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CHAPTER 1
INTRODUCTION
1.1 INTRODUCTION
Inverters are the power electronic circuit, which converts the DC
voltage into AC voltage. The DC source is normally a battery or output of the controlled
rectifier. The output voltage waveform of the inverter can be square wave, quasi-square
wave or low distorted sine wave. The output voltage can be controlled with the help of
drives of the switches. The pulse width modulation techniques are most commonly used
to control the output voltage of inverters. Such inverters are called as PWM inverters.
The output voltage of the inverter contain harmonics whenever it is not sinusoidal. These
harmonics can be reduced by using proper control schemes.
Inverters can be broadly classified into two types. They are-
Voltage Source Inverter (VSI)
Current Source Inverter (CSI)
When the DC voltage remains constant, then it is called voltage inverter
(VSI) or voltage fed inverter (VFI). When input current is maintained constant, then it is
called current source inverter (CSI) or current fed inverter (CFI). Sometimes, the DC
input voltage to the inverter is controlled to adjust the output. Such inverters are called
variable DC link inverters. The inverters can have single phase or three-phase output.
1.1.1 MULTILEVEL INVERTER
Multilevel power conversion was first introducedmore than two decades
ago. The general concept involvesutilizing a higher number of active semiconductor
switches to perform the power conversion in small voltagesteps. There
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are several advantages to this approach whencompared with the conventional power
conversion approach.
The smaller voltage steps lead to the production of higher powerquality
waveforms and also reduce voltage (dv/dt) stress onthe load and the electromagnetic
compatibility concerns.Another important feature of multilevel converters is that
thesemiconductors are wired in a series-type connection, whichallows operation at higher
voltages. However, the series connectionis typically made with clamping diodes, which
eliminatesovervoltage concerns. Furthermore, since the switches are nottruly series
connected, their switching can be staggered, whichreduces the switching frequency and
thus the switching losses.
One clear disadvantage of multilevel power conversion is the higher
number of semiconductor switches required. It should be pointed out that lower voltage
rated switches can be used in the multilevel converter and, therefore, the active
semiconductorcost is not appreciably increased when compared with the twolevelcases.
However, each active semiconductor added requiresassociated gate drive circuits and
adds further complexity to theconverter mechanical layout.
Another disadvantage of multilevel power converters is that the small
voltage steps are typicallyproduced by isolated voltage sources or a bank of
seriescapacitors. Isolated voltage sources may not always be readilyavailable, and series
capacitors require voltage balancing .To some extent, the voltage balancing can be
addressed by usingredundant switching states, which exist due to the high numberof
semiconductor devices. However, for a complete solutionto the voltage-balancing
problem, another multilevel converter may be required .
1.2 NECESSITY
In recent years, industry has begun to demand higher power conversion
equipment,which now reaches the megawatt level. Controlled ac drives in the megawatt
range are usually connected to the medium-voltage network. Today, it is hard to connect
a single power semiconductor switch directly to medium voltage grids (2.3, 3.3, 4.16, or
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6.9 kV). For these reasons, a new family of multilevel inverters has emerged as the
solution for working with higher voltage levels
In excising system multiple bridges are used which in turn increases
the no of switches, and assymetrical supply sources. The supply sources used in each
bridge is doubled for each bridge.The inverter design is difficult to construct and robust
in operation. In single-phase multilevel inverters, the most common topologies are the
cascaded, diode-clamped, and capacitor clamped this types are conventional multilevel
inverter .In practice, bulky transformers either of low or medium frequency are usually
necessary. Difference in the ratings of the switches used is also a major drawback of the
existing topologies.
So a new multilevel inverter topology named reversing voltage
topology is used in this project to reduce the number of componentscompared to
conventional topologies. It is also moreefficient since the inverter has a component
whichoperates the switching power devices at line frequency.Therefore, there is no need
for all switches to workin high frequency which leads to simpler and morereliable control
of the inverter. In this topology 7 leveloutput voltage is produced.
1.3 OBJECTIVES
The main objective of the project is to design and implement a new multilevel
inverter with reverse voltage topology.
To reduce the number of switching devices.
To reduce the total harmonic distortion of the output waveform.
To obtain a 7 level output waveform with approximates a sine wave.
