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Direct current circuit breakers (DCCBs) are extensively utilized in flexible DC transmission systems, with their cost being intricately related to the breaking current. An adaptive current-limiting control structure tailored for half-bridge modular multilevel converters (MMCs) has been developed with the aim of reducing the breaking current of circuit breakers. By examining the short-circuit current characteristics on DC side of MMC, the variation in the input impedance amplitude of the converter station is utilized to indicate the extent of the fault. The coefficient Kf, which defines the fault depth of the converter station, is determined. Kf is incorporated into the control structure of MMC to align it with the reference value of the bridge arm voltage. A proposed method for adaptive current limiting control addresses DC side short circuit fault of MMCs. The model of the half-bridge MMC flexible DC transmission system is created using power systems computer-aided design/electromagnetic transients including DC (PSCAD/EMTDC) platform to model the clearance of DC side short-circuit faults and validate the effectiveness of current limiting control. The simulation results demonstrate that the proposed adaptive current limiting control technique can effectively implement differentiated current limiting control depending on the diverse fault depths of MMC. The breaking current of DCCB is reduced and the fault clearing speed is improved by this approach.

As a kind of clean energy, the exploration and development of coalbed methane (CBM) are of great importance and significance. In this paper, the CBM reservoir parameters of a working area in Western Guizhou Province, China, were predicted by using 3D seismic exploration technology, and the sweet-spot area was evaluated based on the prediction results. In terms of data interpretation, while the traditional technology is extended to the study of CBM, new technologies and methods suitable for CBM were also actively explored, especially in the quantitative prediction methods of parameters such as coal seam thickness, CBM content, coal body structure, in situ stress, etc., and a three-highs quantitative prediction technology of CBM sweet spots characterized by high precision, high resolution, and high coincidence was explored, which is based on logging evaluation and petrophysics. In addition, in the comprehensive evaluation of multiparameter sweet spots, the multiparameter weighted step-by-step evaluation method based on inversion was innovatively proposed, which made the prediction of CBM sweet spots more focused, effective, and practical, and the distribution law of sweet spots was more consistent with the geological law, providing enough basis for subsequent well location deployment. Based on this method, the C409 coal seam was selected as the sweet spot. [ABSTRACT FROM AUTHOR]

Abstract: In this study, a 38mL monolith with homogeneous porous structure was produced by a single polymerization from glycidyl methacrylate (GMA) and ethylene dimethacrylate (EDMA) in the presence of porogens and an initiator. The uniform temperature distribution within the reaction system was achieved by adding reactant mixture continuously and enhancing the heat transfer ability of the polymerization system. Homogeneous porous structure in the monolith was proved by SEM and the pore size distribution profiles measured by mercury intrusion porosimetry. Experimental results from proteins separation indicated that the dynamic capacity and resolution of radial flow monolithic column were independent of flow rates. Furthermore, the pressure drop on the column was linearly dependent on the flow rate and did not exceed 1.7MPa even at a flow rate of 50mL/min, which proved that the prepared monolith could be used in the quick separation and preparation of biopolymers. [Copyright &y& Elsevier]

Hollow magnetic graphitic carbon microspheres (HMGSs) with/without a TiO2nanoparticle layer on their surfaces are designed and synthesized by combining the bubble-template method, assembly of β-cyclodextrin and Fe3O4stabilized by oleic acid and ammonia, and consequent graphitization reaction. HMGSs have potential in adsorption, enrichment, magnetic-assisted separation and drug delivery. [ABSTRACT FROM AUTHOR]

Hollow magnetic graphitic carbon microspheres (HMGSs) with/without a TiO2nanoparticle layer on their surfaces are designed and synthesized by combining the bubble-template method, assembly of β-cyclodextrin and Fe3O4stabilized by oleic acid and ammonia, and consequent graphitization reaction. HMGSs have potential in adsorption, enrichment, magnetic-assisted separation and drug delivery. [ABSTRACT FROM AUTHOR]

A modified QuEChERS method, based on multi-walled carbon nanotubes (MWCNTs), was established for the detection of 10 pyrethroid pesticides (cyfluthrin, flucythrinate, fenpropathrin, bifenthrin, cyhalothrin, permethrin, cypermethrin, etofenprox, fenvalerate, deltamethrin) in tea, in combination with gas chromatography-tandem mass spectrometry (GC-MS/MS). The purification effects and dosages of four carbon nanomaterials, viz. single-walled carbon nanotubes (SWCNTs), MWCNTs, amino-modified MWCNTs, and graphene, were compared. An orthogonal experimental design was used to determine the optimal experimental conditions for sample pretreatment. The experimental factors governing the process were analyzed using variance. The results showed that the optimized sample pretreatment parameters were as follows. Acetonitrile was used as the extraction solvent with ultrasonic extraction for 35 min, while 60 mg MWCNTs, 200 mg PSA, and 200 mg C18, were used as purifiers. The effects of the extraction solvent and the carbon nanomaterials used on the recoveries of the 10 pyrethroid pesticides were significantly different ( p <0.001), and the effect of extraction time on the recoveries was statistically different ( p <0.05). The dosage of carbon nanomaterials had no significant effect on the recoveries ( p >0.05). Good linearities were observed for the 10 pyrethroid pesticides in the concentration range of 0.01-2 mg/L. The limits of detection (LODs) and limits of quantification (LOQs) were in the ranges of 0.001-0.01 mg/kg and 0.005-0.04 mg/kg, respectively. The average recoveries of the pyrethroid pesticides spiked into blank samples of green tea were 91.4%-109.7%, and the relative standard deviations were 0.12%-9.80% ( n =6). Furthermore, the matrix effects (MEs) of scented green tea, green tea, and black tea were evaluated. It was found that the addition of MWCNTs to the purifier can effectively reduce the matrix effect in green tea and black tea matrices. The developed method and the national standard method were used to detect the residues of the 10 pyrethroid pesticides in 120 tea samples available in the market. The results showed that cyfluthrin, deltamethrin, fenvalerate, permethrin, fenpropathrin, cypermethrin, bifenthrin and cyhalothrin were detected, and the contents obtained with the two methods were similar. Although pyrethroids were detected in most tea samples, the contents of all pesticide residues were below the maximum residue limits (MRLs). Therefore, the developed method is suitable for the rapid quantitative analysis of pesticide residues in tea.

Unstirred in-situ polymerization was employed to directly produce glycidyl methacrylate-ethylene dimethacrylate copolymer (poly(GMA-co-EDMA)) monoliths. Mercury intrusion method was adopted to measure some parameters of the monoliths, such as pore size distribution, porosity and specific surface area. Effects of key variables such as composition of pore-forming solvent mixture, temperature and content of cross-linking reagent, divinyl monomer on porous structure of monoliths were studied. The optimization of preparation conditions was achieved. Homogeneous micro porous structure was observed in the monoliths by scanning electron microscope. The effect of flow rate on back pressure was investigated, and good permeability of the monolithic stationary phase was obtained. The monolithic column was also used for the separation of goat serum and five oligonucleotides, and the results proved that the monolithic column is suitable for the separation of biopolymers.