1

A New Topology of Cascaded

Multilevel Converters

With Reduced Number of Components

for

High-Voltage Applications

2

TABLE OF CONTENTS

3

LIST OF TABLES

4

LIST OF FIGURES

5

6

ABBREVIATIONS

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ABSTRACT

A new topology of a cascaded multilevel converter is

proposed. The proposed topology is based on a cascaded

connection of single-phase sub multilevel converter units and full-

bridge converters. Compared to the conventional multilevel

converter, the number of dc voltage sources, switches,

installation area, and converter cost is significantly reduced as

the number of voltage steps increases. In order to calculate the

magnitudes of the required dc voltage sources, three methods are

proposed. Then, the structure of the proposed topology is

optimized in order to utilize a minimum number of switches and

8

dc voltage sources, and produce a high number of output voltage

steps. The operation

and performance of the proposed multilevel converter is verified

by simulation results and compared with experimental results of a

single-phase 49-level converter, too.

CHAPTER-1

INTRODUCTION

Multilevel inverters are commonly used for DC to AC conversion in renewable energy

conversion . The concept of multilevel converter has been introduced since 1975 . Basically, a

multilevel converter is able to achieve higher power by using a series of power switches with

several lower voltage DC sources to perform the power conversion by synthesizing a staircase

voltage waveform . For multilevel inverters, there are three basic types and they are cascaded H-

bridge, diode-clamped, and flying-capacitor converters . The multilevel converter using

cascaded-converter with separate DC sources, which may be obtained from batteries, fuel cells

or solar cells, synthesizes a desired voltage from several independent sources of DC voltage .

Compared with diode-clamped multilevel converters and flying-capacitor multilevel converters,

9

cascade multilevel converters have many advantages: the device load is balanced; the device

switching frequency is consistent; the amount of the devices is the least of the three types; each

level has same structures and it provides the flexibility to increase the number of levels without

introducing complexity into the power stage. Cascaded multilevel inverter reaches the higher

output voltage and power levels and the higher reliability due to its modular topology.

Traditionally, the multilevel converter using cascaded-converter with separate DC sources

synthesizes a desired voltage from several independent sources of DC voltage . For conventional

cascade H-bridge inverter, it will require n energy sources for 2n + 1 levels of output. For many

applications, manipulating so many separate energy sources and switches deter the use of such

inverter for large number of output levels.

To achieve a certain level of total harmonic distortion (THD) from the cascaded H-bridge

converter, either the switching frequency or the number of cascaded H-bridges has to be very

high. If the switching frequency is high, the switching loss will be high. If the number of

cascaded H-bridges is high, the conduction loss will be high because the number of components

involved will be more. The number of separate energy source required will also be more. The

aim of this paper is to reduce the power loss contributed by the inverter with acceptable THD by

reducing the number of cascaded H-bridges and the switching frequency. A single energy source

is introduced and other virtual energy sources are emulated by capacitors. Switching strategies

are derived for the regulation of the virtual energy sources. Experimental results are included to

demonstrate the effectiveness of the proposed inverter.

1.1 Inverter

A   device that converts DC power into AC power at desired output voltage and frequency is called

an Inverter. Phase controlled converters when operated in the inverter mode are called line

commutated inverters. But line commutated inverters require at the output terminals an existing

10

AC supply which is used for their commutation. This means that line commutated inverters can’

t function as isolated AC voltage sources or as variable frequency generators with DC power at

the input. Therefore, voltage level, frequency and waveform on the AC side of the line

commutated inverters can’t be changed. On the other hand, force commutated inverters provide

an independent AC output voltage of adjustable voltage and adjustable frequency and have

therefore much wider application. Based on their operation the inverters can be broadly classified

into

1.Voltage Source Inverters(VSI)

2.Current Source Inverters(CSI)

A voltage source inverter is one where the independently controlled ac output is a voltage waveform.

A current source inverter is one where the independently controlled ac output is a current waveform. Some

industrial applications of inverters are for adjustable- speed ac drives, induction heating, stand by

air-craft power supplies, UPS uninterruptible power supplies) for computers, hvdc transmission

lines etc.

  Essentially, it does the opposite of what a battery charger or "converter" does. DC is usable for some

small appliances, lights, and pumps, but not much else. Some DC appliances are available, but with the exception

of lights, fans and pumps there is not a wide selection. Most other 12 volt items we have seen are expensive and/or

poorly made compared to their AC cousins. The most common battery voltage inputs for inverters are 12, 24, and

48 volts DC - a few models also available in other voltages. There is also a special line of inverters called a utility

intertie or grid tie, which does not usually use batteries - the solar panels or wind generator feeds

directly into the inverter and the inverter output is tied to the grid power. The power produced is

either sold back to the power company or (more commonly) offsets a portion of the power used.

These inverters usually require a fairly high input voltage - 48 volts or more. Some, like the Sunny

Boy, go up to 600 volts DC input.

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1.2 Classification of inverters

There are different basis of classification of inverters. Inverters are broadly classified as

current source inverter and voltage-source inverter. Moreover, it can be classified on the basis of

devices used (SCR or gate-commutation devices), circuit configuration (half-bridge or full-

bridge), nature of outputvoltage (square, quasi-square or sine-wave) and type of circuit (Switch-

mode PWM or resonant converters), etc.

1.2.1 Current-source inverters (CSI)

This type of inverter is fed by a “current source” with high-internal impedance (using

current limiting chokes or inductor in series with a DC source). Therefore, supply current does

notchange quickly. The load current is varied by controlling the input DC voltage to thecurrent-source

inverter. CSI are used in very high-power drives

1.2.2 Voltage-source inverters (VSI)

This type of inverter is fed by a DC source of small internal impedance. Looking from

theAC side, the terminal voltage remains almost constant irrespective of the load currentdrawn. 

Depending on the circuit configurations, the voltage source inverter may beclassified as half-bridge 

and full-bridge inverters. Voltage-source inverters may also beclassified as square-wave inverter and

pulse-width modulated inverter

. (i) Square wave inverter

  A square wave inverter produces a square wave ac voltage of a constant magnitude.

