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

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

YEAR 2012

THE UNIVERSITY OF FAISALABAD, FAISALABAD

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.

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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.

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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.

III

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

IV

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

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CHAPTER NO.1 - INTRODUCTION

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.

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1.5 BLOCK DIAGRAM

Figure 1.1: Block diagram of entire project 2


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

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CHAPTER NO. 2 – AC TO DC CONVERSION

CHAPTER NO. 2 – AC TO DC CONVERSION 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

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CHAPTER NO. 2 – AC TO DC CONVERSION 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] 5

CHAPTER NO. 2 – AC TO DC CONVERSION 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

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CHAPTER NO. 2 – AC TO DC CONVERSION

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)

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CHAPTER NO. 3 – CURRENT LIMITER

CHAPTER NO. 3 – CURRENT LIMITER 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

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CHAPTER NO. 3 – CURRENT LIMITER 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]

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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.

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CHAPTER NO. 3 – CURRENT LIMITER 

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

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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.

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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.

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CHAPTER NO. 4 – DC TO AC CONVERSION

CHAPTER NO. 4 – DC TO AC CONVERSION Converting DC to AC power by switching the DC input voltage (or current) in a predetermined 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.

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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

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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

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CHAPTER NO. 4 – DC TO AC CONVERSION 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.

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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.

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CHAPTER NO. 4 – DC TO AC CONVERSION 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]

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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]

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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

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

Figure 4.9: S1,S2 ON; S3,S4 OFF The output voltage waveform is

Figure 4.10: Positive Half Cycle

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CHAPTER NO. 4 – DC TO AC CONVERSION When S3 and S4 is ON AND S1 and S2 is OFF

Figure 4.11: S3,S4 ON; S1,S2 OFF The output voltage waveform is For t2
Figure 4.12: Negative Half Cycle

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CHAPTER NO. 4 – DC TO AC CONVERSION 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 24

CHAPTER NO. 4 – DC TO AC CONVERSION 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

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CHAPTER NO. 5 – FUTURE ENHANCEMENTS

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

26

applications.

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

http://www.smpspowersupply.com/power-supply.html http://electroschematics.blogspot.com/2011/05/12vdc-to-220v-ac-500winverter-circuit.html http://www.electronics-circuits.com/tech/2006/10/multi-output-instrumentpower-supply/ http://freecircuitdiagram.com/2008/08/27/variable-adjustable-currentlimiter-circuit/ http://en.wikipedia.org/wiki/Boost_converter http://www.ortodoxism.ro/datasheets/maxim/MAX5025-MAX5028.pdf http://en.wikipedia.org/wiki/Power_inverterhttp://www.powerdesigners.com/ InfoWeb/design_center/articles/DC-DC/converter.shtm http://www.interq.or.jp/japan/se-inoue/e_ckt28.htm http://www.elexp.com/t_dc-dc.htm http://www.jaycar.com.au/images_uploaded/dcdcconv.pdf http://www.futurlec.com/News/National/DC_Converter.shtml http://encon.fke.utm.my/notes/inverter-2002.pdf ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-334power-electronics-spring-2007/lecture-notes/ch9.pdf Modified Sine-Wave Inverter Enhanced Page of". Powerelectronics.com. Retrieved 2011-01-10.

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

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