1,674 research outputs found

    Multilevel Converters: An Enabling Technology for High-Power Applications

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    | Multilevel converters are considered today as the state-of-the-art power-conversion systems for high-power and power-quality demanding applications. This paper presents a tutorial on this technology, covering the operating principle and the different power circuit topologies, modulation methods, technical issues and industry applications. Special attention is given to established technology already found in industry with more in-depth and self-contained information, while recent advances and state-of-the-art contributions are addressed with useful references. This paper serves as an introduction to the subject for the not-familiarized reader, as well as an update or reference for academics and practicing engineers working in the field of industrial and power electronics.Ministerio de Ciencia y Tecnología DPI2001-3089Ministerio de Eduación y Ciencia d TEC2006-0386

    A comparative study of methods for estimating virtual flux at the point of common coupling in grid connected voltage source converters with LCL filter

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    Grid connected Voltage Source Converters (VSCs) with LCL filters usually have voltage measurements at the filter capacitors, while it can be important to control the active or reactive power injection at the grid-side of the LCL filter, for instance at a Point of Common Coupling (PCC). Synchronization to the PCC voltage can be obtained by Virtual Flux (VF) estimation, which can also allow for voltage sensor-less operation of VSCs. This paper is presenting a comparative evaluation of methods for estimating the VF at the PCC, considering a VSC connected to the grid through an LCL filter with a Proportional Resonant (PR) controller as the inner current control loop. The VF estimation is achieved by using frequency adaptive dual SOGI-QSGs (DSOGI-VF). The Frequency Locked Loop (FLL) is used in order to keep the positive and negative sequence (PNS) VF estimation inherently frequency adaptive. Three different methods are considered for obtaining the capacitor current needed for estimating the VF at the grid side of the LCL filter which are based on fully estimation by using the voltage sensor-less method, by estimating the capacitor current from the measured voltage or by using additional capacitor current sensors. The results have been compared and validated by simulation studies.Peer ReviewedPostprint (author's final draft

    Class-E rectifiers and power converters: the operation of the class-E topology as a power amplifier and a rectifier with very high conversion efficiencies

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    In the late 70’s, the interest in reducing the value and size of reactive components moved power supply specialists to operate dc-to-dc converters at hundreds of kHz or even MHz frequencies. Passive energy storage (mainly magnetics) dominates the size of power electronics, limiting also its cost, reliability and dynamic response. Motivated by miniaturization and improved control bandwidth, they had to face the frequency-dependent turn-on and turn-off losses associated with the use of rectangular waveforms in the hard-switched topologies of that time. Similar to approaches for RF/microwave power amplifiers (PAs), the introduction of resonant circuits allowed shaping either a sinusoidal voltage or current, with parasitic reactive elements absorbed by the topology in the neighborhood of the switching frequency. The resulting resonant power converters, obtained by cascading a dc-to-ac resonant inverter with a high-frequency ac-to-dc rectifier, first transform the dc input power into controlled ac power, and then convert it back into the desired dc output [1]. This paper provides some historic notes on the operation of the class-E topology, introduced worldwide to the RF/microwave community by Nathan O. Sokal [2], as a power inverter and as a rectifier, with very high conversion efficiencies up to microwave frequencies. Recent research advances and implementations of class-E rectifiers and dc-to-dc converters at UHF and beyond are included. Offering competitive performance in terms of efficiency for RF power recovery, together with a wide bandwidth for low-loss power conversion, their potential for some modern applications is highlighted.The authors would like to acknowledge support in part by the Spanish Ministry of Economy, Industry and Competitiveness (MINECO) through TEC2014-58341-C4-1-R and TEC2017-83343-C4-1-R projects, co-funded with FEDER, and in part by Lockheed Martin Endowed Chair at the University of Colorado

    Class-E rectifiers and power converters

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    This paper reviews the use of the class-E topology for RF-to-DC and DC-to-DC power conversion. After covering its early history, the class-E rectifier is introduced in the context of the time-reversal duality principle, to be then integrated with an inverter in a class-E2 DC/DC converter. Recent examples and applications at UHF and microwave bands are finally presented. A review of RF rectifiers based on Schottky diodes or FET transistors, is followed by a discussion of synchronous and self-synchronous implementations of the double class-E DC/DC converter, using advanced GaN HEMT transistors.This work was supported in part by the Spanish Ministry of Economy and Competitiveness (MINECO) under project TEC2014-58341-C4-1-R, co-funded with FEDER, and in part by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, under Award Number DEAR0000216 and the DARPA MPC program, ONR award N00014-11-1-0931

    A System-based Modeling and Simulation of a Floating Production Storage and Offloading (FPSO) Electric Power Systems and Core Loss Analysis of Synchronous Generator and Induction Motor

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    In this dissertation, a system based modeling and simulation were investigated for FPSO power system components and a few experimental results were evaluated to determine core losses in a synchronous generator under non-sinusoidal current. Modeling and simulation results are presented to predict both the utility and the motor side challenges caused by different harmonic distortion levels. VFD was modeled as a rectifier and an inverter isolated by the DC link in the middle of the rectifier and the inverter in order to have faster and more accurate analysis of harmonics on both the utility and the motor side. Finite element analysis shows that reducing current harmonic level causes lower core losses in the machine. Each component from the simulation, specifically synchronous generator, phase shifting transformer, multipulse rectifier, multilevel inverter, umbilical cable and induction motor, can be replaced or combined with any other topologies which is one of the biggest benefits of the system based modeling and simulation

