This is a complete list of my publications including their abstracts to allow searching and quoting without grabbing the papers. This list appears again as a Geoff Walker's publication list with links but without the abstracts. Finally it appears as a straight publication list without links for neatest printing.
Note that not all publications can be made available for download, due to the copyright restrictions placed on them as part of their publication.
PhD Thesis
G. R. Walker, "Modulation and Control of Multilevel Converters," PhD dissertation, Department of Computer Science and Electrical Engineering, University of Queensland, Brisbane, Australia,1999. pp.178+24.- Abstract: phd_grw_abst (html) phd_grw_abst.pdf
- Table of Contents: phd_grw_toc.pdf
- Bibliography: phd_grw_biblio.pdf
- Complete thesis: phd_grw.pdf (NB 6Meg)
Graduate Certificate in Education (GCEd) Final Report
G. R. Walker, "Using Projects Instead of Pracs for Hands On Learning," Final Report for the Graduate Certificate in Education, 2000, School of Computer Science and Electrical Engineering, University of Queensland, Brisbane, Australia,2000. pp.53- Abstract: GCEd_grw_abst (html)
- Complete report: GCEd_grw.pdf
Journal papers (most recent first)
Note that some of these journal papers are special issue re-publications of our conference papers, so you find the same content in the conference paper of the same name.
Sernia, P. C. and Walker, G. R. (2005) 'Harmonic Quality of Multi-level Cascade Inverters with Random Carrier Phase Pulse Width Modulation', Journal of Electrical & Electronics Engineering, Australia, Vol 2, No3, 233-238.
Walker, G. R. and Sernia, P. C. (2004) 'Cascaded DC-DC converter connection of photovoltaic modules', Power Electronics, IEEE Transactions on, Vol.19, No.4, 1130-1139.
Walker, G. R. (2003) 'A Class B switch-mode assisted linear amplifier', Power Electronics, IEEE Transactions on, Vol.18, No.6, 1278-1285.
Walker, G. R. (2003) 'Digitally-implemented naturally sampled PWM suitable for multilevel converter control', Power Electronics, IEEE Transactions on, Vol.18, No.6, 1322-1329.
Simpson, A. G. and Walker, G. R. (2003) 'A parametric analysis technique for design of fuel cell and hybrid-electric vehicles', SAE 2003 Transactions Journal of Engines, 1883-1893.
Bray, J. T., Walker, G. R., Simpson, A. G., Greaves, M. C. and Guymer, B. D. (2003) 'Brake system performance requirements of a lightweight electric/hybrid rear wheel drive vehicle', International Journal of Vehicle Autonomous Systems, Vol.1, No.3/4, 436-448.
Greaves, M. C., Walker, G. R. and Walsh, B. (2002) 'Design optimization of ironless motors based on magnet selection', Journal of Electrical & Electronics Engineering, Australia, Vol.22, No.1, 43-48.
Walker, G. R. (2001) 'Evaluating MPPT converter topologies using a MATLAB PV model', Journal of Electrical & Electronics Engineering, Australia, Vol.21, No.1, 49-56.
Abstract:
An accurate PV module electrical model is presented based on the Shockley diode equation. The simple model has a photo-current current source, a single diode junction and a series resistance, and includes temperature dependences. The method of parameter extraction and model evaluation in Matlab is demonstrated for a typical 60W solar panel.
This model is used to investigate the variation of maximum power point with temperature and insolation levels. A comparison of buck versus boost maximum power point tracker (MPPT) topologies is made, and compared with a direct connection to a constant voltage (battery) load. The boost con-verter is shown to have a slight advantage over the buck, since it can always track the maximum power point.
Simpson, A. G., Greaves, M. C. and Walker, G. R. (2001) 'Migrating to a sustainable energy system: distributed generation and storage, fuel cells and hypercars', International Journal of Renewable Energy and Sustainable Technologies Australia, Vol.3, No.3, 360-366.
