Induction Generator And AC/DC/AC Converter Model For Wind Energy Conversion
Amir Husin Tanady (09884986)
A thesis submitted for the partial fulfillment of the degree of Bachelor of Engineering (Electrical)
Induction Generator and AC/DC/AC Converter Model for Wind Energy Conversion
Tanady Amir Husin
26th October 2001
Dr. W. W. L. Keerthipala
Bachelor Of Engineering
Electrical
This report documents the analysis and simulation of a selfexcited induction generator and the conversion scheme (AC/DC/AC converter) using PSCAD/EMTDC simulation software. The report basically separated into several parts. First two sections give a brief introduction and the literature review the project. Thirdly, the modelling of the self-excited induction generator and the AC/DC/AC converter was evaluated and analysed. Next, the recommendations on the future work were given. And finally, the conclusion is made.
Induction Generator; Self-Excitation; Power Electronic Converters; AC/DC/AC Converter; Wind Driven Electrical Generators; Wind Energy Conversion; Renewable Energy; PSCAD; EMTDC
Mr. Amir Husin Tanady U31/ 132 Mounts Bay Road West Perth – 6005 Western Australia 26th October 2001
Professor J. L. Hullett School of Electrical and Computer Engineering Curtin University of Technology P. O. Box U1987 Perth, W. A. 6000
Dear Sir,
Re: Final Dissertation For Final Year Project
Please find the enclosed report on the project titled “Induction Generator and AC/DC/AC Converter Model for Wind Energy Conversion”, in partial fulfilment of the requirements of the Electrical Project unit for the award of Bachelor of Engineering (Electrical) – Honours.
Yours Sincerely
Amir Husin Tanady Student No.: 09884986
EXECUTIVE SUMMARY
This report elaborates the analysis and simulation of a self-excited induction generator
and
the
conversion
scheme
(AC/DC/AC
converter)
using
PSCAD/EMTDC simulation software.
The report basically separated into several parts. Firstly, a general introduction of wind power, wind turbine, and the PSCAD/EMTDC software was given. Secondly, the background information including the wind/diesel hybrid system, the Australian wind farms, the potential of offshore wind farms, the wind turbine generators in particular the analysis of the induction machine behaviour, and past investigation that had been conducted by previous students as well as other research papers was summarised. Thirdly, the modelling of the self-excited induction generator and the AC/DC/AC converter was evaluated and analysed. Next, the recommendations on the future work were given. And finally, the conclusion is made.
-i-
ACKNOWLEDGEMENTS
The implementation of this project was possible with the help and advices from many people. The project could not have reached at this stage without the assistance of these people.
Firstly, I would like to thank the project supervisor Dr. W. W. L. Keerthipala for his support, guidance, patience, motivation, and advice throughout the project. Many thanks to Mr. Z. Cielma and M. Fowler for their kindness and support on the technical aspect of the software simulation, induction machine operation and the experiment on the induction machine in the laboratory.
Finally, many thanks to all my friends and everyone for giving me their valuable opinions and support throughout the project.
- ii -
TABLE OF CONTENTS Executive Summary .................................................................................................... i Acknowledgements .................................................................................................... ii Table of Contents ...................................................................................................... iii Nomenclature ............................................................................................................ vi List of Figures .......................................................................................................... vii List of Tables ............................................................................................................ xi 1. Introduction............................................................................................................ 1 1.1 Power From the Wind .......................................................................................1 1.2 Wind Turbine ....................................................................................................3 1.3 PSCAD/EMTDC Software ...............................................................................6 2. Background Information ........................................................................................ 7 2.1 Wind – Diesel Systems .....................................................................................7 2.2 Wind Farms in Australia ...................................................................................8 2.3 Offshore Windfarms .........................................................................................9 2.4 Wind Turbine Generators .................................................................................9 2.4.1 Why Induction Generator ........................................................................10 2.4.2 Induction Machine Analysis ....................................................................12 2.5 Power Electronics ...........................................................................................15 2.5.1 AC to DC Conversion ..............................................................................15 2.5.2 DC to AC Conversion ..............................................................................22 2.5.3 DC – DC Converter .................................................................................23 2.6 Past Investigations ..........................................................................................26 2.6.1 Simulation of Wind-Diesel Hybrid Systems............................................26 2.6.2 Self-excited Induction Generator.............................................................28
- iii -
2.6.3 Problem Formulation ...............................................................................30 3. Modelling of the System...................................................................................... 32 3.1 Self-Excited Induction Generator Model........................................................32 3.2 AC/DC/AC Converter Model .........................................................................41 3.2.1 AC to DC Conversion ..............................................................................41 3.2.2 DC to DC Conversion Using Buck-Boost Converter ..............................49 3.2.3 DC to AC Conversion Using PWM VSI .................................................52 3.2.4 AC/DC/AC Converter..............................................................................59 4. Results Of The Complete System ........................................................................ 71 4.1 System 1..........................................................................................................71 4.2 System 2..........................................................................................................75 4.3 System 3..........................................................................................................78 5. Future Work ......................................................................................................... 82 6. Conclusion ........................................................................................................... 85 References Appendix A - PSCAD Appendix B – Test Machine Appendix C – Diode Rectifier Appendix D – Half-Controlled Rectifier Appendix E – Buck-Boost Converter Appendix F – PWM VSI Appendix G – Diode Rectifier with PWM VSI Appendix H – Half-Controlled Rectifier with PWM VSI Appendix I – Diode Rectifier, Buck-Boost Converter, and PWM VSI Appendix J – Complete System 1 Design
- iv -
Appendix K – Complete System 2 Design Appendix L – Complete System 3 Design
-v-
NOMENCLATURE
FFT – Fast Fourier Transform GTO Thyristors – Gate-Turn-Off Thyristors IGBT – Insulated Gate Bipolar Transistors PSCAD – Power System Computer Aided Design PWM – Pulse Width Modulated RMS – Root Mean Square SEIG – Self-Excited Induction Generator
- vi -
LIST OF FIGURES Fig. 1.1: (a) Main Components of Horizontal-axis Wind Turbine ............................ 5 Fig. 1.1: (b) Cross-section of a Typical Grid-connected Wind Turbine.................... 5 Fig. 1.1: (c) Cross-section of a Nacelle in A Grid-connected Wind Turbine [14] .... 5 Fig. 2.1: Torque vs Speed Characteristics of Squirrel-cage Induction Generator [3] ............................................................................................. 12 Fig. 2.2: Per-Phase Equivalent Circuit of An Induction Machine ........................... 13 Fig. 2.3: Alternative Form for Per-Phase Equivalent Circuit .................................. 14 Fig. 2.4: Power Flow Diagram................................................................................. 14 Fig. 2.5: 3-φ Diode Bridge Rectifier ........................................................................ 16 Fig. 2.6: 3-φ Rectifier Circuit With a Constant DC Current (Ls = 0) ...................... 17 Fig. 2.7: Rectified Voltage Waveform After Diode Bridge..................................... 17 Fig. 2.8: Input AC Line Current of 3-φ Diode Rectifier .......................................... 18 Fig. 2.9: Controlled 6-Pulse Full-Bridge Rectifier .................................................. 20 Fig. 2.10: Effect of α on Vd ..................................................................................... 20 Fig. 2.11: 3-φ Inverter Circuit Diagram ................................................................... 22 Fig. 2.12: Control Signal for DC-DC Converter...................................................... 24 Fig. 2.13: Buck-Boost Converter Circuit Diagram .................................................. 25 Fig. 2.14: Cúk Converter Circuit Diagram .............................................................. 25 Fig. 2.15: Proposed System ..................................................................................... 31 Fig. 3.1: Induction Generator Parameters ................................................................ 33 Fig. 3.2: SEIG Model in PSCAD............................................................................. 34 Fig. 3.7: Simulation Results of Self-Excited Induction Generator Parameters ....... 36 Fig. 3.3: RMS Stator Line Voltage .......................................................................... 37 Fig. 3.4: Stator Frequency........................................................................................ 38 - vii -
Fig. 3.5: RMS Stator Voltage At Higher Compensation Levels.............................. 38 Fig. 3.6: Stator Frequency At Higher Compensation Levels................................... 39 Fig. 3.7: PSCAD Model of Diode Bridge Rectifier ................................................. 42 Fig. 3.8: Voltage and Current At Rectifier Output .................................................. 42 Fig. 3.9: Half-Controlled 6-Pulse Full-Bridge Rectifier Circuit.............................. 43 Fig. 3.10: Thyristor Firing Angle (α) Circuit .......................................................... 44 Fig. 3.11: Alpha Train Pulse Signal Generator Circuit Diagram............................. 45 Fig. 3.12 Comparator Outputs and AND Gate Output For Alpha ........................... 46 Fig. 3.12: Thyristor Train Pulse Waveform............................................................. 47 Fig. 3.13: T3 and T5 Firing Angle Circuit............................................................... 47 Fig. 3.14: VPn, V Nn, and Vd Waveforms................................................................... 48 Fig. 3.15: Buck-Boost Converter Circuit Diagram .................................................. 49 Fig. 3.16: Firing Angle For GTO............................................................................. 50 Fig. 3.17: Gating Signal For GTO ........................................................................... 50 Fig. 3.17: Input Voltage Vd and Output Voltage Vo of Buck Boost Converter...... 51 Fig. 3.18: PWM VSI Supplying Resistive Load...................................................... 52 Fig. 3.19: PWM Circuit ........................................................................................... 53 Fig. 3.20: PWM Pattern For IGBT Switching ......................................................... 53 Fig. 3.21: PWM Patterns in One Leg of VSI........................................................... 54 Fig. 3.22: Inverter Voltage Output With Respect To Zero Potential Between Capacitors on DC Side ............................................................................ 56 Fig. 3.23: Inverter Phase Voltage Output With Respect To DC Neutral................. 56 Fig. 3.24: Inverter Phase Voltage Output With Respect To Neutral Ground .......... 57 Fig. 3.25: FFT of PWM VSI Output Phase Voltage................................................ 58 Fig. 3.27: Rectifier Output Voltage and Current of 1st Design................................ 60
- viii -
Fig. 3.28: Inverter Output Vinva and Vlinvab of 1st Design ......................................... 61 Fig. 3.29: PSIMDemo Model of Diode Rectifier With PWM VSI ......................... 62 Fig. 3.30: FFT of Output Phase Voltage Of Diode Rectifier PWM VSI Converter Design..................................................................................... 62 Fig. 3.31: Half-Controlled Rectifier With PWM VSI Converter Design ................ 63 Fig. 3. 32: Half-Control Rectifier Output Voltage and Current Of 2nd Design ....... 64 Fig. 3.33: Inverter Output Phase Voltage and Line-To-Line Voltage of 2nd Design...................................................................................................... 64 Fig. 3.34: FFT of Inverter Output Phase Voltage Of 2nd Design............................. 65 Fig. 3.35: Diode Rectifier With Buck-Boost Converter and PWM VSI Converter Design (3rd Design)................................................................. 65 Fig. 3.36: (a) Output Voltage Of Diode Rectifier; (b) Output Voltage Of BuckBoost Converter....................................................................................... 66 Fig. 3.37: Inverter Output Phase Voltage And Line-Line Voltage Waveform Of 3rd Converter Design .......................................................................... 67 Fig. 3.38: Buck-Boost Converter Conventional Current Flow Diagram................. 67 Fig. 3.39: Current Flows In Buck-Boost Converter................................................. 68 Fig. 3.40: Modified Buck-Boost Converter ............................................................. 68 Fig. 3.41: Current Flows in Modified Buck-Boost Converter ................................. 69 Fig. 3.42: Inverter Output Phase Voltage And Line-Line Voltage Of 3rd Converter Design..................................................................................... 70 Fig. 4.1: Complete System 1 Circuit Diagram......................................................... 72 Fig. 4.2: RMS Voltage Of SEIG.............................................................................. 72 Fig. 4.3: Rectifier Output Voltage Of System 1 ...................................................... 73 Fig. 4.4: Inverter Output Phase Voltage And Line-to-Line Voltage Of System 1 .. 73
- ix -
Fig. 4.5: FFT of Inverter Output Phase Voltage Of System 1 ................................. 74 Fig. 4.6: Complete System 2 Circuit Diagram......................................................... 75 Fig. 4.7: Modified Alpha Circuit ............................................................................. 76 Fig. 4.8: Half-Control Rectifier Output Voltage Ripple Of System 2 ..................... 76 Fig. 4.9: Inverter Output Line-To-Line Voltage Of System 2................................. 77 Fig. 4.10: FFT Of Inverter Output Phase Voltage OF System 2 ............................. 77 Fig. 4.11: Complete System 3 Circuit Diagram....................................................... 78 Fig. 4.12: Rectifier Output And Buck-Boost Converter Output Voltages Of System 3 .................................................................................................. 80 Fig. 4.13: Inverter Output Line-To-Line Voltage Of System 3............................... 80 Fig. 4.14: FFT Of Inverter Output Phase Voltage Of System 3 .............................. 81 Fig. 5.1: Blanking Time In PWM VSI..................................................................... 83
-x-
LIST OF TABLES Table 3.1: Induction Machine Parameters ................................................................32 Table 3.2: Induction Machine Parameters (Per-Unit Values)...................................33 Table 3.3: Maximum Overvoltages Magnitude ........................................................39
- xi -
1. INTRODUCTION
Wind is abundant almost in any part of the world. Its existence in nature caused by uneven heating on the surface of the earth as well as the earth’s rotation means that the wind resources will always be available.
