Overview Of Thyristor Commutation Techniques In Power Electronics

ABSTRACT

A classification of thyristor commutation methods is presented. The classification is based on whether the commutation pulse is a voltage pulse or current pulse, whether it is connected in parallel or in series with the thyristor to be commutated, whether the pulse is applied with the help of a main or an auxiliary thyristor, and whether the commutation circuit is charged from the main source or from an auxiliary source. Inverters are further classified according to the location of the commutation pulses.

TABLE OF CONTENTS

COVER PAGE

TITLE PAGE

APPROVAL PAGE

DEDICATION

ACKNOWLEDGEMENT

ABSTRACT

CHAPTER ONE

INTRODUCTION

1.1      BACKGROUND OF THE PROJECT

• AIM OF THE PROJECT
• OBJECTIVE OF THE PROJECT
• SCOPE OF THE PROJECT
• SIGNIFICANCE OF THE PROJECT
• APPLICATION OF THE PROJECT

CHAPTER TWO

LITERATURE REVIEW

• OVERVIEW OF POWER ELECTRONICS
• HISTORICAL BACKGROUND OF POWER ELECTRONICS
• OVERVIEW OF THE STUDY
• OVERVIEW OF THE STUDY
• DESCRIPTION OF THYRISTOR
• THYRISTOR LAYERS

CHAPTER THREE

• METHODOLOGY

CHAPTER FOUR

4.1      DYNAMIC TURN OFF SWITCHING CHARACTERISTICS

4.2.     HOW TO TEST AN SCR WITH AN OHMMETER

CHAPTER FIVE

• CONCLUSION
• REFERENCES

CHAPTER ONE

1.0                                                        INTRODUCTION

Power Electronics is the technology associated with efficient conversion, control and conditioning of electric power from its available input into the desired electrical output form. The field of the book is concerned of electrical power processing using electronic devices the key component of which is a switching power converter.

Power electronics has found an important place in modern technology being a core of power and energy control. Almost all the new electrical and electromechanical equipment contain power circuits. The power levels encountered in this field range from less than one watt in supplies for the battery- operated portable equipment to tens, hundreds, or thousands of watts in power units of office equipment, kilowatts to megawatts in variable-speed motor drives, approaching megawatts in the rectifiers and inverters that interface the local transmission lines with the high power systems.

Most of the converter equipment and switch-mode power supplies use power electronics components like thyristors, MOSFET and other power semiconductor devices for high frequency switching operations at high-power ratings. Consider the thyristors that we use very frequently as bistable switches in several applications. These thyristors use switches needed to be switched on and off. For switching on the thyristors, there are some thyristor turn on methods called as thyristor triggering methods. Similarly, for switching off thyristors, there are methods called as thyristors commutation techniques. Before discussing about thyristor commutation techniques, we must know something about the thyristors basics such as thyristor, thyristor operation, different types of thyristors and thyristor turn on methods.

Thyristor can be turned ON by applying appropriate positive gate voltage between the gate and cathode terminals, but it cannot be turned OFF through the gate terminal. The SCR can be brought back to the forward blocking state from the forward conduction state by reducing the anode or forward current below the holding current level.

The turn OFF process of an SCR is called commutation. The term commutation means the transfer of currents from one path to another. So the commutation circuit does this job by reducing the forward current to zero so as to turn OFF the SCR or Thyristor.

To turn OFF the conducting SCR the below conditions must be satisfied.

•  The anode or forward current of SCR must be reduced to zero or below the level of holding current and then,
• A sufficient reverse voltage must be applied across the SCR to regain its forward blocking state.

When the SCR is turned OFF by reducing forward current to zero. There exist excess charge carriers in different layers. To regain the forward blocking state of an SCR, these excess carriers must be recombined. Therefore, this recombination process is accelerated by applying a reverse voltage across the SCR.

1.1                                                   AIM OF THE PROJECT

The main aim of this work is to discussed about thyristor commutation methods or thyristor commutation techniques which are used to turn off.

1.2                                             OBJECTIVE OF THE PROJECT

At the end of this work, the student involved shall be able to know the different thyristor commutation methods and their applications.

1.3                                                 SCOPE OF THE PROJECT

Thyristor can be turned on by triggering gate terminal with low voltage short duration pulse. But after turning on, it will conduct continuous until the thyristor is reverse biased or the load current falls to zero. This continuous conduction of thyristors causes problems in some applications. The process used for turning off a thyristor is called as commutation. By the commutation process, the thyristor operating mode is changed from forward conducting mode to forward blocking mode. So, the thyristor commutation methods or thyristor commutation techniques are used to turn off.

1.4                                          APPLICATION OF THE PROJECT

A thyristor, or silicon controlled rectifier (SCR), is a solid-state component which is used in order to switch and control electric current flow. Due to the robustness of thyristors, they are often used in high current flow applications.

Used in Domestic light dimmers, small motor controls, electric fan speed controls, controlling of small domestic AC power appliances.

1.5                                         SIGNIFICANCE OF THE PROJECT

The goal of this work is to introduce students and working engineers involved in power and energy conversion to the basics of this discipline. The emphasis is on the day-to-day electronic products. The course covers the features and functions of rectifiers, inverters, ac and dc converters. It provides a wealth of technical information and contributes to learning the design of circuits and equipment based on electronic components.