Abstract:
This paper examines a number of issues in sustainable energy generation and distribution, and explores avenues that are available for integration of our society’s energy supplies. In particular, the paper presents a way in which transport vehicle energy supplies could be integrated with distributed generation schemes to achieve synergistic and beneficial outcomes. The worldwide energy system contains fundamental problems that result directly from the use of unsustainable fuels and a lack of energy system integration. There is a need to adopt an integrated, sustainable energy system for our society.
The adoption of distributed generation could result in beneficial restructuring of the energy trade, and a change in the role of energy providers. Inherent benefits in distributed generation schemes would directly combat barriers to installation of renewable generation facilities, which might prove distributed renewable energy sources to be more feasible. The presence of fuel cells, batteries, power electronic inverters and intelligent controls in vehicles of the future provides many opportunities for the integration of vehicle energy supplies into a distributed generation scheme. In such a system, vehicles could play a major role in power generation and storage.
Walker, G. and Ledwich, G. (1999) 'Bandwidth considerations for multilevel converters', IEEE Transactions on Power Electronics, Vol.14, No.1, 74-81.
Abstract:
Multilevel converters can achieve an overall effective switch frequency multiplication and consequent ripple reduction through the cancellation of the lowest order switch frequency terms. This paper investigates the harmonic content and the frequency response of these multimodulator converters. It is shown that the transfer function of uniformly sampled modulators is a bessel function associated with the inherent sampling process. Naturally sampled modulators have a at transfer function, but multiple switchings per switch cycle will occur unless the input is slew-rate limited.
Lower sideband harmonics of the effective carrier frequency and, in uniform converters, harmonics of the input signal also limit the useful bandwidth. Observations about the effect of the number of converters, their type (naturally or uniformly sampled), and the ratio of modulating frequency and switch frequency are made.Index Terms: natural PWM, modulation, multilevel converters, uniform PWM.
Conference papers (most recent first)
Simpson, A. G., Walker, G. R., Greaves, M. C., Guymer, B. D. and Finn, D. A. (2005) ‘Dynamic Simulation of a Light-Weight, Low-Drag, Hybrid-Electric Sports Coupe’, accepted for presentation at the 2005 SAE Future Transport Technology Conference (FTT2005), Sept 7-9 2005, SAE paper number 05FTT-27.
Walker, G. R. (2004) 'Digital PWM Waveform Generation using Adders, Not Counters', Australasian Universities Power Engineering Conference (AUPEC'04), Vol. 1 University of Queensland, Brisbane, Australia, pp. 1-5.
Singer, B. D. and Walker, G. R. (2004) 'A Class E Resonant Inverter for use as Electronic Fluorescent Lamp Ballast', Australasian Universities Power Engineering Conference (AUPEC'04), Vol. 1 University of Queensland, Brisbane, Australia, pp. 1-6.
Sernia, P. C. and Walker, G. R. (2004) 'Harmonic Quality of Multi-level Cascade Inverters with Random Carrier Phase Pulse Width Modulation', Australasian Universities Power Engineering Conference (AUPEC'04), Vol. 1 University of Queensland, Brisbane, Australia, pp. 1-6.
Pierce, J., Walker, G. R., Finn, D. A. and Sernia, P. C. (2004) 'Design Considerations in an Indirectly Coupled Multilevel Motor Controller', Australasian Universities Power Engineering Conference (AUPEC'04), Vol. 1 University of Queensland, Brisbane, Australia, pp. 1-6.
Lister, C. D. and Walker, G. R. (2004) 'Two Stage Unity Power Factor Rectifier Design', Australasian Universities Power Engineering Conference (AUPEC'04), Vol. 1 University of Queensland, Brisbane, Australia, pp. 1-6.
Finn, D. A., Walker, G. R., Sernia, P. C. and Pierce, J. (2004) 'Applications and Equivalent Models for Coupled Inductor Parallel Interleaved Converters', Australasian Universities Power Engineering Conference (AUPEC'04), Vol. 1 University of Queensland, Brisbane, Australia, pp. 1-6.
Croft, M., Walker, G. R. and Hovland, G. (2004) 'A New Efficiency Model of a Regenerative Dynamometer', Australasian Universities Power Engineering Conference (AUPEC'04), Vol. 1 University of Queensland, Brisbane, Australia, pp. 1-6.