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CHAPTER 2
LITERATURE SURVEY
2.1 LITERATURE SURVEY
Several topologies of multi-level inverter system have
beenintroduced in the recent past [2]. The main topologies arediode clamped inverter
system [3], flying capacitor invertersystem and Cascaded H-bridge inverter system ,
[8]in order to generate a high voltage waveform using lowvoltage devices. Each of these
topologies has a differentmechanism for providing the required voltage levels. But
thenumber of main switches of each topology is equal.Comparing with respect to the
other components, for instance,DC-link capacitors having the same capacity per unit,
diodeclamped inverter has the least number of capacitors among thevarious multi-level
inverter system topologies but requiresadditional clamping diodes. Flying capacitor
inverters havethe largest number of capacitors required but need noclamping diode. H
bridge inverters require isolated voltagesources but need no clamping diodes.
Recent research has involved the introduction of novel converter
topologies andunique modulation strategies. However, the most recently usedinverter
topologies, which are mainly addressed as applicablemultilevel inverters, are cascade
converter, neutral-pointclamped(NPC) inverter, and flying capacitor inverter. There are
also some combinations of the mentioned topologies asseries combination of a two-level
converter with a three-levelNPC converter which is named cascade 3/2 multilevel
inverter. There is also a series combination of a three-level cascadeconverter with a five-
level NPC converter which is namedcascade 5/3 multilevel inverter .
Some new approaches have been recently suggested such as the
topology utilizing low-switching-frequency high-powerdevices . Although the topology
has some modification toreduce output voltage distortion, the general disadvantage of
this method is that it has significant low-order current harmonics.
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It is also unable to exactly manipulate the magnitudeof output voltage due to an adopted
pulsewidth modulation.
There is also another topology which requires more switchesthan the
proposed topology for the same number of levels .Some of the proposed topologies suffer
from complexities ofcapacitor balancing . In, the capacitor values usedin the topology are
proportional to the load current, and as theload current increases, a larger capacitor
should be selected.In, the capacitor voltage will affect the output voltage
whenmodulation index reaches near its extreme values, i.e., zeroor one. [3]
The proposed topology in this project is an overview of a new multilevel
invertertopology named reversing voltage (RV). This topology requiresless number of
components compared to conventional topologies.It is also more efficient since the
inverter has a componentwhich operates the switching power devices at line
frequency.Therefore, there is no need for all switches to work in highfrequency which
leads to simpler and more reliable control ofthe inverter. [1]
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CHAPTER 3
CIRCUIT DESCRIPTION
3.1 INTRODUCTION
In conventional multilevel inverters, the power semiconductor
switches are combined to produce a high-frequency waveformin positive and negative
polarities. However, there is noneed to utilize all the switches for generating bipolar
levels.This idea has been put into practice by the new topology.This topology is a hybrid
multilevel topology which separatesthe output voltage into two parts. One part is
namedlevel generation part and is responsible for level generating inpositive polarity.
This part requires high-frequency switches togenerate the required levels. The switches in
this part shouldhave high-switching-frequency capability.The other part is called polarity
generation part and isresponsible for generating the polarity of the output voltage,which
is the low-frequency part operating at line frequency.The topology combines the two
parts (high frequency andlow frequency) to generate the multilevel voltage output.
Inorder to generate a complete multilevel output, the positivelevels are generated by the
high-frequency part (level generation),and then, this part is fed to a full-bridge inverter
(polaritygeneration), which will generate the required polarity for theoutput. This will
eliminate many of the semiconductor switcheswhich were responsible to generate the
output voltage levels inpositive and negative polarities.
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3.2 BLOCK DIAGRAM
Fig 3.2 Block diagram
3.2.1 DESCRIPTION OF VARIOUS BLOCKS
3.2.1.1 AC SUPPLY
The circuit uses standard power supply comprising of a step-down
transformer from 230Vto 12V and 4 diodes forming a bridge rectifier that delivers
pulsating dc which is then filtered by an electrolytic capacitor of about 470μF to 1000Μf.
3.2.1.2 RECTIFIER
A rectifier is an electrical device that converts alternating current (AC),
which periodically reverses direction, to direct current (DC), current that flows in only
one direction, a process known as rectification. Rectifiers have many uses including as
components of power supplies and as detectors of radio signals. Rectifiers may be made
of solid state diodes, vacuum tube diodes, mercury arc valves, and other components.
