Parallel Operation of Generator with Utility Power Supply and Automatic Load Sharing and Load Shedding using PLC

DEPARTMENT OF ELECTRICAL ENGINEERING

Parallel Operation of Generator with Utility Power

Supply and Automatic Load Sharing and Load

Shedding using PLC

A PROJECT REPORT

Session 2012

Submitted by

M. BILAL IRFAN (BEE-FA06-068)

ADNAN MASEEH (BEE-FA08-065)

M. HUSSNAIN RAZA (BEE-FA08-073)

M. FAAZ SHARIF (BEE-SP08-001)

PROJECT SUPERVISOR

PROF. DR. SYED ALI MOHSIN

THE UNIVERSITY OF FAISALABAD, FAISALABAD

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I

DECLARATION

We hereby declare that no portion of the work referred to in this Project Report has been

submitted in support of an application for another degree or qualification to any other

university or other institute of learning. If any act of plagiarism found, we are fully

responsible for every disciplinary action against us depending upon the seriousness of the

proven offence, even the cancellation of our degree by the Disciplinary Committee.

COPYRIGHT STATEMENT

Copyright in text of this report rests with the student authors. Copies (by any

process) either in full, or of extracts, may be made only in accordance with the

instructions given by the authors and lodged in the Library of The University of

Faisalabad. Details may be obtained from the Librarian. This page must form part

of any such copies made. Further copies (by any process) of copies made in

accordance with such instructions may not be made without the permission (in

writing) of the authors.

The ownership of any intellectual property rights which may be described in this

report is vested in the Department of Electrical Engineering, The University of

Faisalabad, subject to any prior agreement to the contrary, and may not be made

available for use by third parties without the written permission of the Department

of Electrical Engineering, The University of Faisalabad, which will prescribe the

terms and conditions of any such agreement.

Further information on the conditions under which disclosures and exploitation

may take place is available in the Library of The University of Faisalabad,

Faisalabad.

II

ACKNOWLEDGEMENTS

All glory to Almighty Allah, the creator of this universe, The Gracious and

compassionate whose bounteous blessings gave us potential thoughts, talented teachers,

helping friends, loving parents, co-operative sisters and brothers and opportunity to make

this humble contribution and all praises to, respect and ‘Darood-O-Salam’ are due to His

Holy Prophet(P.B.U.H) Whose blessings and exaltations flourished my thoughts and

thrived my ambition to have cherished fruit of my modest effort in form of this write-up.

We offer our sincerest words of thanks to our teacher Prof. Dr. Syed Ali Mohsin, from

his inspiring guidance, affectionate supervision and valuable suggestion during the entire

study period. We also like to thank Faisal Fabrics Ltd. and Ahmad Engineering for

helping and supporting us throughout the whole project.

III

ABSTRACT

In this project we use AC-DC and DC-AC converters for parallel operation of generator

with utility power supply and PLC will control automatic load sharing between generator

and utility power supply. This project is about uninterrupted and reliable power supply

and to increase the cost efficiency of power. This project is basically related to large scale

industries which have their own power generation system in addition they have utility

power supply. This project provides the parallel operation of generator with utility power

supply. It also provides user to set how much load should be put on generator. If there is a

fault in generator and it is producing less power than required then the extra load is

automatically shifted on utility power supply, uninterrupted. On the other hand if grid is

overloaded or there is a fault then the extra load is automatically shifted on generator,

uninterrupted. If the load becomes greater than the available supply then PLC will

automatically shed the load according to the set priority to avoid total shutdown of entire

load.

IV

TABLE OF CONTENTS

DECLARATION ..................................................................................................................I

COPYRIGHT STATEMENT ...............................................................................................I

ACKNOWLEDGEMENTS ................................................................................................ II

ABSTRACT ....................................................................................................................... III

TABLE OF CONTENTS ................................................................................................... IV

LIST OF FIGURES ............................................................................................................ V

CHAPTER NO. 1 – INTRODUCTION ............................................................................. 1

1.1 BACKGROUND ........................................................................................................................ 1

1.2 PROBLEMS .............................................................................................................................. 1

1.3 REQUIREMENTS ...................................................................................................................... 1

1.4 OBJECTIVE ............................................................................................................................... 1

