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DESIGN AND IMPLEMENTATION OF AC TO DC CONVERTER

Abstract

Several research activities at KTH are carried out related to Isolated AC/DC converters in order to improve the design and efficiency. Concerning the improvement in the mentioned constraints, losses of the elements in the prototype converter are modeled in this thesis work. The obtained loss model is capable of calculating the losses under different circumstances. The individual contribution of losses for each element at different conditions can be obtained, which is further useful in improving the design and therefore, efficiency. The losses in different elements of the converter, including power semiconductor devices, RC-snubbers, transformer and filter inductor at different operating points can be computed by using the obtained model.The loss model is then validated by comparing the analytical results with the measurements.The results based on developed loss model show consistency with the measured losses. The comparison at different conditions shows that, the difference between measured and analytical results ranges between10% to 20 %. The difference is due to those losses which are disregarded because of their negligible contribution. On the other hand, it is also observed that if the neglected losses are counted, the difference reduces up to 10%.

Chapter 1 – Introduction

1.1            Introduction

The thesis deals with the power loss modelling of “Two Stage Isolated AC/DC Converter” which is suitable for sustainable energy sources. This chapter provides a base for all material presented in the next chapters, which focuses on the loss modelling of different elements in the prototype converter.

AC to DC Converters are one of the most important elements in power electronics. This is because there are a lot of real-life applications that are based on these conversions. The electrical circuits that transform alternating current (AC) input into direct current (DC) output are known as AC-DC converters. They are used in power electronic applications where the power input a 50 Hz or 60 Hz sine-wave AC voltage that requires power conversion for a DC output.

The process of conversion of AC current to dc current is known as rectification. The rectifier converts the AC supply into the DC supply at the load end connection. Similarly, transformers are normally used to adjust the AC source to reduce the voltage level to have a better operation range for DC supply.

1.2            Background and Justification

The thesis is related to the power loss modelling of the two stage isolated AC/DC converter. The power loss modelling of the power electronic converters is of vital importance because of its relation with efficiency, reliability, cost and size.

The prototype converter comprises a Voltage Source Converter (VSC) and a converter coupled by a medium frequency transformer, which are studied in this thesis. The prototype can be operated by applying the soft-switched commutation across all the valves at all the operating points leading to lower switching losses of the power semiconductor devices. The valves of the VSC in the prototype are equipped with lossless snubbers that reduce the

switching losses across the IGBTs and the diodes. The converter in the prototype is equipped with RC-snubbers to reduce the stress on the valves during the commutation and other transients. Furthermore the medium frequency transformer has also lower losses because of its compact size. The concepts applied to the prototype are useful in loss reduction and further can result in higher efficiency and ultimately power density which are the main goals in the design of power electronic converters.

Despite the fact that various loss minimization techniques are employed, the prototype suffers lower efficiency at lower output power. There can be a number of reasons. In order to resolve the issue, power loss modelling of the prototype is required. After obtaining the proper loss model, the losses across the individual elements in the prototype can be obtained, which further helps in improving the design and efficiency.

Alternating current (AC) Alternating Current In alternating current, the current changes direction and flows forward and backward. The current whose direction changes periodically is called an alternating current (AC). It has non-zero frequency. It is produced by AC generator.

Direct Current (DC) Direct current, the current doesn’t change its magnitude and polarity. If the current always flows in the same direction in a conductor then it is called direct current.

1.3    Aim and Objectives of Project/Research

1.3.1     Aim

1. The aim of the project is to implement a AC to DC inverter system.

1.3.2     Objectives

1. To design a ac to dc inverter system.

2. To construct a device that can efficiently change ac voltage to dc voltage level to another within power network.

3. To develop a step down inverter with single input voltage.

4. To be able to describe the losses in all the element that in converted..

1.4            Motivation

AC signals can’t be stored and DC power can’t be stored so we need to convert AC to DC. To convert the electrical energy into DC, we need to store it. AC can be transported over long distances because of it’s frequencies.

1.5     Contribution to Knowledge

Electricity flows in two ways: either in an alternating current (AC) or in a direct current (DC). Electricity or “current” is nothing but the movement of electrons through a conductor, like a wire. The difference between AC and DC lies in the direction in which the electrons flow.

1.6Scope and Limitation of project

Scope

project will make use of technology called Beat Frequency Oscillator (B.F.O). this technology employs two oscillators at resonance and a variation of the frequency.

Limitation

(i) Alternating voltages cannot be used for certain applications e.g. charging of batteries, electroplating, electric traction etc.

(ii) At high voltages, it is more dangerous to work with AC than DC.

You are required to state categorically the coverage and limit of the project.

You are required to state categorically the coverage and limit of the project.                 

1.7Structure of the Report

• Chapter 2 will look at literature and operation of the converter considering the losses are explained in this chapter.
• Chapter 3 This involve adopting engineering methodology to calculate the losses are explained in this chapter. Apart from this the mathematical description and the formulation of the losses is also presented.   
• Chapter 4 This discuss test result of the losses in each element of the converter. The effects of variations of different parameters on the losses are presented.
• Chapter 5 The conclusion, project appraisal, and recommendation.