The conventional ways of generating electricity using non renewable resources such as coal, natural gas, oil and so on, have great impacts on the environment as it contributes vast quantities of carbon dioxide to the earth’s atmosphere which in turn will cause the temperature of the earth’s surface to increase, known as the green house effect [2]. Hence, with the advances in science and technology, ways of generating electricity using renewable energy resources such as the wind are developed. Nowadays, the cost of wind power that is connected to the grid is as cheap as the cost of generating electricity using coal and oil. Thus, the increasing popularity of green electricity means the demand of electricity produced by using non renewable energy is also increased accordingly.
1.1 Power From the Wind
Kinetic energy from the wind is used to turn the generator inside the wind turbine to produced electricity. There are several factors that contribute to the efficiency of the wind turbine in extracting the power from the wind. Firstly, the wind speed is one of the important factors in determining how much power can be extracted from the wind. This is because the power produced from the wind turbine is a function of the
-1-
cubed of the wind speed. Thus, the wind speed if doubled, the power produced will be increased by eight times the original power. Then, location of the wind farm plays an important role in order for the wind turbine to extract the most available power form the wind.
The next important factor of the wind turbine is the rotor blade. The rotor blades length of the wind turbine is one of the important aspects of the wind turbine since the power produced from the wind is also proportional to the swept area of the rotor blades ie. the square of the diameter of the swept area. Hence, by doubling the diameter of the swept area, the power produced will be four fold increased. It is required for the rotor blades to be strong and light and durable [2]. As the blade length increases, these qualities of the rotor blades become more elusive. But with the recent advances in fiberglass and carbon-fibre technology, the production of lightweight and strong rotor blades between 20 to 30 meters long is possible. Wind turbines with the size of these rotor blades are capable to produce up to 1 megawatt of power.
The relationship between the power produced by the wind source and the velocity of the wind and the rotor blades swept diameter is shown below.
Pwind
π = dD 2 v 3wind 8
The derivation to this formula can be looked up in [2]. It should be noted that some books derived the formula in terms of the swept area of the rotor blades (A) and the air density is denoted as ρ.
-2-
Thus, in selecting wind turbine available in the market, the best and efficient wind turbine is the one that can make the best use of the available kinetic energy of the wind.
Wind power has the following advantages over the traditional power plants.
• Improving price competitiveness, • Modular installation, • Rapid construction, • Complementary generation, • Improved system reliability, and • Non-polluting.
1.2 Wind Turbine
There are two types of wind turbine in relation to their rotor settings. They are:
• Horizontal-axis rotors, and • Vertical-axis rotors.
In this report, only the horizontal-axis wind turbine will be discussed since the modeling of the wind driven electric generator is assumed to have the horizontal-axis rotor.
The horizontal-axis wind turbine is designed so that the blades rotate in front of the tower with respect to the wind direction i.e. the axis of rotation is parallel to the wind
-3-
direction. These are generally referred to as upwind rotors. Another type of horizontal axis wind turbine is called downwind rotors which has blades rotating in back of the tower. Nowadays, only the upwind rotors are used in large-scale power generation and in this report, the term “horizontal-axis wind turbine” refers to the upwind rotor arrangement.
The main components of a wind turbine for electricity generation are the rotor, the transmission system, the generator, and the yaw and control system. The following figures show the general layout of a typical horizontal-axis wind turbine, different parts of the typical grid-connected wind turbine, and cross-section view of a nacelle of a wind turbine.
(b)
(a)
-4-
(c) Fig. 1.1: (a) Main Components of Horizontal-axis Wind Turbine (b) Cross-section of a Typical Grid-connected Wind Turbine (c) Cross-section of a Nacelle in A Grid-connected Wind Turbine [14]
As can be seen in figure 1 (c), the nacelle consists of several components. They are the generator, yaw motor, gearbox, tower, yaw ring, main bearings, main shaft, hub, blade, clutch, brake, blade and spinner. Other equipment that is not shown in the figure might include the anemometer, the controller inside the nacelle, the sensors and so on.
The generator is responsible for the conversion of mechanical to electrical energy. Yaw motor is used power the yaw drive to turn the nacelle to the direction of the wind. The gearbox is used to connect the low-speed shaft (main shaft in the figure) to the high-speed shaft which drives the generator rotor. The brake is used to stop the main shaft from over speeding. The blades are used to extract the kinetic power from the wind to mechanical power i.e. lifting and rotating the blades. The tower is made
-5-
from tubular steel or steel lattice and it is usually very high in order to expose the rotor blades to higher wind speed.
1.3 PSCAD/EMTDC Software
In the initial stage, familiarization of the software and understanding the operations involved in the software in order to conduct the project is necessary. The software used in the project is the PSCAD/EMTDC software.
PSCAD is a group of programs which provides a flexible interface to electromagnetic transient simulation based on EMTDC. A more detailed explanation of this software can be looked up in appendix A.
-6-
2. BACKGROUND INFORMATION
2.1 Wind – Diesel Systems
In the early days, diesel generators were used to supply local load in most remote areas. Single diesel generator can be used to supply small load, while for larger load multiple diesel generators are employed. The most reliable and efficient diesel generators are commonly used to supply the base load so that other generators can be shut down at any time in case of regular maintenance, engine failure or other emergencies.
The main advantage of the diesel systems is that they have been proven to be highly dependable in many remote areas if they are maintained correctly. On the other hand, diesel generators should be operated above minimum load in order to maintain their efficiency and to minimize engine wear and fuel consumption. In many instances, diesel generators are incorrectly sized and inefficiently controlled due to the changing consumer loads which regulate the diesel’s operational constraints. The main disadvantage of diesel generators is that they are costly to operate. Their reliance on the diesel fuel proved to be costly since the fuel needs to be transported to the diesel plant in order for the diesel produce electricity and the cost of diesel fuel itself is likely to increase in the future. Moreover, diesel generators like many other generators need to be well maintained particularly when they are not operated under their minimum load. Hence, the cost of the diesel operation and maintenance which
-7-
requires replacing parts of the diesel generator itself is expensive considering the remote location of the diesel plant.
Wind – diesel system provides a solution to this particular problem. The wind-diesel hybrid system is commonly applied in many remote areas where there is a constant wind source. When the demand is low, the wind power is used to supply the load. Diesel generators are only used when the customer demand is high. In this case, the wind power is also used to supply the demand in order to safe the diesel fuel consumption providing there is enough wind to produce electricity. In other case, diesel generators are used when there is a shortfall due to the availability of the wind source.
2.2 Wind Farms in Australia
Wind power is currently used at a number of locations within Western Australia. In 1994, the largest installation within the State is located at Ten Mile Lagoon near Esperance and was the first commercial wind farm within Australia. The wind farm utilizes nine 225 kW Vestas variable pitch wind turbines which is connected to Esperance’s conventional diesel power station via a 33kV power line. The total capacity of the wind farm is 2MW. The wind farm is owned and operated by Western Power.
Another recent wind farm that is now being planned in Western Australia is located at Albany. It is a project that has been in the planning stages for over ten years and in
-8-
29th June 2000 Western Power Managing Director David Eiszele said the $45 million Albany wind farm was given final approval by the State’s Energy Minister, the Hon. Colin Barnett. The completion of the wind farm is predicted to finish in July 2001. The project represented a positive commitment by Western Power and made a significant contribution to the reduction of Australia’s greenhouse gases.
2.3 Offshore Windfarms
Offshore windfarms provides large electric generation since the speed of the available wind is higher compared to the inland wind. The installation of the offshore windfarms is relatively close to the shoreline since the cost of transmission line to transfer the electricity to the inshore is expensive. One of the largest offshore windfarms is the one based in Netherlands. The capacity of electricity generated is 17 MW (each unit of 600 kW).
Major challenges in constructing offshore windfarms are the monitoring of the offshore installation since it is subject to the local weather. The weather plays an important part since the construction cannot be continued if there are any disturbances such as thunderstorm, cyclone, and so on.
2.4 Wind Turbine Generators
Wind turbine generator converts mechanical energy to electrical energy. The electricity generated by the wind generator is usually fed to a transformer outside the
-9-
wind turbine or inside the nacelle (refer to figure 1.1 (c)) to raised the voltage in order to be transmitted to the local grid or power distribution stations. The supply frequency of the wind generator is usually 50 Hz for most part of the world and 60 Hz for the electrical standard in America.
Like many other generators, the wind generator tends to get hot. Thus, a cooling system is introduced in the wind turbine. Some manufacturers use water-cooling system to cool down the generators. The advantage is that it provides some electrical efficiency advantages [25].
In wind turbine industry, two types of generators are used. They are:
• Synchronous generator, and • Asynchronous (Induction) generator.
2.4.1 Why Induction Generator
Induction generator is commonly used in the wind turbine electric generation due to its reduced unit cost, brushless rotor construction, ruggedness, and ease of maintenance. Moreover, induction generators have several characteristics over the synchronous generator. The speed of the asynchronous generator will vary according to the turning force (moment, or torque) applied to it. In real life, the difference between the rotational speed at peak power and at idle is very small approximately 1 percent. This is commonly referred as the generator’s slip which is the difference
- 10 -
between the synchronous speed of the induction generator and the actual speed of the rotor. slip (s) = ns - n
This speed difference is a very important variable for the induction machine. The term slip is used because it describes what an observer riding with the stator field sees looking at the rotor which appears to be slipping backward [35]. A more useful form of the slip quantity results when it is expressed on a per unit basis using synchronous speed as the reference. The expression of the slip in per uni is shown below.
s=
ns
− n r ns
A four-pole, 50 Hz generator will run idle at 1500 rpm according to the following formula.
n s
=
120 f p
If the generator is producing its maximum power, it will be running at 1515 rpm. A useful mechanical property of the generator is that it will increase or decrease its speed slightly if the torque varies and hence will be less tear and wear on the gearbox as well as in the system. This is one of the important reasons to use asynchronous (induction) generator compared to a synchronous generator on a wind turbine.
- 11 -
2.4.2 Induction Machine Analysis
The following figure shows the torque vs speed characteristic of typical squirrel cage induction machine.
Fig. 2.1: Torque vs Speed Characteristics of Squirrel-cage I nduction Generator [3]
In the figure, it can be seen that when the induction machine is running at synchronous speed at the point where the slip is zero i.e. the rotor is spinning at the same speed as the rotating magnetic field of the stator, the torque of the machine is zero. If the induction machine is to be operated as a motor, the machine is to operated just below its synchronous speed. On the other hand, if the induction machine is to be operated as a generator, its stator terminals should be connected to a constant-frequency voltage source and its rotor is driven above synchronous speed (s<0) by a prime mover such as the wind turbine shaft. The source fixes the synchronous speed and supplies the reactive power input required to excite the airgap magnetic field and hence the slip is negative.
