| 3741. |
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| 3742. |
- Xu, Qianwen, 1992-, et al.
(författare)
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A Robust Droop-Based Autonomous Controller for Decentralized Power Sharing in DC Microgrid Considering Large-Signal Stability
- 2020
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Ingår i: IEEE Transactions on Industrial Informatics. - : Institute of Electrical and Electronics Engineers (IEEE). - 1551-3203 .- 1941-0050. ; 16:3, s. 1483-1494
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Tidskriftsartikel (refereegranskat)abstract
- The high penetration of power electronic converter loads in dc microgrid causes system stability issue, or also known as constant power load issue, due to their negative impedance characteristics. The stability concern will be more complicated for a self-disciplined microgrid that allows plug and play of various distributed generations (DGs). This article proposes a robust droop-based controller for decentralized power sharing in a dc microgrid considering large-signal stability. For each DG interface converter subsystem, the interactions with other DG interface converters and loads are estimated by a nonlinear disturbance observer (NDO) utilizing the subsystem's own information to achieve decentralized power sharing and fast voltage regulation. With the uncertainties of circuit parameters modeled as a lumped disturbance term and compensated by an NDO, the proposed controller can significantly enhance the robustness against the uncertainties of circuit parameters. The large-signal stability of the whole interconnected system is proved by the backstepping algorithm and Lyapunov theorem. The efficacy and large-signal stability of the proposed approach are verified by both simulations and experiments.
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| 3743. |
- Xu, Qianwen, 1992-, et al.
(författare)
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An Offset-Free Composite Model Predictive Control Strategy for DC/DC Buck Converter Feeding Constant Power Loads
- 2020
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Ingår i: IEEE transactions on power electronics. - 0885-8993 .- 1941-0107. ; 35:5, s. 5331-5342
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Tidskriftsartikel (refereegranskat)abstract
- The high penetration of power electronic converters into dc microgrids may cause the constant power load stability issues, which could lead to large voltage oscillations or even system collapse. On the other hand, dynamic performance should be satisfied in the control of power electronic converter systems with small overshoot, less oscillations, and smooth transient performance. This article proposes an offset-free model predictive controller for a dc/dc buck converter feeding constant power loads with guaranteed dynamic performance and stability. First, a receding horizon optimization problem is formulated for optimal voltage tracking. To deal with the unknown load variation and system uncertainties, a higher order sliding mode observer is designed and integrated into the optimization problem. Then an explicit closed-loop solution is obtained by solving the receding horizon optimization problem offline. A rigorous stability analysis is performed to ensure the system large signal stability. The proposed controller achieves optimized transient dynamics and accurate tracking with simple implementation. The effectiveness of the proposed controller is validated by simulation and experimental results.
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| 3744. |
- Xu, X., et al.
(författare)
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A vertical transport window of water vapor in the troposphere over the Tibetan Plateau with implications for global climate change
- 2022
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Ingår i: Atmospheric Chemistry and Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 22:2, s. 1149-1157
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Tidskriftsartikel (refereegranskat)abstract
- By using the multi-source data of meteorology over recent decades, this study discovered a summertime "hollow wet pool"in the troposphere with a center of high water vapor over the Asian water tower (AWT) on the Tibetan Plateau (TP), which is indicated by a vertical transport "window"in the troposphere. The water vapor transport in the upper troposphere extends from the vertical transport window over the TP with significant connections among the Arctic, Antarctic and TP regions, highlighting the effect of the TP's vertical transport window of water vapor in the troposphere on global change in water vapor. The vertical transport window is built by the AWT's thermal forcing in association with the dynamic effect of the TP's "hollow heat island". Our study improves the understanding of the vapor transport over the TP with important implication for global climate change. Copyright: © 2022 Xiangde Xu et al.
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| 3745. |
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| 3746. |
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| 3747. |
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| 3748. |
- Yang, C., et al.
(författare)
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Low frequency duct noise control using extended tube liners
- 2020
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Ingår i: AIAA AVIATION 2020 FORUM. - Reston, Virginia : American Institute of Aeronautics and Astronautics (AIAA).
