含分布式电源的城市电网故障分析方法研究

分布式电源大规模接入城市电网会给城市电网的故障特征带来一定的影响,其出力的随机性会引起故障特征的不确定。针对现有故障分析方法未考虑出力随机性的影响,提出一种基于蒙特卡洛法的含分布式电源的城市电网故障分析方法。首先,基于分布式电源出力的概率模型,利用蒙特卡洛法进行随机抽样;然后,通过PQ控制和低电压穿越策略下的故障模型进行计算,形成故障电压、短路电流的概率分布;最后,通过算例进行仿真分析。结果表明,分布式电源接入城市电网后故障特征具有概率特性,该方法计及分布式电源出力随机性的影响,可得故障电压、短路电流的概率分布,可对城市电网故障特性分析以及整定值的选取提供依据。     

The enhanced performance of piezoelectric nanogenerator via suppressing screening effect with Au particles/ZnO nanoarrays Schottky junction

This paper describes a novel strategy to weaken the piezopotential screening effect by forming Schottky junctions on the ZnO surface through the introduction of Au particles onto the surface. With this approach, the piezoelectric-energyconversion performance was greatly enhanced. The output voltage and current density of the Au@ZnO nanoarray-based piezoelectric nanogenerator reached 2 V and 1 μA/cm², respectively, 10 times higher than the output of pristine ZnO nanoarray-based piezoelectric nanogenerators. We attribute this enhancement to dramatic suppression of the screening effect due to the decreased carrier concentration, as determined by scanning Kelvin probe microscope measurements and impedance analysis. The lowered capacitance of the Au@ZnO nanoarraybased piezoelectric nanogenerator also contributes to the improved output. This work provides a novel method to enhance the performance of piezoelectric nanogenerators and possibly extends to piezotronics and piezophototronics.

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A study of Facebook Groups members’ knowledge sharing

There have been many studies focusing on individuals’ knowledge sharing behavior in the organizational setting. With the rapid prevalence of social networking sites, many people began to express their thoughts or share their knowledge via Facebook website. Facebook is an open environment which does not provide any immediate monetary benefits to its users. Its Groups members’ knowledge sharing behavior could be different from the ones in organizations. We proposed a research model to examine factors which promote the Facebook Groups users’ willingness to share knowledge. The factors in the study include extrinsic motivation, social and psychological forces, and social networking sharing culture. We used PLS to test our proposed hypotheses based on 271 responses collected through an online survey. Our results indicated that reputation would affect knowledge sharing attitude of Groups members and sense of self-worth would directly and indirectly (through subjective norm) affect the attitude. In addition, social networking sharing culture (fairness, identification, and openness) is the most significant factor, not only directly affecting knowledge sharing intention, but also indirectly influencing the sharing intention through subjective norm and knowledge sharing attitude.     

An Ontology-Based Approach to Metaphor Cognitive Computation

Language understanding is one of the most important characteristics for human beings. As a pervasive phenomenon in natural language, metaphor is not only an essential thinking approach, but also an ingredient in human conceptual system. Many of our ways of thinking and experiences are virtually represented metaphorically. With the development of the cognitive research on metaphor, it is urgent to formulate a computational model for metaphor understanding based on the cognitive mechanism, especially with the view to promoting natural language understanding. Many works have been done in pragmatics and cognitive linguistics, especially the discussions on metaphor understanding process in pragmatics and metaphor mapping representation in cognitive linguistics. In this paper, a theoretical framework for metaphor understanding based on the embodied mechanism of concept inquiry is proposed. Based on this framework, ontology is introduced as the knowledge representation method in metaphor understanding, and metaphor mapping is formulated as ontology mapping. In line with the conceptual blending theory, a revised conceptual blending framework is presented by adding a lexical ontology and context as the fifth mental space, and a metaphor mapping algorithm is proposed.     