1.5 BLOCK DIAGRAM .................................................................................................................... 2

1.6 HARDWARE COMPONENTS .................................................................................................... 3

1.7 SOFTWARES ............................................................................................................................ 3

CHAPTER NO. 2 – AC TO DC CONVERSION .............................................................. 4

2.1 HOW AC TO DC CONVERSION IS DONE? ................................................................................ 4

2.2 HARDWARE COMPONENTS .................................................................................................... 7

CHAPTER NO. 3 – CURRENT LIMITER ........................................................................ 8

3.1 HOW CURRENT IS LIMITED? ................................................................................................... 8

3.2 FLOWCHART ......................................................................................................................... 10

3.3 HOW LOAD IS SHARED BY PLC? ............................................................................................ 10

3.4 LOAD SHEDDING ................................................................................................................... 13

CHAPTER NO. 4 – DC TO AC CONVERSION ............................................................ 14

4.1 12VDC TO 220VDC CONVERSION ......................................................................................... 14

4.2 DESIGN .................................................................................................................................. 15

4.3 220VDC TO 220VAC CONVERSION ....................................................................................... 20

CHAPTER NO. 5 – FUTURE ENHANCEMENTS ........................................................ 26

5.1 LIMITATIONS IN THE EXISTING CIRCUITS ............................................................................. 26

5.2 ENHANCEMENTS CAN BE MADE .......................................................................................... 26

REFERENCES .................................................................................................................. 27

V

LIST OF FIGURES

Figure 1.1 : Block diagram of entire project. ....................................................................... 2

Figure 2.1 : AC to DC Supply Circuit ................................................................................. 6

Figure 3.1 : Current Limiter ................................................................................................. 8

Figure 3.2 : Flowchart of PLC Operation .......................................................................... 10

Figure 3.3 : Current limiter operated with PLC ................................................................. 11

Figure 4.1 : DC To AC Conversion ................................................................................... 14

Figure 4.2 : Basic layout of boost regulator ....................................................................... 15

Figure 4.3 : Current flow through the converter, depending on the state of the switch .... 16

Figure 4.4 : Inductor current and duty cycle vs. time ........................................................ 16

Figure 4.5 :MAX5026 implementation of a boost converter. ............................................ 18

Figure 4.6 : Duty Cycle ...................................................................................................... 19

Figure 4.7 : Simple Inverter ............................................................................................... 21

Figure 4.8 :Equivalent Circuit............................................................................................ 21

Figure 4.9 : S1,S2 ON; S3,S4 OFF .................................................................................... 22

Figure 4.10 : Positive Half Cycle ....................................................................................... 22

Figure 4.11 : S3,S4 ON; S1,S2 OFF .................................................................................. 23

Figure 4.12 : Negative Half Cycle ..................................................................................... 23

Figure 4.13 : Invertor Output ............................................................................................. 24

Figure 4.14 : Pulse Width Modulation ............................................................................... 25

CHAPTER NO.1 - INTRODUCTION

1

CHAPTER NO. 1 – INTRODUCTION

1.1 BACKGROUND

In large scale industries, utility power supply and industry’s own power generation

systems are used in parallel. According to current circumstances industries decide that on

which power supply load should be transferred and in what proportion. For this purpose

industries use load sharing modules. These modules also synchronize both power systems

in order to use both power systems in parallel.

1.2 PROBLEMS

Modules used in industries are very expensive and use very expensive equipment’s in

order to achieve synchronization and load sharing. These systems are very complicated.

Slightest problem occurring in generation system will make the whole system out of sync

and total system shuts down due to overload.

1.3 REQUIREMENTS

Monitoring the total load in amperes and controlling the current from both sources using

current limiting circuits. In order to operate both power sources in parallel we need two

AC to DC converters and a DC to AC converter.

1.4 OBJECTIVE

To develop a parallel operation, load sharing, and load shedding system which has less

complication, more reliability, and robustness in order to avoid total system shutdown.


