A generalized analytical model based on multistage scattering phenomena for estimating the impact ionization rate of charge carriers in semiconductors

A generalized analytical model based on multistage scattering phenomena has been developed in this paper for estimating the impact ionization rate of charge carriers in semiconductors. The probabilities of impact ionization initiated by electrons and holes have been calculated separately by taking into account all possible combinations of optical phonon scattering and carrier-carrier collisions prior to the impact ionization. Finally the analytical expressions of impact ionization rate of electrons and holes have been developed by using the aforementioned impact ionization probabilities. The impact ionization rates of electrons and holes in 4H-SiC have been calculated within the field range of \(2.5\times 10^{8}\) – \(6.5\times 10^{8}\hbox { V m}^{-1}\) by using the analytical expressions of those developed in the present paper. Those are also calculated by using the analytical expressions developed by some other researchers earlier without considering the multistage scattering phenomena. Finally the theoretical results obtained from the analytical model proposed in this paper and the analytical model developed by earlier researchers within the field range under consideration have been compared with the ionization rate values calculated by using the empirical relations fitted from the experimentally measured data. Closer agreement with the experimental data has been achieved when the impact ionization rate of charge carriers in 4H-SiC are calculated from the proposed model as compared to the earlier one.     

A molecular model to evaluate the impact of aging on space chargesin polymer dielectrics

The understanding of space charge effects depends largely on how concurrent phenomena, such as thermal and electrical aging are understood. A model describing thermal and environmental aging of dielectric polymers is discussed. The model essentially is based on the rate theory associated with the name of Eyring. One basic feature of the model is that the proper energy term is a free energy of activation ΔG, which implies that the entropy change of the process cannot be neglected. Several examples are given to show that ΔG is related directly to some physical or chemical parameter involved in thermal or environmental aging. A model describing the mechanical aging of polymers is also discussed. It is shown that submicrocavities can be generated by mechanical stresses above a given critical level, the value of which depends on the energy of cohesion of the polymer. The size of these defects are in good agreement with the Griffith criterion. Finally, the same model adapted to electrical stresses is shown to describe the electrical aging of polyethylene. It predicts that the most deleterious influence of aging is above a given critical field (similar to the critical stress in mechanical aging), where submicrocavities are induced by the electromechanical forces associated with the ac field. Electrons can then move without scattering within the submicrocavities and this may lead to further degradation. The relations between thermal, mechanical and electrical aging and space charges in polymers are addressed briefly and domains requiring more work are mentioned     

基于瞬时无功功率理论求取电压闪变参数的新方法

基于统计评价的指标只能定量地给出闪变的强弱或严重程度,而无法说明闪变的具体参数特征。提出了一种对电压闪变信号调制的频率和幅值进行计算的有效方法。对单相电压信号向后延迟60°可以得到三相电压,并用三相电压代替三相电流,利用瞬时无功功率理论算出瞬时有功,进而计算出电压信号包络线,利用高精度FFT分析取得的包络信号,可以计算出闪变调制的频率和幅值。仿真结果表明该方法求取平稳和非平稳的电压闪变信号的参数的有效性。     

A molecular model to evaluate the impact of aging on space chargesin polymer dielectrics [comment and reply]

For original paper see J.P. Crine, ibid., vol.4, pp.487-95, 1997. The original paper by Crine provides further consideration and experimental data in support of the already well-known model for the electrical/thermal/mechanical aging of polymeric insulation proposed by the same author some years ago. This paper comments on certain aspects of Crine's model. A reply by Crine is included     

Localization technique research of a pipeline robot based on the magnetic-dipole model

The magnetic field distribution of an emission antenna is studied in this paper. When the slenderness ratio of the emission antenna is high, the emission antenna can be simplified as a magnetic dipole for practical application. The numerical results of the magnetic dipole magnetic field show that the magnetic magnitude distribution has a hump-shape, whose direction is perpendicular with the antenna axis direction. A localization method based on the hump-shape signal detection is presented. The experimental result shows that the precision can reach a value of ±5 cm. The method can be used to localize a pipeline robot working in a metal pipe. __________ Translated from Chinese Journal of Radio Science, 2006, 21(4): 553–557 [译自: 电波科学学报]     

A Bayesian hierarchical model for DCE-MRI to evaluate treatment response in a phase II study in advanced squamous cell carcinoma of the head and neck

Object

Pharmacokinetic parameters from dynamic contrast-enhanced MRI (DCE-MRI) were used to assess the perfusion effects due to treatment response using a tyrosine kinase inhibitor. A Bayesian hierarchical model (BHM) is proposed, as an alternative to voxel-wise estimation procedures, to test for a treatment effect while explicitly modeling known sources of variability.

Materials and methods

Nine subjects from a randomized, blinded, placebo-controlled, multicenter, phase II study of lapatinib were examined before and after treatment. Kinetic parameters were estimated, with an extended compartmental model and subject-specific arterial input function, on a voxel-by-voxel basis.