The outputvoltage of this type of inverter can only be varied by controlling the input dc voltage

.(ii) Pulse width modulated (PWM)

In a PWM inverter, the output has one or more pulses in each half cycle. Varying the width of these

pulses, the output voltage may be controlled .the magnitude of input dc voltage is essential constant in this inverter.

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1.3 Basic design

 In one simple inverter circuit, DC power is connected to a transformer through the centre

tap of the primary winding. A switch is rapidly switched back and forth to allow current to flow

back to the DC source following two alternate paths through one end of the primary winding and

then the other. The alternation of the direction of current in the primary winding of the

transformer produces alternating current (AC) in the secondary circuit.

Fig: 1.1 simple inverter circuit shown with an electromechanical switch.

13

The electromechanical version of the switching device includes two stationary contacts

and a spring supported moving contact. The spring holds the movable contact against one of the

stationary contacts and an electromagnet pulls the movable contact to the opposite stationary

contact. The current in the electromagnet is interrupted by the action of the switch so that the

switch continually switches rapidly back and forth. This type of electromechanical inverter switch, called a

vibrator or buzzer, was once used in vacuum tube automobile radios. A similar mechanism has been

used in door bells, buzzers and tattoo guns.

As they became available with adequate power ratings, transistors and various other types

of semiconductor switches have been incorporated into inverter circuit designs.

1.4 Working

An inverter takes the DC input and runs it into a pair (or more) of power switching

transistors. By rapidly turning these transistors on and off, and feeding opposite sides of a

transformer, it makes the transformer think it is getting AC. The transformer changes this

"alternating DC" into AC at the output. Depending on the quality and complexity of the inverter,

it may put out a square wave, a "quasi-sine" (sometimes called modified sine) wave, or a true

sine wave. Quasi-sine (modified sine, modified square) wave inverters have more circuitry

beyond the simple switching, and put out a wave that looks like a stepped square wave - it is

suitable for most standard appliances, but may not work well with some electronics appliances

that electronic heat or speed control, or uses the AC for clocks or a timer. Also, some of the chargers

used for battery operated tools may not shut off when the battery is charged, and should not be used with

anything but sine wave inverters unless you are sure they will work. Sine wave inverters put out a

14

wave that is the same as you get from the power company – in fact, it is often better and cleaner. Sine

wave inverters can run anything, but are also more expensive.

The quality of the "modified sine" (actually modified square wave), Quasi-sine wave, etc.

can also vary quite a bit between inverters, and may also vary somewhat with the load

1.5 Output waveforms

The switch in the simple inverter described above, when not coupled to an output

transformer, produces a square voltage waveform due to its simple off and on nature as opposed

to the sinusoidal waveform that is the usual waveform of an AC power supply. Using Fourier

analysis, periodic waveforms are represented as the sum of an infinite series of sine waves. The

sine wave that has the same frequency as the original waveform is called the fundamental

component. The other sine waves, called harmonics, that are included in the series have

frequencies that are integral multiples of the fundamental frequency .

Fig.1.2 Output Waveform

15

The quality of the inverter output waveform can be expressed by using the Fourier analysis data

to calculate the total harmonic distortion (THD). The total harmonic distortion is the square root of

the sum of the squares of the harmonic voltages divided by the fundamental voltage.

The quality of output waveform that is needed from an inverter depends on the characteristics of

the connected load. Some loads need a nearly perfect sine wave voltage supply in order to work properly. Other

loads may work quite well with a square wave voltage.

1.6 LITERATURE REVIEW:

1. IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 59, NO. 11, NOVEMBER

2012 Design and Implementation of a New Multilevel Inverter Topology ,Ehsan

Najafi, Member, IEEE, and Abdul Halim Mohamed Yatim, Senior Member, IEEE

New Topology with a Reversing Voltage Component is proposed.

Requires less carrier signals & Gate drives.

Overall cost & Complexity are reduced in Higher Output levels.

A Seven level proposed topology is built and tested to show the performance of the

inverter.

dv/dt output voltage stress are reduced.

2. NATIONAL POWER ELECTRONICS CONFERENCE 2010 Implementation Of A

Five-Level Inverter Using Reversing Voltage Topology: Tekwani, 3.Amar Hinduja

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Department of Electrical Engineering, Institute of Technology, Nirma University,

Ahmedabad, India .

A competitive solution for high power applications.

Reversing voltage topology with five level inverter is proposed.

Require fewer no. of components.

Improve the multi level performance.

Requires half of the required carriers for PWM controller.

3. IEEE Transactions Ind, Appl., vol. 44 no, 4pp 1239 -1248 Jul/Aug 2008. “A Carrier

Based PWM method with Optimal Switching Sequence for a Multi level inverter

Four-leg VSI.” K.Jang-Hwan, S.K. Sul & P.N. Enjeti.

Multi level four-leg VSI used for high power application

To control zero sequence component as well as dq component

Introduced a new offset voltage

It possible for minimization harmonic distortion

PWM method is verified by spectral analysis, simulation and experimental results

4. JPE , Volume 10,No.3,May 2010. “Design of a cascaded H Bridge Multilevel Inverter

based on Power Electronics Building blocks and control for High

Performance” . Young-Min park, Han seong Ryu, Hyun-won Lee, Myung-Gil jung

and Se-Hyun Lee.

CHBM based on PEBB & high performance control scheme is used

To improve current control and increase fault tolerance

It has shown that expansion and modularization characteristics of the CHBM inverter.

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CHAPTER-2

MULTILEVEL INVERTER

2. INTRODUCTION TO MULTILEVEL INVERTERS:

2.1 Inverter

Inverters convert DC power into AC power through waves called either sine waves or

modified sine waves. Sine waves are the waves that are typically found in power from a power

plant. Modified sine waves are made to simulate sine waves. Inverters with modified sine waves

work well for backup power in houses and are much less expensive. Although there are several

types of inverters, all standard inverters use only one switch, or in other words, one power

circuit.

