ABSTRACT
This research studied a circuit topology of a solar based Single Ended Primary Inductance
Converter (SEPIC) system and a battery furnishing a permanent magnet dc motor with current
via a dc grid. The complete model equations of the entire system were derived. Harmonic
balance technique was used to determine the converter steady state and ripple quantities.
Analyses investigating the relationship of the converter state variables as a function of its duty
ratio were developed. Results show that operating the converter between 0 – 0.4 duty ratio give
best system performances. On the other hand, results show that operating the converter above
this range limits the converter optimal performance. The consequence of operating the system
with duty ratios above 0.4 is that the ripples in the voltage and current waveforms increase. More
so, results also show that for a steady state current of 12 A, for the permanent magnet dc motor
in a period of low solar insolation, operating a Bi-directional converter at specific values of duty
ratio causes the battery to discharge much faster compare to operating at duty ratio of 0.78. Two
solar insolation data for Zaria (Lat 11.0670N Long 7.70E) was obtained from National
Aeronautics and Space Administration (NASA) for a period of January to December 2013 and
another ground level captured from Electrical & Computer Engineering Department using the
data acquisition module (model TQ 140876-002) from August 17 to September 24, 2014. These
insolation data from both stations were used to validate the efficiency of the proposed design.
The targeted load demand of approximately 2441.43W was achieved. This represents 97.66 %
of the load demand designed for in this research. The designed solar energy system contributed
70% of the power to the load, while the battery supplied 30% of the load demand power. An
effective hybrid storage system based on solar and battery was achieved, which met the designed
objective of this research.
TABLE OF CONTENTS
Declaration………………………………………………………………………………………….i
Certification……………………………………………………………………………………….ii
Dedication…………………………………………………………………………………………iii
Acknowledgement.……………………………………………………………………………….iv
Table of Contents………………………………………………………………………………….vi
List of Tables.………………………………………………………………………………….…ix
List of Figures………………………………………………………………………………….…..x
List of Appendices……………………………………………………………………………….xiii
Abbreviations and Symbols……………………………………………………………………..xiv
Abstract………………………………………………………………………………………….xvi
CHAPTER ONE: INTRODUCTION…………………………………………………………..1
1.1 Overview………………………………………………………………………………………1
1.2 Statement of Problem. ………………………………………………………………………3
1.3 Aim and Objectives of the Study. …………………………………………………………..3
1.4 Significance of the Study…………………………………………………………………….4
1.5 Thesis Outline. ………. ………………………………………………………………………4
CHAPTER TWO: LITERATURE REIVIEW…………………………………………………………………6
2.1 Introduction…………………………………………………………………………………..6
2.2 Review of Fundamental Concepts……………………………………………………………6
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2.2.1 Definition of Power Converter………………………………………………………………6
2.2.2 Terminologies used to describe converter operations………………………………….…..10
2.2.3 SEPIC basic concept…………………………………………………………………………….15
2.2.4 Bi-directional Converter basic concept………………………………………………….…21
2.2.5 Permanent Magnet dc motor…………………………………………………………………………24
2.3 Review of Similar Works……………………………………………………………………26
2.4 Conclusion…………………………………………………………………………………..30
CHAPTER THREE: MATERIALS AND METHODS…………………………….…………31
3.1 Introduction………………………………………………………………………………….31
3.2 Solar Insolation Data for Zaria …………………………………………………………..31
3.3 MATLAB/Simulink ………………………………………………………………………..32
3.4 Methodology …………………………………………………………………………………34
3.5 Model derivation and Design Considerations…………………………………………….35
3.5.1 Model Derivation of a Complete System…………………………………………………..36
3.5.2 Steady State Equations……………………………………………………………………..41
3.5.3 Design Considerations…………………………………………………………………..…47
3.6 Simulation Equations………………………………………………………………………59
3.7 Conclusion…………………………………………………………………………………..60
CHAPTER FOUR: RESULTS AND ANALYSIS ……………………………………..…….61
4.1 Introduction…………………………………………………………………………………61
4.2 Steady State Analysis……………………………………………………………………….61
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4.2.1 Harmonic Balance Technique Applied to SEPIC System………………………………….61
4.2.2 Steady State Calculations…………………………………………………………….….…63
4.3 Results……………………………………………………………………………………….68
4.4 Conclusion …………………………………………………………………………….…….83
CHAPTER FIVE: CONCLUSION AND RECOMMENDATIONS. ……………………….84
5.1Summary.………………………………………………………………………………….…..84
5.2Conclusion.……………..……………………………………………………………………..84
5.3 Limitation of the Study. ……………………………………………………………………85
5.4 Recommendations for Further Work ……………………………………………………..85
CHAPTER ONE
INTRODUCTION
1.1 Overview
Solar fed systems can be used as an alternative source of power in meeting the electricity requirement of some rural dwellers not connected to the national grid in Nigeria. Their application area is limited to low power level such as street lighting, battery charging, irrigation, etcetera. The application area nevertheless is further limited to difficulty in terrain, mountainous hills, topography of these rural areas just to mention but few. A suggestion for meeting the electricity requirement of these rural dwellers and the need for high power application area is to use a solar fed SEPIC system viz a viz a storage energy system (Thouthong, 2011; Veerachary, 2012). The purpose of the SEPIC is twofold: (i) To buck or boost varying output voltage from the solar module to a voltage suitable for meeting the requirement of the dc grid. (ii) Furnish the dc load with power via a dc grid linking the output of the converter and the dc load. However, the approach in (ii) requires a battery in mitigating the load power demand for short fall in solar insolation. To determine the performance of a dc grid SEPIC based solar electricity for dc load equipped with a battery, the method of harmonic balance technique is used in the analysis of the SEPIC state variables (Sangho et al, 2006). The method is different from that of averaging technique in that it basic application in addition to solving for average values of the SEPIC variables can further be use to:
(i) Predict ripples
(ii) design filter
(iii) predict ripple dynamics
The harmonic balance technique consequent to the fact that it is assumed the converter variables is decomposed into average and fundamental ripple component. This allows for studying the steady state current from the output of the SEPIC. Furthermore, the current
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supplied to the load are greatly influenced by the current supplied from the battery when the
solar insolation is low. It is shown that the current drawn by the load is the sum total of the
current from the SEPIC system and the battery current. For this reason the accompanying
procedure is to study the general influence of the converter duty ratio on the load current.
