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  • 1
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    Frequency Support from a Variable-Speed Wind Turbine Generator Using Different Variable Droop Characteristics (2020)
    Molecular Diversity Preservation International
    Details
    Publication Date: 2020-08-31
    Description: Doubly-fed induction generators (DFIGs) are capable of boosting frequency response capability while preventing the rotor speed from stalling during under-frequency disturbances, by employing variable droop characteristics. However, the frequency response capability during over-frequency disturbances is shortened because the potential for storing the kinetic energy is inversely proportional to the variable droop characteristics used for under-frequency disturbances. This paper designs a frequency control method of a DFIG to boost the frequency response capability during over-frequency disturbances while preserving the frequency response capability during under-frequency disturbances, by employing different variable droop characteristics. The effectiveness of the proposed frequency control method is investigated in a test system. The investigation results under five scenarios with different load variations, wind power penetrations and wind conditions clearly demonstrate that the proposed frequency control method suppresses the maximum system frequency deviations. As such, the proposed frequency control method can provide an effective solution for the frequency control ancillary service of a power system with large integrations of wind energy.
    Electronic ISSN: 1996-1073
    Topics: Energy, Environment Protection, Nuclear Power Engineering
    Published by Molecular Diversity Preservation International
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  • 2
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    Two typical modes in the variabilities of wintertime North Pacific basin-scale oceanic fronts and associated atmospheric eddy-driven jet (2017)
    Wiley
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    Publication Date: 2017-08-19
    Description: The role of oceanic fronts in the midlatitude air–sea interaction remains unclear. This study defines new indexes to quantify the intensity and location of two basin-scale oceanic frontal zones in the wintertime North Pacific, i.e. the subtropical and subarctic frontal zones (STFZ, SAFZ). With these indexes, two typical modes, which are closely related to two large-scale sea surface temperature (SST) anomaly patterns resembling Pacific Decadal Oscillation (PDO) and North Pacific Gyre Oscillation (NPGO), respectively, are found in the oceanic front variabilities as well as in their associations with the midlatitude atmospheric eddy-driven jet. Corresponding to an PDO-like SST anomaly pattern, an enhanced STFZ occurs with a southward shifted SAFZ, which is associated with enhanced overlying atmospheric front, baroclinicity and transient eddy vorticity forcing, thus with an intensification of the westerly jet; and vice versa. On the other hand, corresponding to an NPGO-like SST pattern, an enhanced SAFZ occurs with a northward shifted STFZ, which is associated with a northward shift of the atmospheric front, baroclinicity, transient eddy vorticity forcing, and westerly jet; and vice versa. These results suggest that the basin-scale oceanic frontal zone is a key region for the midlatitude air–sea interaction in which the atmospheric transient eddy's dynamical forcing is a key player in such an interaction. Schematic diagram of two typical modes in the variabilities of wintertime North Pacific basin-scale oceanic fronts and associated atmospheric eddy-driven jet: (a) the enhanced westerly jet mode corresponding to an enhanced STFZ and an southward shifted SAFZ, caused by the PDO-like SST anomaly, and (b) the northward shifted westerly jet mode corresponding to an enhanced SAFZ and an northward shifted STFZ, caused by the NPGO-like SST anomaly. Note that the climatological westerly jet is shaded.
    Electronic ISSN: 1530-261X
    Topics: Geosciences , Physics
    Published by Wiley on behalf of The Royal Meteorological Society (RMetS).
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  • 3
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    Reexamination of the Climatology and Variability of the Northwest Pacific Monsoon Trough Using a Daily Index (2020)
    American Meteorological Society
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    Publication Date: 2020-06-08
    Description: This study developed a daily index to represent the northwest Pacific monsoon trough using westerly related cyclonic vorticity after removing tropical cyclones (TCs) from the reanalysis dataset. This index sufficiently captures the spatial and temporal variations in the monsoon trough. The use of this daily index revealed new features in the monsoon trough, including daily statistical characteristics, the active period over a year, and the main periodicity. A monsoon trough can be identified as active when the daily index is greater than 2.0 × 10−4 s−1. Active monsoon troughs occur during half of the summertime, and these is no monsoon trough on one-third of days, with the remaining days categorized as inactive. The most active month is August, in which approximately 20 days exhibit an active monsoon trough. Using this index, an active monsoon trough period, which is related to vigorous TC activity, was determined by identifying the establishment and decay dates for each year from 1979 to 2016. During most years, the active monsoon trough is established in mid-July and decays in late October, persisting for 3–5 months during the boreal summer. Moreover, spectral and wavelet analyses demonstrated the presence of intraseasonal, interannual, and interdecadal variabilities in the monsoon trough. The dominant periodicity for the interannual variability varied from 1.5 to 4 years in different decades. The relationship between the monsoon trough and TCs is also revealed using this index, showing that approximately 60% of TC formations were related to an active monsoon trough.
