William Dunford


Relevant Degree Programs

Affiliations to Research Centres, Institutes & Clusters


Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - Nov 2020)
Real-time applications of synchrophasor-based wide-area measurement systems (2018)

In this thesis, we focus on two major real-time applications of modern synchrophasor-based wide-area measurement systems , i.e., transient stability assessment (TSA) and fault detection and identification (FDI). First, we develop a tool for real-time TSA based on automatic learning approaches. We use Classification and Regression Tree (CART) as the classification tool and Multivariate Adaptive Regression Splines (MARS) as the regression tool. To train and validate these tools in a practical setting, we conduct test cases on the full Western Electricity Coordinated Council system model, with emphasis on the BC Hydro (BCH) power system. While being mindful of practical field implementations of the proposed methods, our studies assume limited number of phasor measurement units (PMUs) installed, in accordance with existing infrastructure in the BCH system. The trained CART models are tested and show high accuracy rates, and thus, will be able to predict the transient stability issues of the system under study following different contingencies using the synchrophasors obtained from limited number of PMUs in the system. Also, the MARS models, which are proposed to be applied for TSA for the first time, show reasonable prediction accuracy rates. Next, we investigate the possibility of an accurate real-time FDI using synchrophasors received from PMUs installed at the two ends of a transmission line. We apply a new metric called goodness-of- fit (GoF), which is calculated over the time span of measurement and can quantify the credibility of the received synchrophasors. Then, we apply the data to an FDI method to show how accurate and credible the results are. The obtained results show a reasonable relation between the GoF metric, i.e., credibility of the measured sychrophasors, and the accuracy of the obtained results, validating the significance of the proposed method for real-time applications. As it is very rare to have a real power system with all buses and transmission lines equipped with PMUs, we also propose a wide-area real-time FDI approach using a linear observer. Through this wide-area approach, we demonstrate the effectiveness of the proposed method by accurately locating a fault in a small test system.

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Efficient Hydroelectric Generation Using Novel Balance Schemes (2015)

In order to control frequency and interchange schedules in electric power systems, a permanent balance between generation and demand is necessary. Following electric demand has traditionally been realized by control of flexible generation resources. As a consequence, conventional generation units are utilized in lower maximum output power and less efficient operating points. Transition toward increased penetration of intermittent Distributed Energy Resources (DER) requires more balancing capacity in power systems which makes frequency control a more challenging issue. Demand Side Management (DSM) is a main ingredient of Smart Grid (SG)s to improve efficiency and reliability. Some industrial processes have inherent flexibilites making them capable of virtually storing enough energy to immediately and continuously respond to control signals of transmission system operator. These loads, when equipped with advanced metering, communication and control infrastructure, can realize participation of Demand Side Storage (DSS) in sub-hourly time steps of grid balance. In order to fairly distribute the benefits of interconnection among all control areas, frequency control standards are defined and proposed by reliability coordinators e.g. NERC. Once new standards become effective, Balancing Authorities (BA)s modify their Automatic Generation Control (AGC) and real-time balance logic to comply with the new requirements. This research is dedicated to finding novel balance structures in sub-hourly dispatch and real-time operation. The objectives of the proposed balance structures are to increase hydroelectric generation efficiency and reduce unit maneuvering leading to mechanical wear and tear. A new Demand Dispatch (DD) application for industrial flexible loads and a new sub-hourly balance structure based on use of DSS are developed in this thesis. Also in real-time operation, a novel AGC logic is proposed to maximize the benefits of a hydroelectric dominated Balancing Authority based on latest frequency control standards. It is shown through mathematical modeling, static scheduling optimization formulations and dynamic simulations that utilizing 5% of system peak demand as sub-hourly dispatched DSS saves up to 2% in generation efficiency and utilizing the proposed real-time AGC logic leads to generation efficiency saving of up to 1.3%. Both proposed methods also significantly reduce mechanical wear and tear.