- 12 -
The following figure shows the per-phase equivalent circuit of the induction machine. R s
Xs
Xr
+ Is Vt
Iϕ Ic
Ir
Im
R c
Vg
Xm
R r/S
_ Fig. 2.2: Per-Phase Equivalent Circuit of An Induction Machine
In this project, star-connected induction machine is evaluated. All the calculations are in per-phase values. Hence, for a star-connected stator:
V ph
=
Vline 3
; I ph
= I line
In order to analyse the behaviour of an induction generator, the operation of an induction motor must be fully understood. Once, the equivalent circuit parameters have been obtained, the performance of an induction motor is easy to determine.
As shown in Fig. 2.3, the total power Pg transferred across the air gap from the stator is
Pag
= I 2r
R r s
And it is evident from figure 3 that the total rotor loss Prloss is
Prloss
= I 2r R r
Therefore, the internal mechanical power developed by the motor is
Pd
= Pag − Prloss = I 2r
R r s
1 − s
2 − I 2r R r = I 2r R r − 1 = I r R r 1
s
- 13 -
s
From the power point of view, the equivalent circuit of figure 3 can be rearranged to the following figure, where the mechanical power per stator phase is equal to the power absorbed by the resistance R 2(1-s)/s. R s
Xs
Xr
+
Iϕ
Is V _ t
R c
Xm
R r
Ir
1 − s s
Vg
R r
Fig. 2.3: Alternative Form for Per-Phase Equivalent Circuit
The analysis of an induction motor is also facilitated by using the power flow diagram as shown in the following figure in conjunction with the equivalent circuit.
Input per phase
Pag
Pin
Pd
Pshaft Output
PScu Stator copper loss
Pcore Core loss
Prcu Rotor copper loss
Prot Rotational loss
Fig. 2.4: Power Flow Diagram
where, Pag
= Pin − PScu − Pcore
Pd
= Pag − Pcore
Pout
= Pshaft = Pd − Prot
- 14 -
The parameters of an induction generator can be determined by using the no-load test and block rotor test (The steps in calculating the parameters and the test results obtained from a 440V, 4.6A, 2.2kW induction motor are shown in appendix B).
2.5 Power Electronics
The conversion scheme used in this report is the AC/DC/AC converter. The input is an ac sinusoidal source provided by the induction generator. The analysis of the converter was initially carried out using pure ac sinusoidal source to observe the waveform patterns and to ease the analysis of the converter. In latter case, the induction generator is to be connected as the source.
2.5.1 AC to DC Conversion
Ac to dc conversion can be achieved by using the conventional methodology namely the uncontrolled rectifier and the controlled rectifier. An uncontrolled rectifier basically uses 6 diodes as the switches. The operation of such rectifier is discussed in detail in section 2.5.1.1. On the other hand, the controlled rectifier, as the name suggests, uses 6 controllable power switches such as Thyristors, GTOs, and so on. However, only the use of Thyristors is implemented in this project.
- 15 -
2.5.1.1 Uncontrolled 6-Pulse Rectifier
As mentioned before, the uncontrolled 6-pulse rectifier uses 6 diodes as the switches in the rectifier circuit. The typical circuit diagram for a diode rectifier is in Figure 2.5. The ac sinusoidal source is fed into the full bridge diode rectifier as shown in the figure. The top half of the full bridge, only the diodes with its anode at the highest potential will conduct. On the other hand, the bottom half of the full bridge, only the diodes with its cathode at the lowest potential will conduct.
Fig. 2.5: 3-φ Diode Bridge Rectifier
As shown in figure 6, the diode bridge rectifier consists of 6 diodes, a capacitor on the dc side and a load. The input is represented by purely 3-phase ac sinusoidal source each displaced by 120°, and line inductance (Ls). The dc side can be replaced by a constant dc current source Id. It is proven that replacing the dc current Id by a load resistance R load makes little difference in the circuit operation (Mohan, PE304). The sequence of conduction of the diodes is from D1&D2, D3&D4, D5&D6.
- 16 -
First consider an idealised circuit (where Ls = 0) as shown in the following figure.
Fig. 2.6: 3-φ Rectifier Circuit With a Constant DC Current (L s = 0)
The rectified output waveform after the diode bridge (vd) is determined by the difference between vPn and v Nn. The waveform of the rectified voltage vd is shown as follows. vd
2Vline 0
−
Vd0
ωt
π 0 π 6
6
Fig. 2.7: Rectified Voltage Waveform After Diode Bridge
Thus, the average value of vd (Vd0) can be calculated by integrating the area under the curve from -π/6 to π/6 and divide the area with the period of the waveform (π/3). In this case, the frequency of the waveform is determined by the frequency of the fundamental of the ac input. The frequency of vd is exactly 6 times the frequency of the ac input since in one cycle of the ac input waveform corresponds to 6 cycles of vd.
- 17 -
Note that the subscript ‘0’ in Vd0 symbolises zero line inductance (idealised circuit). Hence, Vd0 = 1.35 VLL
In the case of the ac input current waveform, consider only 1 phase (note that the ac source is star connected). The ac input current is shown in the following figure. vs is1
0
120°
is 120° 60°
Fig. 2.8: Input AC Line Current of 3-φ Diode Rectifier
The duty cycle of the input line current is shown to be D = 2/3 Then, the rms value of the input ac line current is Is = √D Id = 0.816 Id and the fundamental frequency component of the input ac line current (IS1) in this idealised case can be found as I s1
=
1
π
6I d
= 0.78I d
The power factor can be found by using the following expression. It can be seen that the power factor depends on the Current Harmonic Factor (CHF) and the Displacement Power Factor (DPF).
- 18 -
PF = DPF ⋅ CHF
DPF is defined as the phase difference between the fundamental-frequency component of the ac sinusoidal phase voltage and the fundamental-frequency component of the ac phase current. CHF is defined as the ratio between the rms value of the fundamental-frequency component of the ac phase current and the total rms value of the phase current itself.
Other effects on the converter such as the current commutation due to line inductances, harmonic components of the input waveform, and so on, are elaborated in great detail in [19].
2.5.1.2 Controlled 6-Pulse Full-Bridge Rectifier
Similar to the operation of the uncontrolled 6-pulse full-bridge rectifiers, the controlled 6-pulse full-bridge rectifiers uses controllable power switches (in this project, only Thyristors are used for uncontrolled rectification).
The aim of using controlled rectification is that it enables the designer to control the output dc voltage to the desired values. This can be achieved by varying the time of “forced” conduction i.e. the “turn on” of the thyristors in such a way that the average value of vPn (VPn) and average value of v Nn (V Nn) adds up to yield the desired Vd at the output. Consider the following circuit diagram of a typical controlled 6-pulse rectifier using Thyristors.
- 19 -
VPn
Vd
V Nn
Fig. 2.9: Controlled 6-Pulse Full-Bridge Rectifier
By adjusting the firing angle of the thyristors (α), the potential difference between the common point of the cathode of the top group of the thyristors to the ground (vPn) and the potential difference between the common point of the anode of the bottom group of the thyristors to the ground (v Nn) can be varied and hence vd can be varied. The effect of α on the average dc output of vd (i.e. Vd) is best described in the following figure. vd
vPn
Vd vref
VPn 0
α
V Nn
v Nn Fig. 2.10: Effect of α on V d
where, Vd = VPn - V Nn.
- 20 -
From the above equation, knowing that VPn and VNn can be varied, then the range of output voltages that can be obtained is
-Vd ≤ Output voltage ≤ Vd
In this project, it is assumed that the input voltage into the controlled rectifier is higher (due to the over-voltages of the self-excited induction generator). Hence, the proposed method of operation of the rectifier is the half-controlled rectifier which utilises 3 thyristors and 3 diodes for the top group and the bottom group respectively.
Operation of the half-controlled 6-pulse rectifier is similar as described in Fig. 2.10. However, the only difference is that only VPn waveform can be varied. Therefore, the range of output voltages that can be obtained is between
0 ≤ Output voltage ≤ Vd
By using the half-controlled 6-pulse rectifier, not only the output can be varied between the range of zero to Vd, but also from the economical and design point of view it will certainly be cheaper to design and easier to implement.
- 21 -
2.5.2 DC to AC Conversion
The operation of a 3-φ inverter is quite similar to that of the 3-φ rectifier. The gating signal pattern (PWM) is produced by the difference between the carrier frequency signal (triangular) and the reference signal (sinusoidal) which is often known as modulation ratio (ma). Moreover, the gating signal for two IGBTs in one leg must be directly opposite of each other. Consider the 3-φ inverter circuit diagram shown below.
Fig. 2.11: 3-φ Inverter Circuit Diagram
In one leg of phase a, when the upper IGBT conducts, the lower IGBT should not conducts and vice versa. This goes to other phases of the inverter as well. The modulation ratio used in this project is ≤ 1 (which is on the linear region of ma). In this case, the peak value of the fundamental frequency component in one of the inverter leg is
VAN1( pk )
= ma
- 22 -
Vd 2
By using simple algebra, the rms value of the fundamental frequency component of line-to-line voltage is
Vline1( rms)
=
3 2 2
ma Vd
The principles and the other aspects on the inverter, such as harmonic contents, line notching, effect of line inductances, and so on, are discussed in great detail in [19]. The waveforms obtained from the simulation in the project is expected to be similar to those described in [19].
2.5.3 DC – DC Converter
The use of the dc to dc converter is to obtain a desired value of dc which differs from the input. There are some types of dc-to-dc converters. They are:
• Step Down (Buck) Converter • Step Up (Boost) Converter • Buck-Boost Converter • Cúk dc-dc Converter • Full-Bridge dc-dc Converter
In these converters, one or more switches are utilised to control the dc output voltage to equal a desired level. This can be achieved by controlling the switch on and off durations i.e. ton and toff which is generated by comparing a signal-level control voltage vcontrol with a sawtooth waveform as shown in the following figure.
- 23 -
v sawtooth vcontrol
t On Control Signal ton
Off
toff ts
Fig. 2.12: Control Signal for DC-DC Converter
The control signal is then fed into the switches. Hence, the duty ratio of the switch can be expressed as follows where |Vst| is the peak of the sawtooth waveform.
D=
t on Ts
=
v control Vst
In this project, it is desirable to have higher or lower level of dc simultaneously and an unidirectional power flow. Hence, only the analysis of both Buck-Boost converter and Cúk converter is considered.
2.5.3.1 Buck-Boost Converter
As the name suggests, the Buck-Boost converter is used in applications where dc output voltage can be either higher or lower than the input voltage. Also, the output voltage has negative polarity with respect to the common terminal of the input voltage. As mentioned in [19], the output-to-input voltage conversion ratio of this converter is
V0 Vd
=D
1 1− D
- 24 -
Hence, this allows the output voltage to be higher or lower than the input voltage depending on the switch duty ratio D. The circuit diagram of the converter is shown in the following figure. Id +
+
Vd
VL
_
IL C
VO
R
L _
_
+ IO
Fig. 2.13: Buck-Boost Converter Circuit Diagram
Basic operation of this converter is discussed in detail in [19].
2.5.3.2 Cúk Converter
The circuit diagram for the Cúk converter is shown in Fig. 2.14. Similar to the BuckBoost converter, the output voltage of the converter has a negative polarity with respect to the common terminal of the input voltage. The capacitor C1 is used as the primary means of storing and transferring energy from the input to the output.
iL1
+
L1 +
VL1
C1 _
+
VC1
L2 _
_
VL2
Vd
iL2
_
+ C
Vo
_
R
+ Io Fig. 2.14: Cúk Converter Circuit Diagram
Analysing the circuit diagram, the average voltage across the capacitor C1 at steady state can be obtained as follows noting the average voltage across the inductors VL1
- 25 -
and VL2 are zero. The equation shows that VC1 is definitely larger than Vd and Vo which is one of the disadvantage of using the Cúk converter.
VC1 = Vd + Vo
Similar to the Buck-Boost converter, the input-output voltage ratio is
V0 Vd
=D
1 1− D
This enables the output voltage to be higher or lower depending on the duty ratio of the switch. The basic operation of the converter is discussed in detail in [19].