2

1.5 BLOCK DIAGRAM

Figure 1.1: Block diagram of entire project


3

1.6 HARDWARE COMPONENTS

Following are the hardware components used in project:

1. Siemens SIMATIC S7-300 CPU 316-2 DP

2. Siemens SIMATIC S7-300 Analog Input Module SM 331

3. Siemens SIMATIC S7-300 Digital Input Module SM 321

4. Siemens SIMATIC S7-300 Digital Output Module SM 322

5. AC to DC converters

6. DC to AC converter

7. Power Transistors

8. Switching Transistors

9. DC Relays

10. AC Relays

11. Shunt Resistors

12. Resistors

13. Diodes

14. LED’s

15. Push Buttons

16. Switches

1.7 SOFTWARES

Following software:

1. Siemens SIMATIC Step 7

2. Proteus

CHAPTER NO. 2 – AC TO DC CONVERSION

4


2.1 HOW AC TO DC CONVERSION IS DONE?

All electronic systems and equipment regardless of their size or function have one thing

in common: they all need a power supply unit (PSU) that converts input voltage into a

voltage or voltages suitable for their circuits. The most common type of today's PSU is

the switch mode power supply (SMPS). There is a wide variety of SMPS topologies and

their practical implementations used by PSU manufacturers. However they all use the

same basic concepts. This explains the principals of operation of a switching mode power

supply. [1]

The required DC power supply is usually obtained by means of a transformer. It is also

possible to have transformer less power supplies. Though the elimination of the

transformer makes the circuit compact, economical and simple, also facilitating quick

assembly and built in short circuit protection, certain drawbacks creep in. These power

supplies are useful only for low current applications. Special safety precautions are to be

followed while using them. Physical contact should be strictly avoided, since the output

terminals are not isolated from AC mains supply. [1]

By suitable modification it is possible to obtain multiple/ fractional dual voltages from a

transformer. Different not-so obvious voltage values can also be obtained from the

transformer by rectification circuits. The output so obtained from a transformer secondary

is unregulated. For good load regulation, the internal impedance of any power supply

should be as low as possible. The regulation can be improved either by resistor zener

method or series regulator method. [1]

However, the three-terminal regulators greatly simplify the power regulation problem.

These regulators need no external components. They employ internal current limiting and

thermal shutdown which make them tough. For simplicity, compactness, convenience and

accuracy the use of three terminal regulators is ideal. These IC voltage regulators are

freely available in various ranges both positive and negative. A functional schematic of a

three terminal regulator is shown in the datasheet. It can be seen that the device is a

complete regulator, with built-in reference, error amplifier, and series pass transistor and

protection circuits. The protection circuits include current limiting, safe area protection to


5

limit dissipation in the series pass transistor and thermal shut down to limit temperature.

[1]

Low power IC voltage regulators of the 78L series used in our measuring instrument are

now so cheap that they represent an economic alternative to simple zener-NPN

stabilizers. In addition they offer the advantages of better regulation, current

limiting/short circuit protection at 1000 mA and thermal shunt down in the event of

excessive power dissipation. In fact, virtually the only way in which these regulators can

be damaged is by incorrect polarity or by an excessive input voltage. Regulators in the

78L series up to the 8V type will withstand input voltages up to about 35v, whilst the 24v

type will withstand 40V. Normally, of course, the regulators would not be operated with

such a large input-output differential as this would lead to excess power dissipation. All

the regulators in the 78L series will deliver a maximum current of 1000mA provided the

input-output voltage differential does not exceed 7V. Otherwise excessive power

dissipation will result, causing thermal shutdown. [1]

Two transformers have been used to step down the voltage from 230-250VAC mains

input. One of the transformers produces an output of 6-0-6V at the secondary terminals.

This output is fed to a full wave rectifier and a capacitive filter. The filtered output is fed

to IC6 which is a 3 pin voltage regulator which gives a regulated output of + 5V. This is

used to activate the DPM circuit. It is also fed to the temperature network as a precision

voltage reference source. [1]

The other transformer produces an output of 12-0-12V at its secondary terminals. The

center tap is grounded like in the previous case. The other two terminals of the secondary

are fed to a bridge rectifier constructed using diodes. The rectified output is filtered by

using capacitor C5 and C6 fed to IC7 and IC. The IC7-8 which is 3 pin voltage regulators

gives an output of ±8V. These two voltages are fed to the signal generator. The -8V

source output is fed to the temperature network, also as voltage reference. It is also

necessary to produce a +12V and -12V supply for application to operational amplifiers.

