Martin Ordonez


Relevant Degree Programs


Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - Nov 2020)
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 (2010 - 2018)
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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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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