FPGA中低噪声CCD时序驱动电路设计

在分析Toshiba公司TCD1209D型CCD工作原理的基础上,分析了驱动时序的关系,详细介绍了驱动电路的设计和实现方法.用Verilog语言设计了TCD 1209D的驱动时序控制电路;选用CyclonelⅣ系列FPGA器件,使用QuartusⅡ软件对设计电路进行了功能仿真,实现了TCD1209D的高速时序驱动;在CycloneⅣ芯片平台上测试了TCD 1209D的实际输出信号.实验结果显示,CCD信号噪声较小,验证了所设计驱动电路的可行性,确定了相关双采样的时刻和位置,为小型CCD测量系统的设计提供了有益参考.     

基于有用时序偏差的时序优化方法

提出了一种利用有用时序偏差来提高电路性能的方法,利用时钟偏差规划算法在时钟树综合之前对时序偏差重新调整规划,以提高电路的性能.使用ISCAS89作为实验对象来验证算法并进行了分析.     

A scalable network forensics mechanism for stealthy self-propagating attacks

Network forensics supports capabilities such as attacker identification and attack reconstruction, which complement the traditional intrusion detection and perimeter defense techniques in building a robust security mechanism. Attacker identification pinpoints attack origin to deter future attackers, while attack reconstruction reveals attack causality and network vulnerabilities. In this paper, we discuss the problem and feasibility of back tracking the origin of a self-propagating stealth attack when given a network traffic trace for a sufficiently long period of time. We propose a network forensics mechanism that is scalable in computation time and space while maintaining high accuracy in the identification of the attack origin. We further develop a data reduction method to filter out attack-irrelevant data and only retain evidence relevant to potential attacks for a post-mortem investigation. Using real-world trace driven experiments, we evaluate the performance of the proposed mechanism and show that we can trim down up to 97% of attack-irrelevant network traffic and successfully identify attack origin.     

UAVs surpassing satellites and aircraft in remote sensing over China

ABSTRACT

Many users are now showing strong interest in UAV RS (Unmanned Aerial Vehicle Remote Sensing) due to its easy accessibility. UAVs have become popular platforms for remote sensing data acquisition. In a number of practical and time constrained circumstances, UAV RS data have been widely used as a substitute for traditional satellite remote sensing data. However, airspace-related regulations are far behind the rapid growth in the number of UAVs and their wide applications. Much effort of the network-based UAV RS have been made by the UAV RS group of the Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences (IGSNRR, CAS), which proposed the concept of UAV RS data carrier. UAV RS data carrier refers to UAV RS data platform with task planning, data storage, image processing, product generation and output products for various UAVs. An ongoing effort to create a nationwide UAV RS network in addition to an existing ground observational network is being carried out in China.     

海潮模型在珠江口潮位预测中的精度评估

结合卫星测高技术的海潮模型能方便、精确地模拟大洋潮汐,可用于缺乏观测资料的地区。为评估该方法在珠江河口潮位预测应用中的精度,本文采用NAO.99b、OTPS和WWT等3种海潮模型预测珠江口不同验潮站的潮位过程,与实测数据对比并计算误差均方根。分析结果表明,海潮模型可以较好地预测珠江河口区潮汐基本时间特征,潮位预测精度在一定区域、一定时段不低于传统的预测方法。口门附近尤其口外的预测精度明显高于口内;在洪水期精度下降但口外开阔区域影响较小;在口门处,潮优型河口湾的预测精度高于径流型河口。3种模型中,推荐使用WWT。     

An iterative HAM approach for nonlinear boundary value problems in a semi-infinite domain

In this paper, we propose an iterative approach to increase the computation efficiency of the homotopy analysis method (HAM), a analytic technique for highly nonlinear problems. By means of the Schmidt–Gram process (Arfken et al., 1985)  [15], we approximate the right-hand side terms of high-order linear sub-equations by a finite set of orthonormal bases. Based on this truncation technique, we introduce the M$M$th-order iterative HAM by using each M$M$th-order approximation as a new initial guess. It is found that the iterative HAM is much more efficient than the standard HAM without truncation, as illustrated by three nonlinear differential equations defined in an infinite domain as examples. This work might greatly improve the computational efficiency of the HAM and also the Mathematica package BVPh for nonlinear BVPs.