    Print ISSN: 0894-8755
    Electronic ISSN: 1520-0442
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 4
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    On the Relationship Between Probabilistic and Deterministic Skills in Dynamical Seasonal Climate Prediction (2018)
    American Geophysical Union
    Details
    Publication Date: 2018-05-27
    Description: Despite the gradually increasing emphasis on assessing the skill of dynamical seasonal climate predictions from the probabilistic perspective, there is a lack of in-depth understanding that an inherent relationship may exist between the probabilistic and deterministic seasonal forecast skills. In this study, we focus on investigating this relationship, through theoretical consideration based on an analytical approach and diagnostic analysis of the historical forecasts produced by multiple dynamical models. The probabilistic forecast skill is gauged in terms of its two different attributes: resolution and reliability, while the deterministic forecast skill is measured in terms of anomaly correlation (AC). Through the theoretical consideration under certain simplified assumptions, a nonlinear, monotonic relationship is analytically derived between the resolution and the AC. Subsequent diagnostic analysis shows that the resolution and AC skills of both the multimodel ensemble and its member single models indeed appear to be approximately monotonically and nonlinearly related, specifically when they are calculated in a zonally aggregated manner by which the impact of finite sample size is reduced. This observed relationship has a specific form that is consistent with what the theory predicts. In short, the theoretical result is well verified by the dynamical model forecasts. Diagnostic analysis also shows that no good relationship exists between the reliability and the AC, signifying the difference of reliability and resolution in nature. A specific application of the proven resolution-AC coherence is also demonstrated. The proved resolution-AC relationship can facilitate comparisons among various assessments of seasonal climate prediction skill from the deterministic or probabilistic perspective alone. ©2018. The Authors.
    Print ISSN: 2169-897X
    Electronic ISSN: 2169-8996
    Topics: Geosciences , Physics
    Published by American Geophysical Union
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  • 5
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    Synoptic-Scale Waves in Sheared Background Flow over the Western North Pacific (2016)
    American Meteorological Society
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    Publication Date: 2016-11-01
    Description: Tropical depression (TD)-type waves are the dominant mode of synoptic-scale fluctuations over the western North Pacific. By applying spatiotemporal filters to the observed OLR data and the NCEP–DOE AMIP-II reanalysis data for 1979–2013, this study reveals the characteristics and energetics of convectively coupled TD-type waves under the effects of different circulation patterns in association with vertical wind shear. Results exhibit that different ambient sheared flows significantly affect the vertical structure of westward-propagating TD-type waves, with a lower-tropospheric mode in an easterly sheared background and an upper-tropospheric mode in a westerly sheared background. Energetic diagnoses demonstrate that when the disturbance is trapped in the lower (upper) level by easterly (westerly) shear, the horizontal mean flow in the lower (upper) level favors wave growth by converting energy from the shear of the zonal mean flow (from the convergence of the meridional mean flow). During the penetration of a westward-propagating synoptic-scale disturbance from a westerly sheared flow into an easterly sheared flow, the upper-level disturbance decays, and the lower-level disturbance intensifies. Meanwhile, the upper-level kinetic energy is transferred downward, but the effect induces the wave growth only confined to the midlevels. Consequently, the low-level growth of the westward-propagating upper-level synoptic-scale disturbance is mainly attributed to the barotropic conversion of horizontal mean flow in the lower troposphere.
    Print ISSN: 0022-4928
    Electronic ISSN: 1520-0469
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 6
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    Fast Frequency Response of a Doubly-Fed Induction Generator for System Inertia Support (2020)
    Molecular Diversity Preservation International
    Details
    Publication Date: 2020-04-23
    Description: Converter-interfaced doubly-fed induction generators (DFIGs) can provide short-term frequency support (STFS) capability by releasing rotating kinetic energy. After arresting the frequency decrease, the rotor speed should return to its initial operating condition. During the rotor speed recovery process, special attention should paid to the performance of the rotor speed restoration duration and size of the second frequency drop (SFD). This paper suggests an enhanced STFS method of DFIGs to preserve better performance of the frequency nadir with less released rotating kinetic energy and accelerate the rotor speed restoration. To this end, a rotor speed-varying incremental power is proposed and is added to the maximum power tracking (MPT) operation reference during STFS, thereby releasing less rotating kinetic energy from DFIGs; afterward, the power reference smoothly decreases to the reference for MPT operation during the preset period. Test results clearly demonstrate that since even less rotating kinetic energy is utilized, the proposed method can preserve better performance of heightening the frequency nadir; furthermore, the proposed method accelerates the rotor speed restoration when the proposed strategy produces the same SFD as the conventional method, thereby improving the power grid resilience.
