Slow dynamics simulation of power systems

This paper investigates the simulation of slow dynamics in two-time-scale power systems. A new approach is proposed to obtain the slow dynamics by projecting the trajectory of the post-fault system onto its slow manifold. This is achieved by a nonlinear projection of the full order system initial condition onto the slow manifold, such that the fast intraarea dynamics are not excited. A projection scheme is developed and applied to two-test power systems.     

Phase dynamics and radiated power of dc SQUIDS

The time variation of the quantum phase difference and the radiated power of two resistively shunted junctions in a superconducting loop are analyzed using a first harmonic approximation method. It is shown explicitly that an output coupling impedance connected across the junctions plays an important role in determining the output power as well as the cutoff frequency in the radiation spectrum. The radiated power of an array of dc SQUIDs is compared with the power output of a synchronized series array in a resonant cavity. WithN superconducting weak links coupled to a fixed load, the SQUID array suffers an output power saturation for largeN; our analysis shows that it is very difficult to obtain radiated power higher than a few microwatts. On the other hand, a cavity-coupled large array gives an output power proportional toN with a narrow linewidth, suggesting that it could be very useful in practice.     

Generation control of interconnected power systems using computational intelligence techniques

Basic power systems are steadily growing and have become large and more complex with interconnections to neighbouring systems. A novel approach is presented for steady-state analysis of generation control, incorporating load and generation characteristics, which gives nearly exact practical solution for large interconnected power systems during normal operating conditions and under disturbance. It also assists planning and operation engineers to observe the steady-state behaviour of a network disturbances, such as loss of generation and loss of load. Adopting decoupled Newton-Raphson method, steady-state frequency deviation (DeltaF) and tie-line error (DeltaP_tie)are estimated for the disturbance. In the paper, the knowledge of frequency deviation and tie-line error are handled to produce the required change in generation using both a fuzzy-logic controller and an artificial-neural-network controller separately.     

Power losses of Nb3Ge-clad tapes in superposed ac and dc magnetic fields

50 Hz power losses of Nb₃Ge-Cu substrate and Cu-coated tapes in parallel ac and dc fields of comparable magnitude have been studied. From the observed minimum in energy dissipation it is shown that the material exhibits a considerable surface barrier for initial flux entry into the superconductor.     

Influence of load diversity exchange on preventive maintenance scheduling in interconnected power systems

Preventive maintenance scheduling in a power system can be performed in a number of ways. One approach is to levelize the risk throughout the study period. This paper presents a practical approach to risk levelization. These concepts are extended to maintenance scheduling in two interconnected systems. Power system interconnection can have a significant effect on the maintenance schedules of the connected systems. This effect will depend on the individual system parameters such as generating unit sizes, forced outage rates, and on the interconnection capabilities and availabilities. The individual load characteristics in each system also have a major impact on both the isolated system and interconnected configuration maintenance schedules. This paper illustrates the influence of load diversity exchange on the preventive maintenance schedule in two interconnected systems using the risk levelization approach. The concepts are examined using the IEEE Reliability Test System.     

ac losses of superconducting composites with 8 μm NbTi filaments in a dc magnetic field with a superimposed ac component at 1 f 500 Hz

Measurements of the ac losses of two superconducting coils, made from low-loss filamentary superconductors, are reported for a coil current consisting of a dc and a superimposed ac component. The matrix losses and hysteresis losses in the superconductor are experimentally and theoretically analysed, especially their frequency and amplitude dependence.     