Weng, L. F. L., Walker, G. R., Dong, Z. Y., Simpson, A. G. and Finn, D. A. (2003) 'Selection of energy storage system for a regenerative dynamic dynamometer', Australasian Universities Power Engineering Conference (AUPEC'03), Vol. 1 (Ed, Duke, R.) University of Canterbury, Christchurch, New Zealand, pp. 1-6.
Walker, G. R., Xue, J. K. and Sernia, P. C. (2003) 'PV string per-module maximum power point enabling converters', Australasian Universities Power Engineering Conference (AUPEC'03), Vol. 1 (Ed, Duke, R.) University of Canterbury, Christchurch, New Zealand, pp. 1-6.
Simpson, A. G. and Walker, G. R. (2003) 'A parametric analysis technique for design of fuel cell and hybrid-electric vehicles', 2003 SAE Future Transportation Technology Conference(Ed, Heffel, J.) SAE International, Warrendale, PA, pp. 1-11.
Sernia, P. C., Finn, D. A. and Walker, G. R. (2003) 'The effects of coupled inductors in parallel interleaved buck converters', Australasian Universities Power Engineering Conference (AUPEC'03), Vol. 1 (Ed, Duke, R.) University of Canterbury, Christchurch, New Zealand, pp. 1-5.
Greaves, M. C., Simpson, A. G., Guymer, B. D., Walker, G. R. and Finn, D. A. (2003) 'Ironless wheel motor for a direct drive vehicle application', Australasian Universities Power Engineering Conference (AUPEC'03), Vol. 1 (Ed, Duke, R.) University of Canterbury, Christchurch, New Zealand, pp. 1-6.
Finn, D. A., Walker, G. R. and Sernia, P. C. (2003) 'Method of extracting switching loss from a high efficiency MOSFET based half bridge converter', Australasian Universities Power Engineering Conference (AUPEC'03), Vol. 1 (Ed, Duke, R.) University of Canterbury, Christchurch, New Zealand, pp. 1-6.
Walker, G. R. and Sernia, P. C. (2002) 'Cascaded DC-DC converter connection of photovoltaic modules', Power Electronics Specialists Conference, 2002. (PESC02), Vol. 1 IEEE, Cairns, Australia, pp. 24-29.
Powerpoint presentation from PESC'02 for this paper (and as a pdf)
Abstract:
New residential scale PhotoVoltaic (PV) arrays are commonly connected to the grid by a single DC-AC inverter connected to a series string of PV modules, or many small DC-AC inverters which connect one or two modules directly to the AC grid. This paper shows that a “converter-per-module” approach offers many advantages including individual module Maximum Power Point Tracking, which gives great flexibility in module layout, replacement, and insensitivity to shading; better protection of PV sources, and redundancy in the case of source or converter failure; easier and safer installation and maintenance; and better data gathering.
Simple non-isolated per-module DC-DC converters can be series connected to create a high voltage string connected to a simplified DC-AC inverter. These advantages are available without the cost or efficiency penalties of individual DC-AC grid connected inverters.
Buck, boost, buck-boost and Cúk converters are possible cascadable converters. The boost converter is best if a significant step up is required, such as with a short string of 12 PV modules. A string of buck converters requires many more modules, but can always deliver any combination of module power. The buck converter will be the most efficient topology for a given cost. While flexible in voltage ranges, buck-boost and Cúk converters are always at an efficiency or alternatively cost disadvantage.
Simpson, A. G., Walker, G. R., Greaves, M. C., Finn, D. A. and Guymer, B. D. (2002) 'The UltraCommuter: A viable and desirable solar-powered commuter vehicle', Australasian Universities Power Engineering Conference (AUPEC'02), Vol. 1 (Ed, Zahedi, A.) Monash University, Melbourne, Australia, pp. 1-6.
Simpson, A. G. and Walker, G. R. (2002) 'Lifecycle costs of ultracapacitors in electric vehicle applications', Power Electronics Specialists Conference, 2002. (PESC02), Vol. 2 Cairns, Australia, pp. 1015-1020 vol.2.
