http://hdl.handle.net/10397/900 Search the Channel search http://repository.lib.polyu.edu.hk/jspui/simple-search http://hdl.handle.net/10397/5704 Title: Applications of complex network science<br/><br/>Authors: Liu, Xiaofan<br/><br/>Abstract: Systems of inter-connected components can be modeled by networks. A network is a collection of "nodes" linked by "edges". The network models of large scale complex systems, e.g., social networks, transportation systems, engineering infrastructures, etc., have shown some non-trivial universal structural properties compared to regular graphs such as lattices or random graphs. These network models are hence referred to as complex networks. In this thesis the author analyzes the structure and dynamics of the complex networks abstracted from different systems, including arts, finance and social systems, in order to describe the functioning mechanisms and to optimize the functionalities of the systems. In Chapter 3, music compositions are analyzed from a complex network perspective. By abstracting music notes to network nodes and the co-occurrence of notes to edges between nodes, it is shown that different genres of music, ranging from pop to classic, western to oriental, all display universal network properties. It is shown that the theme of music is also embedded in the underlying complex networks of music compositions. Computer algorithms for automatic music generation are proposed based on a random walk process in the musical networks. Applying some additional rudimentary rhythmic constraints on the computer algorithms, appealing music can be generated.; In Chapter 4, global stock markets are analyzed from a complex network perspective. The dynamical complex network model considers each stock market as a node and connects each pair of nodes by an edge. Each edge is assigned by a time varying weight which equals the dynamic conditional correlation of the market index return values. The network properties reveal that the global stock market integration exhibits a periodic behavior, and that developed markets show stronger integration than emerging and frontier markets. The network also exhibits strong synchronization when the markets experience volatility. Unlike in the market integration, this volatility spillover phenomenon is more obvious in frontier markets than in developed markets. In Chapter 5, scientific collaborations are analyzed from a complex network perspective. With scientists abstracted by network nodes and the coauthorship of academic papers by edges between nodes, the collaboration network has been found to be intensively evolving in a local scale, where most coauthorships are one time collaboration. The structure of the collaboration network of a scientist depicts his/her influence in the peer collaborators. By examining the impact of unforeseen deaths of active eminent scientists to their collaboration networks, it is found that the fast evolving scientific collaboration networks are very robust to attacks on the hub nodes. It is suggested that future design of engineering systems should imitate the evolution mechanism of human social interactions to create robust systems. In Chapter 5, the consensus process in online social networks is analyzed from a complex network perspective. Users of online social websites form social communities by indicating friendship or interacting with each other. The resulting networks of online social relationship can be considered snapshots of the entire human social communities. By studying the consensus process in an online social network, it is found that a few users with very large number of connections can shift the final consensus decision of the community. Moreover,the compartmental structure of social networks can actually slow down the consensus rate.<br/><br/>Description: xviii, 163 p. : ill. (some col.) ; 30 cm.; PolyU Library Call No.: [THS] LG51 .H577P EIE 2012 Liu http://hdl.handle.net/10397/5528 Title: Nonlinear stability study of DFIG wind energy generation systems<br/><br/>Authors: Li, Zhen<br/><br/>Abstract: With the increasing concern drawn for the environment protection and the conservation of nonrenewable energy, wind energy generation infrastructures have been widely deployed into the power grid system. The integration of the renewable energy such as wind energy into the power grid system, necessitates a more stringent requirement to protect the traditional power grid with single-direction power flow. In light of these ongoing situations, the renewable energy system features the benefit of the bi-directional power flow. Therefore, more concerns should be considered for the mutual interaction between all these generation sources and various loads. Typically, the doubly-fed induction generation wins the most popularity for the wind energy industry due to its variable-speed operation, the maximum power capture within a suitable region, low power losses and the use of fractional power rating converters. The objective of this thesis is to address the stability and performance issues of the grid-connected DFIG wind energy generation system. The stability issue is addressed at first for a standalone negative-sequence-injected DFIG. The corresponding instability phenomenon will be gathered for further studies. Based on the implication from the negative sequence, the interaction between the DFIG and loading with different electrical characteristics will be examined in the light of sustained oscillations at the low-frequency component and the second harmonic of line frequency. Two categories of loadings are adopted to build the grid-connected DFIG wind turbine system. They include unbalanced active loads and passive loads. For each kind of load, by varying different parameters, nonlinear analysis method will be conducted to analyze and predict the occurrence of the inherent bifurcation behaviors. Simulation results will be provided to discover the instability numerically. In addition, the loading spaces will be derived to assist the design-oriented bifurcation analysis in system integration tasks. Furthermore, control schemes will be focused on broadening the system stable operation regions under unbalanced grid conditions. By addressing the critical problems existed in the ongoing control for unbalanced conditions, one new sequence decomposition method will be pointed out to facilitate a windup control