Performance analysis of a full order sensorless control adaptive observer for doubly-fed induction generator in grid connected operation

This paper focuses on the performance analysis of a sensorless control for a Doubly Fed Induction Generator (DFIG) in grid-connected operation for turbine-based wind generation systems. With reference to a conventional stator flux based Field Oriented Control (FOC), a full-order adaptive observer is implemented and a criterion to calculate the observer gain matrix is provided. The observer provides the estimated stator flux and an estimation of the rotor position is also obtained through the measurements of stator and rotor phase currents. Due to parameter inaccuracy, the rotor position estimation is affected by an error. As a novelty of the discussed approach, the rotor position estimation error is considered as an additional machine parameter, and an error tracking procedure is envisioned in order to track the DFIG rotor position with better accuracy. In particular, an adaptive law based on the Lyapunov theory is implemented for the tracking of the rotor position estimation error, and a current injection strategy is developed in order to ensure the necessary tracking sensitivity around zero rotor voltages. The roughly evaluated rotor position can be corrected by means of the tracked rotor position estimation error, so that the corrected rotor position is sent to the FOC for the necessary rotating coordinate transformation. An extensive experimental analysis is carried out on an 11 kW, 4 poles, 400 V/50 Hz induction machine testifying the quality of the sensorless control

Battery losses in a MMC for BEVS application

Objective: This paper analyses the Modular Multilevel Converter (MMC) topology, where each individual Sub Module (SM), in half bridge configuration, is directly fed by an elementary electrochemical cell. Methods: The aim is to investigate how the reference voltages influence the cells currents waveforms, determining how the active powers and the losses are distributed among the cells. Considering a 2-level Voltage Source Inverter (VSI) topology working in the same conditions, the ratio between the MMC total cells losses and VSI total cells losses is calculated. After showing the system architecture and mathematical model, the cells current waveform investigation is presented and detailed both for triangular and sinusoidal voltage reference waveform

Controlled Fault-Tolerant Power Converters for Power Quality Enhancement

Power quality depends generally on the interaction of electrical power with electrical equipments. If electrical equipments operate correctly and reliably without being damaged or stressed, a suitable level of power quality is assured. On the other hand, if the electrical equipment malfunctions, is unreliable, or is damaged during normal usage, power quality is poor and probably the economical loss could be important like the technical one. In the scenario of the Distributed Generation, power quality issues will be moreover important because an higher dissemination of power conditioning equipment will be requested and this obviously increases the sources of vulnerability of the electrical system. In this paper fault tolerant power converters are considered as a viable solution of power quality problems and a suitable control algorithm of them is presented. The control proposed in the paper is based on the model of the power converter reformulated in terms of healthy leg binary variable and the paper shows how this control is able to save the aspect of power quality when the converter works in the linear range. The effectiveness of such an algorithm and of the fault tolerant power converters are finally verified by means of simulations

An HST Imaging Survey of Low-Mass Stars in the Chamaeleon I Star Forming region

We present new HST/WFPC2 observations of 20 fields centered around T Tauri stars in the Chamaeleon I star forming region. Images have been obtained in the F631N ([OI]6300A), F656N (Ha) and F673N ([SII]6716A+6731A) narrow-band filters, plus the Johnson V-band equivalent F547M filter. We detect 31 T Tauri stars falling within our fields. We discuss the optical morphology of 10 sources showing evidence of either binarity, circumstellar material, or mass loss. We supplement our photometry with a compilation of optical, infrared and sub-millimeter data from the literature, together with new sub-mm data for three objects, to build the Spectral Energy Distributions (SED) of 19 single sources. Using an SED model fitting tool, we self-consistently estimate a number of stellar and disk parameters, while mass accretion rates are directly derived from our Ha photometry. We find that bolometric luminosities derived from dereddened optical data tend to be underestimated in systems with high alpha(2-24} IR spectral index, suggesting that disks seen nearly edge-on may occasionally be interpreted as low luminosity (and therefore more evolved) sources. On the other hand, the same alpha(2-24) spectral index, a tracer of the amount of dust in the warmer layers of the circumstellar disks, and the mass accretion rate appear to decay with the isocronal stellar age, suggesting that the observed age spread (~0.5-5 Myr) within the cluster is real. Our sample contains a few outliers that may have dissipated their circumstellar disks on shorter time-scale.Comment: to appear on Astronomical Journal, accepted April 16, 2012 (AJ-10740