Abstract:
The pulse power characteristics of ultracapacitors appear well suited to electric vehicle applications, where they may supply the peak power more efficiently than the battery, and can prevent excessive over sizing of the battery pack due to peak power demands. Operation of ultracapacitors in battery electric vehicles is examined for possible improvements in system efficiency, vehicle driving range, battery pack lifetime, and potential reductions in system lifecycle cost.
The lifecycle operation of these ultracapacitors is simulated using custom-built, dynamic simulation code constructed in Matlab. Despite apparent gains in system efficiency and driving range, the results strongly suggest that the inclusion of ultracapacitors in the electric vehicle does not make sense from a lifecycle cost perspective.
Furthermore, a comparison with results from earlier work shows that this outcome is highly dependant upon the efficiency and cost of the battery under consideration. However, it is likely that the lifecycle cost benefits of ultracapacitors in these electric vehicles would be, at most, marginal and do not justify the additional capital costs and system complexity that would be incurred in the vehicle.
Walker, G. R. (2001) 'Using projects instead of pracs for hands on learning', Australasian Universities Power Engineering Conference (AUPEC'01), Vol. 1 (Ed, Keerthipala, W. W. L.) CRESTA, Perth, Australia, pp. 390-395.
Abstract:
In 1999, power electronics laboratory practicals were isolated two-hour sessions with only nominal assessment. Students were unmotivated, and didn’t prepare for or subsequently review these sessions. The pracs were rushed, and students’ actions task oriented. Learning was shallow at best. In 2000, the practical component was changed to two projects, each spanning four weeks. The projects were larger, linked, real world problems, tackled by groups of three students. Assessment was via individual workbooks kept during the project, a group demonstration of the working project by all members, and a subsequent written report. These projects were highly successful in motivating the students, and achieved the transfer of the theory presented in lectures into personal practical understanding of that material. These outcomes were judged by observations of the class, project and exam marks, and responses to a questionnaire given at the conclusion of the semester.
Simpson, A. G., Sernia, P. C. and Walker, G. R. (2001) 'Simulated lifecycle costs of ultracapacitors in battery electric vehicles', Australasian Universities Power Engineering Conference (AUPEC'01), Vol. 1 (Ed, Keerthipala, W. W. L.) CRESTA, Perth, Australia, pp. 176-181.
Abstract:
The pulse power characteristics of ultracapacitors appear well suited to electric vehicle applications, where they may supply the peak power more efficiently than the battery, and can prevent excessive over sizing of the battery pack due to peak power demands. Operation of ultracapacitors in battery electric vehicles (BEVs) is examined for possible improvements in system efficiency, vehicle driving range, battery pack lifetime, and potential reductions in system lifecycle cost.
The lifecycle operation of these ultracapacitors is simulated using a custom-built, dynamic simulation code constructed in Matlab. Despite apparent gains in system efficiency and driving range, the lifecycle cost benefits as simulated appear to be marginal, and are heavily influenced by the incremental cost of power components. However, additional factors are identified which, in reality, will drive ultracapacitors towards viability in electric vehicle applications.
Sernia, P. C. and Walker, G. R. (2001) 'Multi-converter topology evaluation for connection of low voltage DC sources', Australasian Universities Power Engineering Conference (AUPEC'01), Vol. 1 (Ed, Keerthipala, W. W. L.) CRESTA, Perth, Australia, pp. 495-500.
Abstract:
A design for a cascaded multilevel DC-DC converter is proposed. The applications of a multilevel converter and the design issues involved in changing from a single converter to multiple converters are discussed. Implementation of the multilevel system using multiple Cuk converters is suggested and explanations of design decisions are given. The merits of the proposed design are discussed.
Greaves, M. C., Walker, G. R. and Walsh, B. (2001) 'Design optimization of ironless motors based on magnet selection', Australasian Universities Power Engineering Conference (AUPEC'01), Vol. 1 (Ed, Keerthipala, W. W. L.) CRESTA, Perth, Australia, pp. 216-220.
Abstract:
This paper considers the design of a radial flux permanent magnet ironless core brushless DC motor for use in an electric wheel drive with an integrated epicyclic gear reduction. The motor has been designed for a continuous output torque of 30Nm and peak rating of 60Nm with a maximum operating speed of 7000 RPM.