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Investigation of High-Performance DC-DC Converters for Plug-In Hybrid Electric Vehicle Battery Charging (2014)

Plug-in Hybrid Electric Vehicles (PHEVs) and Electric Vehicles (EVs) are an emerging trend in automotive circles, and consumer interest is growing rapidly. With the development of PHEVs, battery chargers for automotive applications are becoming a large market for the power supply industry. The most common charger power architecture includes an ac-dc converter with power factor correction (PFC) followed by an isolated dc-dc converter. As a key component of a charger system, the dc-dc converter must achieve high efficiency and power density. This dissertation mainly focuses on the dc-dc converter stage only and in order to meet high efficiency, high power density and a cost-effective solution, various dc-dc topologies have been investigated and proposed for battery charging application. In this research work two new full-bridge dc-dc converter topologies (one with inductive and another with capacitive output filter) operating with a trailing edge pulse width modulation (PWM) gating scheme are investigated. Also for higher power (>2 kW) battery charging application, another two new interleaved dc-dc converter topologies using full-bridge with capacitive output filter (one with bridge rectifier diodes and another with voltage doubler rectifier) are also investigated. Detailed operating principle and steady state analysis for different modes of operation, step-by-step design procedure, simulation, experimental results and performance evaluation with various semiconductor devices for each of these topologies are presented in this thesis. The results show that the performance, in terms of efficiency, size and cost for the full-bridge converter with capacitive output filter is superior to that with inductive output filter. Moreover the dc-dc converter with capacitive output filter overcomes some of the major issues such as high voltage ringing on the rectifier diodes and duty-cycle loss, which are present in the converter with inductive output filter.

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An active method for improving the unintentional islanding test in distributed generation systems (2013)

A development of an island stabilizing element (ISE) for use in the IEEE 1547 unintentional islanding test is introduced. The new test setup for non- islanding inverters interconnected with the grid is proposed. The current testing standard uses discrete RLC elements to simulate the test-island. Even though the RLC simulated test-island is useful for its reproducibility, relative scalability and short setup time, as inverter power ratings increase so does the size and cost of the RLC simulated island. The proposed island stabilizing element can represent the function of the resonant part of the test island as well as provide compensation for dynamic changes in power during the test for producing near worst case conditions for an islanding test. This work introduces improvements to the unintentional islanding test. The island stabilizing element is designed and developed. Test cases proved the efficient application of the ISE as means to replace the LC elements in the unintentional islanding test.

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Dynamic modeling, parameter identification, payload estimation, and non-contact arm geometry sensing of the mining cable shovel (2013)

This thesis presents the application of the least squares estimation technique in identification of the cable shovel parameters and monitoring its payload. Both detailed and simplified dynamic models of the cable shovel are derived by modeling the DC motors and analyzing such nonlinear effects as inertia, Coriolis, centripetal, and friction. Mathematical methods, including the interactive Newton-Euler technique, have been used to obtain the kinematic and dynamic equations of the shovel, and establish the relationship between the shovel parameters and the payload inside the bucket. The cable shovel bucket is also referred to as “dipper”. An on-line parameter identification scheme was developed and experimentally verified in order to estimate the cable shovel parameters. A data acquisition system was installed on a P&H2100 cable shovel in the Sarcheshmeh Copper mine, located in Iran, where it logged the cable shovel data for several loading cycles. The payload was dynamically estimated using the simplified dynamic cable-shovel model developed in this thesis. The accuracy and repeatability of the algorithm has been verified based upon the cable-shovel data logged during its normal operation at the mine. In the course of this thesis project, I also devised a novel approach for non-contact sensing of the dynamic arm-geometry of the cable-shovel. A prototype sensor apparatus was designed and assembled that measures the dipper handle angle, the swing angle, and the dipper handle length. Different sensors such as gyroscopes, magnetometers, accelerometers, and a laser sensor are integrated into Arm Geometry Sensor (AGS) apparatus. The AGS apparatus is installed on the saddle block and measures all cable shovel joint variables without having direct physical contact with the links or joints. The AGS apparatus was employed during field trials on the on P&H2100 cable shovel and verified to effectively sense the shovel joint variables with acceptable accuracy.

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Analysis and design of isolated bidirectional DC-DC converter with novel triple phase-shift control (2012)

The bidirectional DC-DC converter is widely used in automobiles, energy storage systems, uninterruptible power supplies and aviation power systems. At present, there are three main problems in this area. The first problem concerns stability of the bidirectional converter when parameters change; the second is maintaining high efficiency of the bidirectional converter over wide load range; the third concerns the sensitivity of the efficiency of the bidirectional converter to parameter changes. This thesis presents a new method to determine the stability of the bidirectional converter using the Lyapunov function method under arbitrary parameter changes. As another new contribution, the stability analysis with eigenvalue method is presented when only the input voltage changes. Although these two methods are used in this thesis to determine the stability of bidirectional dual full bridge DC-DC converter with triple phase-shift control, they can be used to determine the stability of other power converters composed of various power switches and controlled with different control methods. A novel triple phase-shift control method is developed in this thesis to make the bidirectional converter operate at high efficiency and make it robust to parameters changes and output power variations. Simulation results illustrate that the novel control method is better than several other commonly used control methods for the bidirectional converter when component parameters and output power change. The working theory of the bidirectional converter with novel triple phase-shift control method is comprehensively described in the thesis. As another new contribution, the maximum output power of the bidirectional converter is analyzed in detail in the thesis. Simulation studies of this project have provided satisfactory results. Conclusions are made on the presented work and possible future directions in continuing the work are indicated.

