Distributed agent-based control scheme for single-phase parallel inverters in microgrids with photovoltaic systems

In this paper, an agent-based distributed control scheme is presented to control single-phase parallel inverters in solar photovoltaic (PV) systems connected to microgrids. A communication assisted multi-agent framework is developed within microgrids where agents perform their tasks in a distributed manner with an aim of stabilizing load voltage and current under normal and faulted conditions through the asymptotic tracking of the reference current signal. The distributed agent-based control scheme requires information from the neighboring agents through communication network to decide control actions. The proposed control scheme utilizes Ziegler-Nichols (Z-N) tuning approach to design proportional integral (PI) controllers for controlling inverters within the multi-agent system (MAS). A microgrid with parallel inverter-connected solar PV systems is considered for simulations under normal and faulted conditions where results show the excellency of the proposed agent-based scheme in comparison to the conventional scheme without MAS.

Primary voltage control of a single-phase inverter using linear quadratic regulator with integrator

This paper proposes a linear quadratic regulator with integral action, ensuring fast dynamic response and resisting capability of voltage deviation from instantaneous reference grid voltage, to control the inverter voltage that can also be used in a microgrid. The proposed control strategy is based on a linear quadratic regulator, minimizing the cost function of the system, with integral action used to impede voltage degradation from a reference voltage for outside disturbances of the system, such as abrupt load change. The combined integral term assists to recover the voltage difference between grid and reference grid voltage. The validity of the proposed controller has been tested with linear and non-linear loads under various conditions. In both cases, the effectiveness of the controller has been proved. The result of the proposed controller is good to track the instantaneous reference grid voltage.

Mechanochemical synthesis of nanoparticulate ZnO–ZnWO4 powders and their photocatalytic activity

In this study, mechanochemical reaction systems with H₂WO₄ as a precursor were investigated for the synthesis of nanoparticulate powders of WO₃, ZnWO₄, and dual-phase (ZnWO₄)_x(ZnO)_1–x. The objective was to establish whether mechanochemical processing can be used to manufacture high activity photocatalysts in the ZnO–WO₃ system. Milling and heat treatment of H₂WO₄ + 12NaCl was found to result in the formation of irregularly shaped platelets of a sodium tungstate rather than nanoparticles of WO₃. Powders of single-phase ZnWO₄ and dual-phase (ZnWO₄)_x(ZnO)_1–x were successfully synthesised by incorporating H₂WO₄ into the ZnCl₂ + Na₂CO₃ + 4NaCl reactant mixture. The photocatalytic activity of these powders was evaluated using the spin-trapping technique with electron paramagnetic resonance spectroscopy. It was found that the photocatalytic activity decreased with the ZnWO₄ content. This decrease in activity was attributed to the larger average particle size of the ZnWO₄ component compared to the ZnO, which reduced the surface area available for interfacial transfer of the photogenerated charge carriers.

Mechanochemical synthesis of nanocrystalline SnO2-ZnO photocatalysts

Mechanochemical processing of anhydrous chloride precursors with Na₂CO₃ has been investigated as a means of manufacturing nanocrystalline SnO₂ doped ZnO photocatalysts. High-energy milling and heat-treatment of a 0.1SnCl₂+0.9ZnCl₂+Na₂CO₃+4NaCl reactant mixture was found to result in the formation of a composite powder consisting of oxide grains embedded within a matrix of NaCl. Subsequent washing with deionized water resulted in removal of the NaCl matrix phase and partial hydration of the oxide reaction product with the consequent formation of ZnSn(OH)₆. The extent of this hydration reaction was found to decrease in a linear fashion with the temperature of the post-milling heat-treatment over the range of 400–700 °C. For a heat-treatment temperature of 700 °C, the SnO₂ doped ZnO powder was found to exhibit significantly higher photocatalytic activity than either single-phase SnO₂ or ZnO powders that were synthesized using similar processing conditions. The heightened photocatalytic activity of the SnO₂ doped ZnO was attributed to its higher specific surface area and the enhanced charge separation arising from the coupling of ZnO with SnO₂.