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The output from the transformer is fed to the rectifier. It converts A.C. into pulsating
D.C.
Fig 3.2.1.2 Bridge rectifier
3.2.1.3 FILTER
Fig 3.2.1.3 Capacitor filter
Capacitive filter is used in this project. It removes the ripples from the
output of rectifier and smoothens the D.C. Output received from this filter is constant
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until the mains voltage and load is maintained constant. However, if either of the two is
varied, D.C. voltage received at this point changes. Therefore a regulator is applied at its
output .
3.2.1.4 MULTILEVEL INVERTER
This is the main part of the proposed system. It consists of ten high power
switches, which can be operated both in high frequency and low frequency regions. The
switches used here are MOSFETs which have fast switching characteristics. Among the
ten switches six of them take part in level generation process while other four will work
as the polarity generation process. The desired 7 levels are generated from the level
generation part and they are fed into the polarity generation part to reverse its polarity if
needed.
3.2.1.5 PIC MICROCONTROLLER
Fig 3.2.1.5 PIC16F877A Microcontroller
PIC stands for Peripheral Interface Controller
given by Microchip Technology to identify itssingle-chip
microcontrollers. These devices have been very successful in 8-bit
microcontrollers
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Fig 3.2.1.5.1 pin out
PIC 16F877A Specification: PIC16F877 is a 40 pin microcontroller. It has 5 ports port A,
port B,
port C, port D, port E. All the pins of the ports are for interfacing input output devices.
Port A: It consists of 6 pins from A0 to A5
Port B: It consists of 8 pins from B0 to B7
Port C: It consists of 8 pins from C0 to C7
Port D: It consists of 8 pins from D0 to D7
Port E: It consists of 3 pins from E0 to E2
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The rest of the pins are mandatory pins these should not be used to connect input/output
devices.
Pin 1 is MCLR (master clear pin) pin also referred as reset pin.
Pin 13, 14 are used for crystal oscillator to connect to generate a frequency of about
20MHz.
Pin 11, 12 and31, 32 are used for voltage supply Vdd(+)and Vss (-).
3.2.1.6 DRIVER CIRCUIT
It is used to provide 9 to 20 volts to switch the MOSFET
Switches of the inverter. Driver amplifies the voltage from microcontroller which
is 5volts. Also it has an opto coupler for isolating purpose. So damage to
MOSFET is prevented. The driver circuit forms the most important part of the
hardware unit because it acts as the backbone of the inverter because it gives the
triggering pulse to the switches in the proper sequence. The diagram given above gives
the circuit operation of the driver unit.
Fig 3.2.1.6 Driver circuit
+
1000uF/25V
12
1K
D22
LED
IN4007
IC
TLP250
1
2
3
4 5
6
7
8
560E100E
220E
2N2222
31
2
FR 10712
18V ZENER
12
G
0.1uF
12
CK100
31
2
12
GND
Pulse
1K
LED
SW112
S
(230-12)V
V1
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3.2.1.7 CRO
Fig 3.2.1.7 CRO
The cathode ray oscilloscope is an instrument which we use in laboratory
to display measure and analyze various waveforms of various electrical and electronic
circuits. In this project the output waveform of seven level is obtained on the CRO.
3.2.1.8 WORKING
The single phase Ac supply is given to a step down transformer and
therefore 230 V is step down to 12V ac. These are then fed into a bridge rectifier and
filter circuit so that a 12V dc supply is obtained at its output. Another step down
transformer of 230/12v is placed for providing supply for the driver circuit. The driver
circuit will amplifies the voltage from microcontroller which is 5 V. The microcontroller
is supplied by using a 230/9V step down transformer .Since microcontroller works on 5V
supply a 7805 voltage regulator is used to power the controller.
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3.3 CIRCUIT DIAGRAM
Fig 3.3 Circuit diagram
3.3.1 CIRCUIT GENERAL DESCRIPTION
This is a hybrid multilevel topology which separatesthe output voltage
into two parts. One part is named level generation part and is responsible for level
generating in positive polarity. This part requires high-frequency switches to generate the
required levels. The switches in this part should have high-switching-frequency
capability. The other part is called polarity generation part and is responsible for
generating the polarity of the output voltage, which is the low-frequency part operating at
line frequency. The proposed system combines the two parts (high frequency and low
frequency) to generate the multilevel voltage output.
