Martin Ordonez


Relevant Thesis-Based Degree Programs


Graduate Student Supervision

Doctoral Student Supervision

Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.

Bidirectional resonant chargers for e-mobility (2023)

This thesis investigates the development of efficient bidirectional chargers, an integral component for advancing e-mobility. Electric vehicles (EVs), energized by renewable sources, could mitigate the power grid instability by returning stored energy when renewables are less available. Battery voltage in EVs, however, can vary widely based on the EV's application and its battery's State of Charge (SoC). This necessitates employing various bidirectional charger techniques that can handle a broad range of EV battery voltages when connected to the renewable energy grid.Three innovative techniques are proposed. Firstly, this work introduces the Asymmetric Parameters Methodology (APM), a technique that enables the design of asymmetric resonant tanks in bidirectional resonant CLLC DC/DC stages. APM, optimized through a statistical Design of Experiments (DoE) approach, results in a narrower bidirectional switching frequency range, reduced component current stress, and smaller transformer size.The second technique is a unified bidirectional resonant frequency tracking method for the CLLC DC/DC stage of battery chargers, which decreases bidirectional resonance tracking costs and complexity while enhancing efficiency under parameter deviations. It is based on the discovery of two interesting features of the CLLC converters: one, the maximum efficiency for charging and discharging modes occurs at a variable but bidirectionally identical resonant frequency. Two, the voltages at both ends of the resonant tank remain in phase at this frequency for bidirectional operations.This thesis introduces a third technique: a cascaded half-bridge-based multi-level multi-port bridgeless PFC rectifier for the AC/DC stage of chargers. Suitable for low-voltage battery charging, this technique divides high-voltage DC bus voltage into multiple low-voltage ports for the following CLLC DC/DC converter stage. This allows the transformer ratio of the DC/DC converter stage to be designed close to 1. Compared to cascaded full-bridge multi-level PFC, this approach cuts the number of switches per cell by half, while maintaining the same output ports. It also reduces input current ripple due to the reduction in volt-seconds on the boost inductor.

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Fast dynamic transient solutions for three-phase PWM converters (2023)

The ever-growing global energy demand accentuates the importance of integrating renewable energy sources into the grid. Beyond fulfilling escalating energy needs, this integration holds the potential to reduce the dependency on fossil fuels. In this context, power electronics systems assume a key role in the efficient utilization of renewable energy. Particularly, the three-phase pulse-width-modulated (PWM) converter serves as the bridge that facilitates the seamless interaction between the grid, distributed re- sources, and loads. However, as power electronics systems increase in complexity, challenges emerge in terms of modeling, control design, and implementation. Overcoming these challenges necessitates ad- vancements in control design to enhance dynamic performance and optimize the efficiency of integrating renewable energy.This thesis aims to improve the understanding of the dynamic characteristics of three-phase PWM converters, and to develop novel tools for modeling, analyzing and improving their dynamic performance in grid-connected applications. By employing concepts such as normalization, state plane representation, and geometric analysis, a comprehensive large-signal model for the three-phase converter is developed. This model offers an intuitive graphical interpretation of the system’s dynamic behavior by illustrating its evolution in the state plane. Initially, the application of this framework facilitates the identification and characterization of the theoretical limits of dynamic performance. Consequently, it serves as a point of reference for control design engineers to conduct an objective assessment of the converter’s dynamic performance. Furthermore, the introduced analysis enables the development of high-performance control methods, even for challenging scenarios such as controlling active loads and bidirectional operation with wide operating range requirements. These control techniques ensure consistent large-signal behavior, fast transient responses, and low implementation requirements. In this manner, the thesis contributes to advancing power electronics modeling and control through the enhancement of the dynamic performance of three-phase converters.

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High-performance resonant converters for battery chargers : efficiency and dynamics improvement (2023)

As the requirement for clean energy grows, the demand for high-performance power conversion for energy storage and battery charging applications has been soaring. Resonant converters, in particular, LLC or CLLC converters, have been broadly adopted for high-power battery chargers. The purpose of this work is to further improve the performance of the resonant converters from both the efficiency and dynamics aspects.In terms of improving the efficiency, this work focuses on reducing the conduction losses of the output rectifiers using Synchronous Rectification (SR). Conventional SR controllers detect the drain-source voltage during the SR turn-on phase (vds.on) as the control input. However, vds.on is low-magnitude and sensitive to the voltage noise caused by parasitic elements. The distorted vds.on causes SR mis-triggering, undermining the efficiency. With the focus on mainstream LLC resonant converters, this work first introduces a new SR driving strategy based on the resonant capacitor voltage (RCV). Next, a simplified SR method is proposed; it is based on the Volt-Second Product (VSP) of SR drain-source blocking voltage and rectifier current conductiontime. Both methods employ large-magnitude voltages, which are insensitive to the noise generated by parasitic components, reducing SR on-time error. The proposed SR methods are compared with the conventional vds.on based SR to demonstrate the efficiency improvement.Regarding the dynamics aspect, this work focuses on improving the small-signal dynamic model for charge-controlled resonant converters. Charge mode control has been applied to resonant converters to improve the system dynamics, and yet the conventional small-signal model emphasizes only the low-frequency region, which is not suitable for high-bandwidth designs. This work establishes the small-signal modeling methodology based on Extended Describing Functions (EDF) and phasor analysis, which successfully predicts the system frequency response across low- to high-frequency regions, enabling high-bandwidth designs. As the proposed noise-tolerant SR methods improve the efficiency performance, the enhanced small-signalmodel assists to achieve wide loop bandwidth, improving the dynamic performance. This work provides solutions and insights to the design of high-performance resonant battery chargers.