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Investigation of high performance single-phase solutions for AC-DC power factor corrected boost converters (2011)

Plug-in Hybrid Electric Vehicles (PHEVs) and Electric Vehicles (EVs) are an emerging trend in automotive circles, and consumer’s interest is growing rapidly. With the development of PHEVs, battery chargers for automotive applications are becoming a large market for the power supply industries. The improvement of overall charger efficiency is critical for the emergence and acceptance of these vehicular technologies, as the charger efficiency increases, the charge time and utility cost decreases. Additionally, to meet the efficiency and power factor requirements and regulatory standards for the AC supply mains, power factor correction is essential.Due to limited space in vehicle and increasing power consumption, chargers are required to deliver more power with smaller volume. As a key component of a charger system, the frontendAC-DC converter must achieve high efficiency and high power density. In this dissertation, several conventional plug in hybrid electric vehicle charger front end AC-DC converter topologies are investigated and a new bridgeless interleaved and a phase shifted semi-bridgeless power factor corrected converter are proposed to improve the efficiency and performance, which is critical to minimize the charger size, charging time, and the amount and cost of electricity drawn from the utility. A detailed analytical model for these topologies is developed, enabling the calculation of power losses and efficiency. Experimental and simulation results of several prototype boost converter converting universal AC input voltage to 400 V DC at 3.4 kW are given to verify the proof of concept, and analytical work reported in this thesis. The results show a power factor greater than 0.99 from 750 W to 3.4 kW, THD less than 5% from half load to full load and a peak efficiency of 98.94% at 265 V input and 1200 W load.

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Master's Student Supervision (2010 - 2018)
Computing effects of electric and magnetic fields near overhead transmission lines (2017)

Analysis of Electric and Magnetic Fields produced by overhead transmission lines plays a critical role prior to their construction by electrical utilities. They affect the width of the transmission corridor and terrain clearances from overhead conductors. There is substantial interest in learning about the health effects of short and long-term exposure to these fields, as induced currents and voltages from parallel lines pose a hazard to line crews working on a de-energized line.A MATHCAD program has been developed to compute the strengths of Electric and Magnetic Fields and to determine any induction currents and/or voltages due to the coupling effect of these fields. MATHCAD is an engineering software that allows programming and development of engineering calculations using natural mathematical syntax and variables. An overview of the theory of electric and magnetic fields is also provided in this paper. This research tests a few scenarios in the MATHCAD program, and the test results are verified against EPRI (Electric Power Research Institute) and IEEE (Institute of Electrical and Electronic Engineers) guidelines to ensure consistency. The results are also compared with the safety limits published by ICNIRP (International Commission on Non-Ionizing Radiation Protection) and IEEE. We also discuss the practical use of these results, along with potential future updates to the MATHCAD program.

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A Study of State of Health Estimation Methods for Li-Ion Batteries (2016)

Currently, battery management systems are battery chargers, commonly comprised of power electronic circuits, which lack the ability to accurately estimate the state of health of a battery. Since, batteries have a limited lifetime, repeated charge and discharge cycles quickly deteriorate the electrical properties of the battery. With the reduced capacity and several other changes in the state of health of a battery, the electronic device might malfunction. This research is aimed to provide on device upgrade for all battery management systems and battery chargers to include battery health monitoring ability. For the evaluation of state of health of batteries, two approaches are considered in parallel, Electrochemical Impedance Spectroscopy (EIS) and profiling through charge and discharge curves. For EIS, the initial focus of this research is to design and validate the hardware that can perform EIS scans over a desired range of frequencies. Based on the footprints of scan, a state of health classification algorithm is proposed which categorizes batteries according to the set threshold. The main contribution of this project to existing EIS technology is the eradication of the need of battery modeling and parameter estimation from Nyquist plot to find the state of health of a battery. Tests are performed on hardware prototype to validate the designed algorithm that shows State of Health estimation accuracy of almost 90%. Another method considered for State of Health estimation is profiling through charge and discharge curves of the Li Ion batteries. Raw profiling data is examined to decipher the correlation between shape of charge and discharge curves and state of health. From the charging profile of the battery, constant charge current duration parameter is identified to possess promising potential to provide information about state of health of a battery. The behavior of the parameter is investigated in detail with repeated laboratory tests on almost 200 samples gathered from five different battery vendors. This technique showed above 90% classification accuracy.Finally a comparison is drawn between the EIS technology and charge curve profiling method with respective advantages and disadvantages to emphasize the suitability of each technique for different field applications.