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In order to generate a complete multilevel output, the positive levels are
generated by the high-frequency part (level generation), and then, this part is fed to a full-
bridge inverter (polarity generation), which will generate the required polarity for the
output. This will eliminate many of the semiconductor switches which were responsible
to generate the output voltage levels in positive and negative polarities.
3.3.2 SWITCHING SEQUENCE
In Table I, the numbers show the switch according to Fig.3.3 should be
turned on to generate the required voltage level. According to the table, there are six
possible switching patterns to control the inverter. It shows the great redundancy of the
topology. However, as the dc sources are externally adjustable sources (dc power
supplies), there is no need for voltage balancing for this work. In order to avoid unwanted
voltage levels during switching cycles, the switching modes should be selected so that the
switching transitions become minimal during each mode transfer. This will also help to
decrease switching power dissipation. According to fig.1 the aforementioned suggestions,
the sequences of switches (2–3-4), (2-3-5), (2-6-5), and (1, 5) are chosen for levels 0 up
to 3, respectively. These sequences are shown in Fig. 3. As can be observed from Fig.
3.3.2, the output voltage levels are generated in this part by appropriate switching
sequences. The ultimate output voltage level is the sum of voltage sources, which are
included in the current path that is marked in bold. In order to produce seven levels .
Table 1Switching sequences for each level
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Fig 3.3.2 Switching sequence for different level generation
The step form of seven levels of the output waveform is obtained by the
equation 2(N+1) where N is the number of dc sources. So by increasing the number of
sources the level can also be increased. Here three dc sources of 5 V is used in the circuit.
Fig 3.3.2 indicates that for generating the 0 level output only the switches 2,3,5 are turned
on and in the polarity generation part the switches 7 and 10 will be in the on position. For
obtaining the 5 v output level one of the dc source is active in the circuit and the
switches 2,3,5,7,10will be in the ON position.
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For obtaining 10V output step level two of the dc sources will be active and
the switches 2,6,5,8 and 9 will be active. For generating the 15 V level three of the dc
sources will adds up its voltage and the switches 1,5 ,7 and 10 will be active and
therefore desired output level is generated .Similarly for generating the negative polarity
output voltage levels the switches 8 and 9 will be ON in the polarity generation part of
the circuit .
Fig 3.3.2.1 gate signals for level generation
The ON and OFF period of the switches are controlled by generating pulse
signals by PWM technique.Fig 3.3.2.1 shows how the delay time and on period are
calculated. Here there are 12 states for completing one cycle of the wave. So the total
time required for completion one cycle is equal to .02s. So for one switch the time period
is .02/12 which is equal to .00166s.there for the ON percentage is calculated by
(.0016/.02)*100 which is equal to.8333%.
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3.3.3 LIST OF COMPONENTS
The following are the components used in the circuit,
3.3.3.1 INVERTER CIRCUIT DIAGRAM
Bridge rectifier-5Amps
MOSFET IRF840
2 pin PTP
3.3.3.2 CONTROLLERS
PIC16F877A
AC socket
Bridge rectifier 1 amps
Capacitor 470microfrad/25v
Voltage regulator LM7805
LED resistor 330 ohm
Reset switch
Resistor 100 ohm
3.3.3.3 DRIVER BOARD
Diode IN4007
Capacitor 1000microfrad/25v
Transistor(PNP) CK100
Transitor(NPN) IN2222
Resistor 100 ohm
Resisitor 1 k
IC opto coupler MCT2E
Buffer Ic wCD4050
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Zener diode
3.3.4 DECRIPTION OF COMPONENTS
3.3.4.1 MOSFET IRF840
This N-Channel enhancement mode silicon gate power field effect
transistor is an advanced power MOSFET designed, tested, and guaranteed to withstand a
specified level of energy in the breakdown avalanche mode of operation. All of these
power MOSFETs are designed for applications such as switching regulators, switching
converters, motor drivers, relay drivers, and drivers for high power bipolar switching
transistors requiring high speed and low gate drive power. These types can be operated
directly from integrated circuits.
3.3.4. 2 LM7805
FIG 3.3.4.2 LM7805
This series of fixed – voltage integrated-circuit voltage regulators are
designed for a wide range of applications .This applications include on-card regulation
for elimination of noise and distribution problems associated with single point
regulation .Each of these regulators can deliver upto 1.5A of output current.
