Grid Integration of DC Buildings: Standards, Requirements and Power Converter Topologies

Residential dc microgrids and nanogrids are the emerging technology that is aimed to promote the transition to energy-efficient buildings and provide simple, highly flexible integration of renewables, storages, and loads. At the same time, the mass acceptance of dc buildings is slowed down by the relative immaturity of the dc technology, lack of standardization and general awareness about its potential. Additional efforts from multiple directions are necessary to promote this technology and increase its market attractiveness. In the near-term, it is highly likely that the dc buildings will be connected to the conventional ac distribution grid by a front-end ac-dc converter that provides all the necessary protection and desired functionality. At the same time, the corresponding requirements for this converter have not been yet consolidated. To address this, present paper focuses on various aspects of the integration of dc buildings and includes analysis of related standards, directives, operational and compatibility requirements as well as classification of voltage levels. In addition, power converter configurations and modulation methods are analyzed and compared. A classification of topologies that can provide the required functionality for the application is proposed. Finally, future trends and remaining challenges pointed out to motivate new contributions to this topic

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

A GALLIUM NITRIDE INTEGRATED ONBOARD CHARGER

Compared to Silicon metal–oxide–semiconductor field-effect transistors (MOSFETs), Gallium Nitride (GaN) devices have a significant reduction in gate charge, output capacitance, and zero reverse recovery charge, enabling higher switching frequency operation and efficient power conversion. GaN devices are gaining momentum in power electronic systems such as electric vehicle (EV) charging system, due to their promises to significantly enhance the power density and efficiency. In this dissertation, a GaN-based integrated onboard charger (OBC) and auxiliary power module (APM) is proposed for EVs to ensure high efficiency, high frequency, high power density, and capability of bidirectional operation. The high switching frequency operation enabled by the GaN devices and the integration of OBC and APM bring many unique challenges, which are addressed in this dissertation. An important challenge is the optimal design of high-frequency magnetics for a high-frequency GaN-based power electronic interface. Another challenge is to achieve power flow management among three active ports while minimizing the circulating power. Furthermore, the impact of circuit layout parasitics could significantly deteriorate the system interface, due to the sensitivity of GaN device switching characteristics. In this work, the aforementioned challenges have been addressed. First, a comprehensive analysis of the front-end AC-DC power factor correction stage is presented, covering a detailed magnetic modeling technique to address the high-frequency magnetics challenge. Second, the modeling and control of a three-port DC-DC converter, interfacing the AC-DC stage, high-voltage traction battery and low-voltage battery, are discussed to address the power flow challenge. Advanced control methodologies are developed to realize power flow management while maintaining minimum circulating power and soft switching. Furthermore, a new three-winding high-frequency transformer design with improved power density and efficiency is achieved using a genetic-algorithm-based optimization approach. Finally, a GaN-based integrated charger prototype is developed to validate the proposed theoretical hypothesis. The experimental results showed that the GaN-based charging system has the capability of achieving simultaneous charging (G2B) of both HV and LV batteries with a peak efficiency of 95%

Control of distributed power in microgrids: PV field to the grid, islanding operation, and ultra-fast charging station.

Aquesta tesi explora el control de l'energia distribuïda en microxarxes (MG) i aborda diversos reptes relacionats amb el control, l'estabilitat, la compartició d'energia, el disseny del convertidor d'energia, la connexió a la xarxa, la càrrega ultraràpida i el subministrament d'energia renovable. El rendiment dels MG s'analitza tant en modes d'operació connectats a la xarxa com en illa, considerant diferents configuracions i escenaris de flux d'energia. La tesi se centra en diversos reptes clau, com ara maximitzar l'extracció d'energia de matrius fotovoltaiques (PV) en MG que utilitzen convertidors DC-DC, injectar potència MG excedent a la xarxa principal mitjançant inversors de font de tensió DC-AC (VSI) sota càrregues no lineals i desequilibrades, optimitzant el rendiment de MG i la compartició d'energia en mode illa mitjançant VSI, connectant-se a la xarxa principal en el punt d'acoblament comú (PCC) mitjançant transformadors de baixa freqüència (LFT) i transformadors d'estat sòlid (SST) i explorant topologies de convertidors de potència per ultra -càrrega ràpida de CC de vehicles elèctrics (EV). L'ús de SST en lloc de LFT pot millorar la capacitat de MG alhora que redueix el volum i el pes de l'arquitectura elèctrica MG. Aquesta tesi proporciona coneixements i solucions per abordar els reptes esmentats anteriorment, contribuint a l'avenç del control, l'estabilitat, la qualitat de l'energia i la integració eficient de les fonts d'energia renovables i la càrrega dels vehicles elèctrics.Esta tesis explora el control de la potencia distribuida en microrredes (MGs) y aborda diversos retos relacionados con el control, la estabilidad, el reparto de potencia, el diseño de convertidores de potencia, la conexión a la red, la carga ultrarrápida y el suministro de energías renovables. El rendimiento de las MG se analiza tanto en modo de funcionamiento conectado a la red como en modo aislado, considerando diferentes configuraciones y escenarios de flujo de potencia. La tesis se centra en varios retos clave, como la maximización de la extracción de energía de las matrices fotovoltaicas (FV) en las MG utilizando convertidores CC-CC, la inyección del excedente de energía de las MG en la red principal a través de inversores de fuente de tensión CC-CA (VSI) bajo cargas no lineales y desequilibradas, la optimización del rendimiento de las MG y del reparto de energía en modo aislado mediante VSI, la conexión a la red principal en el punto de acoplamiento común (PCC) mediante transformadores de baja frecuencia (LFT) y transformadores de estado sólido (SST), y la exploración de topologías de convertidores de potencia para la carga ultrarrápida en corriente continua de vehículos eléctricos (VE). El uso de SST en lugar de LFT puede mejorar la capacidad de la MG y, al mismo tiempo, reducir el volumen y el peso de la arquitectura eléctrica de la MG. Esta tesis aporta ideas y soluciones para abordar los retos mencionados, contribuyendo al avance del control de la MG, la estabilidad, la calidad de la energía y la integración eficiente de fuentes de energía renovables y la carga de vehículos eléctricos. Traducción realizada con la versión gratuita del traductor www.DeepL.com/TranslatorThis thesis explores the control of distributed power in microgrids (MGs) and addresses various challenges related to control, stability, power sharing, power converter design, grid connection, ultra-fast charging, and renewable energy supply. The performance of MGs is analysed in both grid-connected and islanded modes of operation, considering different configurations and power flow scenarios. The thesis focuses on several key challenges, including maximising power extraction from photovoltaic (PV) arrays in MGs utilizing DC-DC converters, injecting surplus MG power into the main grid via DC-AC voltage source inverters (VSIs) under nonlinear and unbalanced loads, optimising MG performance and power sharing in islanded mode through VSIs, connecting to the main grid at the point of common coupling (PCC) using low-frequency transformers (LFTs) and solid-state transformers (SSTs), and exploring power converter topologies for ultra-fast DC charging of electric vehicles (EVs). The use of SSTs instead of LFTs can enhance MG capability while reducing the volume and weight of the MG electrical architecture. This thesis provides insights and solutions to address the aforementioned challenges, contributing to the advancement of MG control, stability, power quality, and efficient integration of renewable energy sources and EV charging

