ABSTRACT
In the study of the influence of saliency ratio (/) on the performance of a synchronous reluctance generator (SRG), this project investigates two typical generator rotor designs: Generator with cage and without cage otherwise known as cage and cageless rotor respectively. A special attention had been paid to the possible rotor geometries of synchronous reluctance machine, SRM. This ratio can directly influence our insight into the machine’s potential abilities. From the studies of three phase SRG, a modeled direct and quadrature axes equations for both rotor configurations are presented for dynamic simulation. Basic parameters and generator performance, such as phase voltage and current build-up, output power with load current, peak voltage with load current, reactive power with load current are compared for both rotor designs. These analyses from the simulation were carried out by Embedded MATLAB Function. It was observed from capacitor selection that the capacitors ranging from 50 to 120µF produced a suitable voltage build-up in every case without exceeding the current-carrying capacity of the winding coil. Another observation from the result is that, cage-less-rotor can only be excited with much lower capacitor values, between 40 and 65F. This indicates that cage-less-rotor produces a lower voltage, lower load current and yields lower output power. The saliency ratio obtained under rotor geometry modifications does not surpass 7, while the longitudinal magnetization reactance, is reduced by at least 20% with respect to value for uniform air-gap. Different values of saliency ratios were investigated through MATLAB simulation by dividing the measured quadrature axis magnetizing reactance, with arithmetic progressing numbers ranging from 2, 2.5, 3, 3.5 to 6.5 which mathematically increases the direct axis magnetizing reactance value, of saliency ratios by 2, 2, 3, 3.5, to 6.5. The higher the saliency ratio, the higher the power factor and output power. The saliency ratio is the most importance parameter of a synchronous reluctance generator which is directly proportionally to power factor and output power.
TABLE OF CONTENTS
Cover page…………………………………………………………………………………………i
Approval page……………………………………………………………………………………..ii
Title page………………………………………………………………………………………….iii
Certification……………………………………………………………………………………….iv Dedication………………………………………………………………………………………….v Acknowledgement…………………………………………………………………………………vi Abstract……………………………………………………………………………………………viii
Table of contents…………………………………………………………………………………..ix List of Figures…………………..………………………………………………………………….xiv
List of Table………………………………………………………………………………….……xiv
CHAPTER ONE:
INTRODUCTION
1.1 Background of the work……………………………………………………………… 1
1.2 Self excited A.C generator…………………………………………………………… 2
1.3 Saliency ratio of salient pole synchronous machine………………………………… 3
1.4 Improvement of the output power of salient pole synchronous machine……………. 5
1.5 Aims and objectives of the project…………………………………………………… 5
1.6 Motivation for the project………………………………………………………………….. 5
1.7 The outline of the thesis work………………………………………………………….. 5
CHAPTER TWO:
LITERATURE REVIEW
- 1 Introduction on synchronous generator…………………………………………… 7
2.2 The rotor structure…………………………………………………………………. 7
2.2.1 The cylindrical rotor………………………………………………………………. 7
2.2.2 The salient pole rotor…………………………………………………………….. 8
2.3 The stator / armature winding…………………………………………………… 9
2.3.1 Winding types……………………………………………………………………. 9
2.3.1.1 Double layer windings……………………………………………………….….. 9
2.3.2 Number of coils………………………………………………………………….. 9
2.3.3 Conductor design………………………………………………………………… 10
2.3.4 Skewing…………………………………………………………………………. 10
2.4 Rating of synchronous generator (alternator)…………………………………… 10
2.5 Alternator on load……………………………………………………………….. 10
2.6 Voltage regulator……………………………………………………………..…. 11
2.7 Two-reactance concept for salient pole synchronous machine..…………………… 11
2.8 Losses and efficiency………………………………………………………..….. 12
2.9 The synchronous reluctance machine……………………………………..……… 12
2.10 Advantages of synchronous reluctance machine………………………..……… 13
2.11 The working principle of synchronous reluctance generator…………..………. 14
2.12 Saliency ratio optimization…………………………………………….………… 15
2.12.1 The conventional rotor………………………………………………..…………. 15
2.12.2 The segmental rotor…………………………………………………..…………. 16
2.12.3 The channel segment rotor…………………………………………..…………. 17
2.12.4 The flux barrier rotor……………………………………………….…………… 18
2.12.5 The layer type flux barrier………………………………………………………. 19
2.12.6 The axially laminated anisotropic rotor……………………….……………………. 20
2.13 The evolution of anisotropic rotor geometry and classification….……………. 21
