1,720,999 research outputs found
High Efficiency Interfacing Converters for Distributed Energy Systems
Full text linkThe ever-growing amount of renewable energy sources connected to the grid call for a revision of the traditional concept of energy distribution network. In addition, the world climate change issue is becoming a major task for the green energy technology development in the last decades. In the last years, the smart grids are the promising paradigm shift for the low- and medium-voltage grids. The term smart grid indicates the symbiosis of the traditional distribution network with a deeply branched ICT infrastructure. The fundamental benefits of this cooperation is an increase in efficiency, reliability and stability of the whole electrical system, and a minimization of the costs and environmental impacts. The main differences with respect to the traditional distribution grid are the growing arrays of customer-sited distributed energy resources (DERs), including renewable energy sources and storage systems, instead of the large and centralized power plants present in the tra- ditional grid. To control and monitor the overall grid system, the ICT infrastructure must be able to manage the present agents, i.e. DERs, and the latter need to be interfaced with the distribution network through Electronic Power Converters (EPCs) forming a micro-, nano-, pico-grid according to the geographical extension. A typical example of pico-grid is a smart building: renewable energy sources, e.g. photovoltaic (PV) panel or wind generator, and energy storage systems, e.g. battery and/or supercapacitor, are connected together through a common DC Link bus. The interface between DERs and DC Link is demanded to EPCs that manage the power flow, optimize the source utilization or the time-profile generation via the energy storage. The power transfer, besides taking place among the connected DERs, can occur between the DC link and the grid through an AC/DC converter (inverter).
The main aim of the presented work is to look into the EPC design procedure to achieve a deep integration of DERs in the future smart grid. To this end, the efficiency and the
power density are considered the main performance indices to achieve a more effective EPC design.
Since the design process involves the optimization of many performance indices, the first part of this work is dedicated to a general overview of Multi-Objective Optimization problem is discussed to provide the sufficient theoretical background to handle with new methodology. Initially, an example of application of MOO is presented to introduce the notation and show the methodology in a simple case, then the EPC optimization is included.
The central part of the dissertation focuses on the needed background to allow a proper converters modelling. The considered objectives, i.e. efficiency and power density of the considered EPC, are related to the losses, that occurs during the power transfer in the EPC, and the final volume of the converter. Because of this, the estimation of the losses and volume for each electronic components of EPC becomes mandatory. However, the choice of the accuracy of the models used to predict losses and volume is a degree of freedom, since it greatly impacts on computational time. Since this approach is considered in this dissertation a first valuation to make comparison between innovative solutions and the state of the art, the models consider only the main contribution of each loss phenomenon and the main volume contributions. In this way, the MOO analysis doesn’t become a high time- and data-consumption tool. The series resonant converters will be treated in detail because of their many advantages: inherent short circuit protection, higher operation frequency, lower electromagnetic interferences and soft-switching modulation. An innovative mathematical framework is proposed for their steady-state analysis, providing the closed-form solutions of the sampled resonant impedance state, the voltage conversion ratio and the transferred power in a single matrix formulation. This tool has been validated by means of a high efficiency DC/DC topology for renewable source interfacing: the Interleaved Boost with Coupled Inductors (IBCI). The proposed framework has allowed to identify six different operating modes providing the closed-form expressions of the voltage conversion ratios and the operating boundaries. Furthermore, the sampled state variable trajectories lays out some soft switching considerations. The testcase demonstrates the effectiveness of the proposed method also with complex topologies avoiding simulation-based or approximated analysis.