2.6 Past Investigations
2.6.1 Simulation of Wind-Diesel Hybrid Systems
A Simulation of wind-diesel hybrid system was analysed by P. K. T. Tan of Curtin University of Technology in his final year thesis. The simulation of the hybrid system was based on the EMTDC / PSCAD software package.
The report basically separated into two parts. In both parts, theories are first stated to support the principles behind the working of the system. Each part then shows the detailed construction of the relevant components followed by their simulation results. In his project, the induction generator was modelled and analysed by its d and q axis. In his proposed design of the hybrid system, the electricity produced by the wind
- 26 -
turbine is fed into a power electronic converter which rectifies the ac sinusoidal voltage to dc and then inverted again to the ac bus system directly to the consumer load.
The size of the capacitor connected on the dc bus of the converter is unrealistic (100,000
µF) since the size of the capacitor is significant for the relative voltage
rating of the system. Also, the ripple voltage after the diode bridge does not necessarily be ‘ripple free’. So, a specified ripple voltage after the diode bridge is necessary to determine value of the capacitor. TOSHIBA AC/DC/AC 6 pulse converter that is available in the laboratory was used to start the induction motor from a 440V, 50Hz supply (the same machine was used in the experiment of the noload test and the blocked rotor test). The schematic diagram of the converter shows that the capacitance value is 2200 µF for the particular power rating.
Moreover, in his thesis, the gating signal for the inverter side of the AC/DC/AC converter is incorrect. Consider only one leg of the inverter (where the gating signal is G1 and G4). From his result of the simulation, it shows that the gating signal G4 is 180 degrees displaced from G1. Considering that the signal G1 and G4 must be directly opposite of each other, this result is unacceptable. Furthermore, for a 12 pulse inverter, the gating signal for one leg of one phase must be 30 degrees displaced from the gating signal for the other leg of the same phase. This is confirmed by using PSIM demo (power electronic software tool). Hence, his design on the AC/DC/AC converter is inaccurate.
- 27 -
2.6.2 Self-excited Induction Generator
Stand-alone self-excited induction generators provide a significant reduction in system initial costs and they have several advantageous due to the ruggedness and low maintenance requirements of induction machines. Moreover, the self-excitation of the induction generator causes voltage to collapse rapidly when overloaded. This means that the self-excited induction generator is self-protected. On the other hand, there are some drawbacks from using self-excited induction generator. The inability to control the voltage and frequency when the load varies on stand-alone application
Several technical papers had been published in analysing the behaviour of selfexcited induction generator for stand-alone wind power application. Tandon et. al. [4] studied the onset of self excitation and minimum capacitance requirements based on the characteristic polynomial obtained from transient representation of the machine. This method requires sets of results obtained from the numerical solution to the characteristic polynomial satisfying certain criteria before inferring on the minimum capacitance at which self-excitation occurs for a particular speed and load.
Malik and Mazi [20] suggested an indirect procedure, based on the steady state equivalent circuit model to test the self-excitation of an induction generator. The indirect methods involve solutions requiring some initial guess in a trial and error procedure. Al-Jabri and Alolah [5] dealt with different limiting aspects based on per phase stead state T-form equivalent circuit.
- 28 -
Chakraborty et. al. [6] analyses the excitation requirements for stand alone three phase induction generator at different combination of speed, load and excitation capacitance using the inverse-model for the steady state equivalent circuit. Unlike any other methods, this method involve no trial and error procedure and the model used in paper is the inverse-Γ circuit model since the T-form circuit models are actually more complex. Moreover, since the actual rotor variables are not required, the inverse-Γ model can be applied for analysis of self –excited induction generators with no loss of information and accuracy.
These authors [4, 5, 6, 20] only discusses minimum excitation requirement for the induction generator to self-excite. The problems arise from the regulation of the frequency and voltage of the induction generator. Most applications operate at specified voltage and frequency. Tang and Zadavil [17], and Ouhrouche et. al [18] discussed the transient behaviour of the self-excited induction generator in windfarm application when disconnected from the grid. Both papers showed that when the capacitor connected at the terminal of the induction generator is sufficient enough for the self-excitation, the terminal voltage increased from its rated value to a new value which is much higher than its rated voltage (over-voltage). In these cases, both papers showed that the voltage could be as high as ten times its rated value. Moreover, Ouhrouche et. al. [18] showed that the frequency at new operation condition after disconnection from the grid was also increased. In their paper, it is stated that it is not recommended for the induction generator to be connected back to the grid after its disconnection from the grid itself.
- 29 -
2.6.3 Problem Formulation
In stand-alone mode, self-excitation on the induction generator depends on the capacitor size connected on the stator terminals of the machine, speed of operation, and the load [6]. Once self-excitation occurs, new operating conditions of the generator i.e. both the terminal voltage and frequency at new values will be achieved. In order to maintain the voltage and frequency of the generator at a specified value, some means of control has to be implemented.
Feedback control is somehow cannot be implemented into the system since varying the torque or speed of the generator, the self-excitation might not occur since the excitation capacitance is valid only for a particular combination of speed and the load. Realising the problems in this particular area, this paper investigates the feasibility of using power electronic converters in regulating the frequency and voltage of the selfexcited induction generator. In this case, the AC/DC/AC converter is modelled and modified by the addition of dc – dc converter in the dc bus between the rectifier and the inverter. Buck-Boost converter is used in the study since in the Cúk converter as discussed in section 2.5.3.2, average voltage across capacitor C1 is higher as compared to the input and output voltage. Thus, the rating of the capacitor for the design purposes has to be higher and hence the size of the capacitor comes into consideration as the voltage rating goes higher.
- 30 -
The proposed system is shown in the following figure. Temporary voltage source is connected into the system at the initial stage to achieve a steady state operation of the generator before it is isolated or used in the stand-alone mode of operation.
Temporary Voltage Source
Prime Mover
Power Electronic Converter
IG
Self-Excitation Capacitors Fig. 2.15: Proposed System
- 31 -
3-φ Resistive Load
3. MODELLING OF THE SYSTEM
3.1 Self-Excited Induction Generator Model
Self-excited induction generator model is modelled in PSCAD software package. The parameters of the induction machine are based on the recorded results from [6]. Therefore, the machine parameters is
Parameters Voltage Rating 400V Current Rating 10.9A Rated Power 7.5HP (5.593 kW) Stator Resistance 1.23Ω Rotor Resistance 1.105Ω Stator Leakage Reactance 2.756Ω Rotor Leakage Reactance 2.756Ω Mutual Reactance 91.2Ω
Value
Table 3.1: Induction Machine Parameters
Hence, with the information given, the values can be converter into it’s per unit value. Thus, taking the voltage phase rating and the current rating as the base values, the base impedance can be calculated as
Z base
=
V base I base
=
230.94 10.9
= 21.1872Ω
Therefore, knowing the base impedance of the machine, the machine parameters can be converted into its per unit value. The following table summarises the machine parameters.
- 32 -
Parameters Stator Resistance Rotor Resistance Stator Leakage Reactance Rotor Leakage Reactance Mutual Reactance
Per Unit Value 0.058 0.052 0.13 0.13 4.3
Table 3.2: Induction Machine Parameters (Per-Unit Values)
The simulation parameters of the induction machine are shown in the following figure. Some parameters are not provided in [6] and therefore are assumed. For example, the model for the induction machine is assumed to have a single cage and hence the second cage resistance for the PSCAD model is set to be infinite (in this case higher in its per unit value).
Fig. 3.1: Induction Generator Parameters
The machine is operating at 50 Hz and hence the base angular frequency is
ω = 2πf = 2π × 50 = 314.16 rad / s
The SEIG model in PSCAD is shown in Fig. 3.2. It can be seen from the figure that an external voltage source is connected at the stator terminal of the induction generator. The external voltage source is connected until it is in its steady state
- 33 -
operation and then the voltage source is disconnected by sending a tripping signal to the circuit breaker (BRK1).
Fig. 3.2: SEIG Model in PSCAD
The induction machine is driven above its synchronous speed (w = 1.1pu) so that the machine operates as a generator. Moreover, the induction machine is operating on speed control mode (S = 1). A 3-phase resistive load represents the load and the excitation capacitors (86.5µF) are connected at the stator terminals of the induction generator.
- 34 -
In the simulation of the SEIG, the minimum capacitance value to ensure the selfexcitation on the induction generator was found to be 86.5µF in which the machine uses the parameters that are exactly similar to [6]. However, the recorded result from [6] shows that the SEIG machine was able to self-excite at 30µF. The main reason behind this discrepancy is due to the fact that the load is vaguely defined in the paper. In this simulation, the connected load is assumed to be a 3-φ resistive load. Hence, the findings from [6] in this case could not be linked with the result of the simulation.
The induction generator is expected to deliver power both to the load and the voltage source before the circuit breaker operates. At initial stage, the source and the shunt capacitance supply the reactive power to the induction generator. But when the source is disconnected from the system, only the excitation capacitors that supply the reactive power to the generator so that the generator can deliver power to the load. The main use of the capacitors is to maintain the air-gap voltage between the stator and the rotor of the induction generator.
The following figure shows the simulation results obtained from the self-excited induction generator at 3410µF capacitive compensation level. The results show clearly that initially the real power delivered to the load is increased due to the stator overvoltage. In this case, negative value of the output power represents the generated power by the induction generator. On the other hand, the induction generator absorbs the reactive power provided by both the voltage source and the excitation capacitors before the circuit breaker operates.
- 35 -
Source Connected
Islanding
Fig. 3.7: Simulation Results of Self-Excited Induction Generator Parameters
The generator speed is also 1.1 pu as mentioned earlier to run the generator above its synchronous speed as to generate power instead of absorbing power and the mechanical torque is negative since it is driven externally by the wind turbine shaft.
The following figure shows the stator rms voltage at 86.5µF capacitive compensation level. In this simulation, the external voltage source is disconnected from the induction generator at 0.5 seconds. The result shows clearly that, after islanding, the
- 36 -
“theoretical” steady state operation is reached at very high generated voltage levels: more than five times the rated voltage of the induction generator. Moreover, the steady state operation is achieved at approximately 6.5 sec (with transient of 6 seconds). This result agrees with that of [18] in a way that the overvoltage will occur after the disconnection of SEIG from the grid.
Islanding
Fig. 3.3: RMS Stator Line Voltage
Fig. 3.4 also shows the induction generator stator frequency at 86.5 µF capacitive compensation level.
- 37 -
Islanding
Fig. 3.4: Stator Frequency
However, at higher capacitive compensation levels, the transient of the stator rms voltage and the frequency is smaller and the frequency is decreased as shown in Fig. 3.5 and Fig. 3.6 respectively.
1000µF
2000µF Islanding
3000µF
Fig. 3.5: RMS Stator Voltage At Higher Compensation Levels
- 38 -
Islanding 1000µF
2000µF 3000µF
Fig. 3.6: Stator Frequency At Higher Compensation Levels
The simulation results (maximum voltage and its corresponding time), for various capacitive compensation levels, are summarised in Table 3.3. Note that the circuit breaker operates at 0.5 sec . C ( µ F) 86.5 500 1000 2000 3410
Vmax (kV) 2.08 1.672 1.213 0.94 0.783
t (sec) 6.7 0.67 0.615 0.608 0.767
F (Hz) 51.98 49.6 45.58 39.615 35.33
Table 3.3: Maximum Overvoltages Magnitude
The table shows that as the capacitive compensation level increased, the maximum overvoltage magnitude at the stator terminal is decreased. Moreover, steady state operation of the induction generator can be achieved in a shorter period of time. Similarly, as the capacitive compensation level is increased, the operating frequency of the induction generator is decreased.
- 39 -
Hence, in this report, the main objective of this project is in the regulation of the output voltage magnitude and frequency to a specific value by using power electronic converters. The design of such converters is discussed in the next section.
- 40 -
3.2 AC/DC/AC Converter Model
The analysis of the AC/DC/AC converter is divided into three sections. The first section analyses the conversion from ac to dc with uncontrolled 6-pulse full-bridge rectifier or with half-controlled 6-pulse full-bridge rectifier. The second section evaluates the conversion from dc to dc using the Buck-boost converter. The third section examines the conversion from dc to ac using a Pulse Width Modulated (PWM) Voltage Source Inverter (VSI). Finally, the analysis of the whole system combining different converters to achieve regulated output voltage and frequency of SEIG is presented.