However, in considering the dc to dc power converter for this study as a practical example,
under certain conditions of duty ratio variations, the current contribution of the solar and
battery can only be met to some values of duty ratio. The harmonic balance technique permit
the study of the ripple and steady state quantities of the SEPIC duty ratio variations between
[0,1]. Figure 1.1 shows a block diagram of a solar fed SEPIC based system feeding a load via
a dc grid equipped with a storage system. This thesis is focused on modeling and simulation
of a dc grid SEPIC based electricity for a dc load, for a range of duty ratio variation of the
SEPIC. Equations are presented in proper modes for each sub-system. In arriving at the
system model based on different differential equations describing the modes of operation of
the system, it is possible to achieve sufficient understanding of the system and to develop
insight into the general influence of the various parameters on the behavior of the system.
SEPIC
CONERTER
DC Grid
Permanent
Magnet (PM)
Motor
Busbar
Solar Panel
SUN
Battery
Figure 1.1 System Configuration
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1.2 Statement of Problem
The published works have shown techniques in supplying electricity to dc distribution system using varied kind of converter as a link to supply electricity to load. There have been efforts to (i) enhance the power handling capacity of the dc link of a dc distribution system using a variant converters. (ii) enhance the power capabilities of constituent converters through the implementation of various control algorithms (iii) utilize alternative energy sources (mostly solar) to supply current to a dc load in a dc distribution network. However, using a solar based SEPIC system together with a battery system to supply dc load with current via a dc grid system for a range of converter duty ratio variations has not been addressed. In the light of this, there is need for a circuit topology that ensures real time collaboration between the solar PV cells and the battery energy storage system to furnish a dc load with power. This solar fed SEPIC system should be in addition to a storage system (battery) be able to meet part of the load current should there be low solar insolation. It is based on this that the research work is undertaken.
1.3 Aim and Objectives of the Study
The aim of this work is to study the overall system behaviour of a solar fed converter system feeding a dc load i.e permanent magnet (PM) dc motor and battery energy system.
The objectives are as follows:
(i) Development of the mathematical model that describes the dynamic and steady state performance of system.
(ii) Implementation of the principle of harmonic balance technique in finding the steady and ripple quantities of the SEPIC.
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(iii)Determination of the terminal characteristics of the converter under steady state for continuous current mode (CCM) of the converter.
(iv) Determination of the systems parameters and dynamic analysis via simulation using MATLAB.
1.4 Significance of the Study
The current effort is focused on using a SEPIC based solar system and a battery to service a permanent magnet dc motor with power via a dc grid system. The work has been able to show the following:
(i) The contribution of the current delivered to the dc motor for some specific range of duty ratios of the Bi-Directional converter in a case of low solar insolation.
(ii) The steady state average quantities and steady state ripple quantities of the converter variables for a range of duty ratio variations.
1.5 Thesis Outline
The remaining part of this thesis is organized as follows:
Chapter Two presents a detailed literature review for the research. The review is divided into two viz. review of fundamental concepts and review of similar works. The necessity of the present work is highlighted within the context of reviewed works. Chapter Three presents the materials used and method adopted. Data and software tools have been used for the work. A detailed circuit configuration is given and the mode switching operations briefly discussed. Also, certain design considerations necessary for the determination of the parameters of the system are presented. Chapter Four shows the result of the steady state analysis and the Harmonic balance technique study carried out on SEPIC system. Various plots of different
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analysis carried out are presented and discussed. A simulation study of certain aspects of the system is also included. Chapter Five reports the important conclusions of the thesis work. Recommendations are also made for future work. At the end of the thesis, cited references and appendices are provided.
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