This can be conveniently done by means of 12V zener diodes. The output of the bridge

rectifier is clamped to +12V and -12V respectively using two zener diodes. The zener

output is fed to the operational amplifier supply terminals. Since, the supply to the

operational amplifier needed not be very efficiently regulated to + 12V, the use of zener

diodes proves economical. [1]


6

For the testing of electronic components a voltage of above 50V is required. This can be

achieved by means of a voltage quadrupler circuit. It consists of four diodes and four

electrolytic capacitors. The secondary ungrounded terminal of the 12-0-12V is connected

to the quadrupler circuit. The output of the quadrupler circuit is 68V with respect to

ground. [1]

The two transformers can be controlled by the power supply switch PS 1. The switch also

controls a neon lamp, which lights up once the transformer supply is on. The instrument

is prevented against short circuits-excessive voltages by fuses. When the AC power

supply exceeds beyond 250V results in any overload or damage, the fuse F1 blows out

thus saving the rest of the circuit within the instrument. [1]

Figure 2.1 : AC to DC Supply Circuit


7

2.2 HARDWARE COMPONENTS

1: EMI (Electromagnetic Interference) Filter

2: Fuse 13A

3: Rectifier Bridge

4: Transformer T1, T2

5: Resistor (Rs)

6: Capacitor (C1-C7)

7: Diode (D1-D11)

8: Transistors (Q1-Q5)

9: Inductors (L1-L6)

CHAPTER NO. 3 – CURRENT LIMITER

8


Current is limited so that the load on utility and generator can be shared. In our project we

can do load sharing by current limiter. This section explains the working of current

limiter operated with PLC.

3.1 HOW CURRENT IS LIMITED?

A current limiting circuit is use for this purpose. This circuit provides automatic current

limiting up to 8.4A. Unlike current limiter that uses only a resistor, this current limiting

circuit doesn’t drop the voltage, or at least keep the voltage drop at minimum, until a

certain current amount is exceeded. This current amount limit is adjustable from 1.4A to

8.4A using a potentiometer. You can modify the component value to give different

current limiting range. Here is the circuit’s schematic diagram: [4]

Figure 3.1: Current Limiter


9

The resistor R1 is there to sense the current. At R2 potentiometer at minimum resistance

(the center tap connected to R1), if the current drawn by the load reach 1.2A then the

voltage across R1 reach 0.6V and Q2 begin conducting, thus shorting the base voltage of

Q4 to ground. These shorting actions reduce the base current and therefore reduce the

output voltage sensed by the load, and prevent the current to flow further. If you need the

current limiter to limit at lower threshold range, you can change the R1 to 1R and you’ll

get about 0.7A to 4.2A adjustment range. [4]

Because of the power dissipation capability of 2N3055 transistor, at the

worst case that the load is shorted to ground (zero resistance), if you limit the current to

8.4 A then the circuit can handle maximum source voltage of 14V, while limiting the

current at 4.2A can handle up to 27V source voltage. The maximum voltage can be

handled by this circuit is 60 volt, but at that maximum voltage you can only safely set the

current limit at 1.9A in the extreme condition, when the load is shorted to ground. Please

make sure the Q1 transistor has sufficient heat sink. [4]


10

3.2 FLOWCHART

Figure 3.2: Flowchart of PLC Operation

3.3 HOW LOAD IS SHARED BY PLC?

The circuit shown above is modified for operation with PLC. The modifications are:

Three TIP3055 can be connected in parallel for increasing the power

handling capability.


11

The resistor R2 is excluded and base of Q2 is directly connected to emitter

of TIP3055 and R sense.

The output is connected with diodes as freewheeling diode for protection.

R sense can divided into 10 parts to made taps and can be operate when

the signal from PLC is given the relay is activated and resistance is

increased as more relays can be operated from PLC as shown in circuit

below:

Figure 3.3: Current limiter operated with PLC


12

3.3.2 Working of current limiter circuit with PLC

The basic working of current limiter is discussed above. The operation with PLC is

discussed here.

As the signal is given on the base of the transistor Q8-Q11from PLC then

the relay operates and it will add the resistance in parallel to increase the

current limit.

We have 10 relays and one relay can be operated for limit half ampere

approximately. If 5 relays are operated it means the circuit can limit up to

2.5 amperes.