Conducting composite using an immiscible polymer blend matrix

Carbon black (CB) fillers were used to study the feasibility of achieving multiple percolation using an immiscible (polar) polymer blend matrix. By tailoring the morphology of the insulating dual phase matrix it has been shown that the percolation threshold (Ф_c) can be reduced over single-phase matrices. Cocontinuity in the polymer matrix is important in reducing Ф_c by either preferentially isolating the conducting filler at the interface of the two phases or within one particular continuous phase of the matrix thereby forming a continuous conducting network within a continuous network (multiple percolation). Actual melt processing time has been found to influence the dispersion of the fillers and hence Ф_c. Polarity of the matrix as well as the processing method has also been found to influence the dispersion of the filler within the host polymer.

Lithium doped N-methyl-N-ethylpyrrolidiniumbis(trifluoromethanesulfonyl)amide fast-ion conducting plastic crystals

The incorporation of dopant levels of lithium ions (0.5 to 9.3% by mole) in the N-methyl-N-ethylpyrrolidinium bis(trifluoromethanesulfonyl)amide (P₁₂TFSA) plastic crystalline phase results in increases in the solid state ionic conductivity of more than 3 orders of magnitude at 298 K. Conductivities as high as 10^−-4 S cm⁻¹ at 323 K have been measured in these doped plastic crystal phases. These materials can therefore be classified as fast-ion conductors. Higher levels of Li only marginally increase the conductivity, up to around 33 mol%, followed by a slight decrease to 50 mol%. Thermal analysis behaviour has allowed the partial development of the binary phase diagram for the LiTFSA–P₁₂TFSA system between 0–50 mol% LiTFSA, which suggests the presence of a solid solution single phase at concentrations less than 9.3 mol% LiTFSA. There is also strong evidence of eutectic behaviour in this system with a eutectic transition temperature around 308 K at 33 mol% LiTFSA. A model relating ionic conduction to phase behaviour in this system is presented. The increased conductivity upon doping has been associated with lithium ion motion via⁷Li solid state NMR linewidth measurements.

Microstructural characterization and mechanical properties of Mg-Zr-Ca alloys prepared by hot-extrusion for biomedical applications

This paper investigated the microstructural characterization and mechanical properties of Mg-Zr-Ca alloys prepared by hot-extrusion for potential use in biomedical applications. Mg-Zr-Ca alloys were fabricated by commercial pure Mg (99.9%), Ca (99.9%), and master Mg-33% Zr alloy (mass%). The microstructural characterization of the hot-extruded Mg-Zr-Ca alloys was examined by X-ray diffraction analysis and optical microscopy, and the mechanical properties were determined from tensile tests. The experimental results indicate that the hot-extruded Mg-Zr-Ca alloys with 1 mass% Ca are composed of one single phase and those alloys with 2 mass% Ca consist of both Mg2Ca and α phase. The hot-extruded Mg-Zr-Ca alloys exhibit equiaxed granular microstructures and the hot-extrusion process can effectively increase both the tensile strength and ductility of Mg-Zr-Ca alloys. The hot-extruded Mg-1Zr-1Ca alloy (mass%) exhibits the highest strength and best ductility among all the alloys, and has much higher strength than the human bone, suggesting that it has a great potential to be a good candidate for biomedical application.

Real solutions number of the switching angles in the selective harmonic eliminated PWM

With the purpose of solving the real solutions number of the nonlinear transcendental equations in the selective harmonic eliminated PWM (SHEPWM) technology, the nonlinear transcendental equations were transformed to a set of polynomial equations with a set of inequality constraints using the multiple-angle formulas, an analytic method based on semi-algebraic systems machine proving algorithm was proposed to classify the real solution number of the switching angles. The complete classifications of the real solution number and the analytic boundary point of the single phase and three phases SHEPWM inverter with switch points of N=3 and the single phase SHEPWM inverter with switch points of N=4 are obtained. The results indicate that the relationship between the modulation ratio and the real solution number can be demonstrated theoretically by this method, which has great implications for the solution procedure of switching angles and the improvement of harmonic elimination effects of the inverter.