2.14 TLA and ALA comparison…………….…………………………………………. 25
2.15 The parametric effects of saliency ratio on the SRG……………………………… 27
2.15.1 Air gap length…………………………………………………………………….. 27
2.15.1.1 Machine magnetizing inductances………………………………………………… 28
2.15.2 Number of turns of the winding……………………………………………………. 28
2.15.3 Air gap/slot depth ratio……………………………………………………………… 29
2.15.4 Pole arc/pole pitch ratio…………………………………………………………… 29
CHAPTER THREE
MATHEMATICAL MODEL OF SYNCHRONOUS RELUCTANCE GENERATOR
3.1 Modeling of synchronous reluctance generator..………………………………… 30
3.1.1 Cageless-rotor synchronous reluctance generator…….………………………….. 30
3.1.1.1 Voltage equations………………………………………………………………….. 30
3.1.2 Load model equations of synchronous reluctance generator…………………….. 32
3.1.3 Capacitance excitation model equations of synchronous reluctance generator…. 34
3.1.4 Reference frame transformation…………………………………………………… 35
3.2 The steady state at power grid (cageless-rotor synchronous reluctance generator at standstill) ……………………………………………………………. 39
3.3 Cage-rotor synchronous reluctance generator ………………………………………. 40
3.3.1 Voltage equations …………………………………………………………………. 40
3.3.2 The d-q model of cage-rotor SRG…………………………………………………. 41
3.3.3 The steady state of cage-rotor capacitor excited synchronous reluctance generator.. 44
3.3.4 Relationship between 3-phase and orthogonal quantity in stationary and
synchronous reference frame……………………………………………………….. 45
3.4 Power equation…………………………………………………………………….. 47
CHAPTER FOUR
DYNAMIC SIMULATION OF ROTOR CONFIGURATION SYNCHRONOUS RELUCTANCE GENERATOR
4.1 Simulation tools……………………………………………………………………… 49
4.2 Simulation of cage and cageless-rotor synchronous reluctance generator
using Embedded MATLAB Function Block………………………………………… 49
4.3 Simulation Results…………………………………………………………………….. 49
4.4 Analysis of Results…………………………………………………………………… 49
CHAPTER FIVE
Conclusion and Recommendation
5.1 Conclusion and recommendation……………………………………………………. 68
References…………………………………………………………………………… 70
Appendix 1…………………………………………………………………………………… 75
Appendix 2……………………………………………………………………………. 76
Appendix 3……………………………………………………………………………. 77
Appendix 4……………………………………………………………………………. 80
CHAPTER ONE
INTRODUCTION
1.1 Background of the study.
Electrical machines are devices that convert electrical energy to mechanical energy and vice versa. [1]. There are two major types of electrical machines by operation: Electric motors and generators. Electric motors convert electrical energy to mechanical energy while electric generators convert mechanical energy to electrical energy. [1],[2]. Electric generators can be classified into two: Alternating current (A.C) generators also known as “alternator” and direct current (D.C) generators. A.C generator (alternator) can further be classified into two: Induction and synchronous generators. The synchronous generator is one of the first and most well known synchronous machine types. It was in the beginning common in MW-size power range, but is nowadays mainly used for different power range machines. Induction generators need no excitation equipment, as it can be driven from the grid. They are classified as self-excited generators. The synchronous machines have still a number of important advantages which makes them very interesting. To this counts high efficiency, robustness and good controllability. In the upper power range, they are the only option. It can therefore be expected that the synchronous generator will continue to play an important role, also in the future. High speed synchronous generator driven by steam turbines differ considerably in their construction from the slow engine-driven machines and are often described as “flywheel-type”. There are two basic constructions: Machines with a cylindrical rotor and machines with a salient-pole rotor. For mechanical reasons, the cylindrical rotor is preferred for two poles machines because of the large centrifugal forces that arise. The salient-pole rotor is usually the more efficient solution for machines with four poles and upwards, both for cost reasons and performance reasons. The rotor can be made of either laminated steel or solid iron. The solid iron rotor is the dominating solution for machines with low pole numbers, because of its robust mechanical properties. This is the machine type that is studied in this work. For machines of higher pole number, a laminated core is often used. Cylindrical rotor is used for high speed applications while salient-pole rotor is used for low speed applications. The traditional applications for salient-pole machines are pump systems, paper mills, ship propulsion and other applications with moderate dynamic requirements.