In the deisgn of brushless DC motors with a toothed iron stator the peak air-gap magnetic flux density is typically chosen to be close to that of the remanence value of the magnets used. This paper demonstrates that for an ironless motor the optimal peak air-gap flux dentiy is closer to the maximum energy product of the magnets used. The use of a radial flux topology allows for high frequency operation and can be shown to give a high specific power output whicle maintaining a relatively low magnet mass. Two-dimentional Finite Element Analysis is used to predict the air-gap flux density. The motor design is based around commonly available NdFeB bar magent size.
Finn, D. A. and Walker, G. R. (2001) 'A method of switch selection for an electric vehicle drive', Australasian Universities Power Engineering Conference (AUPEC'01), Vol. 1 (Ed, Keerthipala, W. W. L.) CRESTA, Perth, Australia, pp. 287-291.
Abstract:
The key to reducing cost of electric vehicles is integration. All too often systems such as the motor, motor controller, batteries and vehicle chassis/body are considered as separate problems. The truth is that a lot of trade-offs can be made between these systems, causing an overall improvement in many areas including total cost.
Motor controller and battery cost have a relatively simple relationship; the less energy lost in the motor controller the less energy that has to be carried in the batteries, hence the lower the battery cost. A motor controller’s cost is primarily influenced by the cost of the switches. This paper will therefore present a method of assessing the optimal switch selection on the premise that the optimal switch is the one that produces the lowest system cost, where system cost is the cost of batteries + switches.
Walker, G. R. (2000) 'Evaluating MPPT converter topologies using a MATLAB PV model', Australasian Universities Power Engineering Conference, AUPEC'00, Vol. Vol. 1 Brisbane, pp. 138-143.
Abstract:
An accurate PV module electrical model is presented based on the Shockley diode equation. The simple model has a photo-current current source, a single diode junction and a series resistance, and includes temperature dependences. The method of parameter extraction and model evaluation in Matlab is demonstrated for a typical 60W solar panel.
This model is used to investigate the variation of maximum power point with temperature and insolation levels. A comparison of buck versus boost maximum power point tracker (MPPT) topologies is made, and compared with a direct connection to a constant voltage (battery) load. The boost con-verter is shown to have a slight advantage over the buck, since it can always track the maximum power point.
Simpson, A. G., Greaves, M. C. and Walker, G. R. (2000) 'Migrating to a sustainable energy system: distributed generation and storage, fuel cells and hypercars', Australasian Universities Power Engineering Conference, AUPEC'00, Vol. Vol. 1 (Ed, Krivda, A.) Queensland University of Technology, Brisbane, pp. 289-294.01
Abstract:
This paper examines a number of issues in sustainable energy generation and distribution, and explores avenues that are available for integration of our society’s energy supplies. In particular, the paper presents a way in which transport vehicle energy supplies could be integrated with distributed generation schemes to achieve synergistic and beneficial outcomes. The worldwide energy system contains fundamental problems that result directly from the use of unsustainable fuels and a lack of energy system integration. There is a need to adopt an integrated, sustainable energy system for our society.
The adoption of distributed generation could result in beneficial restructuring of the energy trade, and a change in the role of energy providers. Inherent benefits in distributed generation schemes would directly combat barriers to installation of renewable generation facilities, which might prove distributed renewable energy sources to be more feasible. The presence of fuel cells, batteries, power electronic inverters and intelligent controls in vehicles of the future provides many opportunities for the integration of vehicle energy supplies into a distributed generation scheme. In such a system, vehicles could play a major role in power generation and storage.
Walker, G. (1999) 'High Efficiency Auxiliary Power Converters', Australasian Universities Power Engineering Conference, AUPEC'99, Vol. Vol. 1 Darwin, pp. 128-133.
Abstract:
The auxiliary load DC-DC converters of the Sunshark solar car have never been examined. An analysis of the current design reveals it is complicated, and inefficient. Some simple measures to greatly improve the efficiency are present which will achieve an overall worthwhile power saving. Two switch-mode power supply DC-DC converter designs are presented. One is a constant current supply for the LED brake and turn indicators, which allows them to be powered directly from the main DC bus, and switched only as necessary. The second is a low power flyback converter, which employs synchronous rectification among other techniques to achieve good efficiency and regulation
over a large range of output powers.