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Three-phase DC-DC resonant converters for high-power battery charging applications (2023)

Resonant power converters have gained popularity in many application, especially where there are strict requirements on efficiency, power density, and Electromagnetic Compability (EMC). Among different types of resonant converters, LLC and CLLC resonant converters have attracted more attention for uni-directional and bi-directional applications, respectively. This is due to the unique features of these types of converters, such as soft switching. Half-bridge and full-bridge resonant structures have are suitable for low to medium power levels, and three-phase structures are considered for higher power levels. Three-phase resonant converters provide high efficiency, low current ripple, good thermal distribution, and small filter and heat sink size (high power density), that makes them an attractive option for high power battery charger applications.Although there has been substantial research and development on single-phase LLC and CLLC resonant converters for low to medium power levels, the literature investigating three-phase structures has been very limited. Issues to be investigated for three phase resonant converters are: 1) passive components tolerances, which leads to unbalanced behavior of the converter, 2) development of a balancing technique to mitigate the unbalanced behavior of the converter, 3) the natural limited gain range of resonant converter structure, 4) poor light load efficiency of the three-phase resonant converter and possible phase shedding strategies to improve it, and also to increase the reliability of the system. In this work, methods to use three-phase LLC and CLLC resonant converters more efficiently are investigated. This includes: 1) investigation of imbalances in three-phase LLC converters and proposing a novel strategy referred to as Trigonometric Current Balancing (TCB) technique to mitigate the unbalanced behavior of the converter under sever unbalanced situation, 2) investigating phase-shedding techniques and strategies to improve the operation of the converter at light load conditions and extend the gain range of the converter, and 3) investigating the behavior of the three-phase CLLC resonant converter for bi-directional power applications.

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High efficiency single-phase power factor correctors: resonant circuit and flexible topology (2021)

Rectifiers with Alternating Current (AC) input and Direct Current (DC) output are required in many applications to regulate the output and provide a Power Factor Correction (PFC) capability. There are different applications for PFC rectifiers: wide-range and narrow-range. The purpose of this work is to propose two approaches to improve efficiency while keeping the performance high in both wide and narrow range rectifiers. Wide-range applications require PFC converters to support extended ranges of variations in the input voltage. A PFC converter capable of coping with a wide input voltage range, 90VRMS -530VRMS, would significantly decrease costs and streamline development. In this work, a reconfigurable PFC converter is proposed which provides a high and flat efficiency curve throughout the entire operating voltage range. The proposed reconfigurable converter has a flexible bridgeless structure with simple control, low current ripples, low common-mode noise, and startup inrush current handling capabilities. Narrow-range applications are also studied in this work which do not require a wide range of variations in their input. An advanced LLCC resonant structure is proposed in this work which improves the efficiency in narrow-range applications. The operation of this proposed concept is first developed in a switching-time scale and tested in a DC/DC environment, then a modified version is used with an AC input and bridgeless configuration. This converter can provide soft switching for all of the semiconductors without adding extra elements, by just using the passive components in the design as resonant tank. Therefore, the efficiency can be improved which potentially results in lower sizes for the passive elements. This converter can also provide a continuous input current despite using small inductances. This enables an inherent PFC capability with a single loop control architecture in the AC/DC version. This structure has a simple and symmetrical structure with easy control. The proposed converters in this work are theoretically and experimentally analyzed. Their performance is also compared iii with conventional structures. The proposed converters show efficiency improvements as well as better performance in this comparison.

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Advanced monitoring and control of distributed DC systems: an embedded impedance detection approach (2020)

Direct Current (DC) systems, made possible by power electronics technology, are becoming more prevalent due to their advantages when integrating renewable energy sources, energy storage, and DC loads. Microgrids and local area energy systems are instrumental to DC systems, and much progress has been made around them. However, DC microgrids face numerous challenges due to their decentralized nature, such as resource optimization, control, and protection. This thesis focuses on developing a core technology, an embedded impedance detection (EZD) method for DC systems, and its application to five critical challenges in DC systems. The proposed method uses a reference signal of minimal amplitude and high frequency, injected in the control loop of the power electronic converter, and a digital Lock-In Amplifier to extract the incremental behavior of the voltage and current around the DC operating point. These are used to calculate the incremental impedance, which is representative of the reactive part of the system as well as the nonlinear characteristics of the system. The proposed EZD method is applied to address five critical problems in today's DC systems: 1) adaptive control in the presence of active loads - to expand stability and improve transient response; 2) islanding detection - to detect the connection and disconnection of the utility grid and change controllers for autonomous operation; 3) fault location - to detect the distance to a fault and simplify the system restoration; 4) high-impedance fault detection - to accurately distinguish a fault condition from a load increase; and 5) maximum power point tracking of photovoltaic panels - to ensure efficient energy harvesting. For all these applications, the proposed EZD-based solution offers critical benefits and advantages, such as high sensitivity and accuracy at a low system disturbance and fast detection. The work presents a detailed analysis of the proposed EZD technique as well as considerations for its implementation in commercial microcontrollers, followed by simulations to illustrate its capabilities. The thesis also presents a detailed analysis of each DC system application and its particular considerations. The outlined benefits are supported by simulations and validated through experimental results using a real power electronics platform.

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High-efficiency and low noise planar transformers for power converters: paired layers interleaving (2020)