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2.2 Multilevel Inverter

Multilevel inverters are a a source of high power, often used in industrial applications and

can use either sine or modified sine waves. Instead of using one converter to convert an AC

current into a DC current, a multilevel inverter uses a series of semiconductor power converters

(usually two to three) thus generating higher voltage. While with an inverter you would transfer

energy with the flip of one switch, with a multilevel inverter you would have to flip several

switches, each switch requiring a circuit. These multiple switches and circuits usually make

multilevel inverters more expensive than inverters.

2.3 DIFFERENCES BETWEEN INVERTER AND MULTILEVEL INVERTER:

Inverters are often used to provide power to electronics in the case of a power outage or

for activities such as camping, where no power is available. An inverter converts a direct current

(DC) or battery power into an alternating current (AC) or household power. A multilevel inverter

is a more powerful inverter, meaning it does the same thing as an inverter except provides energy

in higher-power situations.

2.4 GENERAL STRUCTURE :

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Fig.2.1 a. Single level Fig.b.Two Level Fig.c. ‘N’ Level

2.5 BLOCK DIAGRAM:

Fig.2.2 Block Diagram

2.6 TYPES OF MULTILEVEL INVERTER:

A multilevel power converter system is a simpler solution than running direct power lines

for different voltages.

There are three structures for multilevel inverters:

Cascaded H-bridges with separate DC sources,

Diode-clamped inverters and

Flying Capacitors

Cascaded H-bridge Inverter:

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In this inverter, each DC power source is connected to an H-bridge inverter. The inverter

has four switches. By using different combinations of switches, the inverter can produce three

different AC voltage outputs.

Diode-Clamped Multilevel Inverter

This inverter uses six diodes and five capacitors to create six voltage levels. The inverter

is suitable for transmission of DC current on an AC transmission line or variable speed motors.

Precise monitoring and control are required to prevent overcharging or discharging.

Control of Cascaded Multilevel Inverters

Research is being conducted that combines the cascaded inverter design with the diode-

clamped design. Such a configuration shows promise in using only one DC power source.

Theory indicates that this configuration could produce seven voltage levels.

Flying Capacitor Multilevel Inverter

This inverter has a similar design to a diode-clamped inverter. However, the clamping

diodes have been replaced with capacitors. The design requires only two switch combinations to

create a voltage output. Tracking the output of all the capacitors is complicated, as is pre-

charging all of the capacitors.

Reverse Topology: .

It is the simulation of new multilevel inverter topology with reverse voltage technique.This

technique helps to reduce the number of power switches used in the power circuit without

compensating the levels.

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Table. 1. COMPARISON BETWEEN MULTILEVEL INVERTERS:

Cascade H-bridge Diode Clamped Flying Capacitor

Voltage Required High voltage rating

required

Medium voltage

required

High voltage required

Device rating Unequal device rating Equal device rating Equal device rating

Voltage balancing Capacitor voltage

imbalance

Capacitor voltage

imbalance

Capacitor voltage

balance

Main switching devices 2(m – 1) 2(m – 1) 2(m – 1)

Main diodes 2(m – 1) 2(m – 1) 2(m – 1)

Less More compared to

cascade

Less compared to

cascade

Output voltage Increase with increasing

levels

Normal output voltage

expected

High output voltage due

to less harmonics results

Switching losses Using PWM techniques

can be avoided

Using PWM techniques

can be avoided

Using PWM techniques

can be avoided

Applications Large Motor drive

applications

Light load applications High power rating

applications

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2.7. CONTROL SCHEMES :

Fig.2.3 Control Schemes

HYSTERESIS CONTROL

Hysteresis control presents an alternative method for producing a sinusoidal ac current

waveform from a dc voltage source. With this method, the controller maintains an output current

that stays within a given tolerance of the reference waveform. The tolerance that the output stays

within is called the “hysteresis band”. Unlike the above described PWM switching technique,

the method of hysteresis control depends on feedback from the output current to control the

inverter system. The closed-loop control method enables the inverter with hysteresis control to

adapt instantly to changes in the output loading.

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2.8 Merits of Multilevel Inverter

Multilevel concept is used to decrease the harmonic distortion in the output waveform

without decreasing the inverter power output.

To increase inverter operating voltage without devices in series.•

To minimize THD with low switching frequencies f sw

To reduce EMI due to lower voltage steps.

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

levels on the DC bus.

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

systems)

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

waveform can be achieved (with stepped modulation technique)

Reduced electromagnetic compatibility (EMC) when operated at high voltage.

Smaller rating of semiconductor devices.

2.9 Demerits of Multilevel Inverter 

The drawbacks are the isolated power supplies required for each one of the stages of the

multi converter and it’s also lot harder to build, more expensive, harder to controlling

software.  

Switching utilization and efficiency are poor for real transmission.

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CHAPTER-3

  PULSE MODULATION SCHEME

3.1 Pulse Amplitude Modulation

Pulse Amplitude Modulation refers to a method of carrying information on a train of

pulses, the information being encoded in the amplitude of pulses. In other words the pulse

amplitude is modulated according to the varying amplitude of analog signal.

3.2 Pulse Width Modulation

Pulse Width Modulation refers to a method of carrying information on a train of pulses, the information

being encoded in the width of the pulses. The pulses have constant amplitude but their duration varies in

direct proportion to the amplitude of analog signal.

3.3 Pulse Position Modulation

The amplitude and width of the pulse is kept constant in the system. The position of each

pulse, in relation to the position of a recurrent reference pulse, is varied by each instantaneous

sampled value of the modulating wave. PPM has the advantage of requiring constant transmitter

power since the pulses are of constant amplitude and duration.

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3.4 Pulse Code Modulation

To obtain PCM from an analog waveform at the source (transmitter), the analog signal

amplitude is sampled at regular time intervals. The sampling rate (number of samples per

second), is several times the maximum frequency of the analog waveform. The amplitude of the

analog signal at each sample is rounded off to the nearest binary level (quantization). The

Number of levels is always a power of 2 (4, 8, 16, 32, 64, ...). These numbers can be represented by two,

three, four, five, six or more binary digits

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Fig3.1(a)Analog signal, s( t). ( b) Pulse-amplitude modulation. ( c) Pulse-width modulation.

