Reduced-order modeling of power electronics components and systems

This dissertation addresses the seemingly inevitable compromise between modeling fidelity and simulation speed in power electronics. Higher-order effects are considered at the component and system levels. Order-reduction techniques are applied to provide insight into accurate, computationally efficient component-level (via reduced-order physics-based model) and system-level simulations (via multiresolution simulation). Proposed high-order models, verified with hardware measurements, are, in turn, used to verify the accuracy of final reduced-order models for both small- and large-signal excitations. At the component level, dynamic high-fidelity magnetic equivalent circuits are introduced for laminated and solid magnetic cores. Automated linear and nonlinear order-reduction techniques are introduced for linear magnetic systems, saturated systems, systems with relative motion, and multiple-winding systems, to extract the desired essential system dynamics. Finite-element models of magnetic components incorporating relative motion are set forth and then reduced. At the system level, a framework for multiresolution simulation of switching converters is developed. Multiresolution simulation provides an alternative method to analyze power converters by providing an appropriate amount of detail based on the time scale and phenomenon being considered. A detailed full-order converter model is built based upon high-order component models and accurate switching transitions. Efficient order-reduction techniques are used to extract several lower-order models for the desired resolution of the simulation. This simulation framework is extended to higher-order converters, converters with nonlinear elements, and closed-loop systems. The resulting rapid-to-integrate component models and flexible simulation frameworks could form the computational core of future virtual prototyping design and analysis environments for energy processing units

Contributions to Modeling, Simulation, and Analysis of Three-Phase Dual Active Bridge Converters for Dc-Grid Integration

RÉSUMÉ Le convertisseur cc-cc bidirectionnel isolé triphasé à deux ponts actifs (3p-DAB) est largement étudié pour son utilisation dans les réseaux à courant continu de nouvelle génération. Son intégration nécessite une modélisation précise de ses caractéristiques petit et grand signal. De plus, sa caractérisation précise en mode dégradé d’opération est un sujet important pour le développement de réseaux fiables et résilients. L’application de stratégies de modélisation bien connues au convertisseur 3p-DAB, tant en fonctionnement normal que dégradé, n’est pas triviale du fait de sa structure cc-ca-cc, des différentes connexions possibles de transformateurs triphasés et du nombre plus élevé d’interrupteurs par rapport à d’autres topologies connues. L’absence de modèles petits signaux précis ainsi que de modèles grands signaux numériquement efficace du 3p-DAB est la principale motivation des développements présentés dans cette thèse. Trois contributions principales se dégagent de ce travail. Cette thèse contribue d’abord à l’identification des limites de l’approche conventionnelle de moyennage en espace d’état (SSA) pour la détermination des impédances d’entrée ZD et ZN du convertisseur 3p-DAB. Ces deux fonctions de transfert sont nécessaires à l’application du théorème d’éléments supplémentaires (EET) de Middlebrook qui est largement utilisé par les concepteurs en électronique de puissance pour prévenir la dégradation des performances dynamiques ainsi que les conditions d’instabilité lors de l’ajout d’un filtre d’entrée. La détermination précise de ZD et ZN est également importante pour déduire les caractéristiques petit signal en boucle fermée du convertisseur à partir de son modèle en boucle ouverte. Bien qu’il soit démontré que l’approche généralisée de moyennage en espace d’état (GSSA) peut être utilisée pour surmonter les limites de la méthode SSA pour l’évaluation de ZD, un nouveau modèle hybride combinant SSA et GSSA est proposé pour le calcul de ZN. La deuxième contribution de cette thèse est le développement d’un modèle moyenné généralisé (GAM) précis et efficace permettant la simulation de réseaux utilisant des convertisseurs 3p-DAB dans des logiciels d’analyses de transitoires électromagnétiques.----------ABSTRACT The three-phase dual active bridge (3p-DAB) isolated-bidirectional dc-dc converter (IBDC) is widely investigated in next-generation dc-grids. Its successful integration requires an accurate representation of its small- and large-signal characteristics. Furthermore, accurate characterization of converters behavior in degraded-mode is an important topic for the development of reliable and resilient dc-grids. The application of well-known modeling strategies to 3p-DAB converters in both normal and degraded operations is not trivial due to its dc-ac-dc structure, the different possible three-phase transformer connections and the higher number of switches compared to other well-known IBDC topologies. The absence of accurate small-signal models as well as computational efficient large-signal models of the 3p-DAB has motivated the developments presented in this thesis. Three main contributions are emerging from this work. This thesis first contributes to the identification of the limitations of the basic state-space averaging (SSA) approach for the determination of the driving point ZD and null driving point ZN input impedances for 3p-DAB converters. These two transfer functions are necessary for the application of Middlebrook’s extra element theorem (EET) which is broadly used by practicing power electronic designers to avoid dynamic performance degradation or instability conditions in the presence of an additional input filter. The accurate determination of ZD and ZN is also important to derive the closed-loop small-signal characteristics of converters from its open-loop characteristics. While it is shown that the generalized state-space averaging (GSSA) approach can be used to overcome SSA limitations to evaluate ZD, a new hybrid SSA and GSSA multi-input multi-output (MIMO) model, which combines SSA and GSSA, is proposed to evaluate ZN. The second contribution of this thesis is the development of an accurate and computational efficient generalized average model (GAM) which enables system-level simulation of dc-grids with 3p-DAB converters in electromagnetic transient type (EMT-type) programs. The proposed model is rigorously compared with alternative modeling techniques: ideal-model, switching-function (SWF) and state-space averaging (SSA). It is concluded that the GAM model provides an optimal solution when accuracy of transient response, reduction in computation time, and wideband response factors are considered