Nowadays, many applications, such as consumer electronics, the automotive industry, and telecoms requirehigh power density and low height power electronics converters. To implement slim power converters, PlanarTransformers (PT) have emerged, featuring low height, low leakage inductance, and low thermal resistance.Despite these benefits, PTs have large parasitic capacitance, which degrades the performance of power converters.Capacitive effects in transformers are divided into two groups: inter-winding and intra-windingcapacitance. Inter-winding capacitance generates large amounts of Common-Mode (CM) noise, creating seriousElectromagnetic Interference (EMI) problems. Intra-winding capacitance affects the performance ofthe converter and can cause loss of voltage regulation in the LLC resonant converter.The inter-winding capacitance can be reduced by separating primary and secondary windings, at the costof increased leakage inductance and AC resistance. On the other hand, interleaved structures minimize ACresistance and leakage inductance but significantly increase the inter-winding capacitance. Therefore, thereis an unfortunate trade-off in the transformer design. In order to resolve this trade-off as well as problemsresulting from PTs large parasitic capacitance, this dissertation develops new design methods that targetthe root cause of the problem. A detailed parasitic capacitance model is developed for PTs that relate thedistributed capacitance of layers to the equivalent circuit of the transformer. Based on this model, the conceptof paired layers is introduced that provides criteria to achieve zero CM noise generation in PTs. Paired layerscan be used to design interleaved structures that not only have low AC resistance and leakage inductance butalso have almost zero CM noise generation. Multiple examples are provided for different types of windings,different turn ratios, and different topologies to show the generality of the method. The proposed method isvalidated using analysis, Finite Element Method (FEM), and experiments.Besides the paired layers method, this dissertation studies the detrimental effects of PTs large intrawindingcapacitance on light-load voltage regulation of LLC resonant converter. It is shown that large intrawindingcapacitance results in loss of voltage regulation. To resolve this, six improved winding layouts withlow intra-winding capacitance are presented to maintain voltage regulation even under no-load condition.

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Large-signal transient control in power electronics: an average-geometric framework (2020)

Switch-mode power converters are a fundamental component of modern power systems; they are ubiquitous in renewable energy applications, electric vehicles, battery chargers, and power supplies. Controllers are an essential component in power converters as they improve the converter's dynamic behaviour during transients and in steady-state. For decades, the power electronics industry has preferred to utilize controllers based on converter small-signal analysis due to their low implementation requirements and in spite of their dynamic performance limitations and global stability issues. On the other hand, excellent dynamic response and global stability are achieved by non-linear boundary controllers based on state-plane analysis, which usually have much higher implementation requirements. This thesis focuses on the dynamic performance improvement of power converters by incorporating state-plane concepts while maintaining low implementation requirements to facilitate the large-scale adoption of the technology. By combining traditional averaging modelling tools with state-plane analysis, the unified Average Natural Trajectories (ANTs) are obtained to accurately model the large-signal dynamic behaviour of the fundamental topologies (buck, boost, and buck-boost). As a result, the proposed framework establishes the foundation for several dynamic performance improvement efforts introduced in this thesis. Employing the ANTs as a large-signal model, the theoretical limits of dynamic performance are defined and used to develop a powerful benchmarking tool, providing great value for design engineers. Furthermore, a unified controller based on the ANTs model is developed for the fundamental topologies in this work. This controller features a predictable large-signal response and outstanding dynamic performance while maintaining low implementation requirements. The ANTs modelling approach is also extended to photovoltaic applications to develop an extremely fast maximum power point tracking method for scenarios that include rapidly changing environmental conditions. Finally, the concept of dual-loop geometric control is introduced by combining state-plane analysis with an industry-standard dual-loop control structure, thereby bridging the gap between industrial applications and state-plane controllers. The concepts introduced in this thesis are supported by thorough mathematical analysis and validated by extensive simulation and experimental results. This thesis significantly contributes to the advancement of the field of modelling and control for power electronics.

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Three-layer control strategy for LLC converters: large-signal, small-signal, and steady-state operation (2019)

Resonant converters, particularly LLC converters, feature low switching losses and electromagnetic interference (EMI), and high power density and efficiency. As a result, they have been widely used in DC/DC applications. Although LLC converters naturally provide soft switching conditions and therefore, produce relatively less switching losses, conduction losses in their rectifier have remained a barrier to achieving higher efficiencies. Moreover, the analysis of LLC converters is complicated since they process the electrical energy through a high-frequency resonant tank that causes excessive nonlinearity. The issue of this complexity becomes even worse since, in reality, the resonant frequency of such converters deviates due to variations in the temperature, operating frequency, load, and manufacturing tolerances. This complexity has caused: a) limited research on large-signal modeling and control of LLC converters to be performed (this leads to uncertain large-signal transient behavior and sluggish dynamic/recovery response), b) limited insight into small-signal modeling of LLC converters (this often leads to low accuracy), c) unregulated LLC converters not to operate in their optimum operating point (this leads to degraded efficiency and gain), d) conduction losses in the LLC rectifier to remain the main challenge to achieve higher efficiency. To address the above concerns, in this dissertation, a three-layer control strategy is introduced. Based on the need, all the three layers or just one of them can be used when implementing the LLC converter. The three-layer control strategy produces accurate and fast dynamics during start-up, sudden load or reference changes with near zero voltage overshoot in the start-up, obtains a near zero steady-state error by employing a second-order average small-signal model valid below, at, and above resonance, improves efficiency by a new synchronous rectification technique, and also tracks the series resonant frequency in unregulated DC/DC applications.

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Advanced control functionalities for photovoltaic and energy storage converters (2017)

Power conversion systems including grid-connected photovoltaic (PV) and electrical energy storage (EES) stages open the prospects for new opportunities to improve the system’s performance in energy production and standards compliance.This dissertation proposes a completely revised state-of-the-art architecture with functionalities integrated within a unified system, which extracts more solar energy, provides safety compliance and grid stability. The first improvement of the power conversion system is focused on inverters' dc voltage extension range, which leads to increased PV energy harvesting. A proposed technique provides a lower voltage limit in the dc-bus utilization with the employment of the new voltage-reactive power control strategy accompanied with a modified zero-sequence modulation. Then, a higher dc-bus voltage limit is obtained by maximizing the utilization of power semiconductors. A graphical comparative analysis approach using I-V and P-V characteristics reflects remarkable PV-converter system behavior, which illustrates the advantages of the wide dc-bus range in 1500V systems. As a result, the maximum power point tracking (MPPT) dc voltage range is extended by an additional 30% improving the systems energy capture capabilities under extreme temperatures beyond the performance of traditional 1000V single-stage inverters. Furthermore, the single-stage conversion was extended to two-stages, with mini-boost rated for a fraction of the nominal power of the converter. Thus, the proposed design concept delivers significantly higher performance whilst reducing system cost at component level.The next proposed improvement of the system focuses on grid fault detection for standards compliance, using a search sequence function. This proposed technique is integrated within the active-reactive power control, MPPT algorithm, and phase-locked loop routine. In addition, the islanding search sequence is synchronized and incorporated within the MPPT (designed with an adaptive strategy to achieve system stability and minimum impact on power quality).Finally, the system’s control functionalities advances into grid support strategies, designed with frequency- and voltage-assist features for network stability. The change in active-reactive power flow is achieved using a responsive gradient to command the transitions between grid-feeding and grid-loading.The proposed system’s combined methods result in a cohesive PV/EES conversion architecture whose improved performance has been confirmed through electronic simulation and experimental results.