( d ) Pulse position modulation

PCM is a general scheme for transmitting analog data in a digital and binary way independent of the

complexity of the analog waveform. With PCM all forms of analog data like video, voice, music and

telemetry can be transferred.

3.5 Advantages of PWM

The output voltage control is easier with PWM than other schemes and can beachieved without any

additional components.

The lower order harmonics are either minimized or eliminated altogether.

The filtering requirements are minimized as lower order harmonics are eliminated and

higher order harmonics are filtered easily.

It has very low power consumption.

The entire control circuit can be digitized which reduces the susceptibility of thecircuit

to interference.

PWM is the most popular method for producing a controlled output for inverters. They

are quite popular in industrial applications.

3.6 Single Phase PWM Inverters

In many industrial applications, to control the output voltage of the inverters is necessary for the following

reasons

To adjust with variations of dc input voltage

To regulate voltage of inverters

To satisfy the contain volts and frequency control requirement There are various

techniques to vary the inverter gain. The most efficient method of Controlling the gain

27

(and output voltage) is to incorporate pulse width modulation (PWM) Control within the

inverters. The commonly used techniques are

a) Single Pulse width Modulation

b) Multiple Pulse width Modulation

c) Sinusoidal Pulse width Modulation

d) Modified sinusoidal Pulse width Modulation

e) Phase-displacement control.

The PWM techniques given above vary with respect to the harmonic content in their output voltages.

3.7 Single Pulse Width Modulation

In this control, there’s only one pulse per half cycle and the width of the pulse is varied to control the

inverter output. The gating signals are generated by comparing a rectangular reference signal of

the amplitude Are with triangular carrier wave of amplitude Ac, the f frequency of the carrier

wave determines the fundamental frequency of output voltage. By varying Ar from 0 to Ac ,the pulse

width can be varied from 0 to 100 percent. The ratio of Arto Ac is the control variable and defined as the

modulation index

3.8Multiple Pulse Width Modulation

The harmonic content can be reduced by using several pulses in each half cycle of outputvoltage. The

generation of gating signals for turning ON and OFF transistors by comparing a reference signal with a

triangular carrier wave. The frequency Fc, determines the number of pulses per half cycle. The

modulation index controls the output voltage. This type of modulation is also known as uniform pulse

width modulation (UPWM).

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3.9 Sinusoidal Pulse Width Modulation

Modulation the width of each pulse is varied in proportion to the amplitude of a sine wave

evaluated at the centre of the same pulse. The distortion factor and lower order harmonics are

reduced significantly. The gating signals are generated by comparing a sinusoidal reference

signal with a Instead of, maintaining the width of all pulses of same as in case of multiple

pulse width triangular carrier wave of frequency Fc. The frequency of reference signal Fr, determines the

inverter output frequency and its peak amplitude Ar, controls the modulation index M, and rms

output voltage Vo. The number of pulses per half cycle depends on carrier frequency.

3.10 Single-phase, full-bridge, voltage-source inverters

The single-phase, half-bridge inverters require only two controlled switches and two

diodes. These inverters can be used only when three-wired dc supply is available. Moreover, the

voltage across the off-state semiconductor device is V , which is double the load voltage. These

drawbacks are removed in full bridge inverters (fig. 2.1). The inverter uses two pairs

of controlled switches (S1S2and S3S4) and two pairs of diodes (D1D2and D3D4). The devices of one

pair operate simultaneously. The gating signals of the switch-pairs S1S2and S3S4areshown in Figs.

2.3 a & b, respectively.

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Fig.3.2: Circuit and gating configurations of single phase full bridge inverter

3.11 Unipolar switching scheme

In unipolar switching scheme, the switch pairs S1S2and S3S4of the full-bridge inverter

of fig.2.1, are not operated as pair. Instead, the switches of the first leg, i.e. S1and S4, are

operated by paring triangular carrier wave (vc) with the sinusoidal reference signal (vref ). The

switches of the other leg, i.e. S2and S3, are operated by comparing vc with –vref . Following

logic is used to operate these switches:1. If vref >vc, S1is on and if vref <vc, S4is on.2. If –vref >vc,

S3is on and if –vref <vc, S2is on .Here V an and V bn, are the potentials of the load terminals A and

B, with respect to the reference point N. The waveform for the unipolar switching scheme,

mf =12 and ma= 0.8, are shown in fig. 2.4. It may be observed that the output voltage fluctuates

from 0 to +V in the positive half-cycle and from0 to –V in the negative half-cycle. Thus the scheme is

called unipolar switching scheme

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CHAPTER-4

CIRCUIT DESCRIPTION

4.1. BLOCK DIAGRAM:

REVERSE TOPOLOGY IN

CASCADE H-BRIDGE CIRCUIT

DC POWER SUPPLY

DRIVER CIRCUIT

OUTPUTCASCADE H-BRIDGE

LEVEL GENERATION UNIT

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BLOCK DIAGRAM DESCRIPTION

DC power supply is taken from battery

Driver circuit drives the motor

15 level reversing voltage topology used

Level generation unit is connected to the H-bridge circuit

Output is taken from the H bridge inverter

4.2 CIRCUIT DIAGRAM:

32

Fig.4.2 General Circuit Diagram for Multilevel inverter

Fig.No.4.3 Detailed Circuit for Multilevel Inverter

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4.3. Inverter with voltage regulation

As mentioned in Section 3, in order to regulate the virtual supply sources which are emulated by

capacitors the switching frequency has to be doubled such that one switching cycle of the

original operation can accommodate two switching operations. An inductor is also added to the

output of the inverter to smooth out the synthesized supply.