3.2.1 AC to DC Conversion
The design of the ac to dc converter is discussed in this section. Ac to dc conversion can be achieved by:
• Uncontrolled 6-pulse full-bridge rectifier • Half-controlled 6-pulse full-bridge rectifier
3.2.1.1 Uncontrolled 6-Pulse Full-Bridge Rectifier
The uncontrolled 6-pulse full-bridge rectifier utilises 6 diodes as its switching devices. In the design of the diode rectifier as well as other converters, a pure sinusoidal voltage with constant magnitude and frequency is used at the input as to ease the design of the converters. Later in the project, the output voltage of the SEIG
- 41 -
is to be used as the input to the converters. The diode bridge rectifier circuit is shown in the following figure.
Fig. 3.7: PSCAD Model of Diode Bridge Rectifier
A 415V, 50Hz input is fed into the diode bridge rectifier. The output voltage and current at the output of the diode bridge rectifier is shown in the following figure.
Fig. 3.8: Voltage and Current At Rectifier Output
- 42 -
The ripple voltage at the output of the rectifier is very much dependent on the capacitor connected at the output of the rectifier. The higher the capacitor value, the smaller the ripple voltage at the rectifier output. In this case, a 500µF capacitor is used at the output of the rectifier which yields a voltage ripple of approximately 25.9V.
3.2.1.2 Half-Controlled 6-Pulse Full-Bridge Rectifier
The half-controlled 6-pulse rectifier utilises 3 thyristors at the top group and 3 diodes at the bottom group as shown in the following figure. One of the bottom group of the diode conducts when the cathode of the diode is at the lowest potential compared to the other two diodes. On the other hand, the thyristors can only conduct when the firing angle (α) of the thyristors is applied.
Fig. 3.9: Half-Controlled 6-Pulse Full-Bridge Rectifier Circuit
- 43 -
The control signal for the thyristors are governed by the following simple equation. It can be seen that the firing angle (α) is a function of the input line-to-line RMS voltage (VLL) and the reference average dc voltage (Vd1) that is specified according to the desired value .
2π Vd1
α = cos −1
3 2 VLL
− 1
PSCAD software enables the manipulation of such equation into the circuit diagram. This is shown in the following figure where b = α.
Fig. 3.10: Thyristor Firing Angle ( α ) Circuit
However, the information given by this equation is in terms of degrees and it cannot be fed into the thyristors since the thyristors only takes a train pulse signal with a magnitude of 1 for a short period of time to turn on. Thus, the output “b” is fed into a more complex circuit diagram to produce a pulse train with a delay angle “b” and a magnitude of 1. The circuit diagram is shown in the following figure.
- 44 -
Fig. 3.11: Alpha Train Pulse Signal Generator Circuit Diagram
The input “b” is used as the phase angle for the sine function block with the magnitude and frequency is set at 1kV and 50Hz respectively. This sinusoidal function is then fed into a comparator block such that if A ≥ B the output is 1, and if A < B the output is zero.
On the other hand, a reference zero phase angle sinusoidal reference must be chosen. In this case, the phase voltage of the sinusoidal input is chosen as the reference. Similar to the previous explanation, this sinusoidal reference is fed into a function block such that if A ≥ B the output is 1, and if A < B the output is zero. Then, the output of both comparators are fed into an AND gate which will only yield 1 if both input are both high (i.e. at 1). The following diagram shows the output of both comparator as well as the output of the AND gate as to aid the explanation.
- 45 -
Bottom Comparator Output
Top Comparator Output
AND Gate Output
Fig. 3.12 Comparator Outputs and AND Gate Output For Alpha
Thus, the output of the AND gate is then fed into an edge detector block which only gives a pulse output if there is a transition (zero to one) at the input of the block. This output is then fed into an integrator which is resetable at 300Hz. The output of this integrator yields the firing angle for T1.
With this circuit implemented by taking “b” as the input yields the pulse train as shown below. It can be seen from the figure that the train signal has a period of 0.02 sec (i.e. 50Hz) and has a magnitude of 1.
- 46 -
α
Fig. 3.12: Thyristor Train Pulse Waveform
The above waveform is applied into thyristor 1 (T1) at the top group of the rectifier. For the other two thyristors (T3 and T5), the waveform of T1 is phase shifted by 120° and 240° for T3 and T5 respectively. Therefore, T1 is fed into two circuits as shown in the following figure to achieve the firing angle for T3 and T5.
Fig. 3.13: T3 and T5 Firing Angle Circuit
After successfully design the circuit for the firing angle of the thyristors, the analysis of the half-controlled rectifier can be conducted. As mentioned earlier, the output of a rectifier (Vd) is a result of the difference between VPn, which is the potential difference between the common point of the cathode of the thyristors, and V Nn, which is the potential difference between the common point of the anode of the
- 47 -
diodes. To evaluate that the circuit operation is valid, 1kV line-to-line voltage source is connected at the input of the rectifier and the desired output of the rectifier is set to be at 750V. The following waveform shows the output voltage of the half-controlled rectifier, VPn, and V Nn.
Fig. 3.14: V Pn , V Nn , and V d Waveforms
It can be seen that the average output of the half-controlled rectifier is approximately equal to 747V which is close to the desired output of 750V.
- 48 -
3.2.2 DC to DC Conversion Using Buck-Boost Converter
Dc to dc converter design is evaluated in this section. The Buck-Boost converter design is mainly discussed in this section because it is assumed that the dc voltage level can be adjusted to any desired value.
The following diagram shows the circuit diagram of a typical Buck-Boost converter. It consists of one switching device (in this case is GTO) which enables to turn on and off depending on the applied gating signal.
Fig. 3.15: Buck-Boost Converter Circuit Diagram
The gating signal for the GTO can be obtained by comparing the sawtooth waveform at high switching frequency (in this case is at 20kHz) with a control voltage at constant magnitude. The control voltage is varied such that if the input to the converter is varied, the output of the converter is maintained at a constant desired value. The magnitude of the control voltage can be obtained by rearranging the following equation. Vd 2 Vd1
=
D 1− D
,
where D =
- 49 -
vcontrol
Vst
where, |Vst| = magnitude of sawtooth waveform, Vd1 = input dc voltage, Vd2 = desired output dc voltage, and D = duty cycle.
Rearranging the above equation yields vcontrol
=
Vst Vd1 +1 Vd 2
The above equation is implemented in PSCAD and is shown in the following figure.
Desired DC Voltage
Fig. 3.16: Firing Angle For GTO
To evaluate the validity of the converter design and operation, a 1kV dc voltage is connected at the input of the Buck-Boost converter and the desired output dc voltage is set to be at 650V. The control voltage and the gating signal for the GTO is shown in the following figure where Vcon is the control voltage, T is the sawtooth waveform, and G is the gating signal for the GTO.
Fig. 3.17: Gating Signal For GTO
- 50 -
It can be seen from the figure that the control voltage is at 0.33 and hence the corresponding gating signal for the GTO is obtained. With this gating signal, the output voltage of the converter is shown in the following figure.
Fig. 3.17: Input Voltage Vd and Output Voltage Vo of Buck Boost Converter
The output of the Buck-Boost converter is approximately 636V which is close to the desired output voltage of 650V. The above figure also shows that the output dc voltage of the Buck-Boost is not entirely ripple “free”. In other words, there is a small ripple at the output voltage which was found to be approximately 20V. This ripple voltage magnitude is very much dependent on the size of the capacitor and inductor values in the converter circuit.
- 51 -
3.2.3 DC to AC Conversion Using PWM VSI
Voltage source inverter using pulse width modulation topology is used in the conversion from dc to ac. The model of this inverter in PSCAD is shown in the following figure. It can be seen that the inverter consists of 6 IGBTs. Each leg of the inverter represents each phase of the inverter output. Moreover, the reference signal in the gating circuit for the IGBTs in each leg of the inverter is displaced by 120°. This can be achieved by shifting the reference signal (sinusoidal) of the PWM.
Fig. 3.18: PWM VSI Supplying Resistive Load
The gating signal for the IGBTs can be created by comparing a sinusoidal reference voltage with a triangular waveform. This is implemented in PSCAD as shown in the following circuit. The triangular waveform has a frequency of 5 kHz and the reference sinusoidal voltage operates at 50 Hz. Basic operation of the circuit is that if the sinusoidal voltage is higher than the triangular waveform, the output of the comparator is one. Similarly if the sinusoidal voltage is lower than the triangular waveform, the output of the comparator is zero.
- 52 -
Fig. 3.19: PWM Circuit
Graphical explanation on the method is shown in the following figure.
Fig. 3.20: PWM Pattern For IGBT Switching
Referring to Fig.3.18, the gating signal for G1a must be directly opposite of the G4a as shown in the following figure. This is to avoid short circuit of the dc source. Keep in mind that the circuits are assumed to be ideal and blanking time is not implemented in the circuit design.
- 53 -
Fig. 3.21: PWM Patterns in One Leg of VSI
The modulation index (ma) of the converter is varied such that if the input voltage to the inverter is varied the magnitude of the fundamental frequency component of the inverter output voltage is fixed. Hence, input output relationship which takes into account the modulation index, i.e. the ratio of the sinusoidal reference magnitude with the sawtooth magnitude, must be formulated. This can be achieved by rearranging the following equation where (VLL1)RMS is the rms value of the fundamental frequency component of the line-to-line voltage, |Vsin| is the magnitude of the sinusoidal reference voltage, and |Vtri| is the magnitude of the carrier voltage i.e. the sawtooth waveform.
(VLL )RMS = 1
3 2 2
m a Vd
, where m a
Rearranging the question yields, Vsin
=
2 2 VLL1 3
RMS
Vd
- 54 -
⋅ Vtri
=
Vsin Vtri
In evaluating the design of this inverter, a 800V dc voltage is used as the input to the inverter and the desired output phase voltage at fundamental frequency is 240V (RMS) or 339V (peak). The inverter output phase voltages with reference to the zero potential between two capacitors are shown in the following figure. It can be seen that the magnitude of the waveforms is half of that of the input voltage i.e. 400 volts dc. This is because the input dc voltage is imposed into the two capacitors on the dc bus. Half of the input voltage is imposed in each capacitor and when the top IGBT is turned on the voltage imposed on the output of one leg is half of the input voltage with respect to the neutral node between the two capacitors and negative half of the input voltage is imposed when the bottom IGBT is turned on. Furthermore, it can be seen from the waveform that each phase is displaced by 120 degrees from each other.
120°
60°
120°
60°
- 55 -
Fig. 3.22: Inverter Voltage Output With Respect To Zero Potential Between Capacitors on DC Side
Similarly, the inverter output voltage with respect to dc negative side are shown in the following figure. In this case, the magnitude of the waveforms is equal to the input voltage. Again, the phase voltages are displaced by 120 degrees. The line voltage of the inverter output can be obtained by taking the difference between the two phase voltages as shown in the following figure. The difference between VaN and V bN creates the line voltage of the inverter output.
Fig. 3.23: Inverter Phase Voltage Output With Respect To DC Neutral
Finally, the phase voltage of the inverter output with respect to neutral ground is shown in the following figure. In this figure, the phase difference between two phase voltages are more obvious compared to that of the phase voltages describes previously.
- 56 -
Fig. 3.24: Inverter Phase Voltage Output With Respect To Neutral Ground
In order to determine the magnitude of the fundamental component of the inverter output phase voltage with respect to the neutral ground, Fast Fourier Transform (FFT) was conducted on the above waveform. The result of FFT on the waveform is shown in the following figure. It can be seen that the magnitude of the fundamental component of the inverter output phase voltage is approximately 340.8V with a Total Harmonic Distortion (THD) of 29%
- 57 -
340.8V
Fig. 3.25: FFT of PWM VSI Output Phase Voltage
- 58 -
3.2.4 AC/DC/AC Converter
In this section, the combinations of the converters discussed in the previous sections will be evaluated. Three designs are presented in this report and they are 1. AC/DC/AC converter utilizing diode rectifier – PWM VSI 2. AC/DC/AC converter utilizing half-controlled rectifier – PWM VSI 3. AC/DC/AC converter utilizing diode rectifier – Buck-boost converter – PWM VSI In the analysis of this converters, a 1kV line-to-line rms three phase voltage source acts as the input for the converters. In the next chapter, the complete system consists of the self-excited induction generator connected to these three converters is presented.