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The internal current-limiting and thermal shutdown features of these
regulators essentially make them immune to overload. In addition to use asfixed –
voltage regulators ,these devices can be used with external component to obtain
adjustable output voltages and currents, and also can be used as the power- pass
element in precision regulators.
3.3.4.3 OPTOCOUPLER
Optocoupler is also termed as optoisolator. Optoisolator a device
which contains a optical emitter, such as an LED, neon bulb, or incandescent bulb, and an
optical receiving element, such as a resistor that changes resistance with variations in
light intensity, or a transistor, diode, or other device that conducts differently when in the
presence of light. These devices are used to isolate the control voltage from the
controlled circuit.
3.3.4.4 BUFFER IC CD4050
The CD4049UBC and CD4050BC hex buffers are monolithic
complementary MOS (CMOS) integrated circuits constructed with N- and P-channel
enhancement mode transistors. These devices feature logic level conversion using only
one supply voltage (VDD). The input signal high level (VIH) can exceed the VDD
supply voltage when these devices are used for logic level conversions. These devices are
intended for use as hex buffers, CMOS to DTL/ TTL converters, or as CMOS current
drivers, and at VDD =5.0V, they can drive directly two DTL/TTL loads over the full
operating temperature range.
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3.3.4.5 IN4007
Diodes are used to convert AC into DC these are used as half wave rectifier
or full wave rectifier. The number and voltage capacity of some of the important diodes
available in the market are as follows: Diodes of number IN4001, IN4002, IN4003,
IN4004, IN4005, IN4006 and IN4007 have maximum reverse bias voltage capacity of
50V and maximum forward current capacity of 1 Amp.
3.3.4.6 RESISTORS
A resistor is a two-terminal electronic component designed to oppose an
electric current by producing a voltage drop between its terminals in proportion to the
current, that is, in accordance with Ohm's law: V = IR Resistors are used as part of
electrical networks and electronic circuits. They are extremely commonplace in most
electronic equipment. Practical resistors can be made of various compounds and films, as
well as resistance wire (wire made of a high resistivity.
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CHAPTER 4
PROJECT PLAN
4.1 OUTPUT FOR PROJECT
The feasibility of the proposed approach is verified using computer
simulations. A model of the seven-level inverter is constructed in MATLAB-Simulink
software. Output waveform is created, resulting in the production of the desired voltage
waveform of the multilevel inverter .The waveform of the proposed multilevel inverter
with an output voltage is 600v peak-peak of a resistive load is 100 Ω is obtained. The
resulting output voltage THD was 34.44% .
Fig 4.1 output waveform
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4.2 DESIGN PROCEDURES
4.2.1 VOLTAGE REGULATOR
As we require 5v we need to use LM 7805 IC
LM 7805 IC ratings:
Input voltage : 7v-35v
Current rating : 1A
Output voltage range : V (max)=5.2v V(min)=4.8v
4.2.2 RECTIFIER CIRCUIT
IN 4007 diodes are used as it is capable of withstanding high reverse voltage.
4.2.3 FILTER CIRCUIT
Let the maximum ripple factor of capacitor input filter be 3%.
Theoretical value of r= 1/(43 f RC)
Power supply frequency f=50 Hz. Assume R=1k,
Then, C=100F, PIC used is 16F877A
4.2.4 MOSFET
IRF840 MOSFETs are used since it can withstand currents upto 8A.
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4.3 TRAINING KNOWLEDGE AND SKILLS NEEDED
MATLAB (matrix laboratory) is a multi-paradigm numerical computing
environment and fourth-generation programming language. Developed by MathWorks,
MATLAB allows matrix manipulations, plotting of functions and data, implementation
of algorithms, creation of user interfaces, and interfacing with programs written in other
languages, including C, C++, Java, Fortran and Python. Although MATLAB is intended
primarily for numerical computing, an optional toolbox uses the MuPADsymbolic
engine, allowing access to symbolic computing capabilities. An additional package,
Simulink, adds graphical multi-domain simulation and Model-Based Design for dynamic
and embedded systems.