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High-order resonant power converters for battery charger applications (2017)

The demand for electric vehicles has expanded rapidly for both industrial and transportation applications. In parallel, new battery technologies have been introduced which are capable of deep-discharge and powering electric vehicles for long periods of time. Due to the increasing complexity of charging algorithms, battery chargers are exposed to demanding operating requirements. In battery charger applications, power converters should not only regulate the battery voltage and power over a wide range, all the way from complete discharge up to the charged floating voltage but also respond to the input voltage variation period. It is also important to work at high efficiency and with low switching noise and charging current ripple. This work studies different problems regarding DC-DC power converters with wide voltage regulation as battery chargers and investigates the application of novel high-order resonant power converters (fourth and fifth-order) and modulation strategies at various power levels. As a solution for high power applications, this work first introduces a modified full bridge LLC resonant power converter driven by both variable frequency and phase shift modulation. The proposed modulation strategy along with the modified resonant circuit exhibits excellent performance for a 3kW resonant power converter, without taking advantage of burst mode strategy. The second part of this work introduces a novel fifth-order L3C2 resonant converter for medium power level applications, that can regulate the battery voltage from near zero output voltage, zero output current to maximum output power. A 950W design example demonstrates a wide output voltage regulation with maximum efficiency of 96%. Finally, a fourth order L3C resonant converter is proposed for electric vehicles with roof-top solar photovoltaic panels, which can not only regulate the battery voltage in a wide range but also track the input voltage variation for extracting the maximum available power from the PV panel. All results from this work have been confirmed experimentally, which highlight the exceptional regulation capability of the proposed resonant power converters and modulation techniques.

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Coreless planar magnetic winding structures for power converters: track-width-ratio (2016)

With the accelerated growth of slim consumer electronics has come the need to reduce the profile of all electronic components. Planar magnetics provide an excellent solution to this problem, where copper strip conductors and flattened planar magnetic cores allow for the height of the components to be severely decreased compared to traditional wire-wound components. Planar magnetics also provide more repeatable characteristics and easier manufacturability. The major design goals for planar windings are low resistance, predictable inductance, and acceptable capacitance. This work investigates the application of a constant ratio between turn widths, called the Track-Width-Ratio (TWR) as a technique to attain these qualities in planar spiral windings. This work introduces the generalized racetrack planar spiral winding, whose low-frequency analysis can be applied to a variety of common winding shapes while accommodating changing track widths. The accompanying dimensional system provides the specification of the novel winding arrangements, including predicting their inductance and resistance. A design example demonstrates an 18% increase in low-frequency performance. The second part investigates the AC resistance from TWR. The proposed technique provides a correction factor based on the most recent models for ac resistance. A winding technique which combines hollow windings with TWR is proposed to increase the quality factor of planar spiral windings at high frequency operation. A design example highlights a change in efficiency from 70% to 90% within a 5W Wireless Power Transfer system. Finally TWR is employed to reduce planar spiral capacitance. Through an inverse TWR winding structure, a significant decrease in capacitance is observed with a moderate reduction in resistance and inductance. A quasi-analytical approach with finite element analysis is employed to determine the winding capacitance. These windings show a 50% decrease in capacitance and a 20% decrease in resistance compared to traditional windings. All results from this work have been confirmed experimentally and highlight the exceptional flexibility which is provided when the turn widths are included in the design of planar spiral windings.

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Master's Student Supervision

Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.

Improved regenerative braking in electric vehicles through switch selection optimization (2022)

Transportation electrification is at the core of the possible solutions to many challenges the world is currently facing. Efficient vehicle electrification has the potential to simultaneously reduce greenhouse gasses emissions and to tackle range anxiety issues. Among different strategies for advancements in Electric Vehicle (EV) efficiency, enhancing Regenerative Braking (REGEN) capabilities is an area with opportunities. As REGEN faces different impediments, upgrades in safety, efficiency, and/or battery quality of life are usually accompanied with further strain in energy management schemes, limiting REGEN performance. Power Electronics (PE) improvements are among the options that have the potential to benefit REGEN and overall efficiency. This work proposes a method to improves REGEN without adding extra stress on the other aspects that limit its performance, by optimizing PE-stage switch selection using openly available, manufacturer-provided data. To do so, the thesis develops a flexible simulation platform capable of: 1) integrating various subsystem modeling approaches, 2) analyzing different EV configurations, architectures, and components, and 3) analyzing the dynamic behavior of the Battery Electric Vehicle (BEV) while maintaining low simulation time. It also adopts a multiobjective optimization approach that gives the user freedom to define the weight of the objectives, as well as to include new objectives at any time - as long as the initial design choices do not change. The combination of a simulation platform suited for model-based design and an optimization formulation yields a method that fits well within the Design Automation (DA) framework. Therefore, the thesis is constructed with the framework as a guideline. The simulations show that proper switch selection can improve REGEN by over 18% and EV range efficiency by over 20%. The solution is corroborated by the results of the sensitivity and the robustness analysis.