Results

The group treated with lapatinib had a decrease in median K ^trans of 0.17min^??, when averaged across all voxels in the tumor ROIs, compared with no change in the placebo group based on nonlinear regression. A hypothesis test of equality between pre- and posttreatment K ^trans could not be rejected against a one-sided alternative (P = 0.09). Equality between median K ^trans in placebo and lapatinib groups posttreatment could also not be rejected using the BHM (P = 0.32). Across all scans acquired in the study, estimates of K ^trans at one site were greater on average than those at the other site by including a site effect in the BHM. The inter-voxel variability is of similar order (within 15%) when compared to the inter-patient variability.

Conclusion

Though the study contained a small number of subjects and no significant difference was found, the Bayesian hierarchical model provided estimates of variability from known sources in the study and confidence intervals for all estimated parameters. We believe the BHM provides a straightforward and thorough interrogation of the imaging data at the level of voxels, patients or sites in this multicenter clinical study.     

基于湍流模型定制的风机塔架结构动力特性分析

介绍湍流风场模型中常用的达文波特功率谱密度模型,根据随机振动理论,确定利用达文波特速度谱模拟风速湍流,并应用有限元软件ANSYS实现风力机塔架模型的建立及湍流风场的数值模拟。通过在风场中对风机塔架进行结构动力特性分析,得到塔架前32阶振动频率及各阶模态、节点速度变化值及节点湍流强度,数值模拟结果符合结构振动规律,可为工程实际提供参考。     

基于等效面磁荷的开放式永磁磁路解析计算与有限元仿真验证

对永磁体外部包括铁磁性试件内的磁场进行空间解析计算在工程应用中具有重要意义,为此研究放置式和穿过式开放永磁磁路模型解析计算.采用等效面磁荷法对上述两种模型中的永磁体进行等效,并应用截断区域特征函数展开法和传递矩阵求解磁标位方程,得到了两种模型中包含铁磁材料和空气在内的全空间磁场分布.为简化计算,对穿过式开放永磁磁路模型中的环形径向磁化永磁体用改进的面磁荷模型进行等效.与有限元仿真结果的对比显示:两种解析模型中磁感应强度沿不同坐标轴方向分量的最小相对误差小于3％,在穿过式开放永磁磁路模型中磁感应强度分量最大相对误差约为20％,表明改进的面磁荷模型是基本可行的.     

Development of an analytical model for generation expansion planning as a tool to provide guidelines for preventing instability in the long‐term electricity market

This paper considers the problem of deciding multiperiod investments for generation expansion planning (GEP) in restructured power systems. This problem has presented a challenge for both market managers and suppliers regarding the stability in the electricity market and minimum income for suppliers over the planning period. In this paper, an analytical model for studying the GEP problem from the viewpoint of a central management entity is presented. The aim of this method is to establish a dynamic balance between energy supply and demand by adjustment of GEP over the horizon of planning so that not only the expected profit is provided for all new generating plants but the long‐term stability in the electricity market is also improved. This analytical model can be utilized by regulatory bodies to obtain some guidelines and thereby to set their policies for improving GEP and preventing instability in the long‐term electricity market. To do so, in this study, the uncertainties of demand and supply have been modeled through two stochastic processes. Furthermore, the market price dynamics and their mutual effects on the GEP's results have been considered. Finally, this nonlinear dynamic optimization problem is solved using a modified genetic algorithm (GA). The efficiency and ability of the proposed method are examined on a test power system. © 2011 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Floquet scattering matrix approach to the phase noise of a single-electron source in the adiabatic regime

We give the basic elements of the Floquet scattering matrix approach (Moskalets in Scattering Matrix Approach to Non-stationary Quantum Transport. Imperial College Press, London, 2011) to the dynamic quantum transport in mesoscopic and nanoscopic conductors. We use the scattering formalism to discuss the noise power spectrum of a single electron source working in the adiabatic regime and emitting particles into a chiral electron waveguide. The noise power is found to be quadratic at low frequencies and exponentially suppressed at high frequencies.     

Influence of space charge buildup on the transition to electricaltreeing in PE under ac voltage

Our investigation is focused on the understanding of electrical aging in polymeric materials under 50 Hz ac voltage. In this report, a needle electrode molded into the insulant to simulate defects producing local field enhancement is used. Special emphasis is given to low density polyethylene. The transition between the discharge-free electroluminescent state to micropartial discharge (PD) state (early electrical tree propagation phase) is studied with a sensitivity reaching 0.01 pC. Optical diagnosis is carried out simultaneously. At the moment of tree initiation, electrical discharges from 0.04 to 0.1 pC occur in the positive half cycle. A very small electrical tree of 5 μm was observed. Using the phase angle of the first measurable PD with respect to the applied voltage offers additional information helping to understand tree initiation. Then a correlation between the magnitude of electrical discharges and the characteristic traces of local breakdown (electrical tree) is reported. We proposed an interpretation based on the similarity with grounding tree experiments in which the initiation of a local breakdown is mainly due to a strong Poisson field radiated by a space charge region in the vicinity of the needle tip     