DC & Hybrid Micro-Grids

This book is a printed version of the papers published in the Special Issue “DC & Hybrid Microgrids” of Applied Sciences. This Special Issue, co-organized by the University of Pisa, Italy and Østfold University College in Norway, has collected nine papers and the editorial, from 28 submitted, with authors from Asia, North America and Europe. The published articles provide an overview of the most recent research advances in direct current (DC) and hybrid microgrids, exploiting the opportunities offered by the use of renewable energy sources, battery energy storage systems, power converters, innovative control and energy management strategies

Power Converter of Electric Machines, Renewable Energy Systems, and Transportation

Power converters and electric machines represent essential components in all fields of electrical engineering. In fact, we are heading towards a future where energy will be more and more electrical: electrical vehicles, electrical motors, renewables, storage systems are now widespread. The ongoing energy transition poses new challenges for interfacing and integrating different power systems. The constraints of space, weight, reliability, performance, and autonomy for the electric system have increased the attention of scientific research in order to find more and more appropriate technological solutions. In this context, power converters and electric machines assume a key role in enabling higher performance of electrical power conversion. Consequently, the design and control of power converters and electric machines shall be developed accordingly to the requirements of the specific application, thus leading to more specialized solutions, with the aim of enhancing the reliability, fault tolerance, and flexibility of the next generation power systems

Three-Phase Unfolding Based Soft DC-Link Converter Topologies for AC to DC Applications

Battery electric vehicles (BEVs) and plugin hybrid electric vehicles (PHEVs) are more efficient than internal combustion-based vehicles. Adaption of EVs will help reduce the carbon emissions produced by the transportation sector. The charging infrastructure has to grow at a rapid pace to encourage EV adaption. Installing higher capacity fast chargers will help alleviate the range anxiety of battery electric vehicle customers. More public charging stations are required for the full adaption of EVs. Utility power is distributed as ‘alternating current.’ A battery requires ‘direct current’ (DC) source to charge it. Hence a power converter that converts AC source to DC source is required to charge an electric vehicle battery. Public transportation is another sector that is adapting electric vehicles at a fast pace. These vehicles require more power to operate and hence have huge battery packs. These vehicles require ultra-high-power charger to keep the charging time reasonable. A 60 Hz stepdown transformer is required at the facility to use the power. The cost and time to install this heavy transformer will inhibit the setting up a charging station. Power converters than can connect to medium voltage directly will eliminate the need for the step-down transformer saving space and cost. Commercially available state-of-the-art fast charging converters are adapted from general purpose commercial and industrial application rectifiers. The efficiencies of these converters tend to be lower (around 94%) due to the two-stage power conversion architecture. All the power that flows from the AC utility grid to charge the battery will be processed and filtered through two power conversion stages. Due to the anticipated increase in demand, there is a renewed interest in developing power converter topologies specific to battery charging applications. The objective here is to develop cheaper and compact power converters for battery charging. This dissertation proposes an innovative quasi-single stage power converter topologies for battery charging applications and direct medium voltage connected converters. The proposed topology fundamentally can achieve higher efficiency and power density than the conventional two-stage based converters. Only one stage requires filtering and incurs power conversion losses. Control burden is usually higher for single stage topologies. Innovative control approaches are presented to simplify the control complexity

Efficiency and Sustainability of the Distributed Renewable Hybrid Power Systems Based on the Energy Internet, Blockchain Technology and Smart Contracts-Volume II

The climate changes that are becoming visible today are a challenge for the global research community. In this context, renewable energy sources, fuel cell systems, and other energy generating sources must be optimally combined and connected to the grid system using advanced energy transaction methods. As this reprint presents the latest solutions in the implementation of fuel cell and renewable energy in mobile and stationary applications, such as hybrid and microgrid power systems based on the Energy Internet, Blockchain technology, and smart contracts, we hope that they will be of interest to readers working in the related fields mentioned above