• Author / Uploaded
• Bijay Poudel

Citation preview

Acknowledgement This project has helped us to get acquainted with the cosmic world of Electrical Engineering. Alongside the sensation of emergent quest, there were incessant grimaces, screw ups, mental loops, success delights and fantasies in the midst of project undertaking. The moment was the one to treasure. The joy of having the project done and the feeling of “Yes! We did it” valued our hard work. We take this opportunity to express our profound gratitude and deep regards to our Department of Electrical Engineering for providing us with this project. We are privileged to have Mr. Mahummad Badrudoza and Mr. Hari Prasad Rimal for their exemplary guidance, monitoring and constant encouragement throughout the course of this project. We also take this opportunity to express a deep sense of gratitude to Prof. Dr. Indraman Tamrakar for his cordial support, valuable information and guidance, which helped us in completing this task through various stages. We are obliged to Anil, Shiva, Surendra, and Saket for the valuable information provided by them in their respective fields. We are grateful for their cooperation during the period of our project. Lastly, we thank all who supported us directly or indirectly during the project without which this project would have been impossible.

Anil Panjiyar Bijay Paudel Rabin Shrestha Rohit Agrawal

ABSTRACT This project “Design and Fabrication of PWM rectifier to Improve Input Power Factor” deals with the development of a controlled rectifier for varied applications, the input p.f. of which is maintained unity. Moreover, one of the important advantages of using PWM technique is the low value of small order harmonics and large high order harmonics that can be filtered easily. In our project, we have implemented a digital controller using PIC microcontroller which is used to generate synchronized control signals, the signals are amplified and are conditioned as per the gate drive requirements of GTO thyristor. The project circuitry includes the control circuit (AVR Atmega16), the gate drive unit and GTO rectifier. The simulations regarding the PIC microcontroller and gate signal generation were done in ‘Proteus’ whereas the response of the system was studied on ‘MATLAB via. Simulink based simulation’. As an additional feature we have provided the system with input switching through which we can operate the rectifier in SPWM and Symmetric mode as well. A comparative study of the system was done by comparing three methods.

LIST OF FIGURES

LIST OF ABBREVIATIONS

TABLE OF CONTENTS

CHAPTER 1 INTRODUCTION 1.1 Overview of the Project AC power source is dominant over DC source in industrial as well as other applications due to it’s flexibility in generation, transmission and distribution. However, DC machines are suited for some applications than AC machines. In the context, rectifiers are used to convert available AC power to DC power supply. Diodes can be used to make rectifiers where we need fixed output but to make controlled rectifiers thyristors or GTO are used. Phase controlled rectifiers are used in many applications like Motor Control, Switched Mode Power Supplies etc. In any electrical system p.f. is a critical issue, similar is the case in PCR. At present time where effective and efficient sysem design is mandatory, the choice of correct alternative is essential. Keeping in mind the various advantages of PWM technique to improve input p.f., we are embarking to develop a rectification system employing this technique. 1.2 Literature Review We have studied several books and papers regarding the use of rectifiers, power factor improvement, concept of project and components to be used. 1. Muhammad M. Rashid, ‘Power electronics Handbook’ We studied about the rectifiers and phase controlled rectifiers. The study helped to realize the necessity of the project. 2. P.S. Bhimbra, ‘Power electronics’ This book is written in simple language and it facilitated system analysis. Waveform analysis, Fourier analysis, Mathematical calculation was done with the help this book. 3. Sedra and Smith, ‘Microelectronic circuit’ Electronics circuits that will be implemented were studied from the book. The studies cover the use of transistor switches, operational amplifiers, and waveform generations. 4. GTO datasheet for ‘Philips BTW58-1500R’ GTO is the device we will be using so the detail information of the device is compulsory. This helps in the design of gate drive circuit of GTO. The features of the GTO are:  Fast Turn-Off With Reverse Gate Pulse  High Voltage= 1000 and 1200 volts  Momentarily Forward Pulse For Turn On  Minimizes Drive Losses  High Surge Capability 5. AVR Atmega handbook/ Datasheet We are using the AVR in order to generate the synchronized control pulses so the skills relating to use of controller was required. Hence, we studied the books related to microcontroller and its programming.

1.3 Objectives The primary objective of the project is to ‘Design and fabricate PWM rectifier to improve input power factor’.