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Optimization-based design of microgrids for critical loads (2022)

With the increasing number of extreme weather events caused by global warming, there is an escalatingneed for reliable and sustainable alternatives for powering up critical loads. In recent years,microgrids with renewable generation have emerged as a promising technology for powering criticalloads as they can achieve high power supply reliability and low carbon footprint. To meet therequirements of critical loads and to justify their economic viability, microgrids should be carefullyplanned and optimized. Existing tools for designing microgrids focus mainly on improving thedesign’s financial aspects by optimally sizing its energy sources; although these tools provide satisfactoryresults for conventional applications, they are not suitable for critical loads as reliabilityis not included in the design process.This work presents a new Vectorial Microgrid Optimization (VMO) design method for criticalloads. The proposed VMO method improves the microgrid design by 1) incorporating the selectionof the microgrid power conversion architecture and the size of the energy sources into a unifieddesign strategy, 2) include the microgrid reliability, net present cost, and energy efficiency as theperformance metrics. Multiobjective optimization is implemented as the decision-making tool tofulfill the critical load power requirements and reach the desire balance between the performancemetrics. To highlight the benefits of the proposed method, an example of a critical load is analyzedto find its optimal microgrid design. The results indicate that the proposed approach produces amicrogrid design with a 100 times lower downtime than those obtained with existing microgriddesign tools while maintaining an energy efficiency of 93.76% and reducing the net present cost by14%.

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Advanced incremental conductance MPPT for small wind turbines (2021)

Small wind turbines (WTs) are robust and commercially viable distributed generation alternatives to photovoltaic (PV) generation in locations deemed unsatisfactory due to low irradiance levels. However, small WTs are susceptible to variable environmental conditions and require sophisticated maximum power point tracking (MPPT) algorithms to ensure the system operates at the maximum power point (MPP) when subjected to fluctuating wind speeds. Optimal relationship based (ORB) and hill-climbing (HC) algorithms are the traditional MPPT methods for small WTs due to their simplicity, yet these strategies suffer from several challenges: ORB algorithms rely on parameterized coefficients that change over time, whereas HC variants are susceptible to algorithm confusion and lack standardized frameworks for choosing the optimal MPPT controller update frequency and perturbation magnitude.This work introduces a novel control-oriented small WT model that facilitates an intuitive approach for analyzing the electromechanical system. This modelling technique enables the development of two incremental conductance (InCond) based MPPT strategies that address the aforementioned challenges. The first presented MPPT strategy tracks the optimal system operating point using an adapted mechanical InCond algorithm and suppresses power oscillations around the MPP. This results in: 1) elimination of algorithm confusion, 2) accurate tracking and detection of the MPP and 3) improved steady state efficiency. The algorithm design requires only electrical sensing, thereby making this method sensorless from a mechanical perspective. The second presented MPPT framework uses the control-oriented model and an online impedance measurement technique to perform a system impedance frequency response analysis. Through this analysis, a small WT equivalent circuit is derived and MPPT controller is developed using a novel system identification (SysID) algorithm to perform InCond control. This methodology offers three advantages over conventional methods: 1) accurate tracking when subjected to erratic wind speeds, 2) optimal MPPT over the system lifetime and 3) a systematic approach for choosing the MPPT update frequency, facilitated by the system impedance frequency response analysis.The presented MPPT methods are supported with detailed mathematical procedures and validated with simulation and experimental results. This thesis significantly contributes to the advancement of small WT modelling and MPPT.

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Accurate power loss models of complex converters (2020)

Power loss estimation is essential for the design and optimization of power converters. Traditionally, the power loss estimation is done using datasheet-level information about the switching devices. However, this information is limited to isolated operating points, compromising the accuracy of the estimation. In addition, the representation of PCB effects and gate driver, which cannot be considered by using datasheet parameters, increase the complexity of the tasks by adding more unknown magnitudes and more complex equations without guaranteeing their correct estimation.In this work, a novel power loss estimation method, which can be applied to any topology, is presented. This proposed method utilizes Design of Experiments (DoE) and Response Surface Methodology (RSM) to model the different types of losses in all the elements in a power converter such as power switches, diodes, and inductors. With RSM, simple equations explain the different types of losses as a function of variables that directly affect them allowing the losses estimation under any operating condition accurately. These models are extensively validated to show the advantage of using the proposed method and significant improvement with respect to datasheet calculations is obtained. For complex topologies such as Power Factor Corrector (PFC), accurate power loss estimation is not only necessary but also a challenge. To show the capabilities of the proposed model, it is applied to Universal PFC, which is a complicated converter in terms of loss calculation and analysis. Using the proposed models, the inductor is designed optimally and the optimal switching frequency is selected to minimize the losses in the different input voltage levels. Experimental validation of the proposed method, prediction, and design impact is presented; the proposed method yields a power loss reduction up to 25%, having the greatest improvement at an input voltage of 110 V, and an output power higher than 500 W.

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Improved sizing of PV-BESS DC microgrids: a mixed-integer optimization (2020)

Remote off-grid communities rely on diesel for generating electricity. In addition to the greenhouse gas emissions, diesel generators require a constant fuel supply leading to transportation and logistics issues over the lifespan of the project. On the other hand, PV-BESS microgrids offer a suitable alternative to provide clean and sustainable energy to the communities due to their decreasing cost, small size, versatility for interconnection and low maintenance. Conventional methods determine the required PV-BESS installed capacity utilizing simple models to assess the financial aspects of the systems. However, they do not provide information about the implementation, thus requiring an additional stage to select available commercial devices and the correct series-parallel interconnection of the PV, BEES and power converters. Consequently, the overall cost of the system increases, while the performance drifts away from the predicted behaviour.In this work, a novel sizing methodology is proposed, reducing the cost of the system by 23%, the installed PV and BESS by 9% and the necessary power converters by 58%. The proposed approach facilitates a comprehensive study of the scenario, incorporating advanced models in the optimization problem, maintaining the financial goals, and adding new goals to a multi-objective optimal energy management strategy (EMS). The proposed optimal solution provides the number of series-parallel interconnected PV and BESS, and the required number of power converters, which eliminates the need for an additional implementation stage. The developed tool provides a comprehensive understanding of standalone microgrids, closing the gap between the optimal solution and industrial implementation.