4.4. Virtual energy sources

Even with voltage regulation for the virtual supply sources, it was shown from Table 3 that

regulations were not supported at some output levels as the provided switching actions are

supposed not to worsen the voltage deviation. Hence capacitors replacing the supply sources

have to be sufficiently large to cope with the changes. According to the local supply rule, the

amplitude of a power supply is allowed for 5% deviation. Hence the virtual supply sources are

allowed for similar deviations from their nominal values under normal operation. The size of the

capacitor is chosen as(4)

(4.1)

where Ci is the size of capacitor for the ith stage H-bridge, Imax is the maximum magnitude of

current handled, ΔT is taken as half of the period of oscillation of the synthesized supply and

ΔEi is the allowable voltage deviation in ΔT. ΔEi is set to 5% of Ei. For the present

implementation, ΔT = 0.01 s,Imax = 7.07 A, ΔE1 = 1.65 V and ΔE2 = 4.95 V which imply that

34

The final capacitors chosen were C1 = 0.1F and C2 = 0.033F such that the voltage levels of

virtual energy sources would still be acceptable even if there was no regulation for half cycle at

peak current.

4.5 Smoothing inductor

Under voltage regulation, some of the switching states are approximated by two switching

operations within one of the original switching cycle. In these cases, the two output levels within

the original switching cycle will be very much different and smoothing is required in order to

retain the THD of the synthesized signal. The size of the added inductor can be chosen as(5)

(4.2)

where L is the inductance of the added inductor, RL is the rated resistive load, fs is the effective

switching frequency of the inverter and ΔE is the maximum difference in the voltage level within

one normal switching cycle under voltage regulation such that the voltage variation appeared

across the rated load will still be less than E. value. If the generated supply is set to 5% above the

nominal value under no-load condition, the total allowable variation from no-load to rated load

will be 10% and (6)

(4.4)

where fv is the frequency of the synthesized signal.

35

Fig. 4.4 shows the final circuit schematic diagram. It should be noted that differential

amplifiers are required to measure the capacitor voltages using the analogue-to-digital converter

(ADC). To avoid the effects of aliasing on the sampled virtual energy sources, lowpass filters

with a cutoff frequency at about 677 Hz are attached to the two capacitors.

Fig.4.5. showed the synthesized waveform of the inverter for 50 Ω loading. The frequency

spectrum of the synthesized voltage was shown in Fig. 4.6. The THD of the raw synthesized

voltage was 4.12%. Fig. 4.7 showed the synthesized voltage after the filtering inductor and  Fig.

4.8 showed the frequency spectrum of the synthesized voltage after filtering. Clearly a much

smoother output was created and the THD of the synthesized waveform reduced to 0.63%. Fig.

4.9 showed the voltages of the two virtual energy sources and they were well regulated at 33 V

and 99 V. Fig. 4.10 showed the synthesized voltage for 200 Ω load and the frequency spectrum

of the raw signal was shown.

Fig.4 . 4SYNTHESIZED VOLTAGE WITH THREE REGULATED DC SOURCES:

36

Fig.4.5. FREQUENCY SPECTRUM OF THE SYNTHESIZED VOLTAGE:

Fig.4.6. RAW SYNTHESIZED VOLTAGE WITH 50 OHM LOAD:

37

Fig.4.7FREQUENCY SPECTRUM OF RAW SYNTHEZIED VOLTAGE WITH 50 OHM LOAD:

38

Fig.4.8. SYNTHESIZED VOLTAGE AFTER FILTERING INDUCTOR WITH 50 OHM LOAD:

Fig.4.9. FREQUENCY SPECTRUM OF THE SYNTHESIZED VOLTAGE AFTER FILTERING INDUCTOR WITH 50

OHM

39

Fig.4.10. VIRTUAL SOURCES E1 AND E2:

Table 2. showed the output voltage and efficiencies of the inverter under different loading

conditions and the efficiencies were above 85% under different loading conditions. Fig.

16 showed a picture of the implemented inverter.

Table 2. Efficiencies and output voltages under different loading conditions.

Loading resistance (Ω) 30.2 44 (nominal) 50 100 200

Output voltage (V rms) 213 220 221 226 233

Rated power (%) 136.6 100 88.8 46.4 24.7

E3 supply current (A) 5.8 4.2 3.72 1.85 1.06

Input power (W) 1722.6 1247.2 1104.8 549.5 314.8

Efficiency (%) 87.2 88.2 88.4 93.0 86.2

40

CHAPTER-5

41

CASCADED MULTILEVEL INVERTER

5.1 BASIC PRINCIPLE OF OPERATION OF CASCADED MULTILEVELINVERTER:

The Cascaded Multilevel Converters (CMC) is simply a number of conventional two-

level bridges, whose AC terminals are simply connected in series to synthesize the output

waveforms. Fig. 1 shows the power circuit for a five level inverter with two cascaded cells. The

CMC needs several independent DC sources which may be obtained from batteries, fuel cells or

solar cells. Through different combinations of the four switches of each cell, each converter level

can generate three different voltage outputs, +Vdc, 0, −Vdc. The AC output is the sum of the individual

converter outputs. The number of output phase voltage levels is defined by n = 2N+1, where N is

the number of DC sources.

42

Fig.5.1. Cascade level inverter

A method is presented showing that a cascade multilevel inverter can be implemented

using only a single DC power source and capacitors. A standard cascade multilevel inverter

requires n DC sources for 2n + 1 levels. Without requiring transformers, the scheme proposed

here allows the use of a single DC power source (e.g., a battery or a fuel cell stack) with the

remaining n-1 DC sources being capacitors. It is shown that one can simultaneously maintain the

DC voltage level of the capacitors and choose a fundamental frequency-switching pattern to

produce a nearly sinusoidal output.

43

5.2 Basics behind this project:

Power-electronic inverters are becoming popular for various industrial drives

applications. In recent years also high-power and medium-voltage drive applications have been

installed. To overcome the limited semiconductor voltage and current ratings, some kind of

series and/or parallel connection will be necessary. Due to their ability to synthesize waveforms

with a better harmonic spectrum and attain higher voltages, multi-level inverters are receiving

increasing attention in the past few years. The multilevel inverter was introduced as a solution to

increase the converter operating voltage above the voltage limits of classical semiconductors.

