solutions to Compensate and Reduce Harmonics

While standards to limit the generation of harmonic currents are under

consideration, harmonic control today relies primarily on remedial techniques. There

are several approaches that can be taken to compensate for or reduce harmonics in the

power system, with varying degrees of effectiveness and efficiency.

1.7 Modulation Topologies for Multilevel Inverter

It is generally accepted that the performance of an inverter, with any switching

strategies, can be related to the harmonic contents of its output voltage [29]. Power

electronics researchers have always studied many novel control techniques to reduce

harmonics in such waveforms. Up-to-date, there are many techniques, which are

applied to inverter topologies.

1.7 .1 Sinusoidal Natural Pulse Width Modulation (SPWM)

Sinusoidal pulse width modulation is one of the primitive techniques, which are

used to suppress harmonics presented in the quasi-square wave [41]. In the modulation

techniques, there are two important defined parameters:

1). The ratio P= known as frequency ratio

2). The ratio M = known as modulation index.

Where is the reference frequency, is the carrier frequency, is reference signal

amplitude, and is carrier signal amplitude. The amplitude of the fundamental

frequency components of the output is directly proportional to the modulation depth.

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The second term of the equation gives the amplitude of the component of the carrier

frequency and the harmonics of the carrier frequency [53]. The magnitude of this term

decreases with increased modulation depth. Because of the presence of sin (m /2),

even harmonics of the carrier are eliminated. Term3 gives the amplitude of the

harmonics in the sidebands around each multiple of the carrier frequency.

Fig.1.5 Three-phase two-level natural SPWM with a triangular carrier wave.

The presence of sin ((m + n) /2) indicated that, for odd harmonics of the carrier,

only even-order sidebands exist, and for even harmonics of the carrier only odd order

sidebands exist. In addition, increasing carrier or switching frequency does not

decrease the amplitude of the harmonics, but the high amplitude harmonic at the carrier

frequency is shifted to higher frequency. Consequently, requirements of the output

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filter can be improved. However, it is not possible to improve the total harmonic

distortion without using output filter circuits.

In multilevel case, SPWM techniques with three different disposed triangular

carrier wave proposed as follows:

1. All the carriers are alternatively in opposition (APO disposition)

2. All the carriers above the zero value reference are in phase among them, but in

opposition with those below (PO disposition)

3. All the carriers are in phase (PH disposition).

1.8 Features of VLSI

Considerable research over the past two decades focused on the design of

parallel machines and many valuable research contributions were made. The

mainstream computer market, however, was largely unaffected by this research. Most

computers today are Uniprocessor and even large servers have only modest numbers (a

few 10s) of processors. In the first conferences, many of the hard problems of parallel

machine design have been solved. The design of fast and efficient networks to connect

arrays of the processors together and mechanisms those allow processors to quickly

communicate.

1) A grid is an intersection of a horizontal and vertical wire [71]. Hence the number of

grids on a chip is the square of the number of wiring tracks that fit along one edge of a

chip. As VLSI chips are limited by wiring, not devices, the number of grids is a better

measure of complexity than the number of transistors.

2) The key here is to match the design of the network to the properties of the

implementation technology rather than to optimize abstract mathematical properties of

the network. Odds of programming parallel machines have been demonstrated.

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Research machines were constructed to demonstrate the technology, provide a platform

for parallel software research, and solve the engineering problems associated with its

realization. The results of this research resulted in numerous commercial machines that

form the core of the high-end computer industry today.

3) MIMD machines are preferable to SIMD machines even for data-parallel

applications. Similarly, general-purpose MIMD machines are preferable to systolic

arrays, even for regular computations with local communication. Bit-serial processors

loose more in efficiency than they gain in density.

4) A good general-purpose network (like a 3-D torus) usually outperforms a network

with a topology matched to the problem of interest (like a tree for divide and conquer

problems). It is better to provide a general-purpose set of mechanisms than to create a

specialized machine for a single model of computation.

While successful at the high end, parallel VLSI architectures have had little impact on

the main stream computer industry. Most desktop machines are Uniprocessor and even

departmental servers contain at most a few 10s of processors. Today‟s mainstream

microprocessor chips are dense enough to hold 1000 of the 8086s or 68000s of 1979,

yet all of this area is used to implement a single processor. By many objective measures

this would clearly be a more efficient architecture.

1.9 Field Programmable Gate Array (FPGA)

The most common FPGA architecture consists of an array of configurable logic

blocks (CLBs), Input / Output blocks and reconfigurable matrix of interconnects CLBs

are used for realization of main logic in FPGA. It typically consists of 4-input Lookup

Tables (LUT), multiplexors and flip-flops. LUT is used for realization of combinational

logic. It is a 16-bit configurable memory which is capable to realize all 4-input

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combination logical functions. The output of the LUT can be registered to realize

sequential functions [22].