    GaN HEMT class-E rectifier for DC+AC power recovery

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    A 915 MHz GaN HEMT-based class-E rectifier is proposed in this paper to be used for DC+AC wireless power recovery. Taking advantage of the time reversal (TR) duality principle, the rectifier was derived from a class-E inverter, whose output network was designed for high-efficiency operation over a wide range of resistive loads. The addition of an appropriate gateside termination allows the device to be turned-on without an additional RF source for gate driving. The rectifier reduced sensitivity to load variation, as well as its capability for efficiently and linearly recovering the envelope of an AM RF excitation, were then characterized. An average efficiency of 82% has been measured for the combined RF-to-DC and RF-to-AC power conversion of a 1.6 W modulated carrier. Frequency multiplexing and frequency modulation alternatives for high-level DC+AC wireless power transmission are finally presented.This work was supported by the Ministry of Economy and Competitiveness (MINECO) through TEC2014-58341-C4-1-R and TEC2014-58341-C4-2-R projects, co-funded with FEDER. The authors wish to thank Prof. Christian Brañas, University of Cantabria, by his assistance with the transformer characterization. David Vegas and José R. Pérez-Cisneros also thank the support provided by the pre-doctoral BES-2015-072203 and the pre-doctoral mobility EEBB-I-15-10447 grants, respectively

    Small signal stability analysis of a four-machine system with placement of multi-terminal high voltage direct current link

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    Inter-area oscillation caused by weak interconnected lines or low generator inertia is a critical problem facing power systems. This study investigated the performance analysis of a multi-terminal high voltage direct current (MTDC) on the damping of inter-area oscillations of a modified two-area four-machine network. Two case studies were considered, utilising scenario 1: a double alternating current (AC) circuit in linking Bus_10 and Bus_11; and scenario 2: a three-terminal line commutated converter high voltage direct current system in linking Bus_6 and Bus_11 into Bus_9. It was found that scenario 2 utilising MTDC link with a robust controller provided quick support in minimising the network oscillations following a fault on the system. The MTDC converter controllers’ setup offered sufficient support for the inertia of the AC system, thus providing efficient damping of the inter-area oscillation of the system

    Optimised design of isolated industrial power systems and system harmonics

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    This work has focused on understanding the nature and impact of non-linear loads on isolated industrial power systems. The work was carried out over a period of 8 years on various industrial power systems: off-shore oil and gas facilities including an FPSO, a wellhead platform, gas production platforms, a mineral processing plant and an LNG plant. The observations regarding non-linear loads and electrical engineering work carried out on these facilities were incorporated into the report.A significant literature describing non-linear loads and system harmonics on industrial power systems was collected and reviewed. The literature was classified into five categories: industrial plants and system harmonics, non-linear loads as the source of current harmonics, practical issues with system harmonics, harmonic mitigation strategies and harmonic measurements.Off-shore oil and gas production facilities consist of a small compact power system. The power system incorporates either its own power generation or is supplied via subsea cable from a remote node. Voltage selection analysis and voltage drop calculation using commercially available power system analysis software are appropriate tools to analyse these systems. Non-linear loads comprise DC rectifiers, variable speed drives, UPS systems and thyristor controlled process heaters. All nonlinear loads produce characteristic and non-characteristic harmonics, while thyristor controlled process heaters generate inter-harmonics. Due to remote location, harmonic survey is not a common design practice. Harmonic current measurements during factory acceptance tests do not provide reliable information for accurate power system analysis.A typical mineral processing plant, located in a remote area includes its own power station. The power generation capacity of those systems is an order of magnitude higher than the power generation of a typical off-shore production facility. Those systems comprise large non-linear loads generating current and voltage interharmonics. Harmonic measurements and harmonic survey will provide a full picture of system harmonics on mineral processing plants which is the only practical way to determine system harmonics. Harmonic measurements on gearless mill drive at the factory are not possible as the GMD is assembled for the first time on site.LNG plants comprise large non-linear loads driving gas compressor, however those loads produce integer harmonics. Design by analysis process is an alternative to the current design process based on load lists. Harmonic measurements and harmonic survey provide a reliable method for determining power system harmonics in an industrial power system

    Power and frequency control of an offshore wind farm connected to grid through an HVDC link with LCC-based rectifier

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    Mención Internacional en el título de doctorThere is an increasing interest in the use of line-commutated converter (LCC) technology to connect large offshore wind farms (OWFs) placed far from the coast by means of a high voltage direct current (HVDC) link. This is due to the better features of LCCs compared to voltage-source converters in terms of cost, reliability and efficiency. However, this technology requires a frequency control in the OWF to allow the operation of both the wind turbine generator systems (WTGSs) and the LCC rectifier. Therefore, this Thesis presents two frequency control proposals. First, a centralized voltage and frequency control for an OWF connected through LCC-rectifier-based HVDC link is proposed. It is derived from an enhanced LCC-rectifier station average-value model which indicates that the active power balance at the point of common coupling drives the OWF voltage while the corresponding reactive power balance drives the OWF frequency. Even though voltage control cannot be applied in case of using a diode rectifier, the voltage magnitude variation is clamped between acceptable values. As a second proposal, a decentralized frequency control for the diode-rectifier-based HVDC link connection of OWFs is also presented. This control is based on a reactive power / frequency droop which allows the WTGSs to reach synchronous operation and equally share the reactive power without the need of communications among the WTGSs. Moreover, the control proposals do not rely on a phase-locked loop, so controls are not subject to grid disturbances or measurement noise. Another important specification of the proposed control strategies is that they do not modify the active power control channel of the WTGSs. Finally, the stability and the simulation results to assess the performance of both control proposals are studied.Programa Oficial de Doctorado en Ingeniería Eléctrica, Electrónica y AutomáticaPresidente: Alireza Nami.- Secretario: Oriol Gomis Bellmunt.- Vocal: Ana Belén Morales Martíne
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