Robust Damping Control In Power Systems

In this thesis, robust damping control design techniques that improve the small disturbance stability of power systems are presented. To ensure power system stability, the controllers used in power systems have to be robust , that is, they must be able to provide sufficient damping to the oscillatory modes under all possible operating conditions. A frequency-domain robust control design based on the {dollar}H\sb{lcub}\infty{rcub}{dollar} optimization technique is used in this thesis to design the damping controllers. The model variations caused by the operating condition changes are treated as model uncertainties, and considered explicitly at the controller design stage.;A robust multivariable controller is designed for an excitation system using the standard {dollar}H\sb{lcub}\infty{rcub}{dollar} mixed-sensitivity formulation. To simplify the design process, a new damping control formulation based on the {dollar}H\sb{lcub}\infty{rcub}{dollar} optimization is proposed, and is applied to the damping control design for a two-machine system with an SVC. Damping controller devices using other FACTS devices such as the TCSC and the UPFC are also investigated in a four-machine two-area system. Nonlinear time domain simulations are performed, for all cases except the UPFC, to verify the damping contribution of the controllers and the robustness of the dosed-loop systems against operating condition variations. Some important issues, such as the derivation of the linearized power system model, the accommodation of model uncertainties in the context of power systems, the selection of the weighting functions, the treatment of pole-zero cancellation, and the selection of feedback signals, are addressed in the thesis.;It is concluded that the proposed new formulation can indeed simplify the selection of the weighting functions and make the controller design process more transparent to the designer. The controllers designed using this new formulation can provide positive damping to the system under all possible operating conditions. It is also demonstrated that the FACTS devices considered in this thesis are able to provide additional damping to the system oscillation modes in addition to their primary functions

Observation of forbidden phonons and dark excitons by resonance Raman scattering in few-layer WS2_2

The optical properties of the two-dimensional (2D) crystals are dominated by tightly bound electron-hole pairs (excitons) and lattice vibration modes (phonons). The exciton-phonon interaction is fundamentally important to understand the optical properties of 2D materials and thus help develop emerging 2D crystal based optoelectronic devices. Here, we presented the excitonic resonant Raman scattering (RRS) spectra of few-layer WS$_2$ excited by 11 lasers lines covered all of A, B and C exciton transition energies at different sample temperatures from 4 to 300 K. As a result, we are not only able to probe the forbidden phonon modes unobserved in ordinary Raman scattering, but also can determine the bright and dark state fine structures of 1s A exciton. In particular, we also observed the quantum interference between low-energy discrete phonon and exciton continuum under resonant excitation. Our works pave a way to understand the exciton-phonon coupling and many-body effects in 2D materials.Comment: 14 pages, 11 figure

Anomalous Frequency Trends in MoS2 Thin Films Attributed to Surface Effects

The layered dichalcogenide MoS2 has many unique physical properties in low dimensions. Recent experimental Raman spectroscopies report an anomalous blue shift of the in-plane E2g1 mode with decreasing thickness, a trend that is not understood. Here, we combine experimental Raman scattering and theoretical studies to clarify and explain this trend. Special attention is given to understanding the surface effect on Raman frequencies by using a force constants model based on first-principles calculations. Surface effects refer to the larger Mo-S force constants at the surface of thin film MoS2, which results from a loss of neighbours in adjacent MoS2 layers. Without surface effects, the frequencies of both out-of-plane A1g and in-plane E2g1 modes decrease with decreasing thickness. However, the E2g1 mode blue shifts while the A1g mode red shifts once the surface effect is included, in agreement with the experiment. Our results show that competition between the thickness effect and the surface effect determines the mechanical properties of two-dimensional MoS2, which we believe applies to other layered materials

Empirical noise performance of prototype active pixel arrays employing polycrystalline silicon thin- film transistors

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/162751/2/mp14321.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/162751/1/mp14321_am.pd