In modern FPGAs are also CLBs with more than 4 inputs integrated.

Multiplexors in CLB can be also used for realization of logic function with more than 4

inputs, they allows combine outputs of LUTs CLBs are used for realization of main

logic in FPGA. It typically consists of 4-input Lookup Tables (LUT), multiplexers and

flip-flops.

LUT is used for realization of combinational logic. It is a 16-bit configurable

memory which is capable to realize all 4-input combination logical functions. The

output of the LUT can be registered to realize sequential functions. In modern FPGAs

there are also CLBs with more than 4 inputs integrated. Multiplexors in CLB can be

also used for realization of logic function with more than 4 inputs, they allow combine

outputs of LUTs FPGAs is higher. On the other hand SRAM memory does not

remember the configuration without the power, thus the configuration must be loaded

to the SRAM before the start of the FPGA function. It increases boot-time and power

consumption.

Nonvolatile FPGAs use antifuses or some type of nonvolatile memory (FLASH,

EEPROM). This solution provides the advantages of lower power consumption and

higher resistance to radiation. Beside basic blocks described above, there are also some

others block integrated in most of the FPGAs. Delay-Locked Loops are used for clock

signal generation. They synchronize clock signal in whole FPGA a can be also used as

clock divider or multiplier. In some application is necessary to use a lot of memory. In

these cases, there are Block RAMs integrated in FPGA. Block RAM is usually a dual-

port RAM with independent control signals for each port. Because FPGAs is widely

used for DSP applications, there are multipliers, adders with Carry chain architecture,

MAC units etc. integrated in some FPGAs to increase speed of computation. The main

advantage of DSP processor in comparison with FPGA is a simple implementation.

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DSP processor can be program in assembler or C language. On the other hand the

computation performance is still limited by the architecture of DSP processor and

program realization.

1.10 Advantages of FPGA

Any technique can prove its success if and only if it is been implemented in real-time.

In order to have a successful hardware implementation, the various constrains viz.

availability of equipment, durability for completion and viability of commercial

transactions should be overcome. Field programmable Gate Arrays (FPGA) are found

to be the most cost-effective and least time consuming with simple solutions for

designers to implement their findings in real-time environment. FPGAs are future-

oriented building blocks, which allow perfect customization of the hardware at an

attractive price even in low quantities.

FPGA components available today have usable sizes at an acceptable price. This makes

them effective factors for cost savings and time-to-market when making individual

configurations of standard products.

Re-design of a board can often be avoided through application-specific integration of

our desired circuit in the FPGA - an alternative for the future, especially for very

specialized applications with only small or medium volumes. FPGA technology is

indispensable wherever long-term availability or harsh industrial environments are

involved. Another important aspect is long-term availability. Many component

manufacturers do not agree on any long-term availability. This makes it difficult or

impossible for the board manufacturer to support his product for more than 10 years.

The remarkable advantage of FPGAs and their nearly unlimited availability lies in the

fact that, even if the device migrates to the next generation, the code remains

unchanged. This is in accordance with norms like the (European standards) EN 50155

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which prescribes that customized parts like FPGAs must be documented to allow

reproduction and that the documentation and the source code must be handed out to the

customer. In order to have a customized function, normally a device is programmed

and is connected to the logic blocks through the transistors as interconnectors. The

major benefit of using FPGA has two fold, one is flexibility in design and the other one

is fast time in completion of the task.

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1.11 Thesis Organization

A brief outline of the various chapters of the thesis is as follows.

Chapter 1: Provides information about the inverters, multilevel inverters (MLI) and

Total Harmonic Distortion. It deals with cascade multi level inverters and real time

application of multilevel inverters, features and challenges in VLSI and advantage of

FPGA.

Chapter 2: Deals with review of literature survey about the generation of PWM, hence

it is applied in power inverters. It gives detailed studies of work before done and it

reviews the PWM and SPWM to be applied in various multilevel inverters.

Chapter 3: Provides information about generation of N-bit counter based PWM using

FPGA and it is applied as triggering pulse for multilevel inverters. A comparison of

Multi Level inverter is performed and analysis the THD values of seven and nine level

inverters using induction motor.

Chapter 4: Design and Analysis of Various PWM Techniques for Symmetric and

asymmetric multilevel inverters using FPGA. PWM pulses are given as triggering input

for multilevel inverters and also provide analyzed results.

Chapter 5: Discussion about the Design of proposed MFCWPT based PWM

techniques for Twenty Seven Level Inverter is presented. MFCPT and MFCWPT based

PWM pulse is given as triggering input for Twenty Seven Level Inverter. Performances

of multilevel inverters were analyzed.

Chapter 6: It deals with the results and discussions of all the methods. Provides

information about application of Twenty Seven Level Asymmetric Inverter.

Chapter 7: Concludes the overall work, which has been done. It provides highlights on

thesis work and suggestions for future work.