221 research outputs found
Reliability Enhancing Control Algorithms for Two-Stage Grid-Tied Inverters
In the photovoltaic (PV) generation system, two types of grid-tied inverter systems are usually deployed: the single-stage grid-tied inverter system and the two-stage grid-tied inverter system. In the single-stage grid-tied inverter system, the input of the inverter is directly connected to the PV arrays, while an additional dc-dc stage is inserted between the PV arrays and the dc-ac inverter in the two-stage design. The additional dc-dc stage could provide a stable dc-link voltage to the inverter, which also enables new design possibilities, including the multi-MPPT operation and solar-plus-storage application. Thus, the two-stage grid-tied inverter has been widely used in the PV generation system.As the core component of the PV generation system, the reliability of the grid-tied inverter determines the overall robustness of the system. The two-stage grid-tied inverter system includes three parts: the dc-dc stage, dc-link capacitor, and dc-ac inverter. Thus, the reliability of the two-stage grid-tied inverter relies on the reliability of each part. The dc-dc stage is used to provide a stable dc-link voltage to the inverter. However, when the inverter stage provides constant power to the grid, the load of the dc-dc stage becomes the constant power load (CPL), which will deteriorate the stability of the dc-dc stage. The dc-link capacitor is used to attenuate the voltage ripple on the dc-link and balance the transient power mismatch between the dc-dc stage and the dc-ac stage. However, during the operation of the inverter system, the degradation of the capacitor will reduce the converter reliability, and even result in system failure. The inverter stage is connected to the grid through the output filter, and the LCL type filter has been commonly used due to its superior performance. The resonance of the LCL filter must be properly damped to enhance the inverter stability. However, the grid-side impedance will lead to the resonant frequency drifting of the LCL filter, which will worsen the stability margin of the inverter. Thus, the control design of the two-stage grid-tied inverter system must consider those reliability challenges. In this work, three control algorithms are proposed to solve the reliability challenges. For the dc-dc stage, an uncertainty and disturbance estimator (UDE) based robust voltage control scheme is proposed. The proposed voltage control scheme can actively estimate and compensate for the disturbance of the dc-dc stage. Both the disturbance rejection performance and the stability margin of the dc-dc stage, especially under the CPL, could be enhanced. For the dc-link capacitor, a high-frequency (HF) signal injection based capacitance estimation scheme is proposed. The proposed estimation scheme can monitor the actual dc-link capacitance in real-time. For the inverter stage, an adaptive extremum seeking control (AESC) based LCL filter resonant frequency estimation scheme is proposed. The AESC-based estimation scheme can estimate the resonant frequency of the LCL filter online. All the proposed reliability enhancing control algorithms could enhance the reliability of the two-stage grid-tied inverter system. Detailed theoretical analysis, simulation studies, and comprehensive experimental studies have been performed to validate the effectiveness
Reliability Enhancing Control Algorithms for Two-Stage Grid-Tied Inverters
In the photovoltaic (PV) generation system, two types of grid-tied inverter systems are usually deployed: the single-stage grid-tied inverter system and the two-stage grid-tied inverter system. In the single-stage grid-tied inverter system, the input of the inverter is directly connected to the PV arrays, while an additional dc-dc stage is inserted between the PV arrays and the dc-ac inverter in the two-stage design. The additional dc-dc stage could provide a stable dc-link voltage to the inverter, which also enables new design possibilities, including the multi-MPPT operation and solar-plus-storage application. Thus, the two-stage grid-tied inverter has been widely used in the PV generation system.As the core component of the PV generation system, the reliability of the grid-tied inverter determines the overall robustness of the system. The two-stage grid-tied inverter system includes three parts: the dc-dc stage, dc-link capacitor, and dc-ac inverter. Thus, the reliability of the two-stage grid-tied inverter relies on the reliability of each part. The dc-dc stage is used to provide a stable dc-link voltage to the inverter. However, when the inverter stage provides constant power to the grid, the load of the dc-dc stage becomes the constant power load (CPL), which will deteriorate the stability of the dc-dc stage. The dc-link capacitor is used to attenuate the voltage ripple on the dc-link and balance the transient power mismatch between the dc-dc stage and the dc-ac stage. However, during the operation of the inverter system, the degradation of the capacitor will reduce the converter reliability, and even result in system failure. The inverter stage is connected to the grid through the output filter, and the LCL type filter has been commonly used due to its superior performance. The resonance of the LCL filter must be properly damped to enhance the inverter stability. However, the grid-side impedance will lead to the resonant frequency drifting of the LCL filter, which will worsen the stability margin of the inverter. Thus, the control design of the two-stage grid-tied inverter system must consider those reliability challenges. In this work, three control algorithms are proposed to solve the reliability challenges. For the dc-dc stage, an uncertainty and disturbance estimator (UDE) based robust voltage control scheme is proposed. The proposed voltage control scheme can actively estimate and compensate for the disturbance of the dc-dc stage. Both the disturbance rejection performance and the stability margin of the dc-dc stage, especially under the CPL, could be enhanced. For the dc-link capacitor, a high-frequency (HF) signal injection based capacitance estimation scheme is proposed. The proposed estimation scheme can monitor the actual dc-link capacitance in real-time. For the inverter stage, an adaptive extremum seeking control (AESC) based LCL filter resonant frequency estimation scheme is proposed. The AESC-based estimation scheme can estimate the resonant frequency of the LCL filter online. All the proposed reliability enhancing control algorithms could enhance the reliability of the two-stage grid-tied inverter system. Detailed theoretical analysis, simulation studies, and comprehensive experimental studies have been performed to validate the effectiveness