Synthesis and characterization of nanocrystalline hexagonal boron carbo-nitride under high temperature and high pressure

A study of the synthesis of hexagonal boron carbo-nitride (h-BCN) compounds via a two-step high-temperature and high-pressure (HTHP) technique using melamine (C 3N 6H 6) and boron oxide (B 2O 3) as raw materials is presented. An amorphous BCN precursor was prepared at 1000K under vacuum in a resistance furnace and then single-phase h-BCN nanocrystalline was synthesized at 1600K and 5.1GPa in a multi-anvil apparatus. X-ray diffraction (XRD) and transmission electron microscopy (TEM) indicated that the final products were pure h-BCN crystals with the lattice constants a ≤ 0.2510nm and c ≤ 0.6690nm. The average grain size was about 150nm. X-ray photoelectron spectroscopy (XPS) results confirmed the occurrence of bonding between C-C, C-N, C-B and N-B atoms. Raman scattering analysis suggested that there were three strong Raman bands centered at 1359, 1596 and 1617cm -1, respectively. The band at 1617cm -1 was considered to be consistent with the characteristic Raman peak of h-BCN.

Plastic crystal phases with high proton conductivity

The addition of up to 4 mol% of the strong acids, trifluoromethane sulfonic acid (TfOH) and bis-trifluoromethanesulfonyl imide [HN(Tf) 2], to the organic ionic plastic crystal (OIPC) [Choline][DHP] has been shown to dramatically increase the ionic conductivity by up to three orders of magnitude whilst still retaining the crystalline structure of the OIPC matrix. This enhanced proton diffusivity led to a significant proton reduction reaction in the electrochemical measurements. Powder XRD and DSC thermal analyses strongly suggest that these mixtures are single phase, crystalline materials. The work here also confirms that an increase in TfOH acid concentration (8 mol% and 12 mol%) results in a higher content of the amorphous phase as previously observed for the H 3PO 4/[Choline][DHP] system. © 2012 The Royal Society of Chemistry.

Microstructure evolution and softening processes in hot deformed austenitic and duplex stainless steels

The microstructure evolution and softening processes occurring in 22Cr-19Ni-3Mo austenitic and 21Cr-10Ni-3Mo duplex stainless steels deformed in torsion at 900 and 1200 °C were studied in the present work. Austenite was observed to soften in both steels via dynamic recovery (DRV) and dynamic recrystallisation (DRX) for the low and high deformation temperatures, respectively. At 900 °C, an "organised", self-screening austenite deformation substructure largely comprising microbands, locally accompanied by micro-shear bands, was formed. By contrast, a "random", accommodating austenite deformation substructure composed of equiaxed subgrains formed at 1200 °C. In the single-phase steel, DRX of austenite largely occurred through straininduced grain boundary migration accompanied by (multiple) twinning. In the duplex steel, this softening mechanism was complemented by the formation of DRX grains through subgrain growth in the austenite/ferrite interface regions and by large-scale subgrain coalescence. At 900 °C, the duplex steel displayed limited stress-assisted phase transformations between austenite and ferrite, characterised by the dissolution of the primary austenite, formation of Widmanstätten secondary austenite and gradual globularisation of the transformed regions with strain. The softening process within ferrite was classified as "extended DRV", characterised by a continuous increase in misorientations across the sub-boundaries with strain, for both deformation temperatures.

Review of transformerless inverter topologies

In the recent years, there has been a widespread trend in the use of the transformerless inverters in the single-phase grid-connected photovoltaic systems. This has been motivated by the low price, minor size, light mass, and great efficiency associated with them. However, when no transformer is used, common-mode voltage appears and leads to ground leakage current through the Photovoltaic parasitic capacitance to the ground due to the non-galvanic isolation configuration. This has lead to a main research focus on ways of minimizing or eliminating these leakage currents without influencing the efficiency of the system. Different topologies have been suggested to eliminate the leakage current in the transformerless grid-connected photovoltaic (PV) systems. In this paper, a review of these transformerless topologies is carried out, in regard to the leakage current for single phase. The benefits, in regard to each scheme, are outlined and their disadvantages explained.

The study of metal nitride layer formation by solid-state reactions

A new solid-state reaction to form metal nitrides has been investigated. It was confirmed that single phase chromium nitride is formed by a solid-state diffusion reaction between iron nitride and chromium chloride powders at temperatures between 570-700°C. The discovered reaction can be applied to form chromium nitride coatings on tool steels for metal forming applications.