10W 0.1 Ohm shunt resistance is used as current sensor for PLC. Sensor

value is given to the analog input of PLC. On the bases of current sensor’s

value PLC operates the relays to share load between two sources.

There are two current limiters used for the parallel operation of utility and

generator.

Priority based current limiting can be done. We can either use generator or

utility as our priority.

If load exceed the limit of priority source then the rest of the load is

transferred on the second priority source.


13

3.3.3 Hardware Used

12VDC Operated Relays

TIP3055 Power Transistor

BD139 Transistor

BC546 Transistor

BC639 Transistor

2W 0.5 Ohm Resistors

4.7K Resistors

10W 0.1 Ohm Resistors

3.4 LOAD SHEDDING

In our project we have set priority based load shedding. We have set the priority of all

departments with respect to the load they are using. When overall load is too much high

and is above from available supply then departments of less priority are shed to avoid

total shutdown and our high priority departments can work uninterrupted.

CHAPTER NO. 4 – DC TO AC CONVERSION

14


Converting DC to AC power by switching the DC input voltage (or current) in a pre-

determined sequence so as to generate AC voltage (or current) output.

The DC to AC can be converted into two steps:

12VDC to 220VDC conversion(CHOPPER)

220VDC to 220AC conversion(INVERTER)

Figure 4.1: DC To AC Conversion

4.1 12VDC TO 220VDC CONVERSION

DC to DC converters offer a method of generating multiple controlled voltages from a

single battery voltage, thereby saving space instead of using multiple batteries to supply

different parts of the device.

A boost converter is simply is a particular type of power converter with an output DC

voltage greater than the input DC voltage. This type of circuit is used to ‘step-up’ a

source voltage to a higher, regulated voltage, allowing one power supply to provide

different driving voltages.

A basic design will be discussed along with a specific application of an integrated circuit

(IC) solution.


15

4.2 DESIGN

A boost converter is part of a subset of DC-DC converters called switch-mode converters.

They generally perform the conversion by applying a DC voltage across an inductor or

transformer for a period of time (usually in the 100 kHz to 5 MHz range) which causes

current to flow through it and store energy magnetically, then switching this voltage off

and causing the stored energy to be transferred to the voltage output in a controlled

manner. The output voltage is regulated by adjusting the ratio of on/off time. As this

subset does not use resistive components to dissipate extra power, the efficiencies are

seen in the 80-95% range. This is clearly desirable, as it increases the running time of

battery-operated devices. [5]

The basic boost converter circuit consists of only a switch (typically a transistor), a diode,

an inductor, and a capacitor. The specific connections are shown in Figure.

Figure 4.2: Basic layout of boost regulator


16

4.2.1 Analysis

Examining the circuit for two cases (switch open and switch closed) is fairly

straightforward, assuming ideal components, and provided that there is constant current

flow through the inductor. This case is referred to as ‘continuous mode operation. [5]

Figure 4.3: Current flow through the converter, depending on the state of the switch

Applying Kirchhoff’s rules around the loops and rearranging terms yields an intuitive

Result:

That is to say, the gain from the boost converter is directly proportional to the duty cycle

(K), or the time the switch is ‘on’ each cycle. Figure graphically demonstrates this. [5]

Figure 4.4: Inductor current and duty cycle vs. time


17

In some cases, the amount of energy required by the load is small enough to be

transferred in a time smaller than the cycle length. In this case, the current through the

inductor falls to zero during part of the period. This is called ‘discontinuous operation.

The only difference, then, is that the inductor is completely discharged at the end of the

cycle. Although slight, the difference has a strong effect on the output voltage equation.

Compared to the expression of the output voltage for the continuous mode, this

expression is much more complicated. Furthermore, in discontinuous operation, the

output voltage not only depends on the duty cycle, but also on the inductor value, the

input voltage, and the output current. [5]

4.2.2 IC Implementation

In order to implement the switching necessary for the converter to work, it is desirable to

find an IC solution. The 5026 chip, from MAXIM, is one such solution. The typical

circuit from the MAX5026 data sheet is shown in Figure 4. In this circuit, the output

voltage, VOUT, is determined by the ratio of fixed resistors R1 and R2. These two

resistors form a voltage divider that feeds a fraction of the output voltage back to the

feedback (FB) pin, creating a closed-loop system. The system is at equilibrium when

VOUT is generating the desired output voltage and the R1 and R2 voltage divider feeds

back 1.25Vto the FB pin. When VOUT is lower than the desired output voltage (the

voltage fed back to FB is below 1.25V), the DC-DC converter IC attempts to deliver

additional power until FB reaches 1.25V. [6]

(

)

( )

Equation 1 is directly from the MAX5026 data sheet. Solving Equation 1 for VOUT

yields Equation 2 where VREF, the FB Set Point, is 1.25V for the MAX5026.