The multilevel voltage source inverter is recently applied in many industrial applications such as

ac power supplies, static VAR compensators, drive systems, etc. One of the significant

advantages of multilevel configuration is the harmonic reduction in the output waveform without

increasing switching frequency or decreasing the inverter power output. The output voltage

waveform of a multilevel inverter is composed of the number of levels of voltages, typically

obtained from capacitor voltage sources. The so-called multilevel starts from three levels. As the

number of levels reach infinity, the output THD (Total Harmonic Distortion) approaches zero.

The number of the achievable voltage levels, however, is limited by voltage unbalance problems,

voltage clamping requirement, circuit layout, and packaging constraints. Multilevel inverters

synthesizing a large number of levels have a lot of merits such as improved output waveform, a

smaller filter size, a lower EMI (Electro Magnetic Interference), and other advantages. The

principle advantage of using multilevel inverters is the low harmonic distortion obtained due to

the multiple voltage levels at the output and reduced stresses on the switching devices used.

Improvements in fast switching power devices have led to an increased interest in voltage source

inverters (VSI) with pulse width modulation control (PWM). It is generally accepted that the

performance of an inverter, with any switching strategies, can be related to the harmonic contents

44

of its output voltage. 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 multilevel inverter topologies. Pulse Width Modulation (PWM) is widely

employed to control the output of static power inverters. The reason for using PWM is that they

provide voltage and/or current wave shaping customized to the specific needs of the application

under consideration. It is lastly performance and cost criteria, which determines the choice of a

PWM method in a specific application. PWM inverters can control their output voltage and

frequency simultaneously. And also they can reduce the harmonic components in load currents.

These features have made them power candidate in many industrial applications such as variable

speed drives, uninterruptible power supplies, and other power conversion systems. However, the

reduction of harmonic components in output currents is still the focus of major interest to

alleviate the influences of electromagnetic interferences or noise and vibrations.

In general, neutral point clamped PWM three-phase inverter which uses four switching elements

in each arm has the five- level voltage waveforms that results in considerable suppression of the

harmonic currents comparing with the conventional full-bridge type three-level PWM inverters.

However, this is not the case of single-phase PWM inverter. In these days, the popular single-

phase inverters adopt the full-bridge type using approximate sinusoidal modulation technique as

the power circuits. The output voltage of them has three values: zero, positive and negative

supply dc voltage levels. Therefore, the harmonic components of their output voltage are

determined by the carrier frequency and switching functions. Moreover, the harmonic reduction

of them is limited to a certain degree. Under these technical backgrounds, is presented in the

project.

45

5.3. Cascaded H-bridge inverter and multilevel output:

A schematic diagram for cascading three H-bridges is shown a single-phase three-level

PWM (Pulse Width Modulation) in Fig. 1. For each H-bridge, it could have three output states

depending on the switch positions. Table 1. shows the switching positions, switching states and

the outputs for different H-bridge inverters. As each H-bridge can have three output levels, a

trinary system is able to form if the voltages across the voltage sources are set appropriately. If

the voltage of an H-bridge is set to three times higher than the previous stage, a maximum of

3n levels inverter with equal intervals can be generated from n-stage H-bridge. If the voltages of

the cascaded H-bridge inverter shown in Fig.1. are set to E1 = E, E2 = 3E and E3 = 9E where E is

any arbitrary voltage level, a maximum of 15 levels with equal intervals can be generated if the

switching states are correctly set. The advantage of the proposed method is only 12 power

switches are needed to generate 15 levels whereas in other schemes the number of level is less

than 11 although proposed a new configuration of cascaded multilevel inverter and the suggested

topology needs fewer switches and gate driver circuits with minimum standing voltage on

switches. Table.2. shows the required switching states for the proposed cascaded H-bridge

inverter, the output of individual inverter and the total output from the cascaded inverter. A

particular switching state for the cascaded H-bridge inverters can be selected if the required

output is half a step within the available output level of the inverter.

46

Fig. 5.2 CASCADE H-BRIDGE INVERTER:

Table 3. Switching tables for the cascaded H-bridge inverter.

Switch position Switching state Output

S1 S2 S3 S4 U1 V1

(a) Switching table for first stage of H-bridge

On Off Off On 1 E1

On On Off Off 0 0

47

Switch position Switching state Output

S1 S2 S3 S4 U1 V1

Off Off On On 0 0

Off On On Off −1 −E1

S5 S6 S7 S8 U2 V2

(b) Switching table for second stage of H-bridge

On Off Off On 1 E2

On On Off Off 0 0

Off Off On On 0 0

Off On On Off −1 −E2

S9 S10 S11 S12 U3 V3

(c) Switching table for third stage of H-bridge

On Off Off On 1 E3

On On Off Off 0 0

Off Off On On 0 0

Off On On Off −1 −E3

48

Table 4. Switching states and output of cascaded H-bridge inverter.

Switching states Inverter output Total output

U3 U2 U1 V3 V2 V1 V = V1 + V2 + V3

1 1 1 9E 3E E 13E

1 1 0 9E 3E 0 12E

1 1 −1 9E 3E −E 11E

1 0 1 9E 0 E 10E

1 0 0 9EE 0 0 9E

1 0 −1 9E 0 −E 8E

1 −1 1 9E −3E E 7E

1 −1 0 9E −3E 0 6E

1 −1 −1 9E −3E −E 5E

0 1 1 0 3E E 4E

0 1 0 0 3E 0 3E

0 1 −1 0 3E −E 2E

0 0 1 0 0 E E

0 0 0 0 0 0 0

49

Switching states Inverter output Total output

U3 U2 U1 V3 V2 V1 V = V1 + V2 + V3

0 0 −1 0 0 −E E

0 −1 1 0 −3E E −2E

0 −1 0 0 −3E 0 −3E

0 −1 −1 0 −3E −E −4E

−1 1 1 −9E 3E E −5E

−1 1 0 −9E 3E 0 −6E

−1 1 −1 −9E 3E −E −7E

−1 0 1 −9E 0 E −8E

1 0 0 −9E 0 0 −9E

−1 0 −1 −9E 0 −E −10E

−1 −1 1 −9E −3E E −11E

−1 −1 0 −9E −3E 0 −12E

−1 −1 −1 −9E −3E −E −13E

5.4 Virtual energy sources and voltage regulation:

As the current drawn from different DC sources are different, the voltages across

different energy sources have to be regulated properly if the energy sources do not have their

own voltage regulation. It will be more problematic if some of the energy sources are replaced

by capacitors. Clearly, the virtual energy sources provided by the capacitors have to be carefully

regulated. Otherwise, the number of levels generated from the inverter will be destroyed and the

