In 2017, LHCb collaboration reported their first observation of the rare decays
B
s
→
ϕ
(
f
0
(
980
)
/
f
2
(
1270
)
→
)
π
+
π
-
and the evidence of
B
0
→
ϕ
(
f
0
(
980
)
/
f
2
(
1270
)
→
)
π
+
π
-
. ...Motivated by this, we study these quasi-two-body decays in the perturbative QCD approach. The branching fractions,
CP
asymmetries and the polarization fractions are calculated. We find that within the appropriate two-meson wave functions, the calculated branching fractions are in agreement with the measurements of LHCb. Based on the narrow-width approximation, We also calculate the branching fractions of the quasi-two-body
B
d
,
s
→
ϕ
(
f
0
(
980
)
/
f
2
(
1270
)
→
)
π
0
π
0
and
B
d
,
s
→
ϕ
(
f
2
(
1270
)
→
)
K
+
K
-
, and hope the predictions to be tested in the ongoing LHCb and Belle II experiments. Moreover, the processes
B
d
,
s
→
ϕ
f
2
(
1270
)
are also analyzed under the approximation. We note that the
CP
asymmetries of these decays are very small, because these decays are either penguin dominant or pure penguin processes.
Grid converters need a synchronization which is often a phase-locked loop (PLL) to get connected to the grid. In the literature, impedance-based models using small-signal technique have been widely ...utilized to describe the behaviors of PLL-synchronized grid converter. However, most of them target at specific problems and are able to represent the phenomenon under certain scenarios. For the analysis during phase transient, like phase jump due to grid fault, an accurate model is still missing. To address this issue, a general impedance-based model using Maclaurin expansions for its trigonometric function approximation has been proposed in this paper, so that it can well represent the system behavior during phase transient. Moreover, different deepness of the general model can be scaled, leading to different models including phasorial model, current source model, impedance-based model with/without PLL effects. A guideline on how to choose a proper model based on the problems to be studied and grid conditions has been provided in this paper. Simulation and experimental results are given to validate the effectiveness of the model and the theoretical analysis.
Repetitive control (RC), which can achieve zero steady-state error tracking of any periodic signal with known integer period, offers active power filters (APFs) a promising accurate current control ...scheme to compensate the harmonic distortion caused by nonlinear loads. However, classical RC cannot exactly compensate periodic signals of variable frequency and would lead to significant performance degradation of APFs. In this paper, a fractional-order RC (FORC) strategy at a fixed sampling rate is proposed to deal with any periodic signal of variable frequency, where a Lagrange-interpolation-based fractional delay (FD) filter is used to approximate the factional delay items. The synthesis and analysis of FORC systems are also presented. The proposed FORC offers fast online tuning of the FD and the fast update of the coefficients, and then provides APFs with a simple but very accurate real-time frequency-adaptive control solution to the elimination of harmonic distortions under grid frequency variations. A case study on a single-phase shunt APF is conducted. Experimental results are provided to demonstrate the validity of the proposed FORC.
Long‐term observations from the Advanced Microwave Sounding Unit‐A (AMSU‐A) onboard polar‐orbiting satellites NOAA 15, 16, 17, and 18 and European Meteorological Operational satellite program‐A ...(MetOp‐A) were intercalibrated using their overlap observations. Simultaneous nadir overpasses (SNOs) and global ocean mean differences between these satellites were used to characterize calibration errors and to obtain calibration coefficients. Calibration errors were found manifesting themselves as certain scatter or temporal patterns of intersatellite biases, such as well‐defined seasonal cycles in the Arctic and Antarctic SNO difference time series or a unique pattern closely correlated to the instrument temperature variability induced by Solar Beta Angle (SBA) variations in global ocean mean difference time series. Analyses of these patterns revealed five different types of biases that need to be removed from existing prelaunch‐calibrated AMSU‐A observations, which include relatively stable intersatellite biases between most satellite pairs, bias drifts on NOAA 16 and channel 7 of MetOp‐A, sun‐heating‐induced instrument temperature variability in radiances, scene temperature dependency in biases due to inaccurate calibration nonlinearity, and biases due to channel frequency shift from its prelaunch measurement in certain satellite channels. Level‐1c time‐dependent calibration offsets and nonlinear coefficients were introduced and determined from SNO and global ocean mean temperature regressions to remove or minimize the first four types of biases. Channel frequency shift in NOAA 15 channel 6 was obtained from the radiative transfer model simulation experiments. The new calibration coefficients and channel frequency values have significantly reduced the five different types of biases and resulted in more consistent multisatellite radiance observations for intercalibrated satellite channels. The intercalibrated AMSU‐A observations have been merged with its precursor, the intercalibrated microwave sounding unit (MSU), to generate the NOAA/Center for Satellite Applications and Research (STAR) version 2.0 upper‐air temperature climate data record (CDR) for climate trend and variability monitoring from 1979 to the present. The intercalibrated AMSU‐A radiance data are expected to further improve accuracies of numerical weather prediction and consistencies in climate reanalysis and CDR developments.