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DC Distribution Systems for Home Application (2016)

Unprecedented expansion of native direct current (DC) powered equipment (computers and consumer electronics) has increased household DC electricity consumption over the past decade. Since power utilities deliver alternating current (AC) rather than DC, the conversion process (rectifier) used to supply DC loads is very inefficient. The research investigates the suitability of employing conventional AC wiring to distribute DC power to supply loads directly, in particular around outlet/switch arcing issues. The problem of arcing in DC system is very predominant and needs to be addressed to meet safety requirements while improving the efficiency of the system. In order to overcome the arcing issues, an alternative flat DC wiring system is proposed which offers improved transient electrical and thermal characteristics for household wiring. The flat wire solution employs the same raw materials and provides improvements in parasitic values associated with arcing while reducing thermal resistance. The proposed flat wire geometry is expected to achieve reduction of arcing and improve the overall efficiency of the distribution system.Simulations of the two preliminary AC and DC systems are provided for typical domestic loads and switching events. These characteristics are verified by conducting similar tests on house wiring system prototype created in the lab. Furthermore, the switching behaviour is observed on loading the system through the outlet.

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Time Efficient State-of-Charge Estimation using Open Circuit Voltage and the Logarithmic Modelling for Battery Management System (2016)

Battery management systems are important devices for protecting batteries in various electrical and electronic applications. One of the most important features of a properly designed battery management system is the diagnostic techniques for estimating the remaining usable charge in batteries in a time effective manner. This thesis presents two time-efficient and reliable techniques that do not require complex electronic hardware for application. One technique concerns the open-circuit voltage characteristics of a battery while the other technique considers the logarithmic modeling of the equivalent circuit of a lithium-ion battery so that the equilibrium OCV can be found within a shorter period of time. The logarithmic modelling method uses the equivalent circuit model and the characteristics of the subject lithium NMC battery and finds the equilibrium OCV. This thesis will include two techniques for the logarithmic modelling method. One technique uses the long time constant in the equivalent model of the battery and predicts the equilibrium with 1% error after the battery relaxes for 150 seconds. The other technique uses the short time constant in the equivalent model and predicts the equilibrium OCV with 2% error after the battery relaxes for 70 seconds. Both techniques of the logarithmic modeling method show improvement for estimating the equilibrium OCV in terms of waiting time compared to the standard methods.

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An Approach to Reduce the Effect of Partial Shading on Photovoltaic Modules (2015)

Partial shading (PS) losses occur when some photovoltaic (PV) cells in a PV array, panel, or module are shaded from the illuminating light source. Such a phenomenon causes a substantial reduction in power generation, and it is seen to be an inevitable problem in maximizing the efficiency of the entire PV system, especially in the case of small-scale urban implementations, where the location choices are often suboptimal and shading obstacles are commonly encountered. Therefore, this thesis proposes a robust technique to reconfigure a PV module’s connection pattern in order to passively reduce the effect of PS by dispersing the shade to other parts of the module without either intensely modifying the circuitry or requiring the introduction of additional electronic devices. An ideal single-diode model of the PV cell is used to simulate and compare the performance of PV modules using different connecting methodologies, including the proposed strategy under extensive PS conditions. The proposed configuration is able to obtain improved maximum power points (MPP(s)), while displaying fewer significant disadvantages that the other shade dispersion methods have exhibited. It also shows fewer scalability limitations after ensuring the PV module is reasonably sufficient in size, as there is no requirement on the scale and the parity combination from its numbers of rows and columns. Moreover, under the placement rules of the proposed strategy, only alternating, and therefore no more than half of the columns in the configuration, are required to be changed, thus making the process modularizable, and reducing the total implementation costs. Furthermore, the resulting PV characteristic curves are shown to have more distinct MPP after utilizing the shade dispersion strategy, which become more convenient for following MPP tracking controls if desired. Finally, a PV module prototype, consisting of 6 x 6 PV cells, is implemented, and the experimental results validate the effectiveness of the proposed shade dispersion approach. The achieved MPP improvements are found to be better than the other tested shade dispersion method if the shade comes with an angle, and significantly better than the typical series-parallel or the total-cross-tied configuration when the shade is substantial in size.