3.2.4.1 Diode Rectifier – PWM VSI
The combination of the diode and PWM VSI yields the AC/DC/AC converter of the 1st design as shown in the following figure. The rectification of the ac voltage using the diode bridge rectifier is then fed into the PWM voltage source inverter. Diode Rectifier
PWM VSI
st
Fig. 3.26: Diode Rectifier With PWM VSI Converter Design (1 Design)
- 59 -
The output voltage and current of the rectifier (DC side voltage and current) is shown in the following figure. The voltage ripple on the output of the rectifier was found to be 18V (utilising 2500µF capacitor on the output of the rectifier) and the current peak at the rectifier output is 230A.
st
Fig. 3.27: Rectifier Output Voltage and Current of 1 Design
The output voltage waveform of the rectifier circuit is fed into the inverter and the resultant output phase voltage with respect to neutral ground, and the output line-toline voltage is shown in the following figure where Vinva is the inverter output phase voltage, and Vlinvab is the inverter output line-to-line voltage.
- 60 -
Fig. 3.28: Inverter Output V inva and V linvab of 1 st Design
The simulation result of the phase voltage waveform differs from the result of the phase voltage of the PWM VSI shown in Fig. 3.24. In order for one to argue that this result is acceptable, the circuit is implemented in PSimDemo Power Electronic Software Simulation as shown in the following figure. The result of the simulation of the phase voltage at the inverter output shows similarity to that in Fig. 3.28. From this finding, one can guarantee that the result of the inverter output phase voltage shown in Fig. 3.28 is correct.
- 61 -
Fig. 3.29: PSIMDemo Model of Diode Rectifier With PWM VSI
The magnitude of the fundamental frequency component of the inverter output phase voltage is found to be 336V with a THD of 50.8% as shown in the following figure.
336V
Fig. 3.30: FFT of Output Phase Voltage Of Diode Rectifier PWM VSI Converter Design
- 62 -
3.2.4.2 Half-Controlled Rectifier – PWM VSI
In this section, the combination of the half-controlled rectifier and PWM VSI yields the AC/DC/AC converter of the 2nd design as shown in the following figure. The rectification of the ac voltage using the half-controlled rectifier is then fed into the PWM voltage source inverter. Half-Controlled Rectifier
PWM VSI
Fig. 3.31: Half-Controlled Rectifier With PWM VSI Converter Design
In this design, the input to the half-controlled rectifier is set to 2kV line-to-line rms three phase voltage source is utilised. The desired output of the rectifier is set to 1.5 kV. Thus, the output voltage and current of the half-controlled rectifier is shown in the following figure. The ripple voltage at the inverter output is 32.14V peak to peak and the current peak is 98A.
- 63 -
Fig. 3. 32: Half-Control Rectifier Output Voltage and Current Of 2nd Design
The output of the half-controlled rectifier is then used as the input to the inverter. The output line-to-line voltage of the inverter is shown in the following figure. As can be seen, the voltage has a period of 0.02 seconds (i.e. 20 ms) which corresponds to a 50Hz output frequency.
nd
Fig. 3.33: Inverter Output Phase Voltage and Line-To-Line Voltage of 2 Design
When FFT is applied on the output phase voltage of the inverter, the magnitude of the fundamental frequency component is 334V with a THD of 61% as shown in the following figure.
- 64 -
334V
Fig. 3.34: FFT of Inverter Output Phase Voltage Of 2nd Design
3.2.4.3 Diode Rectifier – Buck-Boost Converter – PWM VSI
The combination of diode rectifier, buck-boost converter, and PWM VSI yields the AC/DC/AC converter of the 3rd design as shown in the following figure. Basically, the output of the diode rectifier is fed into the buck-boost converter and the output of this converter acts as the input for the inverter. Diode Rectifier
Buck-Boost Conv.
PWM VSI
Fig. 3.35: Diode Rectifier With Buck-Boost Converter and PWM VSI Converter Design (3rd Design)
- 65 -
Since, the output voltage polarity of the buck-boost converter is negative with respect to the input voltage (i.e. from the output of the diode rectifier), the PWM VSI configuration has to be modified such that the positive node of the inverter input is connected to the positive node of the output of the buck-boost converter.
In this design, the input to the diode rectifier is a 1kV line-to-line three phase voltage source, the desired output of the buck-boost converter is set to 800V, and the desired magnitude of the fundamental frequency component of the inverter output phase voltage is set to 339V. The following figure shows the output voltage waveform of the rectifier and the output voltage waveform of the buck-boost converter. The simulation result shows that the buck-boost converter successfully controls the rectified output voltage of 1.4kV to 800V.
800V
(a)
(b)
Fig. 3.36: (a) Output Voltage Of Diode Rectifier; (b) Output Voltage Of Buck-Boost Converter
The output of the buck-boost converter is then fed into the PWM VSI. The inverter output phase voltage and line-to-line voltage is shown in the following figure.
- 66 -
rd
Fig. 3.37: Inverter Output Phase Voltage And Line-Line Voltage Waveform Of 3 Converter Design
Similar to the previous waveform, the line-to-line voltage waveform has a fundamental frequency of 50Hz. However, notice that the output phase voltage of the inverter is in the negative region. This implies that the voltage across the load is also negative. Therefore, detail analysis on this converter has to be done in order to fix the problem. The first step is to analyse the current flows in this converter. Hence, consider the following buck-boost converter conventional current flow diagram.
Id1
Id0
+
+
IL
Vd1
VL
L
_
In
Icap C
Vd2 Id2
_
_
+
Fig. 3.38: Buck-Boost Converter Conventional Current Flow Diagram
It was found that when the GTO of the buck-boost converter is off, there In still flows to the common point of the anode of the diode rectifier which in normal operation there should not be any current flow when the GTO is off. Moreover, Id2 flows in the
- 67 -
negative direction for most of the time during the one switching period as shown in the following figure where G is the gating signal for GTO. Ton
Toff
Fig. 3.39: Current Flows In Buck-Boost Converter
Therefore, knowing the source of this problem, another GTO switch and a diode is implemented in the buck-boost converter as shown in the following figure. Id1
Id0 _
+
+
Vd1
VL
_
_ In
IL
Icap
L
C
Vd2
Id2
+
Fig. 3.40: Modified Buck-Boost Converter
It can be seen that the second GTO is connected at the negative input voltage node of the buck-boost converter to stop the current In to flow when the first GTO is off.
- 68 -
Similarly the diode is connected at the positive output voltage node of the buck-boost to ensure the positive current flow of Id2. The simulation result of this converter is shown in the following figure. It can be seen from the result that when Ga (gating signal for GTO) is off, In is not flowing. Moreover, Id2 flows in the positive direction.
Fig. 3.41: Current Flows in Modified Buck-Boost Converter
Although the current flows in the buck-boost converter is corrected, there is another discrepancy observed. There exist current spikes in the system flowing from Ido to Icap and to Id2. In order to fix this problem, an inductor is connected at the positive output voltage node of the buck-boost converter.
The inverter output phase voltage and line-to-line voltage waveforms after the modification of the buck-boost converter are shown in the following figure.
- 69 -
Fig. 3.42: Inverter Output Phase Voltage And Line-Line Voltage Of 3rd Converter Design
Applying FFT on the output inverter phase voltage yields a magnitude of fundamental frequency component at 341V with a THD of 56.6%.
- 70 -
4. RESULTS OF THE COMPLETE SYSTEM
After the design of the three converters is completed, the next step is to combine individual converters with the self-excited induction generator in order to obtain the result. Thus, there are three complete system that are presented in this section and they are 1. Self-excited induction generator connected to the first converter design (i.e. diode rectifier with PWM VSI) 2. Self-excited induction generator connected to the second converter design (i.e. half-controlled rectifier with PWM VSI) 3. Self-excited induction generator connected to the third converter design (i.e. diode rectifier with buck-boost converter and PWM VSI)
4.1 System 1
The first system consisting of the SEIG and the first design of the converter is elaborated in this section. The following figure shows the first combined system. As can be seen, the unregulated output voltage of the self-excited induction generator is fed into the first converter design which consists of the diode rectifier with PWM VSI. At the initial stage, the generator is supplied by an external voltage source and when the induction generator is at its steady state operation, the voltage source is disconnected from the system.
- 71 -
Fig. 4.1: Complete System 1 Circuit Diagram
The output rms voltage of the induction generator is shown in the following figure. The result shows that when the external voltage source is disconnected at 0.4 seconds, the new steady state operating voltage of the induction generator is 1.18kV which is achieved at 0.8 seconds. The maximum overvoltage occurred is 1.3kV at 0.57 seconds.
Islanding
Fig. 4.2: RMS Voltage Of SEIG
- 72 -
On the other hand, the output of the rectifier, as shown in the following figure, indicates that the maximum overvoltage is 3.16kV at 0.57 seconds. The steady state rectifier output voltage is 2.2kV which is achieved at 1.2 seconds. The dc peak-to peak ripple voltage is 10.68V, which is approximately 0.5% of the peak ripple voltage.
Islanding
Fig. 4.3: Rectifier Output Voltage Of System 1
This rectified voltage is then fed into the PWM VSI and hence the phase voltage and the line-to-line voltage are shown in the following figure. Both inverter output voltages have a period of 20ms which corresponds to a 50Hz output frequency.
Fig. 4.4: Inverter Output Phase Voltage And Line-to-Line Voltage Of System 1
- 73 -
When FFT is applied on the phase voltage of the inverter output, the magnitude of the fundamental frequency component is 334V with a THD of 48%
334V
Fig. 4.5: FFT of Inverter Output Phase Voltage Of System 1
- 74 -
4.2 System 2
Self-excited induction generator connected with the second design of the converter (i.e. half-controlled rectifier with PWM VSI) yields the second system which is shown in the following figure.
Fig. 4.6: Complete System 2 Circuit Diagram
The overvoltage at stator terminal of the SEIG is rectified to a lower value using the half-controlled rectifier and the output of the rectifier is then acts as the input to the inverter. In the previous design of the firing angle circuits for the thyristors (shown in Fig. 3.11), they consider the input to the rectifier is at 50Hz operation. In this case, the stator frequency of the induction generator varies according to the rotor speed, excitation capacitors, and the load (as mentioned in the literature review). This frequency variation has to be taken into consideration as well. Thus, the circuit shown in Fig. 3.11 is modified into the following circuit which takes into account the frequency variation of the SEIG and the disconnection time of the external voltage source that is connected to the SEIG. The reason to consider the disconnection time
- 75 -
of the voltage source is that the firing angle of the thyristors is at zero degrees until the SEIG operates at its steady state. Only when the SEIG new steady state operating condition is reached, then the firing angle obtained from the circuit shown in Fig. 3.10 is applied.
Disconnection Time
Frequency Monitor
Frequency Monitor
Fig. 4.7: Modified Alpha Circuit
The following figure shows the output of the half-controlled rectifier which shows that the dc voltage ripple peak-to-peak is approximately 43.2V which is 2.5% of the peak voltage ripple and the dc voltage level of 1.73kV.
Fig. 4.8: Half-Control Rectifier Output Voltage Ripple Of System 2
- 76 -
With this dc level at the half-controlled rectifier output, the inverter output line-toline voltage is shown in the following figure which shows that the period of the waveform is 20 msec which corresponds to 50Hz output waveform.
Fig. 4.9: Inverter Output Line-To-Line Voltage Of System 2
The magnitude of the fundamental frequency component of the inverter output phase voltage can be obtained by applying FFT to the phase voltage waveform as shown in the following figure. In this case, the magnitude of the fundamental frequency component of the phase voltage is 341V with a THD of 62%
341%
Fig. 4.10: FFT Of Inverter Output Phase Voltage OF System 2
- 77 -
4.3 System 3
The final system is analysed in this section. It consists of the self-excited induction generator connected with the third design of the converter (i.e. diode rectifier connected to buck-boost converter and then connected to PWM VSI) as shown in the following figure.