50

intervals between different levels may not be equal. To maintain the trinary system and 3n levels

of output, the virtual energy voltage sources have to be properly controlled and some switching

states have to be avoided in order not to worsen the deviation of the virtual voltage source from

its nominal value. An increase in voltage for a virtual energy source will depend on the switching

state and inverter current Ic direction. If the inverter current Ic is negative which is equivalent to

current flowing into a capacitor and the switching state is 1, there will be an increase in the

capacitor voltage. If the switching state is 0, there will have no change in the capacitor voltage.

And if the switching state is −1, there will be a reduction in the capacitor voltage.

5.6 APPLICATIONS:

DC power source utilization

An inverter converts the DC electricity from sources such as batteries, solar panels,

or fuel cells to AC electricity. The electricity can be at any required voltage; in particular it can

operate AC equipment designed for mains operation, or rectified to produce DC at any desired

voltage.

Uninterruptible power supplies

An uninterruptible power supply (UPS) uses batteries and an inverter to supply AC

power when main power is not available. When main power is restored, a rectifier supplies DC

power to recharge the batteries.

Induction heating

51

Inverters convert low frequency main AC power to higher frequency for use in induction

heating. To do this, AC power is first rectified to provide DC power. The inverter then changes

the DC power to high frequency AC power.

HVDC power transmission

With HVDC power transmission, AC power is rectified and high voltage DC power is

transmitted to another location. At the receiving location, an inverter in a static inverter

plant converts the power back to AC.

Variable-frequency drives

A variable-frequency drive controls the operating speed of an AC motor by controlling

the frequency and voltage of the power supplied to the motor. An inverter provides the controlled

power. In most cases, the variable-frequency drive includes a rectifier so that DC power for the

inverter can be provided from main AC power. Since an inverter is the key component, variable-

frequency drives are sometimes called inverter drives or just inverters.

Electric vehicle drives

Adjustable speed motor control inverters are currently used to power the traction

motors in some electric and diesel-electric rail vehicles as well as some battery electric

vehicles and hybrid electric highway vehicles such as the Toyota Prius, BYD e6 and Fisker

Karma. Various improvements in inverter technology are being developed specifically for

electric vehicle applications.[4] In vehicles with regenerative braking, the inverter also takes

power from the motor (now acting as a generator) and stores it in the batteries.

52

Air conditioning

An inverter air conditioner uses a variable-frequency drive to control the speed of the

motor and thus the compressor.

53

CHAPTER-6

HARMONICS ANALYSIS OF MULTILEVEL INVERTER

6.1 HARMONICS IN MULTILEVEL INVERTER:

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

fundamental power frequency being 50 or 60Hz (50Hz for European power and 60Hz for

American power). For example, if the fundamental power frequency is 50 Hz, then

the2ndharmonic is 100 Hz, the 3 Rd is 150 Hz, etc. In modern test equipment today harmonics

can be measured up to the 63 rd harmonics. To give an understanding of this, consider a water piping

system. Have you ever taken a shower when someone turns on the cold water at the sink? You

experience the effect of a pressure drop to the cold water, reducing the flow of cold water. The

end result is you get burned! Now imagine that someone at a sink alternately turns on and off the

cold and hot water. You would effectively be hit with alternating cold and hot water! Therefore,

the performance and function of the shower is reduced by other systems. This illustration is similar to an

electrical distribution system with non-linear loads generating harmonics. There are several

industrial applications which may allow a harmonic content of 5% of its fundamental component

of input voltage when inverters are used. Actually, the inverter output voltage may have

harmonic content much higher than 5% of its fundamental component. In order to bring this

harmonic content to a reasonable limit of 5%, one method is to insert filters between the load and

inverter. If the inverter output voltage contains high frequency harmonics, these can be reduced by a

low-size filter. For the attenuation of low-frequency harmonics, however, the size of filter components increases.

This makes the filter circuit costly, bulky and weighty and in addition, the transient response of the

54

system becomes sluggish. This shows that lower order harmonics from the inverter output

voltage should be reduced by some means other the filter.

6.2 Harmonic Optimization Techniques in Multilevel VSI

One of the major problems in electric power quality is the harmonic contents. There are

several methods of indicating the quantity of harmonic contents. The most widely used measure is the total

harmonic distortion (THD). Various switching techniques have been used in static converters to

reduce the output harmonic content. We compare the two harmonic optimization techniques,

known as optimal minimization of the total harmonic distortion (OMTHD) technique and

optimized harmonic stepped-waveform (OHSW) technique used in multi-level inverters with

unequal dc sources. Both techniques are very effective and efficient for improving the quality of

the inverter outputvoltage. First, we describe briefly the cascaded H-bridge multi-level inverter

structure. Multi-level inverter is recently used in many industrial applications such as ac power supplies, static

VAR compensators, drive systems, etc. One of the significant advantages of multi-level structure

is the harmonic reduction in the output waveform without increasing switching frequency or

decreasing the inverter output power. The output voltage waveform of a multi-level inverter is

composed of a number of levels of voltages, typically obtained from capacitor voltage sources.

The so-called multi-level starts from three levels. As the number of levels increases, the output THD approaches

zero.