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Improved power loss estimation for device- to system-level analysis (2019)

Power converters are found nearly everywhere electric power is used and are ubiquitous in renewable energy generation and electric vehicles. All power converters suffer from losses, Modern power converters have very high efficiency, often reaching peak efficiency > 95%. However, the losses in these systems are still significant and must be considered for thermal and financial purposes. To enable maximum loss reduction, accurate estimation of the losses at the design stage is mandatory.Gallium Nitride (GaN) power switches are an emerging technology due to their high efficiency operation and smaller size compared to traditional Silicon devices. To date, simplistic power loss models have been employed for loss predication and thermal management design with Gallium Nitride (GaN). However, these simplistic models do not provide accurate loss prediction, resulting in over-design of the thermal management systems. This work proposes a comprehensive method to predict losses in GaN devices using high-accuracy thermal measurement. The proposed model is validated experimentally and provides a four-fold increase in loss predication accuracy compared to traditional methods.Having established accurate converter-level loss prediction, a higher level of abstraction is then considered. Existing system-level analysis focuses on distribution losses and oversimplifies converter losses by assuming fixed efficiency. In reality, converter losses are highly variable under different operating conditions. In this work, the Rapid Loss Estimation equation (RLEE) is proposed to provide computationally simple loss prediction under all operating conditions. The RLEE extracts detailed loss behavior from multi-domain simulation into a computationally simple parametric equation. Using the RLEE high accuracy and high speed loss estimation is obtained, as demonstrated in a DC microgrid with three different converters.Ultimately, the tools developed in this work improve loss estimation in power converters from the component level up to the system level. The proposed techniques, while explained through specific examples, are widely applicable and can be readily implemented to other devices, topologies and systems. Improved loss estimation is valuable at all levels, from designing thermal management systems for individual devices in a converter to optimizing the financial outcomes of a complex grid with multiple power converters.

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Power loss estimation in LLC MOSFETs: a time interval analysis (2019)

LLC resonant converters have become a mainstream topology in DC/DC power conversion applications such as electric vehicle charging, renewable energy generation and low power energy conversion. This converter presents advantages such as the soft-switching of active devices, which reduces power losses in the energy conversion process, allowing for a more efficient operation when compared to hard-switched topologies.Losses in the MOSFETs of the inverting and rectifying stages of this converter should be accurately determined so to allow for proper heat management design and thermal dissipation. However, determination of losses through simulation can be challenging due to the significant difference between time constants of electrical and thermal phenomena. Moreover, the information presented by the datasheet of power electronic devices is often limited to select operating points, which may compromise the accuracy of power loss estimation.In order to overcome the limitations imposed by the datasheet, a detailed characterization of the main loss mechanisms in the operation of LLC MOSFETs is presented. To avoid the time-consuming and computationally-intensive process of simulation, steady-state time-domain expressions for the converter are developed, based on the electrical behavior of the topology. These equations, based on the Time Interval Analysis, are able to predict key electrical and thermal behaviours based on circuit design considerations and operating conditions, being easily implementable in software such as MATLAB or MS Excel.As verified by simulation and experimental results, estimation of losses using the proposed method is considerably more precise than using the well-established yet oversimplified First-Harmonic Approximation (FHA). In the inverting stage, the observed error in loss determination is reduced from an average of 19%, using FHA, to 2.8% using the proposed method. When it comes to the rectification portion of the circuit, the reduction in error observed is from 12% to 2%. Such improvement in power loss estimation before the converter is built is fundamental for the design of an agile and cost-effective thermal management approach which guarantees the integrity and reliability of the power electronics device.

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Dual-Loop Linear Controller for LLC Resonant Converters (2018)

For the last years, LLC resonant converters have gained wide popularity in a large number of domestic and industrial applications due to their high-efficiency and power density. Common applications of this converter are battery chargers and high efficiency power supplies, which require tight output voltage regulation. In traditional PWM converters, closed-loop controllers based on small-signal models are typically implemented to achieve zero steady-state error and minimize the effects of disturbances at the output. However, traditional averaging techniques employed in PWM converters cannot be applied to LLC's and highly complex mathematical models are required. As a consequence, designing linear controllers for this type of converter is usually based on empirical methods, which require high-cost equipment and do not provide any physical insight into the system. The implementation of current-mode controllers has been vastly developed for PWM converters. Employing an inner current loop and outer voltage loop has shown numerous advantages, such as, tight current regulation, over-current protection, and ample bandwidth. However, this control architecture is not commonly implemented in LLC resonant converters, and conventional single voltage loop controllers are employed.This work proposes a simple and straightforward methodology for designing linear controllers for LLC resonant converters. A simplified second order equivalent circuit is developed and employed to derive all the relevant equations for designing proper compensators. A dual-loop control scheme including an inner current loop and outer voltage loop is proposed. The implementation of the dual-loop configuration provides improved closed-loop performance for the entire operational range. The theoretical findings are supported by detailed mathematical procedures and validated by simulation and experimental results.