Fig. 4.11: Complete System 3 Circuit Diagram
In this system, it was observed from the simulation result that the induction generator never self-excites at any capacitor values. Hence, further analysis was done and it was decided to represent the load at 0.8 power factor lagging as compared to the resistive load only. Hence, in order to achieve a 0.8 power factor lagging, inductors have to be connected in series with the resistor in one phase for the three phases. Thus, to calculate the size of the inductor needed, the following calculation was made. Zload
= R load + jX L = Zload ∠θ°
- 78 -
Manipulating the equation above yields, tan θ =
XL R load
∴ X L = R load ⋅ tan θ = 20 × tan (cos −1 0.8) = 15Ω Therefore, XL
= 2π f L
∴L = =
XL 2π f 15 2π × 50
= 0.0478H = 47.8mH
Thus, by connecting the inductors in series with the resistor, the self-excitation of the induction generator is achieved. The output of the rectifier and the output of the buck boost converter are shown in the following figure. As can be seen from the figure, the dc level of 1.57kV from the output of the rectifier is successfully amplified to 1.8kV at the output of the buck-boost converter. The ripple peak-to-peak voltage at the output diode rectifier is 8.8V which is 0.56% of the peak voltage. In the simulation, the output of the buck-boost as can be seen from the following figure is still in the transient state after 2.8 seconds. However, the steady state is reached at 1.8kV which is at approximately 4 seconds. The steady state waveform cannot be obtained due to the huge number of sample points in the simulation and hence the size of the simulation file and the limitation of the server computer memory size restrict the simulation result.
- 79 -
Fig. 4.12: Rectifier Output And Buck-Boost Converter Output Voltages Of System 3
The inverter output line-to-line voltage is shown in the following figure. It can be seen that the period of the waveform is 20 msec which corresponds to 50Hz frequency waveform. The FFT of the phase voltage yields a voltage magnitude of 337V at fundamental frequency of 50 Hz with a THD of 68% as shown in Fig. 4.14.
Fig. 4.13: Inverter Output Line-To-Line Voltage Of System 3
- 80 -
343V
Fig. 4.14: FFT Of Inverter Output Phase Voltage Of System 3
- 81 -
5. FUTURE WORK
When the objectives of the projects were met, other issues were raised which allows the improvement of this project. However, due to time constraint these issues cannot be implemented and solved during the phase of the project. Thus, future work can be done based on these factors.
Firstly, as can be seen from the results of the simulation across all three complete system designs, the inverter output voltages (whether it is line-to-line voltage or phase voltage) contain high order harmonics at higher magnitude. This can be eliminated by proper selection of harmonic filters such as the low pass filter or a passive filter that is tuned to the specific harmonic that needs to be filtered. Thus, a sinusoidal waveform of 50Hz can be obtained when the filters are implemented at the inverter output.
Secondly, harmonic cancellation can also be implemented in the converter design. This can be achieved by using the following converter topologies.
• 12-pulse rectifier • Multilevel inverters In the 12-pulse rectifier, a wye–wye wye–star transformer is utilised to split the ac voltage and fed into two-cascaded 6-pulse diode rectifier. The addition of both output of these rectifiers makes up the total dc rectified output voltage. On the other hand, the “diode clamped” multilevel inverters utilize combinations of diodes and IGBTs to create a voltage output which represents the sinusoidal voltage at the fundamental
- 82 -
frequency. The PWM VSI design in this report is basically a two level inverter. Other multilevel inverter involves three-level, five-level, and in which the output of the inverter has lower harmonic content (i.e. lower THD) as the level goes up.
Thirdly, the effect of blanking time at the switching of the IGBTs in the inverter operation is not considered in this report since the switches are assumed to be “ideal” as shown in Fig. 3.2.1 which shows the switching pattern for two IGBTs in one inverter leg. The turn-on of a switch in one inverter leg didn’t take into account whether the other switch in the same leg is turned off. In actual situation, one IGBT in one leg can only turned on when the other IGBT on the same leg is turned completely off and vice versa. This is shown in the following figure.
tB
Fig. 5.1: Blanking Time In PWM VSI
- 83 -
It can be seen from the figure that when G1a switch in one inverter leg turns off, the other switch on the same leg (G4a) turns on after a blanking time of tB seconds. Finally, the efficiency of the AC/DC/AC converter is not analysed in this report. The benefit of analysing the efficiency of the converter is that one can use this information to predict how efficient is the converter in regulating the voltage and frequency of SEIG. Also one can do comparisons on the complexity of the converter design and the efficiency of the converter. Power dissipated or loss in the converter circuit generates heat and this information can be further used to determine the requirements of the types of heat sinks, size of the converter, and so on, if the actual hardware implementation is required.
- 84 -
6. CONCLUSION
In this final year project report, the modelling of self-excited induction generator and AC/DC/AC
converter
for
wind
energy
conversion
was
conducted
using
PSCAD/EMTDC software package.
After the brief introduction on the wind energy and the discussion on why induction generator is more suitable in the field of wind energy conversion, the detail analysis and the characteristic of the induction generator was elaborated in detail in which the project objectives were determined through the problem formulation.
The report also looked into the design of power electronic converters in particular the three designs of AC/DC/AC converters. At the beginning of the design phase of the power electronic converters, each converter design namely the diode rectifier, halfcontrolled rectifier, buck-boost converter, and PWM VSI were presented and the proper operation of individual converter was justified before combining them into AC/DC/AC converter. Moreover, the report examines the combinations of three converters namely the combinations of diode rectifier with PWM VSI, the combinations of half-controlled rectifier with PWM VSI, and the combinations of diode rectifier, buck-boost converter, and PWM VSI.
Finally, the complete system consisting of the SEIG connected to individual converters were presented. Satisfactory simulation results of the complete system were obtained at the end of the project.
- 85 -
In conclusion, the self-excitation phenomenon of the induction generator plays an important role in the wind energy conversion. The unregulated stator voltage and frequency of the self-excited induction generator can be regulated by implementing power electronic converters which enable to rectify the ac voltage to dc and then invert the signal back to ac at fixed frequency of 50Hz and a magnitude at the fundamental frequency component of 240V rms (or 339V peak). Recommendations for future work was presented in the previous section which suggested the design of harmonic filters such as the passive low pass filter or tuned filters, and harmonic cancellations by utilizing other converter designs such as the 12-pulse rectifier, multilevel inverters, and so on. Investigations on the hysteresis current controlled voltage source inverter design can be conducted so that comparable results can be done between the voltage controlled voltage source inverter (PWM VSI) and the hysteresis current controlled voltage source inverter.
- 86 -
REFERENCES
[1]
A. Gastli, M. Akherraz, A. Al-Badi, “Modelling and Simulation of a Variable Speed Self-Excited Induction Generator”, International Journal of Renewable Energy Engineering, Vol. 2, No. 3, pp 221-227, December 2000.
[2]
A. Sarre, “Wind “Wind power gathers speed", Australian foundation for science, [Online], Available: http://www.science.org.au/ http://www.science.org.au/nova/037/037ke nova/037/037key.htm, y.htm, 2000.
[3]
A. E. Fitzgerald, C. Kingsley Jr., S. D. Umans, “Electric “Electric Machinery”, Mc Graw Hill Inc. 1992.
[4]
A. K. Tandon, S. S. Murthy, G. J. Berg, “Steady State State Analysis of Capacitor Self-Excited Induction Generators”, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-103, No. 3, pp 612-618, March 1984.
[5]
A. Kh. Al-Jabri, A. I. Alolah, “Limits “Limits on the Performance of the Three-Phase Self-Excited Induction Generators”, IEEE Transactions on Energy Conversion, Vol. 5, No. 2, pp 350-356, June 1990.
[6]
C. Chakraborty, S. S. N. Bhadra, A. K. Chattopadhyay, “Excitation Requirements for Stand Alone Three-Phase Induction Generator”, IEEE Transactions on Energy Conversion, Vol. 13, No. 4, pp 358-365, December 1998.
[7]
C. Grantham, F. Rahman, D. Seyoum, “ A Regulated Self-excited Induction Generator For Use In a Remote Area Power Supply”, International Journal Of Renewable Energy Engineering, Vol 2, No 1, April 2000.
[8]
C. Saunders, “Electrical Machines 303: A Study of Machine Theory”, Curtin University of Technology 2000.
[9]
C. S. Demoulias, P.S. Dokopoulos, “Transient Behaviour and Self-Excitation of Wind-Driven Induction Generator After Its Disconnection From the Power Grid”, IEEE Transactions on Energy Conversion, Vol. 5, No. 2, pp272-278, June 1990.
[10] D. J. De Renzo, “Wind Power: Recent Development”, Noya Data Co. 1979. [11] D. M. Eggleston, F. S. Stoddard, “Wind Turbine Engineering Design”, Van Nostrand Reinhold 1987. [12] D. M. Simmons, “Wind Power”, Noya Data Co. 1975. [13] G. Raina, O. P. Malik, “Wind Energy Conversion Using a Self-Excited Induction Generator”, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-102, No. 12, pp 3933-3936, December 1983. [14] J. Arrillaga, D. B. Watson, “Static Power Conversion From Self-Excited Induction Generators’, IEE Proceedings, Vol. 125, No. 8, pp 743-746, August 1978. [15] J. F. Walker, N. Jenkins, “Wind Energy Technology”, John Wiley & Sons Inc. 1997. [16] L. Ouazene, G. McPherson Jr., “Analysis of The Isolated Induction Generator”, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-102, No. 8, pp 2793-2798, August 1983. [17] L. Tang, R. Zavadil, “ Shunt Capacitor Failures Due to Windfarm Induction Generator Self-Excitation Phenomenon”, IEEE Transactions on Energy Conversion, Vol. 8, No.3, pp 513-519, September 1993.
[18] M. A. Ouhrouche, et. al., “ EMTP Based Simulation of a Self-Excited Induction Generator After its Disconnection From the Grid”, IEEE Transactions on Energy Conversion, Vol. 13, No.1, pp 7-14, March 1998. [19] N. Mohan, T. M. Undeland, W.P. Robbins, “Power Electronics: Converters, Application, and Design”, 2nd ed., John Wiley and Sons Inc., 1995, Canada. [20] N. H. Malik, A. H. Al-Bahrani, “Influence of the Terminal Capacitor on the Performance Characteristics of a Self-Excited Induction Generator”, IEE Proceedings, Vol. 137, Pr. C, No. 2, pp 168- 173, March 1990. [21] N. H. Malik, A. A. Mazi, “Capacitance Requirements for Isolated Self Excited Induction Generators”, IEEE Transactions on Energy Conversion, Vol. EC-2, No.1, pp 62-68, March 1987. [22] N. H. Malik, S.E. Haque, “Steady State Analysis and Performance of an Isolated Self-Excited Induction Generator”, IEEE Transactions on Energy Conversion, Vol. EC-1, No. 3, September 1986. [23] N. P. Cheremisinoff, “Fundamentals of Wind Energy”, Ann Arbor Science Publishers Inc., 1978, Michigan. [24] O. Shtchetinine, “ Voltage Stabilization System for Induction Generator in Stand Alone Mode”, IEEE Transactions on Energy Conversion, Vol. 14, No. 3, pp 298-303, September 1999. [25] P. F. Ryff, “Electric Machinery”, Prentice Hall Inc., 1994, New Jersey [26] R. G. Slemon, “Electric Machines and Drives”, Addison Wesley Publishing Co., 1992, USA. [27] R. Hunter, G. Elliot, 1994, “Wind/Diesel Systems: A Guide To The Technology & Its Implementation”, Cambridge University Press, UK
[28] S. Krohns, “Wind Turbine Generators”, Danish Wind Turbine Manufacturers Association,
[Online],
Available:
http://www.windpower.dk/tour/
wtrb/electric.htm, 2000. [29] S. Krohns, “Asynchronous (Induction) Generators”, Danish Wind Turbine Manufacturers
Association,
[Online],
Available:
http://www.windpower.dk/tour /wtrb/async.htm, 2000. [30] S. Krohns, “Wind Turbines And Power Quality Issues”, Danish Wind Turbine Manufacturers
Association,
[Online],
Available:
http://www.windpower.dk/tour /grid/rein.htm, 2000. [31] S. Rajakaruna, R. Bonert, “ A Technique for the Steady-State Analysis of a Self-Excited Induction Generator with Variable Speed”, IEEE Transactions on Energy conversion, Vol. 8, No. 4, pp 757-761, December 1993. [32] S. Wei, A. I. Maswood, “A Novel Current Source PWM Drive Topology with Specific Harmonic Elimination Switching Patterns”, IEEE Power Engineering Review, pp 53-55, December 2000. [33] S. M. Alghuwainem, “Steady-State Analysis of a Self-Excited Induction Generator Including Transformer Saturation”, IEEE Transactions on Energy conversion, Vol. 14, No. 3, September 1999. [34] S. M. Alghuwainem, “Steady-State Analysis of an Isolated Self-Excited Induction Generator Driven by Regulated and Unregulated Turbine”, IEEE Transactions on Energy Conversion, Vol. 14, No. 3, pp 718-723, September 1999.