The number of achievable voltage levels, however, is limited by voltage unbalance problems, voltage

clamping requirement, circuit layout, and packaging constraints. Therefore, an important key in

designing an effective and efficient multi-level inverter is to ensure that the total harmonic

distortion (THD) in the output voltage waveform is small enough. The well-known multi-level

inverter topologies are: cascaded H-bridge multi-level inverter, diode-clamped multi-level

55

inverter and flying capacitor multi-level inverter. The multi-level inverter composed of cascaded

H-bridges with separate dc sources(SDCSs), hereafter called a cascaded multi-level inverter,

appears to be superior to the other multi-level topologies in terms of its structure that is not only

simple and modular but also requires the least number of components. This modular structure makes it easily

extensible to higher number of output voltage levels without undue increase in power circuit

complexity. In addition, extra clamping diodes or voltage balancing capacitors are not necessary. It is generally

accepted that the performance of an inverter, with any switching strategy, can be related to the

harmonic contents of its output voltage.

6.3.1 Applications in Harmonic Elimination

The present chapter helps us to understand the effects of non-linear loads on the power

system and the implementation of suitable devices to cancel out the harmonics. The use

of inverters in active power filters has been emphasized and the simulated circuits and results

have been described in particular.

6.3.2 Non Linear Loads

A non-linear load on a power system is typically a rectifier or some kind of arc discharge device such as a

fluorescent lamp, electric welding machine, or arc furnace in which current is not linearly related to the voltage.

Because current in these systems is interrupted by a switching action, the current contains frequency

components that are multiples of the power system frequency. This leads to distortion of the current

waveform which in turn distorts the voltage waveform. Distortion power factor is a measure of how

much the harmonic distortion of a load current decreases the average power transferred to the

load.

56

6.3.3 Active Power Filters

The increasing use of power electronics based loads (adjustable speed drives, switch mode power

supplies, etc.) to improve system efficiency and controllability is increasing the concern for

harmonic distortion levels in end use facilities and on the overall power system. The application of

passive tuned filters creates new system resonances which are dependent on specific system conditions.

  In general, passive tuned filters have been used to minimize low-frequency current

harmonics while high-pass units have been connected to attenuate the amplitude of high frequency current

components. However, high-pass filters present disadvantages due to the resistance connected in

parallel to the inductor, which increases the filter losses and reduces the filtering effectiveness at

the tuned frequency. The most critical aspects of passive filters are related to the fact that they

cannot modify their compensation characteristics following the dynamic changes of the

nonlinear load, the performance dependence they present with the power system parameters, and the

probability of series resonances with the power system’s equivalent reactance. Passive filter ratings must

be coordinated with reactive power requirements of the loads and it is often difficult to design

the filters to avoid leading power factor operation for some load conditions. Also, the passive

filter generates at fundamental frequency reactive power that changes the system voltage

regulation, and if the filter is not designed properly or disconnected during low load operating

conditions, over-voltages can be generated at its terminals. A flexible and versatile solution to

voltage/current quality problems is offered by active power filters. Active filters have the

advantage of being able to compensate for harmonics without fundamental frequency reactive

power concerns. This means that the rating of the active power can be less than a conquerable passive filter for

the same nonlinear load and the active filter will not introduce system resonances that can move a

harmonic problem from one frequency to another.

57

Fig.6.1 Generalized block diagram for active pass filters

Figure shows the components of a typical active-power-filter system and their

interconnections. The information regarding the harmonic current, generated by a nonlinear load,

for example, is supplied to the reference-current/voltage estimator together within formation

about other system variables. The reference signal from the current estimator, as well as other

signals, drives the overall system controller. This in turn provides the control for the PWM

switching-pattern generator. The output of the PWM pattern generator controls the power circuit

via a suitable interface. The power circuit in the generalized block diagram can be connected in parallel,

series or parallel/series configurations, depending on the connection transformer used.

58

CHAPTER-7

ANALYSIS OF MULTILEVEL INVERTER

7.1

59

7.1 SIMULINK DIAGRAM FOR PROPOSED 3 LEVEL MULTILEVEL

INVERTER

7.2 OUTPUT WAVEFORM OF 3 LEVEL MULTILEVEL INVERTER

60

CHAPTER-8

CONCLUSION

8.1 CONCLUSION:

A PWM-less 15-level inverter has been implemented based on cascading three H-bridge

inverter and a single energy source. The THD of the synthesized voltage is acceptable by

switching at 2 kHz. Switching strategy has also been successfully implemented for the voltage

regulation of virtual energy sources. With the addition of the inductor and new switching

strategies, a synthesized voltage with acceptable THD is resulted with less number of energy

sources.

In case of Sinusoidal Pulse Modulation the triangular carrier wave is compare with sine wave which

results in switching losses, so square wave inverter is used which reduce the switching losses. Multilevel

inverters are finding increased application in industrial environment with greater demand for

high voltage high power processing techniques with improved efficiency. The essential

advantage of multilevel inverters is the improvement in the output voltage signal quality using

devices of low voltage rating with lesser switching frequency, thereby increasing the overall

efficiency of the system. Multilevel inverters can be applied to utility interface systems and

motor drives. These converters offer a low output voltage THD, and a high efficiency. A

multilevel inverter can reduce the harmonics produced by the inverter and better THD is

obtained when the inverter operated at higher modulation index. The harmonic distortions present in

the load current and voltage waveforms were observed through analysis tool in Matlab/ simulink.

The modulation index m controls the harmonic content of the output voltage waveform. The

magnitude of fundamental component of output voltage is proportional to ma, but ma can never

61

be more than unity. Thus the output voltage is controlled by varying ma and maximum power can also be

achieved.

8.2. FUTURE SCOPE

3 Level inverter can be further implemented into 31 level. It can be used

power consumption. Switching losses will be reduced. . Multilevel inverters are finding

increased application in industrial environment with greater demand for high voltage high power

processing techniques with improved efficiency. The essential advantage of multilevel inverters

is the improvement in the output voltage signal quality using devices of low voltage rating with

lesser switching frequency, thereby increasing the overall efficiency of the system. Multilevel

inverters can be applied to utility interface systems and motor drives. These converters offer a

low output voltage THD, and a high efficiency. A multilevel inverter can reduce the harmonics

produced by the inverter and better THD is obtained when the inverter operated at higher modulation

index. The harmonic distortions present in the load current and voltage waveforms were observed

through analysis tool in Matlab/ simulink. The modulation index m controls the harmonic content of

the output voltage waveform.

62

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