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Feasibility study of inductive heating coil with distributed resonant capacitors (2015)

Steam Assisted Gravity Drainage (SAGD) method is one of the most common methods used in the process of heavy oil recovery. However, the method is economically inefficient, inherently dependent of water availability and has some detrimental impact on the environment. Therefore the need for an efficient, economically feasible and environmental friendly solution to the existing problem is felt vehemently at the core of oil recovery industry. In the light of existing issue, electromagnetic heating technique has emerged as a promising solution and has received greater and greater attention of late. However, most of the studies (especially for inductive heating) have been limited to digital simulations and experiments primarily within the laboratory. In this thesis, preliminary investigations were carried out to confirm the feasibility of a new constructed capacitive compensated inductor coil. Design parameters of the inductor coil were computed and a test model was constructed in the lab for experimental verification of these parameters. The set up comprised a vertical drilling coil design with ferrite core inserted within, thus, creating an intensive electromagnetic field.In this design, a distributed resonating capacitor was proposed to avoid capacitor breakdown due to high voltage. Different coil winding configurations were proposed, constructed and tested by Frequency Response Analysis (FRA) to identify the resonant frequencies. The step response tests and field tests were performed with a square wave supply and were contrasted with the expected induced magnetic field modeled in MatLab. The measurements of the designed coil parameters matched the theoretically computed parameters and the experimental setup verified the advantage of using distributed capacitors, resulting is low breakdown voltage requirement. However, double-layer winding has multiple resonant frequencies and the proposed coil designs suffer from core losses core and winding copper losses. Thus, industrial application of this technology still requires further improvements.

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Implementing Digital Control to Improve Control Bandwidth and Disturbance Rejection on a LLC Resonant DC-DC Power Converter (2015)

In this thesis, the implementation of an adaptive digital control scheme and the development process to implement it for an existing analog controlled LLC resonant converter is presented. The goal is to improve the dynamic performance (aka control bandwidth) and the disturbance rejection ability of the closed loop system using digital control. A brief analysis of the experimented on LLC resonant converter and simulations of its control-to-output frequency response characteristics under different operating conditions are initially performed in order to show its non-linear behavior. Then the design process and requirements for both the digital and analog components to make the existing LLC converter compatible with a digital signal microcontroller is presented in detail. The digital signal microcontroller (DSC), ADC, DPWM, sampling period, interrupt service routine (ISR), and the 2P2Z digital compensator implementation will be overviewed. Analog components such as the voltage/current sensors, the VCO, and other analog interfacing components will also be presented. After that, the complete design process to achieve optimized digital compensators for several different operating points is presented. This design process introduces the method of using either the uncompensated loop-gain frequency response data collected empirically from the physical converter or from a PSIM simulation and then using MATLAB’s System Identification software toolbox to generate an estimated mathematical model based on frequency response data. A digital compensator is then designed based on the estimated mathematical model. A comparison between the PSIM simulation and the empirical data of the LLC converter’s plant frequency response for several different operating conditions is also presented. A digital adaptive compensator algorithm is implemented so that the most optimized compensator design for a given converter operating range is selected. The algorithm uses the output voltage and current to determine the operating point of the converter, which then access a software look-up-table (LUT) for the optimized compensator. A complete prototype is built to experimentally validate the digital design process and the performance results of a classical single compensator design is compared with the adaptive compensator design in order to show the benefits of the adaptive compensator control scheme.