Key Points
Identified 5 different types of calibration errors in AMSU‐A
Developed algorithms to remove these errors
Developed consistent fundamental and thematical climate data record from AMSU‐A
The stability of the grid-connected inverters adopted in distributed energy resources (DERs) highly depends on the characteristics of the grid impedance. Hence, various active damping methods and ...adaptive control algorithms have been proposed for the control of the grid inverters. Differently from those existing solutions, this paper proposes to use a smart transformer (ST) to shape the grid impedance to interact with the controllers of local inverters, aiming at improving the stability of the local controllers as well as the overall grid. A main advantage of this solution is that it offers a pervasive service to all the available DERs, reducing the cost and design effort of the local controllers. Three active damping methods are presented in this paper and the implementation issues in the ST LV-side voltage controller are given. Moreover, the design criteria of the active damping methods considering the control performance of ST and the stability requirements of local DERs are presented. Analysis and experiments are carried out to verify the effectiveness of the proposed solution.
Photocatalytic water splitting using powered semiconductors as photocatalysts represents a promising strategy for clean, low‐cost, and environmentally friendly production of H2 utilizing solar ...energy. The loading of noble‐metal cocatalysts on semiconductors can significantly enhance the solar‐to‐H2 conversion efficiency. However, the high cost and scarcity of noble metals counter their extensive utilization. Therefore, the use of alternative cocatalysts based on non‐precious metal materials is pursued. Nanosized MoS2 cocatalysts have attracted considerable attention in the last decade as a viable alternative to improve solar‐to‐H2 conversion efficiency because of its superb catalytic activity, excellent stability, low cost, availability, environmental friendliness, and chemical inertness. In this perspective, the design, structures, synthesis, and application of MoS2‐based composite photocatalysts for solar H2 generation are summarized, compared, and discussed. Finally, this Review concludes with a summary and remarks on some challenges and opportunities for the future development of MoS2‐based photocatalysts.
Mo and S and Sun for hydrogen: Nanosized MoS2 cocatalyst has attracted great attention as a viable alternate to improve solar‐to‐H2 conversion efficiency due to its superb catalytic activity, excellent stability, low cost and availability. This review presents the recent advances on the development of MoS2 cocatalyst for photocatalytic hydrogen production, the structure, photophysical and electrochemical properties, and catalytic activity of MoS2 cocatalyst are summarized.
Green production of NH3, especially the Li‐mediated electrochemical N2 reduction reaction (NRR) in non‐aqueous solutions, is attracting research interest. Controversies regarding the NRR mechanism ...greatly impede its optimization and wide applications. To understand the electrocatalytic process, we treated Au coated carbon fibrous paper (Au/CP) as the model catalyst. In situ XRD confirmed the transformation of lithium intermediates during NRR. Au greatly improved electron transfer kinetics to catalyze metallic Li formation, and accordingly highly accelerated spontaneous NRR. The Faradaic efficiency of NRR on Au/CP reached 34.0 %, and NH3 yield was as high as 50 μg h−1 cm−2. Our research shows that the key step of Li‐mediated non‐aqueous NRR is electrocatalytic Li reduction and offers a novel electrocatalyst design method for Li reduction.
The key step of Li‐mediated non‐aqueous NRR is electrocatalytic Li reduction. Gold greatly improved Li adsorption energy and thus highly accelerated the domino‐like nitrogen reduction reaction (NRR).
Smart Transformer-Fed Variable Frequency Distribution Grid Zhi-Xiang Zou; De Carne, Giovanni; Buticchi, Giampaolo ...
IEEE transactions on industrial electronics (1982),
2018-Jan., 2018-1-00, 20180101, Volume:
65, Issue:
1
Journal Article
Peer reviewed
Open access
The smart transformer (ST), a solid-state transformer with control and communication functionalities, that interfaces medium voltage (MV) and low voltage (LV) grids, enables the control of the ...frequency in the LV grid independently from the MV one. In a ST-fed distribution grid, the ST can interact with the droop controllers of local generators and loads frequency characteristic to control the LV power demand. However, most of the existing controllers for power converters cannot guarantee good harmonic control under variable frequency condition. To address this issue, a frequency-adaptive control scheme based on the fractional-order repetitive control and the frequency-adaptive phaselocked loop are proposed in this paper. The proposed scheme provides fast online parameter tuning capability in order to be highly adaptive to variable frequencies, and it can be easily implemented in the power converters controllers of a ST-fed distribution grid. Moreover, the stability analysis of the frequency-adaptive system considering the effect of synchronization has been investigated in this paper. Simulation and experiments have been carried out to verify the effectiveness of the proposed scheme as well as the considered scenarios.
In this work, we study the (
S
,
P
and
D
)-wave
K
+
K
-
contributions to
B
→
K
K
K
decays in the perturbative QCD approach at leading order. Within the two-meson wave functions describing the ...nonperturbative dynamics in the kaon-pair for different waves, we calculate the branching fractions and the direct
CP
asymmetries of these decay modes in the corresponding resonance regions. Most of our numerical results are well consistent with the current measurements. We note that the narrow-width approximation is invalid in the quasi-two-body decays
B
→
K
f
0
(
980
)
→
K
K
K
. For other decays, under the narrow-width approximation we can extract the branching fractions of the corresponding two-body decays involving the intermediate resonant states, and the related branching fractions agree with the current experimental data well. Furthermore, we also predict the branching fractions of the corresponding quasi-two-body decays
B
→
K
π
+
π
-
, which are expected to be tested in the ongoing LHCb and Belle-II experiments.
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