1,720,960 research outputs found
Design and Stability Analysis of a Three-Phase Triple-Stage Solid-State Transformer
Full text linkThe electrical distribution system is experiencing a profound evolution process triggered by the increasing integration of Renewable Energy Sources (RES) and Distributed Generation (DG), alongside the widespread use of Electric Vehicles (EVs), the related charging stations, and the growing adoption of Energy Storage Systems (ESSs). The behavior of such loads and sources, interfaced with the grid via an increasing number of power electronics converters and often intermittent in nature, together with a bidirectional power flow requirement, poses new challenges for the reliable and safe operation of the distribution system. In this context, the concept of Internet of Energy (IoE), or Energy Internet (EI), has emerged and is nowadays widely discussed in the literature as a new paradigm shift to address the growing demand for modernization of the current distribution network. The goal in the IoE scenario is reshaping the current distribution grid into an intelligent and flexible active network, both through a radical informatization process that involves the renewal of the grid communication infrastructure and the addition of distributed monitoring points and via the implementation of advanced energy management and control functionalities to enable the safe, robust, effective, and efficient integration of intermittent sources and loads.
At the core of this future smart grid scenario, the Solid-State Transformer (SST) is envisioned as the best candidate due to its flexibility and advanced control features. This is because the SST is a power electronic-based transformer capable of providing advanced services and grid-supporting features, besides galvanic isolation and voltage adaptation, through its control system, and therefore is intended for replacing conventional Line Frequency Transformers (LFTs) at strategic nodes of the grid. Moreover, the core isolation stage of the SST operates at high frequencies and, therefore, it enables volume and weight reduction of the whole system compared to traditional and bulky LFTs.
In the IoE scenario, the most suitable SST configuration is the triple-stage one, which consists of three conversion stages. Due to the large number of stages, the SST control is intrinsically complex. It has been shown in the literature how the coupling among controllers makes the design of the overall control system challenging and, additionally, multistage cascaded converters are significantly prone to instability due to interaction between converters. Moreover, even if the SST is stable as a standalone system, it may become unstable when connected to the grid because of dynamic interactions with other grid-connected converters, leading to the so-called harmonic instability phenomenon.
In this context, this thesis aims to explore the SST stability issue from both the DC-link and grid-connection perspectives. To do so, in the first part of this work, the SST suitable topologies and their conversion stages are reviewed. Once the SST architecture is selected, the main ratings and parameters are designed according to the presented IoE application requirements. An average model of the converter, that enables faster simulations and physical insights into the SST dynamics, is then derived. Through it, the small-signal model of the SST can be obtained. Based on that, the SST control system is presented and designed and the related impedance model is derived. The latter is selected as assessment tool to evaluate the DC-link and grid-connection stability of the SST under investigation. The results obtained provide support during the design phase of the SST and its control strategy, with the aim to achieve a stable grid-connected operating system.The electrical distribution system is experiencing a profound evolution process triggered by the increasing integration of Renewable Energy Sources (RES) and Distributed Generation (DG), alongside the widespread use of Electric Vehicles (EVs), the related charging stations, and the growing adoption of Energy Storage Systems (ESSs). The behavior of such loads and sources, interfaced with the grid via an increasing number of power electronics converters and often intermittent in nature, together with a bidirectional power flow requirement, poses new challenges for the reliable and safe operation of the distribution system. In this context, the concept of Internet of Energy (IoE), or Energy Internet (EI), has emerged and is nowadays widely discussed in the literature as a new paradigm shift to address the growing demand for modernization of the current distribution network. The goal in the IoE scenario is reshaping the current distribution grid into an intelligent and flexible active network, both through a radical informatization process that involves the renewal of the grid communication infrastructure and the addition of distributed monitoring points and via the implementation of advanced energy management and control functionalities to enable the safe, robust, effective, and efficient integration of intermittent sources and loads.
At the core of this future smart grid scenario, the Solid-State Transformer (SST) is envisioned as the best candidate due to its flexibility and advanced control features. This is because the SST is a power electronic-based transformer capable of providing advanced services and grid-supporting features, besides galvanic isolation and voltage adaptation, through its control system, and therefore is intended for replacing conventional Line Frequency Transformers (LFTs) at strategic nodes of the grid. Moreover, the core isolation stage of the SST operates at high frequencies and, therefore, it enables volume and weight reduction of the whole system compared to traditional and bulky LFTs.
In the IoE scenario, the most suitable SST configuration is the triple-stage one, which consists of three conversion stages. Due to the large number of stages, the SST control is intrinsically complex. It has been shown in the literature how the coupling among controllers makes the design of the overall control system challenging and, additionally, multistage cascaded converters are significantly prone to instability due to interaction between converters. Moreover, even if the SST is stable as a standalone system, it may become unstable when connected to the grid because of dynamic interactions with other grid-connected converters, leading to the so-called harmonic instability phenomenon.
