在电网换相换流器高压直流输电(linecommutated converter based high voltage direct current,LCC-HVDC)逆变侧交流母线处并联静止同步补偿器(staticsynchronous compensator,STATCOM),可有效调节交流母线电压从而抑制换相失败。然而,...在电网换相换流器高压直流输电(linecommutated converter based high voltage direct current,LCC-HVDC)逆变侧交流母线处并联静止同步补偿器(staticsynchronous compensator,STATCOM),可有效调节交流母线电压从而抑制换相失败。然而,连接弱受端交流电网时,STATCOM与LCC控制系统之间的耦合作用会对整个系统的小扰动稳定性产生负面影响,甚至导致系统小信号失稳。该文采用小信号分析方法,将含STATCOM的LCC-HVDC非线性系统在稳定运行点处线性化,而后推导出能够定量衡量系统小扰动稳定性的二次型指标,并以此为目标函数,利用蒙特卡洛(Monte Carlo)方法对多组关键控制参数进行优化。二次型指标、特征根分析及PSCAD/EMTDC的电磁暂态仿真结果表明,采用优化后的控制参数,系统在低短路比下能够稳定运行,小扰动稳定性明显增强。该文提出的控制参数优化思路,同样适用于其他系统,具有一定的学术研究和工程应用价值。展开更多
Elegant Ince-Gaussian beams, which are the exact solutions of the paraxial wave equation in a quadratic-index medium, are derived in elliptical coordinates. These kinds of beams are the alternative form of standard In...Elegant Ince-Gaussian beams, which are the exact solutions of the paraxial wave equation in a quadratic-index medium, are derived in elliptical coordinates. These kinds of beams are the alternative form of standard Ince-Gaussian beams and they display better symmetry between the ]nce-polynomials and the Gaussian function in mathematics. The transverse intensity distribution and the phase of the elegant Ince Gaussian beams are discussed.展开更多
Aims Understanding the relative importance of historical and environ-mental processes in the structure and composition of communities is one of the longest quests in ecological research.Increasingly,researchers are re...Aims Understanding the relative importance of historical and environ-mental processes in the structure and composition of communities is one of the longest quests in ecological research.Increasingly,researchers are relying on the functional and phylogeneticβ-diversity of natural communities to provide concise explanations on the mechanistic basis of community assembly and the drivers of trait variation among species.The present study investigated how plant functional and phylogeneticβ-diversity change along key environmental and spatial gradients in the Western Swiss Alps.Methods Using the quadratic diversity measure based on six functional traits-specific leaf area,leaf dry matter content,plant height,leaf carbon content,leaf nitrogen content and leaf carbon to nitrogen content alongside a species-resolved phylogenetic tree-we relate variations in climate,spatial geographic,land use and soil gradients to plant functional and phylogenetic turnover in mountain commu-nities of the Western Swiss Alps.Important Findings Our study highlights two main points.First,climate and land-use factors play an important role in mountain plant community turnover.Second,the overlap between plant functional and phy-logenetic turnover along these gradients correlates with the low phylogenetic signal in traits,suggesting that in mountain land-scapes,trait lability is likely an important factor in driving plant community assembly.Overall,we demonstrate the importance of climate and land-use factors in plant functional and phyloge-netic community turnover and provide valuable complementary insights into understanding patterns ofβ-diversity along several ecological gradients.展开更多
In this paper we study the relation of trajectories and limit cycles between index-inverse differential systems, especially, index-inverse quadratic differential systems.