1.2 Self excited A.C generators
With today’s trend for distributed generation and the need for alternative and renewable energy sources, self-excited induction and synchronous reluctance generators have attracted more attention for wind, tidal and hydro power generation applications. Compared to synchronous DC generators, they have the advantages of brushless, robustness, low cost with no need for a DC excitation. There has been substantial research conducted in system modeling and control schemes of these generators, especially for induction types. Self-excited induction generator (SEIG) was one of the earliest types of self-excited AC generator and has its inherent advantage of low unit price, robust, brushless structure with squirrel-cage rotor, reduced size, no DC excitation and better transient performance, etc. However, both magnitude and frequency of the output voltage and current are load dependent, which makes performance prediction difficult and complicates the control strategy [4],[5]. Compared with self-excited induction generator (SEIG), self-excited synchronous reluctance generator (SESRG) not only has the advantages of simplicity and ruggedness, but can also give high efficiency over a wide range of operation [5]. Moreover, its output frequency is determined only by the prime mover speed, rather than by both load and the prime mover speed as in an induction generator, so it can be easily integrated with a power electronic converter to control the output [6]. Furthermore, at the same power output level, for permanent magnet synchronous generator, aging of the magnets at elevated temperature may totally demagnetize the magnets. Such problem can also happen under high armature reaction on load, or short circuit fault. For the synchronous reluctance generator, the operating temperature is limited only by winding insulation. So with appropriate design consideration, the power/weight ratio can be improved and comparable to permanent magnet synchronous generator. Moreover, with appropriate compensation techniques using inverter-battery supply connected with the load, performance can be greatly improved. These merits outweigh its disadvantage of low iron utilization. The operation of all self excited generators depends on flux saturation. So the usual analysis and design methods, which are based on the assumption of infinite permeability of the iron, can only predict the characteristic for its unsaturated operation. It is the purpose of this study to provide a method for the analysis of the self-excited synchronous reluctance generator (SERG) neglecting saturation of the iron. Sufficient attention has not been paid to three-phase, self-excited synchronous reluctance generators. Their balance loads make the analysis simple and comprehensive unlike single-phase unsymmetrical self excited reluctance generator (SERG) which is very cumbersome. Since they are good alternatives to three-phase induction generators, they are investigated in this research and a general methodology is suggested for no-load and load response prediction, steady state performance analysis with special emphasis on influence of saliency ratio.
1.3 Saliency ratio of salient pole synchronous machine.
From elementary synchronous machine studies, the output power of an m-phase salient-pole synchronous machine is given by:
Pout. = sin + ( -) sin (1.1)
In (1.1), E is the excitation, V is the terminal voltage, is the load angle, is the direct-axis inductance and is the quadrature-axis inductance. It is observed from above equation (1.1) that the above expression consists of two terms:
- First term represents power due to field excitation; i.e
Pe = sin (1.2)
- Second term gives the reluctance power, (that is power due to saliency);
For a synchronous reluctance machine, there is no field excitation, which means that E = 0, then, equation (1.1) becomes
Prel. = ( -) sin (1.3)
In (1.3), Prel. is the reluctance power. It is clear that if is reduced to a very small value, the reluctance power will be high [7]
1.4 Improvement of the output power of salient pole synchronous reluctance machine.
The output power of an m-phase salient-pole synchronous reluctance machine given in equation (1.3) can be rewritten as:
Prel. = ( sin (1.4)
From equation (1.4), the following observations can be made [8]:
- If the d-axis reactance is equal to the q-axis reactance, the output power will be zero.
- If the q-axis reactance is greater than d-axis reactance, the axis of operation of the machine will shift by 90
- If the q-axis reactance is kept constant and the d-axis is increased, the output power will not increase because of the reciprocal of the d-axis reactance, .
- If the d-axis reactance is kept constant and q-axis is reduced, the output power will increase.
- If the q-axis reactance is equal to zero, the output power will tend to infinity for a given load angle,
- It has been shown that torque is directly proportional to the d-axis and q-axis reactance differences (–) while power factor is dependent on d-axis and q-axis reactance ratio (/). In fact, maximum power factor. is given by :
= (1.5)
Where k = /.
It can be seen that as k increases, also increases.
1.5 Aims and objectives of the thesis
The primary goal of this work is to investigate the influence of saliency (Ld/Lq) ratio on the performance of a synchronous generator through development of a simulation model for synchronous reluctance generator both for cage-rotor and cage-less rotor designs.
1.6 Motivation for the project
These are motivation for the project:
- The knowledge and understanding of a synchronous reluctance generator has shown that saliency (/) ratio has most significant influence on the machine performance indices like output power and power factor.
- There has been considerable interest of study on synchronous reluctance machine operating as a motor unlike the generator operation.
- A possible replacement of induction generator and permanent magnet synchronous generator with a synchronous reluctance generator due to the market cost economy.
1.7 Outline of the thesis.
This thesis is organized as follows:
After the introductory concept of chapter one, chapter two deals with the literature review, the brief and fundamental information of a synchronous machine generator, synchronous generators, synchronous reluctance generator, its application and principles of operation of a synchronous reluctance generator, the history of synchronous reluctance generator, the classifications of rotor geometry, the structure and rotor development of the synchronous reluctance generator.
In chapter three, we derive the synchronous reluctance generator (both cage and cage-less) mathematical model through synchronous generator, its analysis, the steady-state equations, its analysis and the power equation of the synchronous reluctance generator.
Chapter four mainly deals with the simulation of cage and cage-less rotor synchronous reluctance generator.
In chapter five, the analysis of simulation results and conclusion of the machine performance are also presented, followed by appendices and references.
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