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Solid state transformer is a new type of power equipment which is expected to replace the traditional distribution transformer. It can satisfy the custom power demand of the renewable energy utilization and the high reliability of power supply. In intelligent distribution network with a large number of flexible equipment and impact load, the parallel operation of solid state transformers not only needs to consider the power distribution problem, but also needs to consider the power quality problems caused by these harmonic sources. An integrated droop control method is proposed in this paper, which not only realizes the active and reactive power distributed according to the capacity ratio, but also effectively suppress the harmonic current caused by the nonlinear load and ensure the power quality of the low-voltage AC bus. Matlab/Simulink simulation results show the validity and advantage of the proposed method, compared with the classic droop control method.

In recent years, engineering constructions increase rapidly in western and central areas of China, where expansive soil widely distributes. Since expansive soil is sensitive to water content, the characterization of its shear strength should be carefully conducted. For simplicity and ease of use, the Mohr-Coulomb criterion is often adopted to describe the shear strength of expansive soil. In this paper, the physical meaning of the cohesion and frictional strength of expansive soil are explained, and the variations of the strength parameters with water content are investigated. By fitting to the experimental results from direct shear test and triaxial tests, the changing characteristics of cohesion and friction angle with water content are obtained.

Many investigations have reported threshold switching (TS) effects in amorphous semiconductors, but it is rarely observed in BiFeO3 nanoparticles. Here, the effects of cobalt doping on the electrical conductivity of Bi0.9Nd0.1Fe1−xCoxO3 (x = 0, 0.01, 0.03, 0.05) nanoparticles were investigated. Leakage current density (J) versus applied voltage (V) curves were measured and it was found that the leakage current increases with increases of the cobalt dopant concentration. With the increase of the maximal applied voltage, the samples with cobalt doping exhibit nearly symmetric TS behaviors at room temperature. With the increase of cobalt dopant concentration, the optical band gap is decreased, and the smallest optical band gap value (∼2.01 eV) is observed for the 3% Co doped BiFeO3 sample. X-ray photoelectron spectroscopy of the samples shows that the cobalt dopant induces an enhancement of the oxygen vacancy concentration. Possible mechanisms for the threshold switching effects are discussed on the basis of a conductive filament model. The mobile charge carriers align to form conducting filaments when the threshold voltage (VTH) is achieved, corresponding to the transition from a high-resistance state to a low-resistance state, and the rupture of the conductive path takes place when the applied voltage is decreased. The leakage current decreases dramatically and eventually returns to the initial high-resistance state.

The structural and conductive characteristics of La 0.2 Bi 0.8 Fe 1 − x Mg x O 3 ( x = 0, 0.01, 0.02, 0.03, 0.04, 0.05) ceramics synthesized by a modified rapid sintering process are investigated and discussed. There is no any impure phase even in La 0.2 Bi 0.8 Fe 0.95 Mg 0.05 O 3 ceramic, meanwhile, all samples show rhombohedral structure and the SEM micrograph shows that the ceramics are getting compact with the increase of Mg substitution. The AC impedance measurement reveals an obvious trend of transition from insulator to semiconductor upon doping Mg. When x = 0.01–0.04, a threshold switching effect can be observed and the threshold voltage shows strong dependence on the Mg dopant level. This work suggests an effective method to modulate the conductivity of BiFeO 3 -based multiferroic materials with purephase structure by codoping with La and Mg at Bi and Fe site respectively.

Highly aliovalent Na1+ ions were selected as the dopant to substitute Bi3+ ions in BiFeO3 (BFO) and Bi1−xNaxFeO3 (x = 0, 0.01, 0.03, 0.05) nanoparticles prepared via a facile sol–gel method. Weak ferromagnetism and an obvious exchange bias (EB) phenomenon without field cooling were observed in the samples. To establish the presence of EB in the nanoparticles, training effect (TE) data were analyzed using Binek's model. Moreover, with the increase in Na1+ content, the band gap was decreased, while interestingly, the leakage current density was significantly reduced and the smallest leakage current density (∼10−7 A cm−2) was observed for the 3% Na-doped BFO. The electrical conduction mechanism of samples was investigated by plotting log J versus log E. Oxygen vacancies decreased with the increase of Na content analyzed through X-ray photoelectron spectroscopy (XPS) measurements. To further explain the decrease of band gap and leakage current density with the increase of Na content, the interplay of oxygen vacancies and holes was analyzed and a phenomenological qualitative model based on the electronic energy band proposed.

The charge defective structure in Bi1−x Ca x FeO3 (CBFO, x = 0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3) nanoparticles (NPs) ranging from 140 to 25 nm as well as their relations to band gap and leakage current behavior are investigated. It is demonstrated that Ca doping effectively narrows the band gap from ~2.16 to ~2.02 eV, due to the appearance and accumulation of oxygen vacancy. Subsequently, enhanced electrical conductivity was obtained in these CBFO NPs, which leads to the appearance of a distinct threshold switching behavior in Ca-doped BFO NPs with higher conductivity at room temperature. Possible mechanisms for Ca doping effects on the electric conduction were discussed upon the interplay of NPs’ size effect and mobile charged defects on the basis of reduced particle size and the increased density of oxygen vacancy analyzed through X-ray photoelectron spectrum.

Single phase Bi 0.97 Ba 0.03 Fe 1-x Ta x O 3 ceramics with x = 0, 0.01, 0.03, 0.05 were synthesized by modified rapid sintering process method. The formation of rhombohedral perovskite-like structure was confirmed by X-ray diffraction investigation for all the samples. Dielectric and leakage current measurements indicated that the content of the oxygen vacancy in the samples decreased as a function of the substitution of Ta 5+ ions. A distinct threshold switching behavior was observed in the leakage current density. The impedance measurements suggested that the grain effect made a major contribution to the resistance. The changes in dielectric, multiferroic properties and resistance modulation of the Ta 5+ and Ba 2+ co-doped BiFeO 3 ceramics could have a huge potential for material application.

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