This paper is concerned with the effects of redundant control inputs on the quadratic optimal performance index in finite-time optimal control.Actually,the effects of redundant control input on quadratic performance i...This paper is concerned with the effects of redundant control inputs on the quadratic optimal performance index in finite-time optimal control.Actually,the effects of redundant control input on quadratic performance index is equal to study the effects of the redundant control input on the solution of Riccati differential equation(RDE).Hence,some sufficient conditions are presented to strictly decrease the solution of RDE with the column of input matrix increased.Especially,a necessary and sufficient condition is derived to strictly decrease the solution of the homogeneous RDE after control input extensions.Moreover,one sufficient condition is obtained,which guarantees that the minimum control energy index strictly decreases with input extension.It is shown that redundant control inputs can improve the system performance based on these results.Some examples are taken to illustrate the effectiveness of related problems.展开更多
Suppose F = Q(√-p1 pt) is an imaginary quadratic number field with distinct primes p1,..., pt,where pi≡ 1(mod 4)(i = 1,..., t- 1) and pt ≡ 3(mod 4). We express the possible values of the 8-rank r8 of the class grou...Suppose F = Q(√-p1 pt) is an imaginary quadratic number field with distinct primes p1,..., pt,where pi≡ 1(mod 4)(i = 1,..., t- 1) and pt ≡ 3(mod 4). We express the possible values of the 8-rank r8 of the class group of F in terms of a quadratic form Q over F2 which is defined by quartic symbols. In particular,we show that r8 is bounded by the isotropy index of Q.展开更多
文摘在电网换相换流器高压直流输电(linecommutated converter based high voltage direct current,LCC-HVDC)逆变侧交流母线处并联静止同步补偿器(staticsynchronous compensator,STATCOM),可有效调节交流母线电压从而抑制换相失败。然而,连接弱受端交流电网时,STATCOM与LCC控制系统之间的耦合作用会对整个系统的小扰动稳定性产生负面影响,甚至导致系统小信号失稳。该文采用小信号分析方法,将含STATCOM的LCC-HVDC非线性系统在稳定运行点处线性化,而后推导出能够定量衡量系统小扰动稳定性的二次型指标,并以此为目标函数,利用蒙特卡洛(Monte Carlo)方法对多组关键控制参数进行优化。二次型指标、特征根分析及PSCAD/EMTDC的电磁暂态仿真结果表明,采用优化后的控制参数,系统在低短路比下能够稳定运行,小扰动稳定性明显增强。该文提出的控制参数优化思路,同样适用于其他系统,具有一定的学术研究和工程应用价值。
基金Project supported by the National Natural Science Foundation of China (Grant Nos.10904041 and 10674050)the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No.20094407110008)the Specialized Research Fund for Growing Seedlings of the Higher Education in Guangdong Province,China (Grant No.C10087)
文摘Elegant Ince-Gaussian beams, which are the exact solutions of the paraxial wave equation in a quadratic-index medium, are derived in elliptical coordinates. These kinds of beams are the alternative form of standard Ince-Gaussian beams and they display better symmetry between the ]nce-polynomials and the Gaussian function in mathematics. The transverse intensity distribution and the phase of the elegant Ince Gaussian beams are discussed.
基金Fellowship grant from the Faculty of Biology and Medicine University of Lausanne,Switzerland.
文摘Aims Understanding the relative importance of historical and environ-mental processes in the structure and composition of communities is one of the longest quests in ecological research.Increasingly,researchers are relying on the functional and phylogeneticβ-diversity of natural communities to provide concise explanations on the mechanistic basis of community assembly and the drivers of trait variation among species.The present study investigated how plant functional and phylogeneticβ-diversity change along key environmental and spatial gradients in the Western Swiss Alps.Methods Using the quadratic diversity measure based on six functional traits-specific leaf area,leaf dry matter content,plant height,leaf carbon content,leaf nitrogen content and leaf carbon to nitrogen content alongside a species-resolved phylogenetic tree-we relate variations in climate,spatial geographic,land use and soil gradients to plant functional and phylogenetic turnover in mountain commu-nities of the Western Swiss Alps.Important Findings Our study highlights two main points.First,climate and land-use factors play an important role in mountain plant community turnover.Second,the overlap between plant functional and phy-logenetic turnover along these gradients correlates with the low phylogenetic signal in traits,suggesting that in mountain land-scapes,trait lability is likely an important factor in driving plant community assembly.Overall,we demonstrate the importance of climate and land-use factors in plant functional and phyloge-netic community turnover and provide valuable complementary insights into understanding patterns ofβ-diversity along several ecological gradients.
文摘In this paper we study the relation of trajectories and limit cycles between index-inverse differential systems, especially, index-inverse quadratic differential systems.
基金supported by the National Natural Science Foundation of China under Grant Nos.61174038 and 61203129
文摘This paper is concerned with the effects of redundant control inputs on the quadratic optimal performance index in finite-time optimal control.Actually,the effects of redundant control input on quadratic performance index is equal to study the effects of the redundant control input on the solution of Riccati differential equation(RDE).Hence,some sufficient conditions are presented to strictly decrease the solution of RDE with the column of input matrix increased.Especially,a necessary and sufficient condition is derived to strictly decrease the solution of the homogeneous RDE after control input extensions.Moreover,one sufficient condition is obtained,which guarantees that the minimum control energy index strictly decreases with input extension.It is shown that redundant control inputs can improve the system performance based on these results.Some examples are taken to illustrate the effectiveness of related problems.
基金supported by China Postdoctoral Science Foundation(Grant No.2013M541064)National Natural Science Foundation of China(Grant No.11371043)National Basic Research Program of China(Grant No.2013CB834202)
文摘Suppose F = Q(√-p1 pt) is an imaginary quadratic number field with distinct primes p1,..., pt,where pi≡ 1(mod 4)(i = 1,..., t- 1) and pt ≡ 3(mod 4). We express the possible values of the 8-rank r8 of the class group of F in terms of a quadratic form Q over F2 which is defined by quartic symbols. In particular,we show that r8 is bounded by the isotropy index of Q.