18

Figure 4.5:MAX5026 implementation of a boost converter.

4.2.3 Results

The output voltage obtained during this study was not a full 220V. The actual output was

approximately 218V. The discrepancy is most likely due to losses in the board, as well as

to non-ideal devices (most notably the inductor). [6]

In the analysis above, all components were assumed ideal. It was assumed that the power

is transmitted without losses from the input voltage source to the load. However,

parasitic resistances exist in all circuits, due to the resistivity of the materials they are

made from. Therefore, a fraction of the power managed by the converter is dissipated by

these parasitic resistances. This is why the efficiencies are not at a perfect 100%.For the

sake of simplicity, the inductor is assumed the only non-ideal component, and that it is

equivalent to an inductor and a resistor in series. This is reasonable because an inductor is

made of one long wound piece of wire, so it is likely to exhibit a non-negligible parasitic

resistance. Furthermore, current flows through the inductor both in the on and the off

states, so any non-ideal effects will be more pronounced. Reworking the earlier equations

with the added inductor resistance (RL) changes the gain equation to the following: [6]

( )

Even without the full derivation, the equation makes intuitive sense. If the inductor

resistance is zero (an ideal inductor), the equation above becomes equal to the ideal case;

however, as RL increases, the voltage gain of the converter decreases compared to the

ideal case. Also, the effect of RL increases with the duty cycle, K.


19

Figure displays these effects graphically. As the inductor becomes less ideal, the possible

gain drops off sharply from the theoretical value, especially as the duty cycle grows

above 50%. [6]

Figure 4.6: Duty Cycle

4.2.4 Conclusions

DC-DC converters are an excellent way to get the most use out of a single power supply.

Though the total power must remain constant, one can efficiently tradeoff between

current strength and voltage levels to power a variety of sub-circuits without costly extra

batteries. [6]


20

4.3 220VDC TO 220VAC CONVERSION

A power inverter, or inverter, is an electrical device that changes direct current (DC) to

alternating current (AC); the converted AC can be at any required voltage and frequency

with the use of appropriate transformers, switching, and control circuits.

The conversion of a high DC source to an AC waveform using pulse-width modulation.

Power inverters are devices which can convert electrical energy of DC form into that of

AC. They come in all shapes and sizes, from low power functions such as powering a car

radio to that of backing up a building in case of power outage. Inverters can come in

many different varieties, differing in price, power, efficiency and purpose. The purpose

of a DC/AC power inverter is typically to take DC power supplied by battery, and

transform it into a 220 volt AC power source operating at 50 Hz, emulating the power

available at an ordinary household electrical out let. [7]

4.3.1 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.

4.3.2 Basic designs

A normal ac inverter has three parts:

1. An input converter to rectify ac power to dc power. It is normally called the source

bridge.

2. An energy storage device which separates the input from the output and allows each to

operate independently from the other. It is usually called a link filter.

3. A dc-to-ac inverter in the output stage. It is called an inverter. It generates the desired

ac output voltage and frequency. [12]


21

4.3.3 Operation of simple square-wave inverter

To illustrate the concept of AC waveform generation:

Figure 4.7: Simple Inverter

This can be shown in the equivalent circuit form as

Figure 4.8:Equivalent Circuit

This circuit has two modes of operation:

When S1 and S2 is ON AND S3 and S4 is OFF

When S3 And S4 is ON AND S1 and S2 is OFF


22


Figure 4.9: S1,S2 ON; S3,S4 OFF

The output voltage waveform is

Figure 4.10: Positive Half Cycle


23


Figure 4.11: S3,S4 ON; S1,S2 OFF

The output voltage waveform is

For t2<t<t3

Figure 4.12: Negative Half Cycle


24

The Combine output voltage wave form is

Figure 4.13: Invertor Output

The output voltage wave form of full bridge is shown above with the peak value of 200V.