Phase space contraction and quantum operations

We give a criterion to differentiate between dissipative and diffusive quantum operations. It is based on the classical idea that dissipative processes contract volumes in phase space. We define a quantity that can be regarded as ``quantum phase space contraction rate'' and which is related to a fundamental property of quantum channels: non-unitality. We relate it to other properties of the channel and also show a simple example of dissipative noise composed with a chaotic map. The emergence of attaractor-like structures is displayed.Comment: 8 pages, 6 figures. Changes added according to refferee sugestions. (To appear in PRA

The Solar Twin Planet Search II. A Jupiter twin around a solar twin

Through our HARPS radial velocity survey for planets around solar twin stars, we have identified a promising Jupiter twin candidate around the star HIP11915. We characterize this Keplerian signal and investigate its potential origins in stellar activity. Our analysis indicates that HIP11915 hosts a Jupiter-mass planet with a 3800-day orbital period and low eccentricity. Although we cannot definitively rule out an activity cycle interpretation, we find that a planet interpretation is more likely based on a joint analysis of RV and activity index data. The challenges of long-period radial velocity signals addressed in this paper are critical for the ongoing discovery of Jupiter-like exoplanets. If planetary in nature, the signal investigated here represents a very close analog to the solar system in terms of both Sun-like host star and Jupiter-like planet.Comment: 8 pages, 5 figures; A&A accepted; typos corrected in this versio

The temporal evolution of neutron-capture elements in the Galactic discs

Important insights into the formation and evolution of the Galactic disc(s) are contained in the chemical compositions of stars. We analysed high-resolution and high signal to noise HARPS spectra of 79 solar twin stars in order to obtain precise determinations of their atmospheric parameters, ages ($\sigma$$\sim$0.4 Gyr) and chemical abundances ($\sigma$$<$0.01~dex) of 12 neutron-capture elements (Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, and Dy). This valuable dataset allows us to study the [X/Fe]-age relations over a time interval of $\sim$10 Gyr and among stars belonging to the thin and thick discs. These relations show that i) the $s$-process has been the main channel of nucleosynthesis of $n$-capture elements during the evolution of the thin disc; ii) the thick disc is rich in $r$-process elements which suggests that its formation has been rapid and intensive. %; iii) a chemical continuity between the thin and thick discs is evident in the abundances of Ba. In addition, the heavy (Ba, La, Ce) and light (Sr, Y, Zr) $s$-process elements revealed details on the dependence between the yields of AGB stars and the stellar mass or metallicity. Finally, we confirmed that both [Y/Mg] and [Y/Al] ratios can be employed as stellar clocks, allowing ages of solar twin stars to be estimated with an average precision of $\sim$0.5~Gyr

Chemical inhomogeneities in the Pleiades: signatures of rocky-forming material in stellar atmospheres

The aim of Galactic archaeology is to recover the history of our Galaxy through the information encoded in stars. An unprobed assumption of this field is that the chemical composition of a star is an immutable marker of the gas from which it formed. It is vital to test this assumption on open clusters, group of stars formed from the same gas. Previous investigations have shown that unevolved stars in clusters are chemically homogeneous within the typical uncertainties of these analysis, i.e. 15$\%$ of the elemental abundances. Our strictly differential analysis on five members of the Pleiades allows us to reach precisions of 5$\%$ for most elements and to unveil chemical anomalies within the cluster that could be explained by planet engulfment events. These results reveal that the evolution of planetary systems may alter the chemical composition of stars, challenging our capability of tagging them to their native environments, and also paving the way for the study of planetary architectures and their evolution, through the chemical pattern of their host stars.Comment: 10 pages, 2 figures. Accepted for publication in Ap