Parallel processing in dynamic simulation of power systems

The dynamic behaviour of a large interconnected electric power system is characterized by a simultaneous set of nonlinear algebraic and ordinary differential equations. The solution is obtained by numerical methods and the simulation of the transient behaviour for a few seconds after a fault is the standard analytical procedure used in planning and operational studies of the system. The need for on-line simulation in near real time for more efficient operation has encouraged the search for faster solution methods and the use of parallel computers for this purpose has attracted the attention of many researchers. The success of parallelization depends on three factors: the problem structure, the computer architecture, and the algorithm that takes maximum advantage of both. In this problem, the generator equations are only coupled through the electrical network providing some parallelization in (variable) space, and a solution is needed at each time step leading to some parallelization in time (waveform relaxation). However, since the problem formulation is not completely decoupled, parallel algorithms can only be developed by trading off any relaxation with a degradation in convergence. The fastest sequential algorithm used today is the combination of implicit trapezoidal integration with a dishonest Newton solution. The Newton algorithm is not parallel at all but has the fastest convergence while a Gauss-Jacobi algorithm is completely parallel but converges very slowly. A relaxation of the Newton algorithm appears to be a good compromise. As for the parallel hardware, the coupling seems to require significant communication between processors thus favouring a data-sharing architecture over a message-passing hypercube. Special architectures to match the problem structure have also been an area of investigation. This paper elaborates on the above issues and assesses the present state-of-the-art.     

Structural origin of enhanced slow dynamics near a wall in glass-forming systems

Spatial confinement is known to induce a drastic change in the viscosity, relaxation times, and flow profile of liquids near the glass (or jamming) transition point. The essential underlying question is how a wall affects the dynamics of densely packed systems. Here we study this fundamental problem, using experiments on a driven granular hard-sphere liquid and numerical simulations of polydisperse and bidisperse colloidal liquids. The nearly hard-core nature of the particle-wall interaction provides an ideal opportunity to study purely geometrical confinement effects. We reveal that the slower dynamics near a wall is induced by wall-induced enhancement of 'glassy structural order', which is a manifestation of strong interparticle correlations. By generalizing the structure-dynamics relation for bulk systems, we find a quantitative relation between the structural relaxation time at a certain distance from a wall and the correlation length of glassy structural order there. Our finding suggests that glassy structural ordering may be the origin of the slow glassy dynamics of a supercooled liquid.     

Recent advances in simulation of power electronics converter systems

This paper presents a comprehensive state-of-the-art of the recent advances in simulation of power electronic converter systems. Knowing the importance of simulation, this paper reviews the various methods of modelling, circuit analysis approaches, numerical techniques etc. Several general purpose simulators and dedicated power electronic simulators have been discussed. A few demonstrative examples of simulation of power electronic converters by using different simulators are provided. Practical difficulties in simulation, challenges, new developments and scope for future work are also discussed.     

Virtual simulation of five-axis machine tool with consideration of CNC interpolation,servo dynamics,friction, and geometric errors

Abstract

A virtual machine tool (VMT) simulation system which considers tool center point (TCP) interpolation, geometric errors, servo dynamics, and friction effects for a five-axis machine tool is developed in this paper. A novel five-axis interpolation method is proposed to ensure that maximum velocity constraints for each axis can be satisfied. The geometric error model, including lead screw, straightness, angular and squareness errors is built to analyze the volumetric errors within the working space. The model which includes rigid body motion, friction model and servo control loops is utilized to evaluate servo dynamics and non-linear effects. The errors caused by the locations, servo dynamics, and friction effects are integrated into the VMT simulation program. Simulation results of TCP trajectory are represented by small line segments to generate NC codes. Then the NC codes are fed into VERICUT software to perform virtual cutting. To evaluate the interpolation design, cutting experiments are carried out on the five-axis engraving machine with a PC-based controller. The performances using the proposed interpolation method are comparable with the commercial CNC controller from Heidenhain. The effects of different error sources on the surface are demonstrated by cutting the sculpture of a human face. Path overcut caused by servo dynamics is found at sharp corners, and volumetric errors cause obvious tool marks and poor surface roughness. The proposed methodology can serve as a useful tool in evaluating the behaviors of error sources during the design stage.     