基于层次分析法的可中断负荷模型参与备用市场的研究

阐述了可中断负荷的基本概念,讨论了影响可中断负荷成本的因素,提出通过层次分析法来建立可中断负荷的模型,并以电网公司的角度从安全性和经济性两方面来考虑选择可断负荷,通过算例证明了该模型的合理性。     

基于改进IMC的磁头微焊点寿命预测模型研究

磁头无铅微焊点寿命预测方法主要采用疲劳模型,由于磁头焊点十分小,传统的寿命预测方法很难取得较好效果,本文融合金属间化合物生长动力学理论,提出了一种新的用于预测磁头无铅微焊点寿命的生长模型。对比结果表明,新模型预测磁头无铅微焊点寿命与实际结果更加接近。     

Verification and expansion of a method of transient analysis of voltage collapse based on the induction motor load model

Voltage instability is now under intensive study in Japan, and some dynamic methods for analyzing voltage collapse have already been presented. However, the suitability of load models used in the methods and the generality of the results have not been verified. In this paper, useful voltage collapse data acquired in real power systems are presented. The measured data are compared with the result of dynamic digital simulation to verify the suitability of an induction motor load model. In the latter half of this paper, dynamic simulations of voltage collapse with consideration of generator dynamics are carried out. An interconnected power model system with 10 nodes is analyzed. The generality of basic characteristics concerning voltage collapse which have already been obtained without considering generator dynamics is verified by such simulations.     

电磁场对高速运动电荷的作用力-洛伦兹力公式的新形式

百年以前,由洛伦兹建立的电磁场对电荷的作用力公式－－洛伦和公式,是在低速情况下得到的,后来被推广到了任意运动速度,本文根据一般电磁场理论信了运动电荷的电磁场特性,认为该公式只适用于低速情况,对高速运动电荷不再适用。并在此基础上导出了适用于任意速度的新的洛伦兹力公式,它在低速情况下转化为通常的洛伦兹力公式。     

基于改进PSO算法的Logistic模型在饱和负荷预测中的应用

将改进的粒子群(PSO)算法应用到饱和电力负荷预测中,通过与Logistic时间序列预测模型相结合,对Logistic曲线函数进行优化参数求解。建立了基于该优化算法的Logistic时间序列饱和负荷预测模型,利用某地区电网历史数据进行Logistic时间序列分析。仿真结果表明,该改进算法收敛速度快,全局寻优能力强,克服了传统PSO算法局部搜索能力较差、容易陷入局部最优的缺点。利用它得到的Logistic拟合曲线,相对于传统PSO算法和Marquardt迭代算法的拟合结果,精度有明显的提高,说明该模型能够很好地反映电力负荷整体变化趋势。另外,运用该模型和人均用电量法分别对某地区电网饱和全社会用电量进行预测,结果显示两者预测结果较为接近,而人均用电量法在饱和电力负荷预测中运用已较为成熟,因此可以证明该模型应用到饱和电力负荷预测中是可行的。     

Risk assessment of urban network planning in china based on the matter-element model and extension analysis

The practice of urban power network planning in China is facing more and more complex risks. The nature of the risks themselves is unclear and uncertain. In order to ensure that urban power networks are planned in a scientific manner, a reasonable risk assessment needs to be performed. Based on extension analysis and the matter-element extension theory, a model of risk evaluation designed to be applied to urban power network planning is put forward in this paper. This model has the advantage that it conveniently quantifies the qualitative indices. At the same time, it also overcomes the main limitation of the matter-element method, namely, that if the risk index exceeds the risk grading standard, it can not be calculated using the correlation function. Finally, taking a specific urban power network as an example, the empirical analysis results show that this model is credible and practical.     

基于模糊聚类的神经网络模型及其在渗流分析中的应用

本文采用模糊聚类理论方法对因子集进行模糊聚类 ,然后利用神经网络的方法建立样本因子集类别变量特征值与样本观测值之间的预测模型 ,提出了将模糊聚类、模糊模式识别以及神经网络三者有机结合的预测理论。并通过某大坝渗流计算实例对传统的统计预报模型和基于模糊聚类的神经网络预测模型进行了比较 ,结果表明后者的预报精度比前者要高。     

基于竞价上网的发电侧与售电侧峰谷分时电价联合优化模型

电价联动是继续推进电力市场化改革迫切需要解决的问题之一。针对当前峰谷分时电价的实施现状,在综合考虑需求侧响应及有关主体利益平衡的基础上,以优化负荷曲线为目标函数,建立了发电侧与售电侧峰谷分时电价联合设计的优化模型。