[12]

4.3.4 Pulse-Width-Modulated Inverters

Pulse-Width-Modulated Inverters (PWM) is referred to as time ratio control. From a

constant DC input voltage, we get a variable output voltage and frequency by varying the

percentage of time that the power control switch is closed. The output voltage will

increase by increasing the percentage of time the switch is closed. The switch is either

open or closed. PWM is used extensively as a means of powering alternating current

(AC) devices with an available direct current (DC) source or for advanced DC/AC

conversion. Variation of duty cycle in the PWM is gnarl to provide a DC voltage across

the load in a specific pattern will appear to the load as an AC signal, or can control the

speed of motors that would otherwise run only at full speed or off. This is further

explained in this section. The pattern at which the duty cycle of a PWM signal varies can

be created through simple analog components, a digital microcontroller, or specific PWM

integrated circuits. Analog PWM control requires the generation of both reference and

carrier signals that feed in to a comparator which creates output signals based on the

difference between the signals. The reference signal is sinusoidal and at the frequency of

the desired output signal, while the carrier signal is often either a saw tooth or triangular

wave at a frequency significantly greater than the reference. When the carrier signal

exceeds the reference, the comparator output signal is at one state, and when the reference


25

is at higher voltage, the output is at its second state. This process is shown in with the

triangular carrier wave in red, sinusoidal reference wave in blue and modulated and UN

modulated sine pulse. [12]

Figure 4.14: Pulse Width Modulation

CHAPTER NO. 5 – FUTURE ENHANCEMENTS

26

CHAPTER NO. 5 – FUTURE ENHANCEMENTS

5.1 LIMITATIONS IN THE EXISTING CIRCUITS

In the existing circuits there are some power losses. Transistors used can work only on

low voltage (60VDC maximum) which means more amperes and more power dissipation

in the circuit. These circuits are not capable handling high power.

5.2 ENHANCEMENTS CAN BE MADE

We can replace bipolar transistors with MOSFETs in current limiter circuit which can

operate at higher voltages and they have high switching speed which can make the circuit

more efficient in limiting the current. We can use 400VDC rated MOSFETS and desired

power rating can be choose according to power requirements.

The AC-DC and DC-AC converters use voltage step down and step up circuits which can

be removed in order to make project more efficient and less complicated. Without

stepping down the voltages in AC-DC converter will give us 311VDC approximately.

These voltages will be given to current limiter circuit which will be now being using

MOSFETs. After passing the current limiter circuit voltages are fed to DC-AC converter

and without the need of any stepping up the voltages the converter will make 220VAC at

output.

These enhancements will reduce the power losses in the project and project will be more

cost effective. The project will become less complicated and more efficient and capable

of handling high power applications.

27

REFERENCES

[1] http://www.smpspowersupply.com/power-supply.html

[2] http://electroschematics.blogspot.com/2011/05/12vdc-to-220v-ac-500w-

inverter-circuit.html

[3] http://www.electronics-circuits.com/tech/2006/10/multi-output-instrument-

power-supply/

[4] http://freecircuitdiagram.com/2008/08/27/variable-adjustable-current-

limiter-circuit/

[5] http://en.wikipedia.org/wiki/Boost_converter

[6] http://www.ortodoxism.ro/datasheets/maxim/MAX5025-MAX5028.pdf

[7] http://en.wikipedia.org/wiki/Power_inverterhttp://www.powerdesigners.com/

InfoWeb/design_center/articles/DC-DC/converter.shtm

[8] http://www.interq.or.jp/japan/se-inoue/e_ckt28.htm

[9] http://www.elexp.com/t_dc-dc.htm

[10] http://www.jaycar.com.au/images_uploaded/dcdcconv.pdf

[11] http://www.futurlec.com/News/National/DC_Converter.shtml

[12] http://encon.fke.utm.my/notes/inverter-2002.pdf

[13] ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-334-

power-electronics-spring-2007/lecture-notes/ch9.pdf

[14] Modified Sine-Wave Inverter Enhanced Page of". Powerelectronics.com.

Retrieved 2011-01-10.

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