Practical results from both converters, and an indication of the overall improvement in system efficiency will be offered.
Walker, G. (1999) 'A DC Circuit Breaker for an Electric Vehicle Battery Pack', Australasian Universities Power Engineering Conference, AUPEC'99, Vol. 1 Darwin, pp. 116-121.
Abstract:
Electric vehicle battery packs require DC circuit breakers for safety. These must break thousands of Amps DC at hundreds of Volts. The Sunshark solar racing car has a 140V 17Ahr battery box which needs such a breaker. A static design using 200V MOSFETs to interrupt the fault current is presented. The design specification, decisions and proposed solution circuit are given. The current sensing technique, MOSFET overvoltage protection, and DC bus capacitor precharging scheme are specific focuses. Simulation results are presented and discussed.
Walker, G. and Ledwich, G. (1996) 'Implementing Natural PWM Digitally', Australasian Universities Power Engineering Conference, AUPEC'96, Vol. Vol. 1 Melbourne, pp. 169-174.
Abstract:
Natural sampled PWM is the best choice for applications which require closed loop, wide bandwidth modulation such as active power filtering. It does not attenuate or distort the modulating signal, even when the frequency of that signal is similar to the switch frequency. Carrier based PWM is also easily adapted to multilevel converter modulation by phase shifting the carriers.
A digital implementation is preferred for multilevel modulation. Switching edges with crystal accuracy and more importantly repeatability are needed to give the best carrier cancellation in a multilevel converter. Digital control is more easily modularised and is more noise immune.
Re-sampled uniform PWM is a digital implementation which approaches the frequency and transient response of natural PWM. Both hardware and software multilevel implementations are presented, and the improvement over uniform PWM is demonstrated.
Walker, G. and Ledwich, G. (1996) 'An Isolated MOSFET Gate Driver', Australasian Universities Power Engineering Conference, AUPEC'96, Vol. Vol. 1 Melbourne, pp. 175-180.
Abstract:
Traditional methods of isolated MOSFET/IGBT gate drive are presented, and their pros and cons assessed. The best options are chosen to meet our objective --- a small, high speed, low cost, low power isolated gate drive module.
Two small ferrite bead transformers are used for isolation, one transmits power at 2.5MHz, the other sends narrow set reset pulses. On the secondary these pulses drive a transistor totem pole to ensure high current drive, and the value is held by CMOS buffers with positive feedback. An alternative design for driving logic level devices uses only an HC buffer on the secondary.
Double sided SMD construction (primary one side, secondary on the other) yields an upright module 40x18x5mm. Propagation delay was 20ns, and rise/fall time 15ns with a 1nF load. The design places no limits on frequency of operation or duty cycle. Power supply requirements were 5V @ 20mA for operation below 100kHz, dominated by magnetising current.
Walker, G. and Ledwich, G. (1994) 'Bandwidth Considerations for Multiple Modulator Converters', Australasian Universities Power Engineering Conference, AUPEC'94, Vol. Vol. 3 Adelaide, pp. 514-519.
Abstract:
Multi-bridge or multilevel converters can achieve an overall effective switch frequency multiplication and consequent ripple reduction through the cancellation of the lowest order switch frequency terms. This paper investigates the harmonic content, and the effective frequency response, of these multi-modulator converters. It is shown that the effective bandwidth is limited in uniformly sampled converters by a bessel function associated with the inherent sample and hold process. In naturally sampled converters the modulator's frequency response is flat, but as the modulating frequency approaches the carrier frequency, multiple switchings per switch cycle will occur unless the input is slew-rate limited.
Lower sideband harmonics of the effective carrier frequency and, in uniform converters, input signal harmonics also limit the useful bandwidth. Observations about the effect of the number of converters, their type -- naturally or uniformly sampled, and the ratio of modulating frequency and switch frequency are made.
Geoff Walker / School of Information Technology and Electrical Engineering / walkerg@itee.uq.edu.au