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On the Possibility of a Small Nuclear Reactor Application for Energy Supply of Isolated Communities in Northern Canada (2015)

Isolated northern communities in Canada are currently satisfying their needs in energy using ineffective and environmentally unfriendly diesel generators. There has been long desire to find an alternative solution which would provide these settlements with electrical energy and heat inexpensively and with the least environmental impact. One possibility is to adopt a well-developed nuclear submarine reactor technology which has been used for the past 60 years. Most modern nuclear submarines use uranium-235 enriched up to 98% and allow operating the nuclear reactor without refueling for up to 25 years. The second option which offers many benefits, such as abundance and availability of cheaper fuel, greater anti-proliferation benefits as well as better safety, is the thorium option, a new approach, which has not been extensively researched and tested yet but which could offer attractive benefits once implemented. This thesis investigates a possibility of a small nuclear reactor application based on the research in the field of nuclear submarine technology for peaceful purposes and molten salt reactor experiment conducted by the Oak Ridge Lab, and compares uranium and thorium options with the currently used diesel generator technology. Fossil fuel scarcity and greenhouse effect of their use require finding alternative energy sources, and, nuclear technology provides such opportunity, especially, when highly enriched uranium is not available or difficult to obtain due to proliferation concerns. Thorium which is abundant, and is not currently in high demand or use, could be a great opportunity for the following reasons: (1) thorium is significantly cheaper and requires less processing than uranium; (2) thorium fuel could be used as a circulating liquid mixed with molten fluoride salts instead of using solid fuel elements which need special preparation; (3) the fluoride salts can be used as a reactor coolant due to their better chemical properties.

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A Nonlinear Circuit Model for Lithium-Ion Batteries (2014)

A lithium ion battery model is required for proper design of battery powered systems such as marine applications. The various existing models lack of the nonlinear aspect of the batteries being studied, suffer from high complexity and are not quite suitable for system level simulation. Therefore the scope of this thesis is to produce a battery model capable of capturing the nonlinearity of the battery for system level design. The Randle circuit model is selected as the base model to build upon and the electrochemical impedance spectroscopy is chosen as the fundamental test method for extracting the model parameters. The nonlinearity is accounted for by defining the equivalent circuit model elements’ parameters functions of the state of charge. Since lithium-ion batteries are prone to be affected by the changes in temperature, all the experiments are done under controlled temperature. The proposed model is a modified Randle circuit model in time domain implemented in MatLAB/SIMULINK. The model’s validity is verified using test data. It is concluded that this final version of model proposed in this thesis can be used directly in system level simulation while offering reasonable accuracy. However the proposed model does suffer from its dependency on the cell chemistry, which will limit its applications.

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Hydrokinetic turbine power converter and controller system design and implementation (2014)

Due to environmental considerations and decline of fossil fuels, searching for the viable energy alternatives is a pressing need. In order to meet the energy demand globally, research into the renewable energy technologies must be pursued. Wind energy has emerged as the leader of new energy source, while other types of energy sources continue to be investigated. Hydrokinetic power as a potential opportunity to harvest energy is being explored recently. Because the hydrokinetic turbines are still in an early stage of development, study on the hydrokinetic system is an active topic of academic research. Although water speed is instable and unpredictable, the hydrokinetic system can still operate at the peak power point. For this reason, the power converter and rotor speed controller are the most important components in this study. In the first two chapters some background studies and some system components are introduced. In this project, the maximum power point tracking is realized by the hill-climb searching method and the lookup table method, plus the analysis and comparison of these two methods are presented. In terms of rotor speed controller design, the Field Oriented Control strategy is applied and discussed. The presented hydrokinetic system is a stand-alone system which throws excessive energy to a load resistor. In order to demonstrate the feasibility of the system, the hydrokinetic system model is simulated in the PSIM software. A number of cases are simulated with the PSIM model to validate the feasibility of this hydrokinetic system. Finally, the project objectives are achieved and some of the hardware is tested. Future work is still required, such as the bench testing with a real PMSG, a variable frequency drive, and the optimization of the maximum power point tracking method and rotor speed controller with the hardware test results.

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LLC resonant converter modelling (2014)

Many of today's power converters use pulse-width modulation(PWM) techniques to regulate the circulating currents and voltages. A significant problem with most dc-dc converters is the increased power loss during switching. These devices typically operate in hard-switching mode which results in switching losses. Resonant converters have been used to minimize or even eliminate this problem. Although LLC resonant converters have shown significant gains in terms of efficiency, their modeling is still a challenge. LLC converters are designed to function in a specific mode and region of operation. It has been difficult to design a stable and robust controller with consistent bandwidth and disturbance rejection for every application. The complexity of the control design is magnified when the LLC converters are controlled using embedded digital control techniques. Recent developments in micro-controllers, including processing speed, power, and memory management, make possible the use of innovative non-linear or adaptive control algorithms, in order to produce high performance LLC circuits. Accurate modeling of the hardware is the key to an effective solution. This thesis presents several modeling techniques of the LLC resonant converter. Previous research is discussed and relevant techniques are used as reference for deriving the models presented here. A new approach will be used to describe the characteristics of the LLC within the operating region. This approach is derived using the method of Least Squares of errors. The method estimates the coefficients of the plant transfer function, which then help to calculate control coefficients in the instantaneous operating condition of the LLC resonant power converter.