    Electronic ISSN: 2076-3417
    Topics: Natural Sciences in General
    Published by Molecular Diversity Preservation International
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  • 7
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    Dynamic Frequency Support from a DFIG-Based Wind Turbine Generator via Virtual Inertia Control (2020)
    Molecular Diversity Preservation International
    Details
    Publication Date: 2020-05-13
    Description: As the penetrated level of wind in power grids increases, the online system inertia becomes weak. Doubly-fed induction generator (DFIG)-based wind turbine generators (WTGs) are required to provide virtual inertia response to support system frequency. The present inertia control strategy with fixed control gain is not suitable and may cause stall of the DFIG-based WTG, as the virtual inertia response potential from the DFIG-based WTG is different with various wind speed conditions. This paper addresses a virtual inertia control method for the DFIG-based WTGs to improve the system frequency stability without causing stalling of the wind turbine for various wind speed conditions. The effectiveness of the proposed virtual inertia control method is investigated in a small system embedded with the DFIG-based WTG. Results demonstrate that the proposed virtual inertia strategy improves the frequency stability without causing the rotor speed security issue. Thus, the proposed control strategy can secure the dynamic system frequency security of power systems under the scenarios of full and partial loads, and, consequently, the proposed method provides a promising solution of ancillary services to power systems.
    Electronic ISSN: 2076-3417
    Topics: Natural Sciences in General
    Published by Molecular Diversity Preservation International
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  • 8
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    Midlatitude atmospheric transient eddy feedbacks influenced ENSO-associated wintertime Pacific teleconnection patterns in two PDO phases (2020)
    Springer
    Details
    Publication Date: 2020-01-29
    Print ISSN: 0930-7575
    Electronic ISSN: 1432-0894
    Topics: Geosciences , Physics
    Published by Springer
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  • 9
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    Characteristics and Mechanisms of the Subseasonal Eastward Extension of the South Asian High (2015)
    American Meteorological Society
    Details
    Publication Date: 2015-09-01
    Description: This study investigates the features of eastward extension of the South Asian high (SAH) and its connection with diabatic heating and rainfall over eastern Asia on subseasonal time scales. The causes of SAH’s eastward extension are examined by potential vorticity (PV) diagnosis with emphasis on the joint role of diabatic heating feedback and midlatitude wave train. The SAH’s eastward extension features eastward propagation of a wave train across Eurasia. Among the wave train, the migration of weak high from the western flank of the Tibetan Plateau (TP) to the east of TP contributes to the SAH’s eastward extension at the early stage. When the SAH approaches its easternmost position, a strong negative PV (positive geopotential height) center prevails to the east of the TP at 200 hPa. The associated anomalies in diabatic heating and rainfall include the anomalous heating and above-normal rainfall over the South China Sea (SCS) and subtropical western Pacific occurring 12 days before the SAH’s easternmost stretch, and then anomalous cooling and below-normal rainfall over the southern foot of the TP and southern China and heating and above-normal rainfall over the northern TP and northern China a week later. The anomalous heating and ascending motion over the northern TP and northern China act to increase negative PV locally at 200 hPa. The cooling and descending induce positive PV over southern China. The north–south dipolar structure of PV anomaly with the climatological northerly flow is favorable to southward advection of a negative PV anomaly at 200 hPa. The anomalous heating over the SCS–western Pacific helps to develop a below-normal rainfall condition over southern China via inducing a lower-level anomalous cyclone over coastal region. These processes are conducive to the SAH’s eastward extension at its later stage.
    Print ISSN: 0894-8755
    Electronic ISSN: 1520-0442
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 10
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    Information-based potential predictability of the Asian summer monsoon in a coupled model (2012)
    American Geophysical Union
    Details
    Publication Date: 2012-02-15
    Print ISSN: 0148-0227
    Electronic ISSN: 2156-2202
    Topics: Geosciences
    Published by American Geophysical Union
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