In the latter part of the work, the input stage of an AC/DC converter is presented to validate the usage of the MOO approach: the Power Factor Correction (PFC) Boost stage design in a Medium Voltage Solid State Transformer . The analysis is carried out to obtain the concrete benefits introduced by the implementation of a proposed solution to reduce the losses in the switches
Extremum Seeking Control for the Efficiency Optimization of a Multi-stage Converter
No full textThe article introduces an extremum seeking control (ESC) technique for determining the optimal operating points of a multi-stage converter named twin-bus buck converter (TBB) [8]. The TBB converter features several modulation parameters, such as the switching frequencies of the CLLC stage and of the TBB post-regulator, which have a notable impact on the overall conversion efficiency. The benefit of off-line optimization techniques is limited due to the difficulties in modeling the converter behavior and the dependence on the actual operating point. Then, a model-free online search method based on the ESC technique is investigated and applied herein to find the optimal switching frequency of the TBB converter stage. The originality of this work lies in the utilization of small frequency perturbations that generate minimal effects on the objective variable, allowing for system optimization. The effectiveness of the proposed search technique is verified through experimental validation using a prototype rated 10 kW
Three-phase Four-wire Bidirectional Y-converter for an Enhanced Interface between the AC Grid and the Unipolar DC Microgrid
No full textAn Unbalance and Power Controller Allowing Smooth Islanded Transitions in Three-Phase Microgrids
Full text linkThis article introduces a power controller for three-phase inverters in microgrids that can be used in three-phase three-wire and three-phase four-wire systems. The controller enables active and reactive power tracking and unbalanced current control during grid-tied operation, while also allowing seamless transitions into islanded operation. The proposal is motivated by the compelling need in forthcoming power-electronics-dominated grids to provide ancillary services for the main grid and to support grid-forming functionalities for the microgrid, especially in case of islanded operation. The control is developed on the symmetrical components framework. Power tracking is achieved by dedicated control loops that, exploiting and droop laws applied on positive-sequence powers, accommodate output-power control during grid-tied operation as well as grid-forming capabilities during islanded operation. The controller also includes synchronous -frame control for negative-sequence current regulation for providing unbalanced current compensation. The proposed solution addresses the challenge of simultaneously providing concurrent grid-tied control features, such as output power tracking, during grid-tied operation as well as grid-forming capabilities during islanded operation. The related challenge stems from the intrinsically different control actions involved in the two modes of operation, namely, grid tied and islanded. The proposal is verified on an experimental setup composed of converters rated 3 kW
An Energy-Based Model of Four-Switch Buck-Boost Converters
Full text linkThe four-switch buck-boost (FSBB) topology is often used in combination with other isolated converters to extend the voltage range capability of the overall structure. In such applications, the duty-cycles of the two legs of the FSBB are independently controlled, and a phase shift is typically used to shape the inductor current ripple and, thus, achieve zero voltage switching. This article proposes a nonlinear average model, and the corresponding linearized small-signal model for the FSBB operating in the described way. Notably, the proposed approach is based on the modeling of the inductor energy, which allows the correct description of the dynamics related to the phase shift, in addition to those related to the duty-cycles. The derived models are shown to be in excellent agreement with simulation results and are also validated by measurements on an experimental prototype
Model-Free Optimization of Isolated DC-DC Triple Active Bridge Converters for ZVS and Reduced RMS Current Operation
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Enhancing DC microgrids integration with three-phase AC grids using multiport converters
Full text linkDecentralized Impedance Specifications for Grid-Connected Converters in AC Grids
No full textTo analyze the small-signal stability of systems with numerous Power Electronics Converters (PECs) connected to the same grid, the impedance-based method is widely applied. Due to the complexity of the grid, the possibility to provide independent impedance specifications to each PEC, referred to as decentralized specifications, has many advantages, especially when PECs are originating from multiple vendors. Decentralized specifications provide the possibility of individually designing the PEC connected to the grid while maintaining the system’s small-signal stability. This paper firstly summarizes the already established decentralized specifications for DC distribution grids where the small-signal impedances are modeled as SISO transfer functions. Secondly, it outlines the problems of extending the specifications for AC grids, where impedances are usually modeled as MIMO transfer functions. Finally, this article proposes the solution for providing such specifications based on estimating the position of eigenvalues with the Gershgorin theorem
Efficient high step-up topology for renewable energy source interfacing
No full textThe paper presents the complete analysis and design procedure for an interleaved, isolated resonant boost converter. The topology is inherently capable of high step-ratios and soft-switching operation in a wide input voltage and load range. Thanks to resonant operation, the performance level is further improved with respect to similar, non resonant topologies, achieving higher conversion efficiency and lower switch current stress. Overall, the converter represents an excellent solution to interface low voltage, high current renewable energy sources with the distribution grid through a high voltage DC bus. Topology characteristic equations in different operating modes are provided, based on which a design procedure for the power stage components is derived. A single photovoltaic module interface converter, rated for 300 W output power, is considered as a case study to validate the analysis and the design procedure. Both are fully validated by measurements taken on the laboratory prototype
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