The grid-connected inverters may experience excessive current stress in case of unbalanced grid voltage fault ride through (FRT), which significantly affects the reliability of the power supply ...system. In order to solve the problem, the inherent mechanisms of the excessive current phenomenon with the conventional FRT solutions are discussed. The quantitative analysis of three-phase current peak values is conducted and a novel current-limited control strategy is proposed to achieve the flexible active and reactive power regulation and successful FRT in a safe current operation area with the aim of improving the system reliability under grid faults. Finally, the simulation and experiments of traditional and proposed FRT solutions are carried out. The results verify the effectiveness of the proposed method.
In this paper, a novel cascaded seven-level inverter topology with a single input source integrating switched-capacitor techniques is presented. Compared with the traditional cascade multilevel ...inverter, the proposed topology replaces all the separate dc sources with capacitors, leaving only one H-bridge cell with a real dc voltage source and only adds two charging switches. The capacitor charging circuit contains only power switches, so that the capacitor charging time is independent of the load. The capacitor voltage can be controlled at a desired level without complex voltage control algorithm and only use the most common carrier phase-shifted sinusoidal pulse width modulation strategy. The operation principle and the charging-discharging characteristic analysis are discussed in detail. A 1-kW experimental prototype is built and tested to verify the feasibility and effectiveness of the proposed topology.
System modeling and stability analysis is one of the most important issues of inverter-dominated microgrids. It is useful to determine the system stability and optimize the control parameters. The ...complete small signal models for the inverter-dominated microgrids have been developed, which are very accurate and could be found in literature. However, the modeling procedure will become very complex when the number of inverters in microgrid is large. One possible solution is to use the reduced-order small signal models for the inverter-dominated microgrids. Unfortunately, the reduced-order small signal models fail to predict the system instabilities. In order to solve the problem, a new modeling approach for inverter-dominated microgrids by using dynamic phasors is presented in this paper. Our findings indicate that the proposed dynamic phasor model is able to predict accurately the stability margins of the system, while the conventional reduced-order small signal model fails. In addition, the virtual ω-E frame power control method, which deals with the power coupling caused by the line impedance X/R characteristic, has also been chosen as an application example of the proposed modeling technique.
Flying capacitor photovoltaic (PV) inverters have been widely discussed in the literature. However, the relevant leakage current issues have not received much attention. In this paper, the modulation ...techniques for a transformerless three-phase flying capacitor PV inverter are investigated for the leakage current suppression. First, the theoretical analysis of the system common mode model and leakage current is presented. Second, three kinds of conventional carrier-based modulation techniques are evaluated, and results indicate that they fail to effectively suppress the leakage current. Third, a new carrier modulation and logic function is presented. It is able to maintain the constant common mode voltage, and consequently, the leakage current can be significantly reduced. Finally, the experimental tests are conducted on a transformerless three-phase flying capacitor PV inverter to verify the effectiveness of the proposed method.
Eliminating the leakage current is one of the most important issues for transformerless three-phase photovoltaic (PV) systems. In this paper, the leakage current elimination of a three-phase four-leg ...PV inverter is investigated. With the common-mode loop model established, the generation mechanism of the leakage current is clearly identified. Different typical carrier-based modulation methods and their corresponding common-mode voltages are discussed. A new modulation strategy with Boolean logic function is proposed to achieve the constant common-mode voltage for the leakage current reduction. Finally, the different modulation methods are implemented and tested on the TMS320F28335 DSP +XC3S400 FPGA digital control platform. The experimental results verify the effectiveness of the proposed solution.
Perivascular fat attenuation index (FAI) was thought to be an indicator of active vessel inflammation surrounding coronary plaques. However, whether this index can be reduced by statin treatment ...remains unknown. We aimed to investigate the serial change of lesion-specific perivascular FAI as quantified by coronary computed tomography (CCTA) after statin treatment.