and a power oscillatory control. With this new combined control method, two critical objectives such as the elimination of DC-link voltage fluctuation and torque pulsation can be achieved simultaneously with easy discrete-time domain implementation.; This thesis consists of five chapters. The first chapter provides an introduction of the wind energy system and an integral overview of the DFIG technologies. The second chapter presents elaborations on the basic knowledge about DFIG, such as the transformation basis and the DFIG modeling as well as the commonly-used conventional vector control in detail. The remaining parts will be devoted to report the contribution of the tasks in this project. Throughout the tasks, they are compliant with one common objective to facilitate the design-oriented bifurcation analysis for the power system by means of various parameter spaces in light of ensuring stability issues. Furthermore, a numerical analysis procedure is presented so that a better understanding of system behavior can be achieved with control scheme improvement. For each task, the simulation verification and theoretical analysis will also be conducted accordingly. It is expected that this thesis can offer a useful reference of practice for the wind energy system engineering.<br/><br/>Description: xxvi, 143 p. : ill. (some col.) ; 30 cm.; PolyU Library Call No.: [THS] LG51 .H577P EIE 2012 Li http://hdl.handle.net/10397/5527 Title: Fabrication and characterization of nano/microhole Si photovoltaic cells<br/><br/>Authors: Zhao, Yingqi<br/><br/>Abstract: A simple and cost-effective technique to fabricate wafer-scale silicon nanohole radial p-n junction PVCs has been improved. By combining thermal annealing of Ag thin films and metal catalyzed electroless etching we can fabricate silicon micro/nano holes with controllable size, depth and distribution. We have conducted detailed investigations on the influence of the Ag film thickness, annealing time, annealing temperature, and silicon wafer surface property on the morphology of thermal annealing formed Ag islands. Both P-type Si wafers with both (100) and (111) orientation used in the PVC fabrication. After MCEE the (100) wafers demonstrated lower reflectance, indicating better light trapping property. The PVC with best performance was also based on (100) wafer. The moderate cell performances demonstrate the potential of the technique in large scale fabrication of cost-effective nanostructure PVCs. In addition, the cell performances are investigated in detail as functions of the nanohole dimensions and doping conditions.<br/><br/>Description: x, 87 leaves : ill. ; 30 cm.; PolyU Library Call No.: [THS] LG51 .H577M EIE 2012 ZhaoYQ http://hdl.handle.net/10397/5463 Title: Diversely polarized antenna-array signal processing<br/><br/>Authors: Yuan, Xin<br/><br/>Abstract: The dissertation is composed of three distinct but related components, which relate to direction finding and/or polarization estimation with diversely polarized antenna arrays. The three parts are briefly summarized below: (1) "Vector cross-product direction-finding" with an electromagnetic vector-sensor of six orthogonally oriented but spatially non-collocating dipoles / loops. Direction-finding capability has recently been advanced by synergies between the customary approach of interferometry and the new approach of "vector cross product" based Poynting-vector estimator. The latter approach measures the incident electromagnetic wavefield for each of its six electromagnetic components, all at one point in space, to allow a vector cross-product between the measured electric-field vector and the measured magnetic-field vector. This would lead to the estimation of each incident source's Poynting-vector, which (after proper norm-normalization) would then reveal the corresponding Cartesian direction-cosines, and thus the azimuth-elevation arrival angles. Such a "vector cross product" algorithm has been predicated on the measurement of all six electromagnetic components at one same spatial location. This physically requires an electromagnetic vector-sensor, i.e., three identical but orthogonally oriented electrically short dipoles, plus three identical but orthogonally oriented magnetically small loops all spatially collocated in a point-like geometry. Such a complicated "vector-antenna" would require exceptionally effective electro-magnetic isolation among its six component-antennas. To minimize mutual coupling across these collocated antennas, considerable antennas-complexity and hardware cost could be required. Instead, Chapter 2 shows how to apply the "vector cross-product" direction-of-arrival estimator, even if the three dipoles and the three loops are located separately (instead of collocating in a point-like geometry). This new scheme has great practical value, in reducing mutual coupling, in simplifying the antennas hardware, and in extending the spatial aperture to refine the direction-finding accuracy by orders of magnitude. (2) Various triad-compositions of collocated dipoles/loops, for direction finding & polarization estimation. To form a collocated triad of orthogonally oriented dipole(s) and/or loop(s), 20 different compositions are possible and these compositions are investigated in Chapter 3. For each such composition: (i) closed-form formulas are produced here to estimate the azimuth-elevation direction-of-arrival and the polarization-parameters, or (ii) reasoning is given why such estimation is inviable. (3) Polarization estimation with a dipole-dipole pair, a dipole-loop pair, or a loop-loop pair of various orientations. Chapter 4 aims to estimate the polarization of fully polarized sources, given prior knowledge of the incident sources' azimuth-elevation directions-of-arrival, using a pair of diversely polarized antennas two electrically small dipoles, or two small loops, or one each. The pair may be collocated, or spatially separated by a known displacement. Each antenna may orient along any Cartesian coordinate. Altogether, fifteen antenna/orientation configurations are thus possible. For each configuration, Chapter 4 derives (i) the closed-form polarization-estimation formulas, (ii) the associated Cramer-Rao bounds, and (iii) the associated computational numerical stability.<br/><br/>Description: 101 p. : ill. ; 30 cm.; PolyU Library Call No.: [THS] LG51 .H577P EIE 2012 Yuan