Low-dose megavoltage cone-beam CT imaging using thick, segmented scintillators

Megavoltage, cone-beam computed tomography (MV CBCT) employing an electronic portal imaging device (EPID) is a highly promising technique for providing soft-tissue visualization in image-guided radiotherapy. However, current EPIDs based on active matrix flat-panel imagers (AMFPIs), which are regarded as the gold standard for portal imaging and referred to as conventional MV AMFPIs, require high radiation doses to achieve this goal due to poor x-ray detection efficiency (~2% at 6 MV). To overcome this limitation, the incorporation of thick, segmented, crystalline scintillators, as a replacement for the phosphor screens used in these AMFPIs, has been shown to significantly improve the detective quantum efficiency (DQE) performance, leading to improved image quality for projection imaging at low dose. Toward the realization of practical AMFPIs capable of low dose, soft-tissue visualization using MV CBCT imaging, two prototype AMFPIs incorporating segmented scintillators with ~11 mm thick CsI:Tl and Bi 4 Ge 3 O 12 (BGO) crystals were evaluated. Each scintillator consists of 120 _ 60 crystalline elements separated by reflective septal walls, with an element-to-element pitch of 1.016 mm. The prototypes were evaluated using a bench-top CBCT system, allowing the acquisition of 180 projection, 360° tomographic scans with a 6 MV radiotherapy photon beam. Reconstructed images of a spatial resolution phantom, as well as of a water-equivalent phantom, embedded with tissue equivalent objects having electron densities (relative to water) varying from ~0.28 to ~1.70, were obtained down to one beam pulse per projection image, corresponding to a scan dose of ~4 cGy–-a dose similar to that required for a single portal image obtained from a conventional MV AMFPI. By virtue of their significantly improved DQE, the prototypes provided low contrast visualization, allowing clear delineation of an object with an electron density difference of ~2.76%. Results of contrast, noise and contrast-to-noise ratio are presented as a function of dose and compared to those from a conventional MV AMFPI.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90773/1/0031-9155_56_6_001.pd

Monte Carlo investigations of megavoltage coneâ beam CT using thick, segmented scintillating detectors for soft tissue visualization

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134940/1/mp8957.pd

Effects of x-ray irradiation on polycrystalline silicon, thin-film transistors

The effects of x-ray irradiation on the transfer and noise characteristics of excimer-laser-annealed polycrystalline silicon (poly-Si) thin-film transistors (TFTs) have been examined at dose levels up to 1000 Gy1000Gy. Parameters including mobility, threshold voltage, subthreshold swing, and leakage current, as well as flicker and thermal noise coefficients, were determined as a function of dose. In addition, the physical mechanisms of the observed changes in these parameters are analyzed in terms of radiation-generated charge in the gate oxide, at the Si–SiO2Si–SiO2 interface, and at the grain boundaries. The results of the studies indicate that poly-Si TFTs exhibit sufficient radiation tolerance for the use in active-matrix flat-panel imagers for most medical x-ray applications.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87455/2/064501_1.pd

Retrospective analysis of main and interaction effects in genetic association studies of human complex traits

<p>Abstract</p> <p>Background</p> <p>The etiology of multifactorial human diseases involves complex interactions between numerous environmental factors and alleles of many genes. Efficient statistical tools are demanded in identifying the genetic and environmental variants that affect the risk of disease development. This paper introduces a retrospective polytomous logistic regression model to measure both the main and interaction effects in genetic association studies of human discrete and continuous complex traits. In this model, combinations of genotypes at two interacting loci or of environmental exposure and genotypes at one locus are treated as nominal outcomes of which the proportions are modeled as a function of the disease trait assigning both main and interaction effects and with no assumption of normality in the trait distribution. Performance of our method in detecting interaction effect is compared with that of the case-only model.</p> <p>Results</p> <p>Results from our simulation study indicate that our retrospective model exhibits high power in capturing even relatively small effect with reasonable sample sizes. Application of our method to data from an association study on the catalase -262C/T promoter polymorphism and aging phenotypes detected significant main and interaction effects for age-group and allele T on individual's cognitive functioning and produced consistent results in estimating the interaction effect as compared with the popular case-only model.</p> <p>Conclusion</p> <p>The retrospective polytomous logistic regression model can be used as a convenient tool for assessing both main and interaction effects in genetic association studies of human multifactorial diseases involving genetic and non-genetic factors as well as categorical or continuous traits.</p

Power estimation for gene-longevity association analysis using concordant twins.

Statistical power is one of the major concerns in genetic association studies. Related individuals such as twins are valuable samples for genetic studies because of their genetic relatedness. Phenotype similarity in twin pairs provides evidence of genetic control over the phenotype variation in a population. The genetic association study on human longevity, a complex trait that is under control of both genetic and environmental factors, has been confronted by the small sample sizes of longevity subjects which limit statistical power. Twin pairs concordant for longevity have increased probability for carrying beneficial genes and thus are useful samples for gene-longevity association analysis. We conducted a computer simulation to estimate the power of association study using longevity concordant twin pairs. We observed remarkable power increases in using singletons from longevity concordant twin pairs as cases in comparison with cases of sporadic proband. A similar power would require doubled sample sizes for fraternal twins than for identical twins who are concordant for longevity suggesting that longevity concordant identical twins are more efficient samples than fraternal twins. We also observed an approximate of 2- to 3-fold increase in sample sizes needed for longevity cutoff at age 90 as compared with that at age 95. Overall, our results showed high value of twins in genetic association studies on human longevity.Peer Reviewe