双馈风电机的虚拟惯性控制优化策略

电力系统在面对不同扰动时,双馈风力电机采用传统虚拟惯性控制不能充分利用转子动能参与调频,提出一种改进的虚拟惯性控制策略。对现有的风机传统虚拟惯性控制方法中的控制增益进行改进,即将控制增益与频率偏差建立耦合关系,使双馈风力电机根据系统频率偏差实时调整调频增发功率,减缓频率跌落速度,提高频率最低点。基于EMTP-RV仿真平台搭建含高比例风电渗透率的电力系统模型,验证改进方法的有效性。仿真结果表明,提出的改进虚拟惯性控制方法在不同扰动下均可大幅改善频率稳定性,且随扰动增大,改善效果越明显。     

Internal electrical fault detection techniques in DFIG-based wind turbines: a review

The keys factor in making wind power one of the main power sources to meet the world’s growing energy demands is the reliability improvement of wind turbines (WTs). However, the eventuality of fault occurrence on WT com ponents cannot be avoided, especially for doubly-fed induction generator (DFIG) based WTs, which are operating in severe environments. The maintenance need increases due to unexpected faults, which in turn leads to higher operating cost and poor reliability. Extensive investigation into DFIG internal fault detection techniques has been carried out in the last decade. This paper presents a detailed review of these techniques. It discusses the methods that can be used to detect internal electrical faults in a DFIG stator, rotor, or both. A novel sorting technique is presented which takes into consideration different parameters such as fault location, detection technique, and DFIG modelling. The main mathematical representation used to detect these faults is presented to allow an easier and faster under standing of each method. In addition, a comparison is carried out in every section to illustrate the main differences, advantages, and disadvantages of every method and/or model. Some real monitoring systems available in the market are presented. Finally, recommendations for the challenges, future work, and main gaps in the field of internal faults in a DFIG are presented. This review is organized in a tutorial manner, to be an effective guide for future research for enhancing the reliability of DFIG-based WTs.     

Aqueous solution of Oxone as new oxidizing agent for multi-walled carbon nanotube functionalization

In this work, multi-wall carbon nanotube (MWCNT) was successfully modified using aqueous solution of Oxone as a new oxidant. The effect of oxidation temperature on various characteristics of the treated MWCNTs was also investigated. FTIR and titration analysis proved the formation of carboxyl, carbonyl and epoxide groups at the surface of MWCNTs. The concentration of the functional groups increased as the modification temperature increased. The presence of such oxygen containing groups at the surface of MWCNTs justified the long time stability of the treated MWCNTs suspensions in water and methanol. The modified MWCNTs showed higher entanglement compared to row MWCNT due to the cross-links adjacent effect of pendant functional groups. Finally, it was concluded that Oxone oxidation process destroys the structure of the MWCNTs, but not severe enough to unzip the MWCNTs.     

A Phase‐Separation Route to Synthesize Porous CNTs with Excellent Stability for Na+ Storage

Porous carbon nanotubes (CNTs) are obtained by removing MoO₂ nanoparticles from MoO₂@C core@shell nanofibers which are synthesized by phase‐segregation via a single‐needle electrospinning method. The specific surface area of porous CNTs is 502.9 m² g^?1, and many oxygen‐containing functional groups (C? OH, C?O) are present. As anodes for sodium‐ion batteries, the porous CNT electrode displays excellent rate performance and cycling stability (110 mA h g^?1 after 1200 cycles at 5 A g^?1). Those high properties can be attributed to the porous structure and surface modification to steadily store Na⁺ with high capacity. The work provides a facile and broadly applicable way to fabricate the porous CNTs and their composites for batteries, catalysts, and fuel cells.     

Wave Groups in Uni-Directional Surface-Wave Models

Uni-directional wave models are used to study wave groups that appear in wave tanks of hydrodynamic laboratories; characteristic for waves in such tanks is that the wave length is rather small, comparable to the depth of the layer. In second-order theory, the resulting Nonlinear Schrödinger (NLS) equation for the envelope of the wave group contains the dispersion of the group velocity multiplying the linear term and a gen-coefficient that results from mode generation multiplying the nonlinear term. The signs of these coefficients determine whether experimentally relevant wave groups are possible or not. If the dispersion is modelled in such a way that it is correct for all wave lengths for infinitesimal waves, relevant wave groups are obtained consisting of constituent waves with a certain maximal wave length; other models for the dispersion (such as in the KdV-equation) lead to different results.     

针对隐性需求的产品设计研究

随着经济、技术的发展,消费者的需求变得日渐复杂。很多深层次的需求,连消费者自身都不能明确表达。本文就是在这样的背景下,研究消费者隐性需求产生的原因和不同阶段,提出了借鉴群体文化学的方法,并对其步骤进行了分析,希望设计能更好的满足人的需要。     

分组移动对象的区域偏离监视算法

以大量可分组移动对象的区域偏离监视问题为研究背景,扩展了基于区域覆盖的空间索引结构——虚拟网格四分树,提出了一种新的索引结构VGQ_G。基于VGQ_G,实现了区域偏离监视算法RDMA;给出了监视算法的性能对比实验。实验结果表明,在移动对象数量足够大的时候,设计的监视算法比朴素的监视算法在响应时间上提高了一个数量级。     

On properties not inherited by monoids from their Schützenberger groups

We give an example of a monoid with finitely many left and right ideals, all of whose Schützenberger groups are presentable by finite complete rewriting systems, and so each have finite derivation type, but such that the monoid itself does not have finite derivation type, and therefore does not admit a presentation by a finite complete rewriting system. The example also serves as a counterexample to several other natural questions regarding complete rewriting systems and finite derivation type. Specifically it allows us to construct two finitely generated monoids M and N with isometric Cayley graphs, where N has finite derivation type (respectively, admits a presentation by a finite complete rewriting system) but M does not. This contrasts with the case of finitely generated groups for which finite derivation type is known to be a quasi-isometry invariant. The same example is also used to show that neither of these two properties is preserved under finite Green index extensions.     

Soft groups and normalistic soft groups

Soft set theory, proposed by Molodtsov, has been regarded as an effective mathematical tool to deal with uncertainties. In this paper, first we correct some of the problematic cases in a previous paper by Akta? and Çag˜man [H. Akta?, N. Çag˜man, Soft sets and soft groups, Inf. Sci. 177 (2007) 2726-2735]. Moreover, we introduce the concepts of normalistic soft group and normalistic soft group homomorphism, study their several related properties, and investigate some structures that are preserved under normalistic soft group homomorphisms.     

Understanding risk-taking behavior of groups: A “decision analysis” perspective

Past research has extensively investigated the effect of media, especially focusing on how anonymity increases risk-related behaviors of groups when using computer-mediated communication (CMC). This study extends prior research by examining the differences in group risk-taking behaviors between face-to-face groups and completely non-anonymous CMC groups (i.e., groups working in a fully identified, synchronous CMC environment similar to popular instant messaging systems utilized widely within organizations). Drawing on the “decision analysis” perspective, a key framework for understanding organizational decision-making, the study also examines the effects of the firm's risk preferences as well as the type of information distribution among group members (i.e., full information known to all group members versus partial information know by only some of the members) on the groups' risk-taking behaviors. Results from a laboratory experiment using student subjects found no differences in risk-taking behaviors between CMC and face-to-face groups; additionally, no differences were found related to how information was distributed among group members. A significant effect was found, however, for the risk preference of the firm, showing that risk-neutral firms influenced groups to make riskier decisions than groups from risk-averse firms. Finally, groups within risk-neutral firms receiving partial information made riskier decisions than groups receiving full information. The implications of these results for future research and practice are examined.