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Improving dynamic performance in dc microgrids using trajectory control (2018)

Direct-current (dc) microgrids interconnect dc loads, distributed renewable energy sources, and energy storage elements within networks that can operate independently from the main grid. Due to their high efficiency, increasing technological viability and resilience to natural disturbances, they are set to gain popularity. When load-side converters in a microgrid tightly regulate their output voltages, they are seen as constant power loads (CPLs) from the standpoint of the source-end converters. CPLs can cause instability within the network, including large voltage drops or oscillations in the dc bus during load transients, which can lead to dc bus voltage collapse.Traditionally, the stability of CPL-loaded dc microgrids relies on the addition of passive elements, usually leading to dc-bus capacitance increase. In this scenarios, source-end converters controllers are usually linear dual proportional-integral (PI) compensators. The limited dynamic response of these controllers exacerbates the CPL behavior, which leads to the use of larger passive elements.Recent contributions focus on implementing control modifications on the source-end converter in order to improve the system performance under CPLs. Particularly, the use of state-plane based controllers has been studied for the case of a single dc-dc power converter loaded by a CPL, showing fast and robust transient performance. However, the microgrid problem, where these faster converters interface with others of a slower response has not been studied thoroughly. This work proposes the use of a fast state-plane controller to replace one of the system’s source-end converters controllers in order to improve three aspects of the microgrid operation: resiliency under CPL's steps, load transient voltage regulation, and voltage transient recovery time.Since the converter is operating within a microgrid, the controller incorporates a traditional droop rule to enable current sharing with the rest of the converters of the network. The small-signal stability improvement of the whole system obtained by the addition of a single faster controller is analyzed for a linear model, and a parametric analysis demonstrates the improvements in a detailed model.Simulations and experimental results of a microgrid with three converters feeding a CPL prove the effectiveness of the technique for large-signal transients.

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Optimization of a grid-tied inverter: an application-oriented for designing multilevel converters (2016)

Recent developments in energy systems, including the rapid adoption of renewable energy sources and expansion of microgrids have been introducing new challenges and opportunities for the power electronics industry. Of particular interest to this thesis is the increase of grid-connected DC systems. As a means to reduce cost of copper infrastructure these systems favour the utilization of higher DC link voltages. To accommodate higher voltages, a variety of multilevel converters have been proposed, which generally can be built without specialized components, present lower dV/dt losses and synthesize AC signals with better power quality. Irrespective of the application, the power electronics industry has traditionally relied on generic rules and practices to quickly design converters. Rarely does the development cycle allow for thorough investigation of the converter design, which could enhance performance and give an edge over competitors. This thesis proposes using optimization techniques to aid power electronics engineers in the design of multilevel converters. The Neutral Point Clamped (NPC) with its variant the Active NPC (ANPC) were selected for the exercise presented. Chapter 2 of this thesis explains the operation and modulation of the topologies. From the analysis, the conduction and switching losses of each device can be predicted. A description of three semiconductor technologies is presented with their characteristics and source of losses. Lastly an equation to size the filter inductor is introduced. All this information is packaged into a model used in the optimization. Chapter 3 introduces the optimization strategy. Given the complex nature of power electronics, four objective functions were adopted: efficiency, loss distribution, inductance and cost. These functions were combined through a weight system which allows priorities to be asserted. Next, design variables are introduced along with their respective impacts on the objective functions. Experiments performed with a hardware platform showed the model closely predicts the impact of the design variables on the objective functions. Confident in the model, the optimization was carried out for various scenarios. Single objective optimization led to converters that excel in one aspect but were often not practical. When optimizing with multiple objectives a good compromise was reached with a practical converter.

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Discrete Signal Processing Techniques for Power Converters: Multi-Carrier Modulation and Efficient Filtering Techniques (2014)

Digital control has become ubiquitous in the field of power electronics due to the ease of implementation, reusability, and flexibility. Practical engineers have been hesitant to use digital control rather than the more traditional analog control methods due to the unfamiliar theory, relatively complicated implementation and various challenges associated with digital quantization. This thesis presents discrete signal processing theory to solve issues in digitally controlled power converters including reference generation and filtering.First, this thesis presents advancements made in the field of digital control of dc-ac and ac-dc power converters. First, a multi-carrier PWM strategy is proposed for the accurate and computationally inexpensive generation of sinusoidal signals. This method aims to reduce the cost of implementing a sine-wave generator by reducing both memory and computational requirements. The technique, backed by theoretical and experimental evidence, is simple to implement, and does not rely on any specialized hardware. The method was simulated and experimentally implemented in a voltage-controlled PWM inverter and can be extended to any application involving the digital generation of periodic signals.The second advancement described in this thesis is the use of simple digital filters to improve the response time of single-phase active rectifiers. Under traditional analog control strategies, the bandwidth of an active rectifier is unduly restricted in order to reduce any unwanted harmonic distortion. This work investigates digital filters as a proposed means to improve the bandwidth, and thereby create a faster, more efficient ac-dc power converter. Finally, a moving average filter is proposed, due to its simple implementation and minor computational burden, as an efficient means to expand the bandwidth. Since moving average filters are well known and widely understood in industry, this proposed filter is an attractive solution for practicing engineers.The theory developed in this thesis is verified through simulations and experiments.

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Electric Vehicle Power Trains: High-Performance Control for Constant Power Load Stabilization (2014)

The development of sustainable transport systems has experienced great improvements inthe last 15 years. As a result, electric vehicles, namely hybrid electric vehicles (HEVs) andall-electric or battery electric vehicles (BEVs), are slowly starting to coexist with regularinternal combustion vehicles around the world. The complex powering structure of automotive electric systems can be described as a distributed multiconverter architecture. In pursuit of performance, constant-power behavior of tightly regulated downstream converters has raised as an important challenge in terms of system stability and controllability. The first part of this work presents the theory and experimental validation of the unstable behavior introduced by constant-power loads (CPLs) in power converters, more precisely in a Buck+Boost cascade converter as the battery charge/discharge unit. The second part of this work presents the derivation of the Circular Switching Surfaces (CSS) and the implementation of the CSS-based control technique for CPL stabilization. The analysis shows that the constant-power load trajectories and the proposed CSS present a wide, stable operating area and near-optimal transient response. Furthermore, impedance analysis of the converter in close-loop control shows advantageous reduced output source impedance. This extremely high dynamic capability prevents the use of bulky DC capacitors for bus stabilization, and allows the implementation of metal-film capacitors, which have reliability advantages over commonly employed electrolytic capacitors, as well as reduced ESR to improve system efficiency. Beyond the improved stabilization properties of the proposed CCS-based controller, a comparison with traditional compensated linear controller and nonlinear SMC highlights significant improvements in terms of dynamic response for sudden CPL changes. Simulation and experimental results are provided to validate the work. The last part of this thesis work presents the design, construction, and testing of a high-power 3-phase converter. This platform is intended for electric motor driving and is able to manage 20kW of power flow and above, making it suitable for high power traction system development. The platform features an Intelligent Power Module (IPM) to provide with flexibility allowing for changing the power module according to the requirements of the development. Testing of the platform was done in a 0.5HP AC induction motor drive controlled with Voltz-per-Hertz control technique. The integration of the BCDU and the high-power 3-phase motor drive platform conform a high-power bidirectional motor drive platform for the development and testing of control techniques for energy management in EV.