Knowledge Prompting for Few-shot Action Recognition
Few-shot action recognition in videos is challenging for its lack of
supervision and difficulty in generalizing to unseen actions. To address this
task, we propose a simple yet effective method, called knowledge prompting,
which leverages commonsense knowledge of actions from external resources to
prompt a powerful pre-trained vision-language model for few-shot
classification. We first collect large-scale language descriptions of actions,
defined as text proposals, to build an action knowledge base. The collection of
text proposals is done by filling in handcraft sentence templates with external
action-related corpus or by extracting action-related phrases from captions of
Web instruction videos.Then we feed these text proposals into the pre-trained
vision-language model along with video frames to generate matching scores of
the proposals to each frame, and the scores can be treated as action semantics
with strong generalization. Finally, we design a lightweight temporal modeling
network to capture the temporal evolution of action semantics for
classification.Extensive experiments on six benchmark datasets demonstrate that
our method generally achieves the state-of-the-art performance while reducing
the training overhead to 0.001 of existing methods
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Job Satisfaction Among Methadone Maintenance Treatment Clinic Service Providers in Jiangsu, China: A Cross-sectional Survey.
ObjectiveService providers' job satisfaction is critical to the stability of the work force and thereby the effectiveness of methadone maintenance treatment (MMT) programs. This study aimed to explore MMT clinic service providers' job satisfaction and associated factors in Jiangsu, China.MethodsThis secondary study used baseline data of a randomized interventional trial implemented in Jiangsu, China. A survey was conducted among 76 MMT service providers using the computer-assisted self-interview (CASI) method. Job satisfaction responses were assessed via a 30-item scale, with a higher score indicating a higher level of job satisfaction. Perceived institutional support and perceived stigma due to working with drug users were measured using a 9-item scale. Correlation and multiple linear regression analyses were performed to identify factors associated with job satisfaction.ResultsCorrelation analyses found a significant association between job satisfaction and having professional experience in the prevention and control of HIV, other sexually transmitted infections, or other infectious diseases (Pβ=β0.046). Multiple regression analyses revealed that working at MMT clinics affiliated with Center for Disease Control and Prevention sites was associated with a lower level of job satisfaction (Pβ=β0.014), and perception of greater institutional support (Pβ=β0.001) was associated with a higher level of job satisfaction.ConclusionJob satisfaction among MMT clinic service providers was moderate in our study. Our findings suggest that institutional support for providers should be improved, and that acquisition of additional expertise should be encouraged
Observation of Valley Zeeman and Quantum Hall Effects at Q Valley of Few-Layer Transition Metal Disulfides
In few-layer (FL) transition metal dichalcogenides (TMDC), the conduction
bands along the Gamma-K directions shift downward energetically in the presence
of interlayer interactions, forming six Q valleys related by three-fold
rotational symmetry and time reversal symmetry. In even-layers the extra
inversion symmetry requires all states to be Kramers degenerate, whereas in
odd-layers the intrinsic inversion asymmetry dictates the Q valleys to be
spin-valley coupled. In this Letter, we report the transport characterization
of prominent Shubnikov-de Hass (SdH) oscillations for the Q valley electrons in
FL transition metal disulfide (TMDs), as well as the first quantum Hall effect
(QHE) in TMDCs. Our devices exhibit ultrahigh field-effect mobilities (~16,000
cm2V-1s-1 for FL WS2 and ~10,500 cm2V-1s-1 for FL MoS2) at cryogenic
temperatures. Universally in the SdH oscillations, we observe a valley Zeeman
effect in all odd-layer TMD devices and a spin Zeeman effect in all even-layer
TMD devices.Comment: 20 pages, 4 figure
Molecular state interpretation of charmed baryons in the quark model
Stimulated by the observation of by the Belle
Collaboration, the -wave pentaquark systems
with = 0, = are
investigated in the framework of quark delocalization color screening
model(QDCSM). The real-scaling method is utilized to check the bound states and
the genuine resonance states. The root mean square of cluster spacing is also
calculated to study the structure of the states and estimate if the state is
resonance state or not. The numerical results show that
cannot be interpreted as a molecular state, and cannot be
explained as the molecular state with . can
be interpreted as the molecular state with and the main
component is . can be interpreted as the
molecular state with and the main component is
. is likely to be interpreted as a
molecular state with , and the main component is . Besides,
two new molecular states are predicted, one is the
resonance state with the mass around 3140 MeV, another one is the
with the mass of 3188.3 MeV.Comment: 12 pages, 3 figure
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