石墨纳米片/聚乙烯复合物分子动力学模拟

通过分子动力学模拟对石墨纳米片（GNP）/聚乙烯（PE）复合物的结构、力学和气体输运性质进行计算研究,分析其随模拟温度和GNP填充量的变化规律,探讨纳米界面形成、复合机制及结构与特性的关系。GNP/PE复合物呈现二维结构,GNP趋向于平面取向排列并通过范德华力和纳米石墨片层表面上的碳氢-π键使周围几个原子尺度内的PE分子固化为有序原子层,而PE基体仍然为各向同性的无定形结构。GNP/PE界面上纳米复合作用使体系能量降低,与PE体系相比,GNP/PE的杨氏模量和泊松比分别显著增高和降低。GNP平面取向导致GNP/PE的力学特性表现出二维各向异性的弹性常数张量,在石墨纳米片层平面方向上的杨氏模量明显增高,并且随温度的降低和GNP填充量的提高而增大,填充GNP有效改善了GNP/PE的力学性质。GNP/PE复合物的气体输运性质明显受到填充GNP的气体阻隔和取向的影响并且对3种气体渗透没有明显的选择性。GNP与基体的纳米复合导致N₂、O₂和CO₂的分子输运呈现二维各向异性,随着石墨纳米颗粒填充量的增加,取向GNP层面方向的扩散系数比垂直方向高5~8倍,可用于气体分子屏障与渗流控制。     

Practical molecular dynamic simulation of monolayer graphene with consideration of structural defects

Molecular dynamic simulation method has been employed to consider the mechanical properties of pristine and defected armchair and zigzag monolayer graphene sheets under tension and compression. Effects of Stone Wales (SW), Cell deformed (CD) and cell vacancy (CV) defects on ultimate stress and strain of graphene sheets have been considered. Obtained results indicate that zigzag graphene sheet has higher elastic modulus and stiffness than armchair type. Three SW defects have the highest destructive effect on ultimate stress and strain of zigzag graphene sheet while CV defect reveals the highest reducing effect on those of armchair type.     

计算流体力学对膜生物反应器水力学特征的模拟研究

通过计算流体力学(CFD)中的FLUENT软件,对膜生物反应器内流场和流态进行模拟,定量给出了其流速与剪切力的分布.研究了膜组件距离反应器底部不同距离时的水力学情况和水力学条件.对比了膜生物反应器中,玻璃纤维编织管式膜组件与平板膜组件,由于构型不同所导致的水力学条件差异.结果表明,从雷诺数和质量流速率参数分析,玻璃纤维编织管式膜组件的水力学条件更好,更有利于膜污染的防治.探讨了单个与多个曝气器情况下的流速分布,结果表明,后者流速分布更为均匀.由本文研究结果可知,计算流体力学可以作为一个有力的工具应用于MBR及其膜污染控制研究中.     

碳纳米管/聚乙烯复合物分子动力学模拟研究

采用分子动力学方法模拟了碳纳米管/聚乙烯复合物的结构、热力学和力学特性,分析其随模拟温度和碳纳米管填充率的变化。模拟结果表明,碳纳米管/聚乙烯复合物为各向同性的无定形结构,聚乙烯和碳纳米管通过较强的范德华作用结合在一起,在聚乙烯基体作用下,碳纳米管壁上的碳原子排列的周期性下降,出现弯曲和褶皱。从能量上看,填充率较高的复合物更加稳定。碳纳米管/聚乙烯复合物具有比聚乙烯体系更高的等容热容和与聚乙烯体系相反的负值热压力系数,热容随碳纳米管填充率的变化较小,但随温度的升高而明显减小,具有显著的温度效应;热压力系数随温度的变化较小,温度稳定性比聚乙烯更好,但随填充率增加而减小。碳纳米管/聚乙烯复合物的力学特性表现出各向同性材料的弹性常数张量,弹性模量和泊松比比纯聚乙烯体系高得多,并且都随温度的升高和碳纳米管含量的降低而减小,说明加入碳纳米管可显著改善聚乙烯的力学性质。     

Modelling and transient simulation of CO2-refrigeration systems with Modelica

This paper presents the current results of the development of a Modelica library for CO₂-refrigeration systems based on the free Modelica library ThermoFluid. The development of the library is carried out in a research project of Airbus Deutschland and the TUHH and is focused on the aim to obtain a library for detailed numerical investigations of refrigeration systems with the rediscovered refrigerant carbon dioxide (CO₂). A survey of the concept of an integrated on-board cooling system of airliners, the modelling language Modelica™ and the CO₂-library is given and the modelling of CO₂-heat exchangers is described. A comparison with steady state results of heat exchangers shows a fair agreement. The presented transient simulation results are compared with experimental data showing also a fair agreement.     