[35] S. S. Murthy, O.P. Malik, A. K. Tandon, “Analysis of Self-Excited Induction Generators”, IEE Proceedings, Vol. 129, Pt. C, No. 6, pp 260-265, November 1982. [36] T. F. Chan, “Steady-State Analysis of Self-Excited Induction Generators”, IEEE Transactions on Energy Conversion, Vol. 9, No. 2, pp 288-294, June 1994. [37] T. F. Chan, “Self-Excited Induction Generators Driven by Regulated and Unregulated Turbines”, IEEE Transactions on Energy Conversion, Vol., 11, No. 2, pp 338-343, June 1996. [38] V. C. Toro, “Electric Machines And Power Systems”, Prentice Hall Inc., 1985, New Jersey [39] Z. A. Yamayee, J. L. Bala Jr., “Electromechanical Energy Devices And Power Systems”, John Wiley & Sons Inc., 1994, Canada
APPENDIX A - PSCAD
®
®
PSCAD / EMTDC Software
The behaviour of a power system under certain condition plays an important part in the design of the system itself. For example, the behaviour of a power system network under severe overvoltages due to a fault in the network is critical in the design of the protection system in the network, and so on. In certain cases, these conditions are best analysed before they occur in the power system network. That is why the simulation using computer softwares comes into the picture.
PSCAD® stands for Power Systems Computer Aided Design. It is used as a GUI (Graphical User Interface) for EMTDC. It is a powerful graphical user interface which increases productivity in undertaking simulation of electromagnetic transients studies of electrical power systems. PSCAD has many advanced features to boost productivity, including intelligent data forms, interactive control and feedback, upto-date documentation of the circuit along with plots and comments, context sensitive help, hierarchical designs, and multiple levels of zooming. The software also serves as the front end for the RTDS (Real Time Digital Simulator).
EMTDC® stands for ElectroMagnetic Transient for DC. It was first engineered by Dennis Woodford in 1975 since at that time the existing tools were not comprehensive and flexible to study the Manitoba Hydro Nelson River HVDC Power System in Manitoba, Canada. The well-known Dommel Algorithm is used in EMTDC software. The program structure has been developed so as to allow the user
great flexibility in modelling power systems and their associated controls in great detail.
An interpolation algorithm has been included to permit accurate modelling of switching devices such as thyristors and GTO's. The solution of very larger power systems has been made efficient by allowing networks to be separated into smaller sub-networks connected by transmission lines. Solutions of these individual subsystems have been made efficient by use of Gaussian Elimination sparse matrix techniques.
PSCAD / EMTDC is a software simulation tool for studying the transient behaviour of electrical networks. This software was first developed in 1976. The software comes with a comprehensive library of electrical models consisting of all aspects of AC and DC power systems and controls. Users are also able to create their own model and libraries using the built-in graphical Component Workshop in the software. Moreover, EMTDC supports models written in FORTRAN / C/C++ which made the construction of models easy. The resources of user’s computer are the only limitation to the size of the circuit to be simulated in PSCAD / EMTDC software. The software supports all aspects of conducting a simulation including circuit assembly, run-time control, analysis, and reporting. In 1988 PSCAD V1 was first introduced on Apollo workstations. The rapid development of the software brought the introduction of PSCAD V2 in 1994, which operates under Unix system. In 1999 PSCAD V3 was released and it operates under Windows platform.
The scope of simulation studies that can be conducted using PSCAD/EMTDC software is as follows.
• General power system electromagnetic transient studies, • Overvoltages in a power system due to a fault or breaker operation, • DC transmission systems and detailed controls, • Synchronous and induction machine torsional effects and self-excitation, • Static Var Compensators, • Harmonic interactions, • Relay protection studies, • and many more.
FILEMANAGER
The main window that appears when the user runs PSCAD is the File Manager software module. The user’s database were presented in a project / case / file hierarchy so that the usual database maintenance tasks such as file editing, copying, deleting, and so on can be performed easily and directly from the File Manager. Moreover, other PSCAD software modules can be initiated by clicking on the appropriate button on the top right hand screen of the File Manager and each individual modules that are currently running is shown as icons on the right hand side of the File Manager screen under ‘process icons’ label.
DRAFT
The Draft module is where the users are able to graphically create their power system models which are to be simulated. Simulation using either EMTDC or the RTDS can be performed in the Draft module.
Electrical components of power systems are located on the library palette which is on the right hand side of the Draft window. Users copy the desired components and place them onto the canvas which is on the left hand side of the Draft window followed by the appearance of the parameter menu of that particular component.
In the case where a large number of components are to be implemented onto the canvas, users are able to create a sub-circuit which can be link together by exporting the variables that are needed. Once the user has completed the layout, it can be printed either by a PostScript based laser printer or plotter capable of accepting HPGL commands.
RUNTIME
The analysis of the user’s circuit can be carried out in the Runtime module. Users are allowed to load, start, stop and analyse the specific parameters of their circuit in a form of on-line plotting of data as it is generated in the Runtime module. Valuable feedback from the simulation can be obtained regarding the transient or dynamic behaviour such as set-point changes, breaker action and fault.
MULTIPLOT
The generated data by either EMTDC or RTDS can be plotted and analysed in Multiplot module. It also enables scaling and general formatting of data. Furthermore, Fourier analysis can be directly performed. Plots obtained from the Runtime module can be combined and manipulated into a single page for the ease of comparison. Moreover, some other features includes labelling, text editing, date and so on can be added into the plots.
APPENDIX B – TEST MACHINE
Three Phase Squirrel Cage Induction Motor Parameters From No-Load and Locked Rotor Tests
Objective
The purpose of this experiment is to conduct no-load and locked rotor test in order to obtain the induction machine parameters.
Equipment List
• 440 V, 4.6 A, 2.2 kW, 1420rpm, delta-connected induction motor • 2 x Nanovip power meters • 1 x Multimeter • Autotransformer
Circuit Diagram
Nanovip 1 A 440 V 3-Phase
B
50Hz C Nanovip 2
Procedure
No-load Test
The autotransformer panel was set to 440 V. Then the motor was uncoupled from the dc dynamometer by removing a rubber bar that connects the shafts together and the motor was switched on to allow the motor to warm up (approximately 5mins). The line voltage, line current, and total input power were obtained. After the motor was switched off, the stator winding was disconnected and the resistances of each stator winding were obtained using the multimeter. Thus, the stator resistance per phase R s was calculated by using the average of the three readings. Determine the rotational losses by subtracting the input power and the stator copper losses (i.e. 3Is2R s). Lastly, by assuming the stator core loss and the friction and windage losses are equal in magnitude, the values of stator core loss R c was calculated.
Locked Rotor Test
Recouple the motor to the dc dynamometer and lock the rotor. The autotransformer was set to 90V. Then, quickly measure the line voltage, line current, total power input and torque as the motor was switched on. After the measurements were obtained, the motor was switched off. Because of the locked rotor, there are no frictional loss and at the reduced voltage, the core loss is negligible. Thus, it can be assumed that entire power input is used to supply the stator and rotor copper loss. Rr was then calculated from the reading as well as Xe (Xe = Xs + Xr ). It can be assumed
that stator leakage reactance Xs and rotor leakage reactance Xr are equal in magnitude. Measurement
No-Load Test
Parameters
Nanovip 1
Nanovip 2
Vl
413 V
416 V
Il
2.27 A
2.43 A
Pin
427 W
559 W
R a
11.5 Ω
-
R b
11.7 Ω
-
R c
11.4 Ω
-
R ave (R s)
11.53 Ω
-
P.F.
-0.45
0.55
Locked Rotor Test
Parameters
Nanovip 1
Nanovip 2
Vl
89.2 V
90.1 V
Il
4.15 A
3.94 A
Pin
9W
310 W
T
0.875 Nm
-
Calculations
No-Load Test
The following figure shows the circuit diagram for no-load test. R s
Inl
jXs
Vnl
jXm
I nl
Vnl = 414.5 V
=
Il 3
= 1.357A
Il = 2.35 A Pnl = 559 – 427 = 132 W Hence, the rotational loss can be calculated as follows.
Prot
= Pnl − Pcu = Pnl − 3I 2nl R s = 132 − 3(1.357) 2 (11.53) = 68.32W
It is assumed that the stator core loss and friction and windage losses are equal in magnitude. Hence Psc = Prot = 68.32 W There are some other information that could be obtained:
PF = cos θ =
Pnl 3Vnl I nl
=
132 3(414.5)(1.357)
= 0.078
∴ θ = 85.51° lagging And,
Z nl
=
Vnl I nl
=
414.5 1.357∠ − 85.51°
= 305.453∠85.51° = 23.91 + j304.516
Hence, the no load resistance and reactance can be found as R nl = 23.91 Ω, and Xnl = 304.516 Ω Moreover,
Ic
=
Psc 3Vnl
=
68.49 3(414.5)
= 0.0551 A
Thus,
R c
=
Vnl Ic
=
414.5 0.0551
= 7.5227 k Ω
Locked Rotor Test
The following figure shows the equivalent circuit diagram of the locked rotor test of the induction machine. I bl
R s
R r
jXs
jXr
V bl
I bl
=
Il 3
=
V bl = 89.65 V
P bl = 301 W
Il = 4.045 A
T = 0.875 Nm
4.045 3
= 2.335 A
and
R bl
=
P bl 2 bl
3I
=
301 3(2.335) 2
= 18.4 Ω
The rotor resistance referred to the stator R r can be found once the locked rotor test resistance is found. R bl = R s + R r
Hence, R r = R bl – R s = 18.4 – 11.53 = 6.87 Ω The locked rotor reactance can be found as
= Z bl2 − R bl2 = 38.394 2 − 18.4 2 = 33.7 Ω
X bl Thus,
Xs = Xr = X bl / 2 = 16.85 Ω Moreover, the magnetising reactance can be found once the leakage reactance of the stator is found. Xnl = Xs + Xm and, Xm = Xnl – Xs = 304.516 – 16.85 = 287.67 Ω
Summary of Results
Base values: V base = 440 V, I base = 2.656 A, Z base = 165.67 Ω.
Parameters
Values
p.u.
R s
11.53 Ω
0.0696
Xs = Xr
16.85 Ω
0.102
R r
6.87 Ω
0.0415
R c
7.5227 k Ω
45.41
Xm
287.67 Ω
1.7364
APPENDIX C – DIODE RECTIFIER
APPENDIX D – HALF-CONTROLLED RECTIFIER
APPENDIX E – BUCK-BOOST CONVERTER
APPENDIX F – PWM VSI
APPENDIX G – DIODE RECTIFIER WITH PWM VSI
APPENDIX H – HALF-CONTROLLED RECTIFIER WITH PWM VSI
APPENDIX I – DIODE RECTIFIER, BUCK-BOOST CONVERTER, AND PWM VSI
APPENDIX J – COMPLETE SYSTEM 1 DESIGN
APPENDIX K – COMPLETE SYSTEM 2 DESIGN
APPENDIX L – COMPLETE SYSTEM 3 DESIGN