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Photovoltaic Maximum Power Point Tracker with Zero Oscillation and Adaptive Step (2014)

Maximum Power Point Tracking (MPPT) strategies in Photovoltaic (PV) systems ensure efficient utilization of PV arrays. Among different strategies, the Perturb and Observe (P&O) algorithm has gained wide popularity due to its intuitive nature and simple implementation. However, such simplicity in P&O introduces two inherent issues, an artificial perturbation that creates losses in steady-state operation and a limited ability to track transients in changing environmental conditions. This work develops and discusses in detail an MPPT algorithm with zero oscillation and slope tracking to address those technical challenges. The strategy combines three techniques to improve steady-state behavior and transient operation: 1) idle operation on the Maximum Power Point (MPP), 2) identification of the irradiance change through a natural perturbation and 3) a simple multi-level adaptive tracking step. Two key elements, which form the foundation of the proposed solution, are investigated: the suppression of the artificial perturbation at the MPP and the indirect identification of irradiance change through a current-monitoring algorithm which acts as a natural perturbation. The Zero-oscillation, Adaptive step Perturb and Observe (ZA-P&O) MPPT strategy builds on these mechanisms to identify relevant information and produce efficiency gains. As a result, the combined techniques achieve superior overall performance while maintaining simplicity of implementation. Simulations and experimental results are provided to validate the proposed strategy and illustrate its behavior in steady state and transient operation.

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Reduction of Current/Torque Ripple in Low Power Grid-Tie PMSG Wind Turbines (2014)

Small-scale Wind Energy Conversion Systems (WECS) are becoming an attractive option for distributed and renewable energy generation. In order to be affordable, WECS must have low capital and maintenance costs. This leads to the increasing penetration of Permanent Magnet Synchronous Generators (PMSG) operating at variable frequency with connections to the power grid through a rectifier, and grid-tie inverter. Because PMSGs lack brushes and can be directly coupled to wind turbines, the capital and maintenance costs are greatly reduced. A direct connection to the grid further reduces system costs by removing the requirement of large battery banks.The loading produced by grid-tie inverters on the DC bus is different than more typical constant-current or constant-power loads. They are characterized by large input ripple currents at twice the inverter's grid frequency. These ripple currents are reflected through the DC bus into the PMSG causing increased heating in the stator, and ripple torques which lead to premature bearing failure and increased maintenance costs. To mitigate this problem, manufacturers typically add large amounts of capacitance on the DC bus to partially absorb these ripples at the expense of system size, cost, and reliability.In this work, the effects of the grid-tie inverter load are explored using system behavioural models which provide insight into the low frequency behaviour of the PMSG, rectifier, DC bus, and inverter. The swinging bus concept is presented and analysed in the time and frequency domains. A control philosophy is developed which allows the DC bus to swing, thus removing the effects of the grid-tie inverter on the PMSG while keeping the DC bus capacitor small. A solution consisting of a Moving Average Filter (MAF) is presented as an integral part of the control strategy.Full simulations of a complete system are developed and investigated to verify the ripple torque reduction technique. Finally, a prototype is developed and experimental results are presented for a 2.5kW PMSG turbine generator. The simulation and experimental results are compared to a traditional controller showing tangible improvements in ripple current and torque in the PMSG, while improving the dynamic response of the system.

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Novel concepts for power electronics control: Introducing the dynamic physical limits, the average natural trajectories, and the centric-based controller (2013)

Controllers are an essential component in power conversion systems that have a significant impact on characteristic features such as performance, efficiency, size, and cost, among many others. During the last four decades, countless efforts have been made to find better controllers for power electronics systems in order to improve the converters steady state and dynamic behaviour, increase power densities and reduce losses in the system.Small-signal based linear controllers have been the preferred alternative during decades. This technique features fixed switching frequency and low computation/sensing requirements, while the dynamic response can be improved to only a limited extent and the global stability cannot be ensured. On the other hand, excellent dynamic performances and global stability are achieved by boundary controllers, in which the switching frequency is variable and faster sensors are required.The first part of this work presents a practical tool which allows to objectively quantize improvements made by the controllers to the performance of power converters. The theoretical optimal dynamic behaviour of buck converters is determined, analyzed, and characterized using closed-form mathematical expressions, setting a strong benchmark point for the performance evaluation.Taking the physical limits of dynamic performance into account, and merging the advantages of linear and boundary techniques, a novel control scheme is developed for buck converters. The proposed controller is based on a large-signal model introduced here: the Average Natural Trajectories (ANTs). Enhanced dynamic performance and global stability are achieved while low sensing and computational requirements are maintained, which makes the technique very appealing for use in high-volume production applications.Due to the outstanding results in the basic buck converter, and in order to illustrate the application of the ideas introduced in this work for different topologies, the ANTs and the centric-based controller are developed for boost converters. The obtained results confirm the enhanced dynamic response and fixed frequency operation as natural advantages of the proposed control scheme.The theoretical findings are supported by detailed mathematical procedures and validated by experimental results, which highlight the practical usefulness of the concepts introduced in this work.

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