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Konferensbidrag (refereegranskat)abstract
- This work investigates the noise reduction effect of extended tube liners which have the potential to shift the resonance to lower frequencies as compared with the conventional perforated liner which is constrained by the quarter-wavelength rule. In order to better utilize this technique for duct noise control, the surface impedance of the extended tube liner has to been known. However, existing models are mainly applicable to the normal incidence case and the influence of grazing flow effect is often ignored. In the current paper, an impedance model is developed based on the transfer matrix method and the grazing flow effect on the surface impedance is taken into account in terms of the end corrections. To examine the validity of the proposed model, liner samples are tested on a flow duct facility and the liner impedances are obtained from an impedance eduction method. The proposed model shows a reasonable agreement with the educed data. Then, geometry parameters including extended tube length, hole diameter and grazing flow speed are investigated. It is found that the frequency for maximum attenuation is shifted to lower frequency for a longer extended tube or smaller hole diameter but it is nearly unchanged with respect to the grazing flow speed.
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| 3749. |
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| 3750. |
- Yang, Chentao, 1988, et al.
(författare)
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SUNRISE: The rich molecular inventory of high-redshift dusty galaxies revealed by broadband spectral line surveys
- 2023
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 680
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Tidskriftsartikel (refereegranskat)abstract
- Understanding the nature of high-redshift dusty galaxies requires a comprehensive view of their interstellar medium (ISM) and molecular complexity. However, the molecular ISM at high redshifts is commonly studied using only a few species beyond 12C16O, limiting our understanding. In this paper, we present the results of deep 3 mm spectral line surveys using the NOrthern Extended Millimeter Array (NOEMA) targeting two strongly lensed dusty galaxies observed when the Universe was less than 1.8 Gyr old: APM 08279+5255, a quasar at redshift z = 3.911, and NCv1.143 (H-ATLAS J125632.7+233625), a z = 3.565 starburst galaxy. The spectral line surveys cover rest-frame frequencies from about 330 to 550 GHz for both galaxies. We report the detection of 38 and 25 emission lines in APM 08279+5255 and NCv1.143, respectively. These lines originate from 17 species, namely CO, 13CO, C18O, CN, CCH, HCN, HCO+, HNC, CS, C34S, H2O, H3O+, NO, N2H+, CH, c-C3H2, and the vibrationally excited HCN and neutral carbon. The spectra reveal the chemical richness and the complexity of the physical properties of the ISM. By comparing the spectra of the two sources and combining the analysis of the molecular gas excitation, we find that the physical properties and the chemical imprints of the ISM are different: the molecular gas is more excited in APM 08279+5255, which exhibits higher molecular gas temperatures and densities compared to NCv1.143; the molecular abundances in APM 08279+5255 are akin to the values of local active galactic nuclei (AGN), showing boosted relative abundances of the dense gas tracers that might be related to high-temperature chemistry and/or the X-ray-dominated regions, while NCv1.143 more closely resembles local starburst galaxies. The most significant differences between the two sources are found in H2O: the 448 GHz ortho-H2O(423 - 330) line is significantly brighter in APM 08279+5255, which is likely linked to the intense far-infrared radiation from the dust powered by AGN. Our astrochemical model suggests that, at such high column densities, far-ultraviolet radiation is less important in regulating the ISM, while cosmic rays (and/or X-rays and shocks) are the key players in shaping the molecular abundances and the initial conditions of star formation. Both our observed CO isotopologs line ratios and the derived extreme ISM conditions (high gas temperatures, densities, and cosmic-ray ionization rates) suggest the presence of a top-heavy stellar initial mass function. From the ~330-550 GHz continuum, we also find evidence of nonthermal millimeter flux excess in APM 08279+5255 that might be related to the central supermassive black hole. Such deep spectral line surveys open a new window into the physics and chemistry of the ISM and the radiation field of galaxies in the early Universe.
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