Averaged Behavior Model of Current-Mode Buck Converters for Transient Power Noise Analysis

Accurate Evaluation and Simulation of Power Noise is Critical in the Development of Modern Electronic Devices. However, the Widely Used Target Impedance Fails to Predict the Low-Frequency Noise Generated in a Device Due to the Existence of the Dc–dc Converter, Whose Output Impedance Can Change under Different Loading Conditions. a Physical Circuit Model is Then Desired to Replicate the Behavior of a Voltage Regulator Module, and the Average Technique is an Efficient Method to Estimate the Noise of a Pulse Width-Modulated (PWM) Converter. with the Emergence of Converters with Adaptive On-Time (AOT) Controllers, More Complex Averaging Methods Are Required, But None of Them Supports Transient Simulation. a General, Efficient, and Accurate Modeling Technique is Presented in This Article, Whose Framework Supports Both Current-Mode PWM and AOT Controllers. in Addition, a Novel Two-Step Parameter Extraction Method is Proposed, Which Can Be Used to Evaluate the Equivalent Values of Internal Feedback Parameters of an Encrypted Simulation Model or from Measurement. the Modeling Method is Validated by Both Simulation and Measurement

Control and Stability of Residential Microgrid with Grid-Forming Prosumers

The rise of the prosumers (producers-consumers), residential customers equipped with behind-the-meter distributed energy resources (DER), such as battery storage and rooftop solar PV, offers an opportunity to use prosumer-owned DER innovatively. The thesis rests on the premise that prosumers equipped with grid-forming inverters can not only provide inertia to improve the frequency performance of the bulk grid but also support islanded operation of residential microgrids (low-voltage distribution feeder operated in an islanded mode), which can improve distribution grids’ resilience and reliability without purposely designing low-voltage (LV) distribution feeders as microgrids. Today, grid-following control is predominantly used to control prosumer DER, by which the prosumers behave as controlled current sources. These grid-following prosumers deliver active and reactive power by staying synchronized with the existing grid. However, they cannot operate if disconnected from the main grid due to the lack of voltage reference. This gives rise to the increasing interest in the use of grid-forming power converters, by which the prosumers behave as voltage sources. Grid-forming converters regulate their output voltage according to the reference of their own and exhibit load sharing with other prosumers even in islanded operation. Making use of grid-forming prosumers opens up opportunities to improve distribution grids’ resilience and enhance the genuine inertia of highly renewable-penetrated power systems. Firstly, electricity networks in many regional communities are prone to frequent power outages. Instead of purposely designing the community as a microgrid with dedicated grid-forming equipment, the LV feeder can be turned into a residential microgrid with multiple paralleled grid-forming prosumers. In this case, the LV feeder can operate in both grid-connected and islanded modes. Secondly, gridforming prosumers in the residential microgrid behave as voltage sources that respond naturally to the varying loads in the system. This is much like synchronous machines extracting kinetic energy from rotating masses. “Genuine” system inertia is thus enhanced, which is fundamentally different from the “emulated” inertia by fast frequency response (FFR) from grid-following converters. Against this backdrop, this thesis mainly focuses on two aspects. The first is the small-signal stability of such residential microgrids. In particular, the impact of the increasing number of grid-forming prosumers is studied based on the linearised model. The impact of the various dynamic response of primary sources is also investigated. The second is the control of the grid-forming prosumers aiming to provide sufficient inertia for the system. The control is focused on both the inverters and the DC-stage converters. Specifically, the thesis proposes an advanced controller for the DC-stage converters based on active disturbance rejection control (ADRC), which observes and rejects the “total disturbance” of the system, thereby enhancing the inertial response provided by prosumer DER. In addition, to make better use of the energy from prosumer-owned DER, an adaptive droop controller based on a piecewise power function is proposed, which ensures that residential ESS provide little power in the steady state while supplying sufficient power to cater for the demand variation during the transient state. Proposed strategies are verified by time-domain simulations

