189,046 research outputs found

    Power electronics options for large wind farm integration : VSC-based HVDC transmission

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    This paper describes the use of voltage source converter based HVDC transmission (VSC transmission) system for grid integration of large wind farms over long distance. The wind farms can be based on either doubly-fed induction generator (DFIG) or fixed speed induction generator (FSIG). The paper describes the operation principles and control strategies of the proposed system. Automatic power balancing during network AC fault is achieved without communication between the two converters. PSCAD/EMTDC simulations are presented to demonstrate the robust performance and to validate the proposed system during various operating conditions such as variations of generation and AC fault conditions. The proposed VSC transmission system has technical and economic advantages over a conventional AC connection for integrating large wind farms over long distanc

    Magnetic Interactions in BiFeO3_3: a First-Principles Study

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    First-principles calculations, in combination with the four-state energy mapping method, are performed to extract the magnetic interaction parameters of multiferroic BiFeO3_3. Such parameters include the symmetric exchange (SE) couplings and the Dzyaloshinskii-Moriya (DM) interactions up to second nearest neighbors, as well as the single ion anisotropy (SIA). All magnetic parameters are obtained not only for the R3cR3c structural ground state, but also for the R3mR3m and R3ˉcR\bar{3}c phases in order to determine the effects of ferroelectricity and antiferrodistortion distortions, respectively, on these magnetic parameters. In particular, two different second-nearest neighbor couplings are identified and their origins are discussed in details. Moreover, Monte-Carlo (MC) simulations using a magnetic Hamiltonian incorporating these first-principles-derived interaction parameters are further performed. They result (i) not only in the accurate prediction of the spin-canted G-type antiferromagnetic structure and of the known magnetic cycloid propagating along a direction, as well as their unusual characteristics (such as a weak magnetization and spin-density-waves, respectively); (ii) but also in the finding of another cycloidal state of low-energy and that awaits to be experimentally confirmed. Turning on and off the different magnetic interaction parameters in the MC simulations also reveal the precise role of each of them on magnetism

    Ant-Colony-Based Multiuser Detection for MC DS-CDMA Systems

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    In this contribution we present a novel ant colony optimization (ACO) based multi-user detector (MUD) designed for synchronous multi-carrier direct sequence code division multiple access (MC DSCDMA) systems. The operation of the ACO-based MUD is based on the behaviour of the ant colony in nature. The ACO-based MUD aims for achieving the same bit-error-rate (BER) performance as the optimum maximum likelihood (ML) MUD, without carrying out an exhaustive search of the entire MC DS-CDMA search space constituted by all possible combinations of the received multi-user vectors. We will demonstrate that the system is capable of supporting almost as many users as the number of chips in the spreading sequence, while searching only a small fraction of the entire ML search space. It will also be demonstrated that the number of floating point operations per second is a factor of 108 lower for the proposed ACO-based MUD than that of the ML MUD, when supporting K = 32 users in a MC DS-CDMA system employing 31-chip Gold codes as the T-domain spreading sequence

    Control of DFIG based wind generation systems under unbalanced network supply

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    This paper develops a dynamic model and control scheme for DFIG systems to improve the performance and stability under unbalanced grid conditions. A dynamic DFIG model containing the positive and negative sequence components is presented using stator voltage orientation. The proposed model accurately illustrates the active power, reactive power and torque oscillations, and provides a basis for DFIG control system design during unbalanced network supply. Various control targets such as eliminating the oscillations of the torque, active/reactive power are discussed and the required rotor negative sequence current for fulfilling different control targets are described. Performance of a DFIG-based wind turbine under unbalanced condition using the proposed control method is evaluated by simulation studies using Matlab/Simulink. The proposed control scheme significantly attenuates the DFIG torque or active power oscillations during network unbalance whereas significant torque/power oscillations exist with the conventional control schemes

    Genetic iterative search-centre-shifting K-best sphere detection for rank-deficient SDM-OFDM systems

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    A generic sphere-detection (SD) scheme is proposed, which substantially reduces the detection complexity by decomposing it into two stages, namely the generic iterative search-centre-update phase and the reduced-complexity search around it. This two-stage philosophy circumvents the high complexity of channel-coded soft-decision aided SDs

    Ant-colony-based multiuser detection for multifunctional-antenna-array-assisted MC DS-CDMA systems

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    A novel Ant Colony Optimization (ACO) based Multi-User Detector (MUD) is designed for the synchronous Multi-Functional Antenna Array (MFAA) assisted Multi-Carrier Direct-Sequence Code-Division Multiple-Access (MC DS-CDMA) uplink (UL), which supports both receiver diversity and receiver beamforming. The ACO-based MUD aims for achieving a bit-error-rate (BER) performance approaching that of the optimum maximum likelihood (ML) MUD, without carrying out an exhaustive search of the entire MC DS-CDMA search space constituted by all possible combinations of the received multi-user vectors. We will demonstrate that regardless of the number of the subcarriers or of the MFAA configuration, the system employing the proposed ACO based MUD is capable of supporting 32 users with the aid of 31-chip Gold codes used as the T-domain spreading sequence without any significant performance degradation compared to the single-user system. As a further benefit, the number of floating point operations per second (FLOPS) imposed by the proposed ACO-based MUD is a factor of 108 lower than that of the ML MUD. We will also show that at a given increase of the complexity, the MFAA will allow the ACO based MUD to achieve a higher SNR gain than the Single-Input Single-Output (SISO) MC DS-CDMA system. Index Terms—Ant Colony Optimization, Multi-User Detector, Multi-Functional Antenna Array, Multi-Carrier Direct-Sequence Code-Division Multiple-Access, Uplink, Near-Maximum Likelihood Detection

    Crystallization kinetics and glass-forming ability of bulk metallic glasses Pd40Cu30Ni10P20 and Zr41.2Ti13.8Cu12.5Ni10Be22.5 from classical theory

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    Due to their scientific significance and potential engineering applications, bulk metallic glasses are among the most intensively studied advanced materials. Understanding the glass-forming ability (GFA) of these metallic alloys is a long-standing subject. While a large number of empirical factors have been proposed to correlate with GFA of the alloys, a full understanding of GFA remains a goal to achieve. Since glass formation is a competing process against crystallization, we have performed a systematic analysis on the crystallization kinetics of two known best metallic glass-formers Pd40Cu30Ni10P20 (in at. %) and Zr41.2Ti13.8Cu12.5Ni10Be22.5 based on classical nucleation and growth theory. Our results show that there is a dramatic difference between the two alloys in their nucleation behavior although they possess comparable GFA. Particularly, an extremely sharp nucleation peak (~10^18/m^3 s) is found for Pd40Cu30Ni10P20 around 632 K with a very small half maximum width of 42 K, implying that this alloy is an excellent candidate for nanocrystallization studies. Moreover, we have also found that the GFA of these alloys can be calculated to a high accuracy and precision based on the classical theory, suggesting that the classical theory may be sufficient to account for glass formation mechanism in these metallic alloys
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