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Reference model based power smoothing for stand-alone hybrid PV-diesel micro-grid (2014)

Photovoltaic (PV) generator generates clean energy but also brings active power fluctuation to the network. The thesis investigates the frequency stability issue of a MW level stand-alone hybrid micro grid which contains PV generator, diesel generator, storage unit and loads. The PV generator can only generate as much power as the sun provides. The resulting power mismatch between PV generation and load demand needs to be compensated. The slow responding diesel generator is designed to compensate for the steady state power mismatch. The battery, as the fast responding storage unit, is set to reject the power transients. A battery control method based on the micro grid frequency feedback and PV output feed-forward is presented to satisfy the requirement of active power compensation in transients. It will be shown that the method keeps the stand - alone micro grid frequency within a specified region and provides the diesel generators more margin of time to adjust their output for better diesel efficiency.

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Impact of photovoltaic generators and electric vehicles on a weak low voltage distributiion grid (2013)

In this thesis, the behaviour of a weak power distribution grid at the Point of Common Coupling (PCC) in the presence of a Photovoltaic (PV) inverter and Electric Vehicles (EV) as loads is analyzed. The grid connected to PV and EV has high impedance. The impact at PCC when the injected power varies in conjunction with the frequency and voltage deviation with a delay in inverter fed power is elaborated. Various measures such as peak shaving, coordinated charging, voltage drop correction have already been developed to mitigate the impact at PCC. These measures are observed on a combined EV and PV setup. In general, the grid tied inverter injects power based on an average grid voltage calculation at stable synchronization with the grid. If an error (for instance, an error in average grid voltage calculation) persists in such a case causing a loss in synchronization between the PV inverter and the grid, then a delay in the power injected may result in an oscillation at the PCC. A simple two bus system is considered to analyze the result of transportation delay. The delay and droop parameters of the PV inverter are altered whose results are quantitatively analyzed. The model abides the grid codes for active power reduction and static voltage support requirements. Further, the impact of a fault along with an inverter delay is analyzed. Simulative analysis is performed in the DIgSILENT PowerFactory software. To reduce the impact at PCC, performance criteria are analyzed whose parameters could be measured and altered. Scenarios are developed to analyze EV‟s impact in the presence and absence of storage and Distributed Generator (DG) that can be extended onto the micro grids.

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Modelling and control of the LLC resonant converter (2013)

To achieve certain objectives and specifications such as output voltage regulation, any power electronics converter must be coupled with a feedback control system. Therefore, a topic of considerable interest is the design and implementation of control systems for the LLC resonant converter. Additionally, with the current trend of smaller, more cost effective and reliable digital signal processors, the implementation of digital feedback control systems has garnered plenty of interest from academia as well as industry. Therefore, the scope of this thesis is to develop a digital control algorithm for the LLC resonant converter. For output voltage regulation, the LLC resonant converter varies its switching frequency to manipulate the voltage gain observed at the output. Thus, the plant of the control system is represented by the small signal control-to-output transfer function, and is given by P(s) =V_o/f. The difficulty in designing compensators for the LLC resonant converter is the lack of known transfer functions which describe the dynamics of the control-to-output transfer function. Thus, the main contribution of this thesis is a novel derivation of the small signal control-to-output transfer function. The derivation model proposes that the inclusion of the third and fifth harmonic frequencies, in addition to the fundamental frequency, is required to fully capture the dynamics of the LLC resonant converter. Additionally, the effect of higher order sideband frequencies is also considered, and included in the model. In this thesis, a detailed analysis of the control-to-output transfer function is presented, and based on the results, a digital compensator was implemented in MATLAB. The compensator's functionality was then verified in simulation. A comparison of the derivation model and the prototype model (based on bench measurements) showed that the derivation model is a good approximation of the true system dynamics. It was therefore concluded that both the bench measurement model and the derivation model could be used to design a z-domain digital compensator for a digital negative feedback control system. By using the derivation model, the main advantages are reduced computational power and the requirement for a physical prototype model is diminished.

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