4.4 SCHEDULE OF THE PROJECT
SL NO: SCHEDULE OF WORK MONTH
1 Project selection August
2 Design of circuit August
3 Basic introduction to MATLAB August - September
4 Simulation September - October
5 Further improvements in circuit design December-January
6 Hardware implementation February Table 1 Schedule of the project
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4.5 PROTOTYPE COST
SL
NO
COMPONENT NAME
COST(in Rs)
INVERTER CIRCUIT DIAGRAM
1 P2 pin PTP-1 connector 4
2 Bridge rectifier-5Amps 98
3 MOSFET IRF840 26
4 2 pin PTP 4
CONTROLLERS
5 PIC16F877A 250
6 AC socket 40
7 Bridge rectifier 1 A 52
8 Capacitor 479 micro farad/25V 5
9 Voltage regulator LM7805 35
10 LED resistor 330 ohms 2
11 Reset switch 2
12 Resistor 100 ohms 2
DRIVER BOARD
13 Diode IN4007 4
14 Capacitor 5
15 Transistor (NPN) IN2222 5
16 Resistor 100 ohm 2
17 Resistor 1k 2
18 IC MCT2E 34
19 Buffer IC WCD4050 65
20 Zener diode 7
21 PCB layout board for PIC microcontroller 750
22 PCB layout board for driver board 870
23 Transformer 230/12V 220
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24 General purpose soldering board 50
TOTAL 2534
Table 2 prototype cost
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CHAPTER 5
CURRENT STATUS OF THE PROJECT
5.1 CURRENT STATUS OF THE PROJECT
The feasibility of the proposed approach is verified using computer
simulations. A model of the seven-level inverter is constructed in MATLAB-Simulink
software. Output waveform is created, resulting in the production of the desired voltage
waveform of the multilevel inverter.
Simulation completed
Designed Multilevel inverter
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CHAPTER 6
APPLICATIONS
1. DC power source utilization
An inverter converts the DC electricity from sources such as batteries or
fuel cells to AC electricity.
2. Uninterrupted powers source utilization
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.
3. Electric motor speed control
Inverter circuits designed to produce a variable output voltage range are
often used within motor speed controllers. The DC power for the inverter section can be
derived from a normal AC wall outlet or some other source. Control and feedback
circuitry is used to adjust the final output of the inverter section which will ultimately
determine the speed of the motor operating under its mechanical load.
4. Power grid
Grid-tied inverters are designed to feed into the electric power distribution
system. They transfer synchronously with the line and have as little harmonic content as
possible. They also need a means of detecting the presence of utility power for safety
reasons, so as not to continue to dangerously feed power to the grid during a power
outage.
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5. Solar
A solar inverter is a balance of system (BOS) component of a
photovoltaic system and can be used for both, grid-connected and off-grid systems. Solar
inverters have special functions adapted for use with photovoltaic arrays, including
maximum power point tracking and anti-islanding protection.
6. HVDC
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. The inverter must be synchronized
with grid frequency and phase and minimize harmonic generation.
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CHAPTER 7
CONCLUSION
In this project, a new inverter topology has been proposed which has
superior features over conventional topologies in terms of the required power switches
and isolated dc supplies, control requirements, cost, and reliability. It is shown that this
topology can be a good candidate for converters used in power applications such as
FACTS, HVDC, PV systems, UPS, etc. In the mentioned topology, the switching
operation is separated into high- and low-frequency parts. This will add up to the
efficiency of the converter as well as reducing the size and cost of the final prototype. In
this phase of the project the feasibility of the prototype model has been designed using
MATLAB and the desired output level of seven levels for a resistive load of 100 ohms
is obtained.
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CHAPTER 8
REFERENCES
[1] EhsanNajafi, Member, IEEE, and Abdul Halim Mohamed Yatim, Senior Member,
IEEE,“Design and Implementation of a New Multilevel Inverter Topology”, IEEE
TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 59, NO. 11, NOVEMBER
2012.
[2] K. Jang-Hwan, S.-K. Sul, and P. N. Enjeti, “A carrier-based PWMmethod with
optimal switching sequence for a multilevel four-leg voltagesource inverter,” IEEE
TRANS. IND. APPL., VOL. 44, NO. 4, PP. 1239–1248,JUL./AUG. 2008.
[3] X. Yun, Y. Zou, X. Liu, and Y. He, “A novel composite cascade multilevel
converter,” IN PROC. 33RD IEEE IECON, 2007, PP. 1799–1804.
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