In this context, this thesis aims to explore the SST stability issue from both the DC-link and grid-connection perspectives. To do so, in the first part of this work, the SST suitable topologies and their conversion stages are reviewed. Once the SST architecture is selected, the main ratings and parameters are designed according to the presented IoE application requirements. An average model of the converter, that enables faster simulations and physical insights into the SST dynamics, is then derived. Through it, the small-signal model of the SST can be obtained. Based on that, the SST control system is presented and designed and the related impedance model is derived. The latter is selected as assessment tool to evaluate the DC-link and grid-connection stability of the SST under investigation. The results obtained provide support during the design phase of the SST and its control strategy, with the aim to achieve a stable grid-connected operating system
Kalman filter estimation method for battery cell parameters in Reconfigurable Cascaded Multilevel Converter
No full textDirect AC charging of EV Reconfigurable Cascaded Multilevel Converter
No full textThis paper presents the Reconfigurable Cascaded Multilevel Converter (RCMC), employed for EV powertrain applications, in charging configuration. The converter is directly connected to an AC three-phase power system and the battery modules are charged by dynamically controlling the reconfigurable battery modules. The intrinsic structure of the converter gives the advantage to implement different charging algorithms, according to customizable requirements, without the need of extra middle power stages. Two charging methods are proposed and explained in detail: the first one prioritizes the SOC balancing between the battery modules, the second one the power losses reduction. Finally, a charging time estimation is computed for the RCMC and for a classic battery pack, assuming to increase their initial state of charge of 20%. The computation results show that the Reconfigurable Cascaded Multilevel Converter requires 40% less time than the battery pack
Impact of the DC-DC Stage on Grid-Connection Stability in Solid-State Transformer
No full textThis paper addresses the impact of the dc-dc stage on the grid-connection stability of a three-phase Solid-State Transformer based on the Cascaded H-Bridge and Dual Active Bridge topologies. In particular, the analysis aimed to reveal and discuss the impact of the isolated bidirectional dc-dc stage on the input dq-frame impedance matrix properties in terms of its passivity. For this purpose, the d-axis input impedance has been mainly addressed in this work. Its low-frequency simplified expression has been derived, by means of which its real part and the negative-resistance region upper limit can be analytically deduced, enabling a passivity-oriented design procedure. It is demonstrated that the dc-dc converter lowers that limit thus enhancing the grid-connected Solid-State Transformer passivity. The analysis performed in this work can be applied to a generic ac-dc-dc topology
Negative Voltage Sequence Control for an Electric Arc Furnace Power Supply based on a Multilevel AC-AC Converter
No full textThis paper proposes a novel control approach for a back to back multilevel AC-AC converter in Electric Arc Furnace (EAF) power supply applications. The study presents a feasibility analysis about the use of the chosen converter structure for EAF applications, along with considerations for control mechanisms. In particular, the effect of both positive and negative voltage sequences on the system is explored. Through comprehensive analysis, the paper introduces a novel control method utilizing negative voltage sequence, aiming to enhance the overall system performances. The results are presented using both a simplified model and the Cassie-Mayr (CM) model for the EAF
Stability Assessment Study for a Triple-Stage Three-Phase Solid-State Transformer
No full textThis paper proposes an accurate stability and performance analysis for a triple-stage three-phase modular solid-state transformer (SST), based on the cascaded H-bridge (CHB) and on the dual active bridge (DAB) topologies. The control strategy of this system consists of a complex multi-layer structure, mainly because of the need to balance the various dc-links of the converter. Such a complex system is highly prone to instability, both for the cascaded topology and for the control structure. In this paper, the small-signal modelling approach and the Middlebrook's criterion are used to address the stability issues of the converter. For this purpose, the small signal model for each converter stage is developed and through those the SST impedance transfer functions are derived. To validate the theoretical impedance equations, a numerical simulation has been carried out in order to map the frequency response of the converter stages. Then, the stability of the whole system is discussed, pointing out what parameters may cause instability in the analyzed SST system. Four relevant case studies corresponding to four different operating modes are assessed and verified via Simulink simulations
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Power Electronics Converters for the Internet of Energy: A Review
Full text linkThis paper presents a comprehensive review of multi-port power electronics converters used for application in AC, DC, or hybrid distribution systems in an Internet of Energy scenario. In particular, multi-port solid-state transformer (SST) topologies have been addressed and classified according to their isolation capabilities and their conversion stages configurations. Non-conventional configurations have been considered. A comparison of the most relevant features and design specifications between popular topologies has been provided through a comprehensive and effective table. Potential benefits of SSTs in distribution applications have been highlighted even with reference to a network active nodes usage. This review also highlights standards and technical regulations in force for connecting SSTs to the electrical distribution system. Finally, two case studies of multi-port topologies have been presented and discussed. The first one is an isolated multi-port bidirectional dual active bridge DC-DC converter useful in fast-charging applications. The second case of study deals with a three-port AC-AC multi-level power converter in H-Bridge configuration able to replicate a network active node and capable of routing and controlling energy under different operating conditions
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