High Power, Medium Frequency, and Medium Voltage Transformer Design and Implementation

Many industrial applications that require high-power and high-voltage DC-DC conversion are emerging. Space-borne and off-shore wind farms, fleet fast electric vehicle charging stations, large data centers, and smart distribution systems are among the applications. Solid State Transformer (SST) is a promising concept for addressing these emerging applications. It replaces the traditional Low Frequency Transformer (LFT) while offering many advanced features such as VAR compensation, voltage regulation, fault isolation, and DC connectivity. Many technical challenges related to high voltage stress, efficiency, reliability, protection, and insulation must be addressed before the technology is ready for commercial deployment. Among the major challenges in the construction of SSTs are the strategies for connecting to Medium Voltage (MV) level. This issue has primarily been addressed by synthesizing multicellular SST concepts based on modules rated for a fraction of the total MV side voltage and connecting these modules in series at the input side. Silicon Carbide (SiC) semiconductor development enables the fabrication of power semiconductor devices with high blocking voltage capabilities while achieving superior switching and conduction performances. When compared to modular lower voltage converters, these higher voltage semiconductors enable the construction of single-cell SSTs by avoiding the series connection of several modules, resulting in simple, reliable, lighter mass, more power dense, higher efficiency, and cost effective converter structures. This dissertation proposes a solution to this major issue. The proposed work focuses on the development of a dual active bridge with high power, medium voltage, and medium frequency control. This architecture addresses the shortcomings of existing modular systems by providing a more power dense, cost-effective, and efficient solution. For the first time, this topology is investigated on a 700kW system connected to a 13kVdc input to generate 7.2kVdc at the output. The use of 10kV SiC modules and gate drivers in an active neutral point clamped to two level dual active bridge converter is investigated. A special emphasis will be placed on a comprehensive transformer design that employs a multi-physics approach that addresses all magnetic, electrical, insulation, and thermal aspects. The transformer is designed and tested to ensure the system’s viability

Modeling and Analysis of Power Processing Systems (MAPPS), initial phase 2

The overall objective of the program is to provide the engineering tools to reduce the analysis, design, and development effort, and thus the cost, in achieving the required performances for switching regulators and dc-dc converter systems. The program was both tutorial and application oriented. Various analytical methods were described in detail and supplemented with examples, and those with standardization appeals were reduced into computer-based subprograms. Major program efforts included those concerning small and large signal control-dependent performance analysis and simulation, control circuit design, power circuit design and optimization, system configuration study, and system performance simulation. Techniques including discrete time domain, conventional frequency domain, Lagrange multiplier, nonlinear programming, and control design synthesis were employed in these efforts. To enhance interactive conversation between the modeling and analysis subprograms and the user, a working prototype of the Data Management Program was also developed to facilitate expansion as future subprogram capabilities increase

On Enhancing Microgrid Control and the Optimal Design of a Modular Solid-State Transformer with Grid-Forming Inverter

abstract: This dissertation covers three primary topics and relates them in context. High frequency transformer design, microgrid modeling and control, and converter design as it pertains to the other topics are each investigated, establishing a summary of the state-of-the-art at the intersection of the three as a baseline. The culminating work produced by the confluence of these topics is a novel modular solid-state transformer (SST) design, featuring an array of dual active bridge (DAB) converters, each of which contains an optimized high-frequency transformer, and an array of grid-forming inverters (GFI) suitable for centralized control in a microgrid environment. While no hardware was produced for this design, detailed modeling and simulation has been completed, and results are contextualized by rigorous analysis and comparison with results from published literature. The main contributions to each topic are best presented by topic area. For transformers, contributions include collation and presentation of the best-known methods of minimum loss high-frequency transformer design and analysis, descriptions of the implementation of these methods into a unified design script as well as access to an example of such a script, and the derivation and presentation of novel tools for analysis of multi-winding and multi-frequency transformers. For microgrid modeling and control, contributions include the modeling and simulation validation of the GFI and SST designs via state space modeling in a multi-scale simulation framework, as well as demonstration of stable and effective participation of these models in a centralized control scheme under phase imbalance. For converters, the SST design, analysis, and simulation are the primary contributions, though several novel derivations and analysis tools are also presented for the asymmetric half bridge and DAB.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201