1.4 Methodology The method adopted for the advancement of project activities can be explained under following headings: Study of Related Theory First of all we studied the theories related to the project. We consulted different books to know about rectifiers, controlled rectifiers, p.f. improvement requirement and PWM control method to improve input power factor. From his study we confirm system circuitry and its analysis.  Matlab and Proteus Simulation  Modeling of different components in Simulink.  Learn and practice different Simulink models and be familiar with tools and techniques required.  Design Simulink model for PWM rectifier.  Electronic circuit design for control signal generation.  Programming AVR Atmega16.  Testing in Proteus. 



Fabrication of the proposed system The system incorporating the control circuit was designed and fabricated in PCB. Modular design was preferred. The control circuit was interfaced with the rectifier unit to get the desired outputs. Testing in lab The system outputs for the control circuits and rectifier were viewed by the help of oscilloscope.

The detailed information will be provided on the following chapters.

CHAPTER 2 THEORITICAL STUDY

2.1 Theoretical Study Rectifiers and Controlled Rectifiers A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The process is known as rectification. Rectifiers have many uses, but are often found serving as components of DC power supplies and highvoltage direct current power transmission systems. In a controlled rectifier or a phase-controlled rectifier, the diodes in the rectifier circuit are replaced by SCRs. These circuits are produced a variable DC output voltage whose magnitude is varied by phase control, that is, by controlling the duration of the conduction period by varying the point at which a gate signal is applied to the SCR. Unlike a diode, an SCR will not automatically conduct when the anode to cathode voltage becomes positive-a gate pulse must be provided. If we adjust the delay time of the gate pulse, and if this process is done repeatedly, then the rectifier output can be controlled. This process is called phase control.

Power factor as an important issue Lets us take a look at single phase semi-convertor shown in the figure below:

Fig: half controlled single phase bridge rectifier

(a) waveforms of input voltage and input current

From the above waveform we see that current lags voltage by some phase angle(α), So that there is lagging power factor. The PF of phase controlled converters depends on delay angle, and is general low, especially at the low output voltage range. Following methods can be implemented to improve input power factor:1) 2) 3) 4)

Extinction angle control Symmetric angle control PWM control Sinusoidal PWM control

PWM Control Method In PWM control, the converter switches are turned on and off several times during a half-cycle and the output voltage is controlled by varying the width of pulses. The gate signals are generated by programming microcontroller. The lower order harmonics can be eliminated or reduced by selecting the number of pulses per half-cycle. However, increasing the number of pulses would also increase the magnitude of higher order harmonics, which could easily be filtered out.

Fig: PWM rectifier

Fig: Output waveform of PWM rectifier

One other advantage of PWM rectifier is low value of lower order harmonics and high value of higher order harmonics. It is advantageous as the high order harmonics can be filtered out easily.

GTO Gate turn-off (GTO) thyristors are able to not only turn on the main current but also turn it off, provided with a gate drive circuit. Unlike conventional thyristors, they have no commutation circuit, downsizing application systems while improving efficiency. They are the most suitable for high-current, high speed switching applications, such as inverters and chopper circuits.

Fig: GTO

GTO OPERATION GTO can be turned on by applying a positive gate signal and can be turned off by applying a negative signal in its gate. The waveforms can be shown as:

SIMULATION Matlab Simulation After learning general Matlab tools, we model our system in Matlab as shown in following diagram:

Fig: Simulink Based Model for PWM rectifier The output observed after running the simulation is also shown below:

Fig: Output waveform for PWM rectifier

This Matlab simulation works fine, however there’s a drawback. The GTO used in the simulation process doesn’t require negative pulses to turn off, which is not practical. So, for actual implementation, the gate drive signal must compose of positive pulse as well as negative pulse. This pulse can be generated by using a suitable signal conditioning circuit, its matlab model and simulation results shown below:

Fig: GTO gate drive circuit The output waveforms were observed as:

Fig: Output waveform of GTO gate drive circuit

RC1 and RC2 are signals generated from two pins of microcontrollers. One thing noticed in above diagram is the magnitude of negative current and voltage being greater than that of positive current and voltage. This has been made due to the reason that GTO require almost twice more current to turn off rather than to turn on (as per datasheet). Capacitor has been used for the purpose. In absence of negative another circuitry (H-bridge) might be implemented, however former one is preferred due to its simplicity and economic considerations.

PROTEUS SIMULATION In proteus, we have interfaced PIC microcontroller with zero level detector and input interface (button/Keypad). Zero crossing detectors comprises of the optocouplers and transformer that performs dual function of isolation and zero crossing detection. Buttons or Keypads are used so that the conduction angle of GTO can be varied to get variable voltage output. The circuit diagram used in proteus is: