Search

Montmorillonite is a major mineral present in ion-adsorption rare earth ores, and the microscopic adsorption states of rare earth ions on its surface are of a great significance for the efficient exploitation of ion-adsorption rare earth ores. In this article, density functional theory calculations were used to investigate the adsorption mechanisms and bonding characteristics of hydrated Pr, Mg and NH4 ions on the (001) surface of montmorillonite. Pr3+ exhibited a directed tendency geometry with Pr(H2O)103+, which was adsorbed onto montmorillonite by hydrogen bonding with an adsorption energy of −1182 kJ/mol, and one coordinated H2O ligand was separated from the first hydration layer of Pr. Both hydrated Mg and NH4 ions were adsorbed onto the montmorillonite surface through hydrogen bonds, and the adsorption energies were −206 and −188 kJ/mol, respectively, indicating that the adsorption stability of the hydrated Mg ion was slightly higher than that of the hydrated NH4 ion, but both were lower than that of hydrated Pr (−1182 kJ/mol). Hence, higher concentrations of Mg and NH4 ions than rare earth ions would be necessary in the leaching process of ion-adsorption rare earth ores. Additionally, desorption experiments revealed that the recovery of Pr3+ by Mg2+ with a concentration of 38 mmol/L is 80%, while it is only 65% with the same concentration of NH4+, and the concentrations of Mg2+ and NH4+ were much higher than that of Pr3+ in lixivium, which is consistent with the DFT calculations.

We consider a mass conservative type method for semiconductor device problem by employing mixed finite element method (FEM) for electric potential equation, mass conservative characteristic FEM for both electron and hole density equations. The boundedness of numerical solution without certain time step restriction and optimal L^2 error estimates of full discrete scheme are proved. Numerical experiment is presented to verify the effectiveness and unconditional stability of the proposed method.

In this paper, a multi-point flux mixed-finite-element decoupled method was considered for the compressible miscible displacement problem. For this compressible problem, a fully discrete backward Euler scheme was proposed, in which the velocity and pressure equations were decoupled by a multi-point flux MFE method using BDM1 elements combined with a trapezoidal quadrature rule. The concentration equation was handled by a standard FE method. The error analysis for velocity, pressure, and concentration were rigorously derived. Numerical experiments to verify the convergence rates and simulate the miscible displacement problem of a water–oil system were presented.

Once an asymmetrical fault occurs on the AC side of the receiving-end of a high-voltage direct current (HVDC) transmission system, the current reference will be affected by the control regulation on the DC inverter side and the commutation voltage asymmetry. In this case, the advance firing angle will fluctuate periodically, causing security threats to the system. If the fault cannot be cleared in time, the effect may be even more serious. However, the traditional proportional-integral (PI) controller cannot effectively suppress the periodic components in the input error signal, which is an important cause of continuous commutation failure. Thus, the system requires more time to recover from the fault. Motivated by this, a self-adaptive auto-disturbance rejection PI controller is proposed in this study. The controller has the advantages of fast response speed and strong anti-interference ability of the auto-disturbance rejection controller. On one hand, it can automatically adjust PI, and the parameters can maintain the system's adaptive ability. On the other hand, the discretization process satisfies the computer simulation requirements. By applying the proposed controller to a system under constant current control and extinction angle control, the dynamic response speed can be improved and the robust performance of the system can be ensured when dealing with a wide range of perturbations. Finally, simulation results show that the proposed algorithm can effectively suppress the continuous commutation failure of DC transmission systems.

In the existing research, most of the heterogeneous catalysts applied in the activation of persulfate to degrade organic pollutants were synthesized from chemical reagents in the laboratory. In this paper, we have obtained a spent iron ore (IO) residue directly collecting from the iron ore plants, and efficiently activating peroxydisulfate (PS) to produce reactive free radicals. The experimental results demonstrated that the IO could effectively activate PS to degrade tetracycline hydrochloride (TCH), with TCH removal rate reaching up to 85.6% within 2 h at room temperature. The TCH removal rate was increased with increasing iron ore dosage, while the more acidic pH condition would be favorable to TCH removal process. The material characterization results demonstrated that the dominant components of IO were Fe

Iron-based catalysts are widely used in water treatment and environmental remediation due to their abundant content in nature and their ability to activate persulfate at room temperature. Here, eggshell biochar-loaded natural iron slag (IO@ESBC) was successfully synthesized to remove tetracycline hydrochloride (TCH) by activated persulfate. The morphology, structure and chemical composition of IO@ESBC were systematically characterized. The IO@ESBC/PS process showed good performance for TCH removal. The decomposition rate constant (k) for IO@ESBC was 0.011 min−1 and the degradation rate was 3690 mmol/g/h in this system. With the increase of PS concentration and IO@ESBC content, the removal rate of TCH both increased. The IO@ESBC/PS process can effectively remove TCH at pH 3–9. There are different effects on TCH removal for the reason that the addition of water matrix species (humic acid, Cl−, HCO3−, NO3− and HPO42−). The IO@ESBC/PS system for degrading TCH was mainly controlled by both the free radical pathway (SO4•−, •OH and O2•−) and non-free radical pathway (1O2). The loading of ESBC slows down the agglomeration between iron particles, and more active sites are exposed. The removal rate of TCH was still above 75% after five cycles of IO@ESBC. This interesting investigation has provided a green route for synthesis of composite driving from waste resources, expanding its further application for environmental remediations.

GEO optical remote sensing satellites need to avoid sunlight entering the camera optical system at midnight hours. For the existing evasion methods such as thermal doors and posture planning, nearly 15% of the satellite in-orbit life cannot be reasonably utilized due to the passive evasion methods. To solve this problem, a method based on mission planning is proposed. When evasion begins, the satellite changes the orbital coordinate system into the inertial coordinate system and maintains the safe attitude in inertial space. If space missions are planned such as camera calibrations and celestial bodies observations, the algorithm can determine the compliance of the missions with attitude constraints and energy constraints, and the eligible missions are executed. When evasion ends, the satellite maneuvers toward the Earth proceeding with Earth observation missions. The mathematical simulation indicates the method is feasible, ensuring the safety of the optical camera and executing the space missions meanwhile. The mission planning method extends relatively the service life of the satellite and makes the expansion of the space observation field possible.

Herein, we discuss a mixed finite element method applied to the Brinkman equation of fluid motion in porous medium, which covers a field of situation from the Darcy equation to the Stokes problem associated with various perturbation parameters. The finite element based on staggered meshes is shown to be stable and effective with each case as the corresponding error estimates for both velocity and pressure are established. Finally, we present numerical examples confirming the theoretical analysis and the stability of the finite spaces approximation.

Considering the investment cost and operation safety, an improved fault current limiter (FCL) for dc distributed network based on self-clearing converters is proposed in this paper. Before self-clearing converters being blocked, dc fault current can be automatically limited, which provides enough time for selective protection to identify fault lines and reduce the action speed requirements on mechanical switches. During normal operation, the proposed FCL does not deteriorate the transient response characteristics of dc system. The circuit design, control strategy and protection are simple. The threshold values on the voltage, current and capacity of power electronic devices are low, which greatly reduces the cost and conducting loss, because the power of dc reactor is divided from the dc cable.

In the case of faults, the output current of the inverter interfaced distributed generator (IIDG) is related to the type of fault, the location of the fault, and the control strategy of the IIDG. This will cause the traditional current protection scheme to be unsuitable for distribution networks containing IIDG. To solve this problem, this paper proposes a quick-break protection scheme combining distance protection and adaptive current protection. The downstream line of IIDG and adjacent feeder adopt adaptive current protection. The upstream line of IIDG uses distance protection. The scheme does not require the use of IIDG information, and the protection setting can be achieved only by using the information of the protection point. Finally, the correctness of the protection scheme is verified through simulation.

It is one of the main development directions of wind power generation in the future that wind farms are connected to the grid using VSC-HVDC. VSC-HVDC system can supply power to passive networks. When the wind farm withdraws from operation due to a fault, the wind power grid-connected system can be regarded as the power supply from the power grid to the passive network via the flexible system. Based on the different tasks undertaken by the grid-side converter station and the wind farm-side converter station, this paper analyzes and designs the control strategies of the converter stations on both sides. Besides, the control strategy of the converter station on the wind farm side is improved because of the system oscillation problem caused by the initial wind power grid connection and the power overrun problem that may occur during the process of the system changing to the passive network power supply state. Finally, the corresponding simulation model is built based on the Matlab/Simulink simulation platform. It can be seen from the simulation results that the VSC-HVDC system can be smoothly switched from the wind power grid connection state to the passive network power supply state. The above results prove the effectiveness of the control strategy used in this paper and provide a certain reference value for related research.

In this paper, a worst-case margin algorithm is proposed to deal with the stability margins of Multi-Input Multi-Output(MIMO) linear time-invariant interval systems. Firstly, the Kharitonov’s Theorem and V. Krishnamurthi’s corollary on Routh criterion are used to prove the stability of MIMO linear interval systems with affine parametric uncertainty structure. In particular, it can be used to analyze the stability regions of interval systems with parameters as well Secondly, each eigenvalue function of the MIMO linear interval systems is regarded as a Single-Input Single-Output(SISO) linear time-invariant system. Then the frequency domain approach and optimization algorithms are applied to develop a computationally tractable algorithm for calculating the stability margins. Moreover, this method can be extended to the non-minimum phase system. Finally, the engineer relevance examples are presented to validate the effectiveness and accuracy of the proposed method.

The combination of photovoltaic (PV) power generation and voltage source converter-based high voltage direct current (VSC-HVDC) system is one of the development directions of new energy power generation and its power transmission in the future. Power grid failure or maintenance will lead to power outage. In order to reduce the economic loss caused by this, the PV-VSC-HVDC coordinated control system is used to continuously supply power to loads in island state to maintain its normal operation. By analyzing the mechanism and cause of the islanding effect, a coordinated control strategy suitable to PV-VSC-HVDC system is proposed. Finally, a PV-VSC-HVDC grid-connected system was built based on Matlab/Simulink, and a detailed simulation analysis of the operating characteristics of the system is made. The simulation results show that this control strategy can realize the smooth switch from grid-connected state to island state, ensuring the stable operation of the loads and reducing the economic loss caused by power failure.

We consider a numerical scheme for incompressible miscible displacement problem in porous media. A multipoint flux mixed finite element method is used to handle the velocity–pressure equation. The standard finite element method is used to approximate the concentration equation. Error estimates for pressure and velocity and concentration are presented. Numerical experiments show that the convergence rates of this scheme are in agreement with the theoretical analysis.

σ phase is one of the main precipitates affecting the toughness of austenitic stainless steel, V-notch impact test, SEM, EDS and TEM analysis were conducted on the newly developed 22Cr15Ni3.5Cu stainless steel after 650°C aging. Precipitation mechanism of σ phase and its effect on the toughness of the material were analyzed. The test results show that toughness of the material decreases to 25.6J after 300h aging, σ phase started to precipitate along the grain boundary after 500h aging, and in the crystal after 1000h aging. The precipitation spacing is about 100 nm, forming a gradually increasing size from crystal to grain boundary. As the precipitation time 500h of σ phase was later than the critical aging time of ductile brittle transition, it can be inferred from the test result that σ phase is not the main precipitation phase affecting the toughness of 22Cr15Ni3.5Cu.

The conventional synchronous generator (SG) can regulate the active and reactive power for frequency and voltage stability, when the system fluctuates. In renewable energy generation system (RES), the stability problem of RES can be solved by adding virtual inertia and damping in the control strategy of the grid-connected inverter. The grid-connected inverter and renewable energy system with this kind of control strategy, called as virtual synchronous generator (VSG), acts as a synchronous generator. In this paper, the topology of grid-connected PV array and energy storage system is proposed. The VSG used in grid-connected inverter, is modelled as stator and rotor model by stator voltage equation and rotor motion equation, respectively. Meanwhile, the VSG control mechanism, based on droop control and model of VSG, can be divided as active power-frequency and reactive power-voltage control. Besides, the system stability is analyzed based on small signal model, for the design of inertia and damping. The simulation model is designed by MATLAB/Simulink for the implementation of VSG. In simulation, it is compared that the dynamic regulation performance of VSG with different control coefficient, such as virtual inertia, virtual damping, droop coefficient and integral coefficient. The optimization for regulation performance in simulation provides an appropriate selection method of the control coefficient for VSG.

Based on the theory of information entropy concerning nonlinear errors, the growth rules for the nonlinear errors of the Lorenz system and its predictable components are studied. The results show that the impact of the uncertainties, both in the initial error and in the system itself, needs to be considered in a quantitative estimation of the system predictability. The nonlinear error growth is related to the magnitude of the initial error, and to the spatial distribution of the initial error vectors. Even if these initial errors have the same magnitude but different directions, there are also differences in the nonlinear error growth. The predictability of nonlinear error growth is related to the error component, but not related to the ratio of these components. The component with the highest/lowest rate of contribution does not necessarily have the greatest/least predictability. The different components have different predictabilities, and in different time periods, the different predictable components also have different predictabilities.

In recent years, many distributed generators (DGs) and DC loads have been continuously connected to power grid. The DC distribution network (DCDN) has become the trend of power grid development in the future because of its strong controllability, high reliability, and the ability to coordinate the access of DGs and multiple loads. In this paper, typical DGs are modeled and the typical net structure and working strategy are introduced. Based on the PSCAD platform, a radial DC net model with typical DGs is built. The operating states of various devices are analyzed by example analysis.

Combing demonstration project of medium and low voltage DC distribution and consumption system in Suzhou, protection configuration scheme and fault location method for DC distribution system is studied. First, according to the grid structure, the protection areas of DC power distribution and consumption system are partitioned. Next, based on the fault characteristics of DC power distribution system, the protection configurations of each protected area are proposed. Finally, according to the characteristics of DC loop system, the protection criterion and fault location method are presented. The comprehensive fault location method based on differential protection and networked multi-point information is proposed. The correctness of protection strategy and fault location method is verified by RTDS.

In a modular multilevel converter (MMC) system, the grid asymmetry on AC side will cause problems like non-sinusoidal 3-phase current on AC side of MMC converter, fluctuation of active and reactive power, and large fluctuation of double frequency current and voltage on DC side. In order to solve these problems, a differential-based positive and negative sequence separation is proposed, which has limited delay and high precision after grid asymmetry occurring. Then the fluctuation form on AC-side power is analyzed according to the instantaneous power theory. Secondly, in conjunction with the control objectives of suppressed negative sequence current and instantaneous active power, theoretical analysis on the bridge arm circulating current is carried out, which obtains the expression of given current in the inner loop under the corresponding control objectives. Finally, MMC simulation model is established based on PSCAD/EMTDC, then the simulation verification for two control objectives under asymmetric grid is carried out respectively.

The Budyko hypothesis has been widely used to describe precipitation partitioning at the catchment scale. Many empirical and analytical formulas have been proposed to describe the Budyko hypothesis. Based on dimensional analysis and mathematic reasoning, previous studies have given an analytical derivation, i.e., the Mezentsev-Choudhury-Yang (MCY) equation. However, few hydrological processes are involved in the derivation. Note that similar to electrical circuits and atmospheric motions, this study tried to give a new derivation of the Budyko hypothesis based on an analogy of the Ohms-type approach and the homogeneity assumption. The derived equation has the same form as the MCY equation but has a more physical explanation than the mathematic reasoning proposed in previous studies. In addition, under conditions without the homogeneity constraint, a more general expression is E=P(b+kE)0) [Pn+(b+kE)0)n](1/n), where E, E0 and P are evaporation, potential evaporation and precipitation, respectively, and n, k and b are constants.