Analysis and implementation of an active snubber dc/dc converter with series?parallel connection in primary and secondary sides

An active snubber dc/dc converter to achieve zero voltage switching (ZVS) on power switch is presented. In the proposed converter, the primary windings of two transformers are connected in series so that the primary currents of the two transformers are equal. The secondary sides of the isolated zeta converters are connected in the parallel to share the load current and reduce the current stresses on the secondary windings of the two transformers. A boost type of active snubber is connected in parallel with the main switch to recycle the energy stored in transformer leakage and magnetizing inductors and to limit voltage stress of the main switch. During the transition interval between the active switch and the auxiliary switch, the resonance based on the resonant inductor and the output capacitor of the power switch will allow the switch to turn on at ZVS. The principle of operation, steady-state analysis and design consideration of the proposed converter are provided. Finally, experimental results for a 360 W (12 V/30 A) prototype circuit with 150 kHz switching frequency were given to demonstrate the circuit performance and verify the feasibility of the proposed converter.     

Dependence of dc and ac conduction on the pre-sintering temperature in sol–gel derived Ba0.5Sr0.5TiO3 thin films

The effect of pre-sintering temperature on the activation energies of dc and ac conduction in sol–gel spin-coated Ba_0.5Sr_0.5TiO₃ (BST) thin films is reported in this paper. Layer by layer coating of BST sol on Pt/TiN/SiO₂/Si substrates was done with varying pre-sintering temperatures (drying temperature for individual layers). Dominant mechanism of dc conduction was identified as Schottky process whereas ac conduction was governed by trap controlled hopping process. The activation energy for dc conduction decreased from 0.45 to 0.38 eV with increase in the pre-sintering temperature from 400 to 600 °C; whereas, that for ac conduction increased from 0.17 to 0.21 eV. The results have been correlated with grain growth phenomena and reduction of oxygen vacancies in the films with increase in the pre-sintering temperature.     

Stability theory of differential/algebraic models of power systems

Lyapunov stability results are given for differential/algebraic models of power systems which include the effect of generator damping and nonlinear loads. The global dynamical structure of such a system is studied in terms of multivalued energy functions defined on so-called ‘voltage causal regions’ where voltage behaviour is predicted from angle behaviour. These regions are separated by ‘impasse surfaces’ related to singularity in the load flow equations.     

含有柔性电缆的复杂系统装配仿真

将装配单元划分为“刚性组件”和“柔性电缆”,从装配对象建模、装配工艺规划和装配过程模拟三个方面对复杂系统的装配仿真方法进行系统的研究。首先,根据刚性组件和柔性电缆的不同特点,给出相应的建模方法,即针对刚性组件的CAD三维建模方法和针对柔性电缆的虚拟布线方法;然后,将装配过程视为拆卸过程的逆过程,提出含有柔性电缆的复杂系统的装配工艺规划方法,其中考虑了电缆形变过程及相应的模拟方法;最后,以装配工艺为基础对仿真对象进行装配仿真。实例表明,所提出的方法可以用于含有柔性电缆的复杂系统装配仿真。     

硅锗超晶格薄膜界面热传导的分子动力学模拟

采用非平衡态分子动力学方法(NEMD)对超晶格薄膜导热系数与界面晶格失配的关系进行了研究,并就晶格失配使导热系数产生极小值的原因进行了分析.模拟是在加温度梯度的系统中进行的,固定周期长度;其中采用热流修正,分层考虑的思想使系统稳定.模拟结果揭示了,当温度固定时,界面处晶格失配对超晶格薄膜热传导性能的下降有贡献.结果中也发现,当失配程度增大到一定值后,热传导系数出现极小值,经初步分析为声子传播模式不同引起.