Consecutive patients with chest pain and intermediate likelihood of coronary artery disease were referred for baseline CCTA. Patients were retrospectively included if they were treated medically and underwent follow-up CCTA at 1-year to 1.5-year interval. Lesion-specific perivascular FAI, as well as other plaque features, were measured at baseline and follow-up.
One hundred and eight patients (mean age 67.7 ± 11.1, 76 males) were included. A significant reduction of the FAI value was found for non-calcified plaques and mixed plaques (−68.0 HU ± 8.5 HU Vs. -71.5 HU ± 8.1 HU, p < .001 and − 70.5 HU ± 8.9 HU Vs. -72.8 HU ± 9.0 HU, p = .014). However, this improvement was not observed for calcified plaques (−70.6 HU ± 9.7 HU Vs. −71.7 HU ± 9.9 HU, p = .258). For non-calcified and mixed plaques, the volumes of non-calcified as well as low attenuation component was significantly reduced whereas total plaque volume and volume of calcified component increased. For calcified plaque, total plaque volume also demonstrated remarkable increase after statin treatment
Lesion-specific perivascular FAI decreased at mid-term follow-up after statin treatment for non-calcified and mixed plaques. Perivascular FAI can be a potential imaging biomarker to monitor the anti-inflammation response to statin treatments.
•FAI decreased after statin treatment for non-calcified and mixed plaques.•LAP volume was reduced for non-calcified and mixed plaques.•FAI was not significantly changed for calcified plaques.
A Current-Fed Isolated Bidirectional DC-DC Converter Sun, Xiaofeng; Wu, Xiaoying; Shen, Yanfeng ...
IEEE transactions on power electronics,
2017-Sept., 2017-9-00, 20170901, Letnik:
32, Številka:
9
Journal Article
Recenzirano
This paper proposes a current-fed isolated bidirectional dc-dc converter (CF-IBDC) that has the advantages of wide input voltage range, low input current ripple, low conduction losses, and soft ...switching over the full operating range. Compared with conventional CF-IBDCs, the voltage spikes of the low-voltage (LV) side switches in the proposed converter can be eliminated without additional clamp circuits. The converter adopts the pulse width modulation plus hybrid phase-shift control scheme such that the bus voltage can match the output voltage by means of the transformer. Thus, the current stresses and conduction losses of the converter become lower. In addition, the practical zero voltage switching (ZVS) of the secondary-side switches can be realized by adjusting the phase-shift angle within the secondary side when in light load or no load condition. The operating principles and characteristics including the power transfer, root-mean-square (RMS) current, and soft switching are investigated in detail. Then the design guidelines of inductors are also given. Finally an experimental prototype with 30-60 V input and 400 V/2.5 An output is built to verify the correctness of theoretical analyses.
Power oscillation and current quality are the important performance targets for the grid-connected inverter under unbalanced grid faults. First, the inherent reason for the current harmonic and power ...oscillation of the inverter is discussed with a quantitative analysis. Second, a new control strategy is proposed to achieve the coordinate control of power and current quality without the need for a phase-locked loop (PLL) or voltage/current positive/negative sequence extraction calculation. Finally, the experimental tests are conducted under unbalanced grid faults, and the results verify the effectiveness of the propose method.
Porous ceramics based on additive manufacturing have great application potential in many industries, including filtration, catalysis, and heat insulation. In this research, we propose a method for ...manufacturing porous ceramics with connected channels structure through ceramic digital light processing (DLP) and organic fiber decomposition. The crossed fibers in the green body, working as a pore-forming agent, were decomposed and removed to form connected channels in ceramic. It was confirmed that ball milling changed the fiber morphology during slurry preparation, which was beneficial to promote fibers crossing. Besides, we focused on the influence of the “Sponge Compression effect” during the DLP process, which affected the fibers distribution. The existence of fibers in the green body resulted in uneven pressure distribution during the debinding process, providing a potential source of cracks. Results show that this method can produce channels with a diameter of 100 μm and high connectivity, providing great potential in fabricating high connectivity porous ceramics with complex shapes and structures.
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