Browsing by Subject "Frequency"
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Item Energy storage sizing for low-inertia microgrids, and lessons learned from a potential microgrid(2016-05) Toliyat, Amir; Baldick, Ross; Kwasinski, Alexis; Arapostathis, Aristotle; Hallock, Gary; Uriarte, FabianThe coordinated control of multiple distributed generators in a microgrid and the preservation of adequate system inertia in real-time operations are some of the principal technical challenges for stable microgrid operation. One issue in particular pertains to grid-tied inverters, which, as mandated by present standards, are only permitted to operate at unity power factor, thereby requiring the microgrid’s synchronous generators to operate at a low power factor. This behavior accordingly introduces ramifications by limiting the generator’s active power output, which would compromise frequency and voltage stability margins. Consideration is also given to the effect of line impedances, since interconnecting microgrid lines can be described by a variety of X/R ratios that affect the control and flow of active and reactive power. Moreover, the absence of a stiff grid presents control challenges for grid-tied inverters due to the inverters’ tendency to regulate the voltage at the point of common coupling. These same inverters also jeopardize microgrid stability due to their low equivalent inertia as traditional forms of generation (i.e., spinning sources) become displaced by inertia-less inverters. Because of this low microgrid inertia, fluctuations in the output power of renewable energy sources or changes in local load levels may lead to power quality or frequency/voltage stability concerns. Therefore, energy storage sizing is investigated in this dissertation, as it is closely related to the stability analysis of microgrids. Furthermore, an existing residential community (in Austin, TX) described by a moderate penetration of photovoltaic sources and electric vehicle charging is considered, and the implications of said community being retrofitted to a microgrid are examined.Item Fluidic Tuning of a Four-Arm Spiral-Based Frequency Selective Surface(2011-08-08) Wells, Elizabeth ChristineFrequency selective surfaces (FSSs) provide a variety of spatial filtering functions, such as band-pass or band-stop properties in a radome or other multilayer structure. This filtering is typically achieved through closely-spaced periodic arrangements of metallic shapes on top of a dielectric substrate (or within a stack of dielectric materials). In most cases, the unit cell size, its shape, the substrate parameters, and the inter-element spacing collectively impact the response of the FSS. Expanding this design space to include reconfigurable FSSs provides opportunities for applications requiring frequency agility and/or other properties. Tuning can also enable operation over a potentially wider range of frequencies and can in some cases be used as a loading mechanism or quasi-ground plane. Many technologies have been considered for this type of agility (RF MEMS, PIN diodes, etc.). This includes the recent use of microfluidics and dispersions of nanoparticles, or fluids with controllable dielectrics, which have entered the design space of numerous other EM applications including stub-tuners, antennas, and filters. In this work they provide a material based approach to reconfiguring an FSS. An FSS based on a four-arm spiral with tunable band-stop characteristics is presented in this work. A thin colloidal dispersion above each element provides this tuning capability. The radial expansion and contraction of this dispersion, as well as the variable permittivity of the dispersion, are used to load each element individually. This design incorporates thin fluidic channels within a PDMS layer below the substrate leading to individual unit cells that provide a closed pressure-driven subsystem that contains the dispersion. With the capability to individually control each cell, groups of cells can be locally altered (individually or in groups) to create gratings and other electromagnetically agile features across the surface or within the volume of a radome or other covering. Simulations and measurements of an S-band tunable design using colloidal Barium Strontium Titanate dispersed Silicone oil are provided to demonstrate the capability to adjust the stop-band characteristics of the FSS across the S-band.Item Rio Grande wild turkey diets in the Texas Panhandle(2007-12) Petersen, Brian Earl; Ballard, Warren B.; Wallace, Mark C.; Holdstock, Derrick P.Many diet studies have been conducted throughout the range of the wild turkey (Meleagris gallopavo), and have described wild turkeys as opportunistic. However, only a few diet studies have been conducted on the Rio Grande wild turkey (M. g. intermedia) in Texas and none have been conducted in the Texas Panhandle. Our objectives were to determine seasonal and annual diets of wild turkeys in the Texas Panhandle. We conducted our study on 3 study sites, Gene Howe Wildlife Management Area, Matador Wildlife Management Area, and private lands surrounding the Salt Fork of the Red River. We harvested 70 birds, 35 male and 35 female, from all the study sites over 4 seasons. We analyzed crop contents to determine diet. We removed, dried, and examined samples macroscopically to identify plants (to species) and insects (to order) eaten. Hackberries (Celtis spp.) (24.7%), domestic peanuts (10.7%), woolybucket bumelia (Bumelia lanuginosa) (9.6%), corn (8.2%), Chickasaw plum (Prunus angustifolia) fruit (6.9%), and grasshoppers (6.1%) were all important components of the annual diet by volume. Annually, hackberries were found in the greatest frequency (62.9%) followed by beetles (55.7%), grass vegetation (47.1%), and grasshoppers (44.3%). Winter diets were composed primarily of hackberries (56.4%), peanuts (14.1%), and western wheatgrass (Agropyron smithii) (10.5%). Spring diets consisted primarily of peanuts (41.6%), dense-flower bladderpod (Lesquerella densiflora) (24.6%), and grasses (11.4%). Summer diets were dominated by Chickasaw plum fruit (28.1%), grasshoppers (20.0%), and western soapberry (Sapindus drummondii) fruit (19.2%). Finally, the most abundant foods during the autumn were woolybucket bumelia (32.1%), hackberry (18.7%), corn (16.9%), and western ragweed ( ) (7.3%). We found no significant differences between foods consumed by male or female wild turkeys. We found that animal matter was consumed most during summer (P = 0.0004), mast was consumed least during spring (P = 0.0065), and miscellaneous items were consumed in greater amounts in the winter (P = 0.0011). We also found that anthropogenic food was consumed the most at the Salt Fork study area (P < 0.0001), mast was consumed the least at the Salt Fork study area (P = 0.0067), and miscellaneous items were consumed in the greatest amounts at the Matador study area (P = 0.0022). Wildlife managers can use this information to evaluate wild turkey habitat for management planning. Future research should focus on determining the effect of food sources on wild turkey population dynamics, movement, and distribution, especially the role of anthropogenic foods on these parameters. Chapter I is a comprehensive review of all literature relating to wild turkey history, ecology and diet. In the first part, I outline the history of the wild turkey in North America, the differences between the subspecies, and basic habitat associations of the Rio Grande wild turkey. In the second part, I compare and contrast different methods for determining diet, and review what previous diet studies on wild turkeys have found. The intent of the first chapter is to give a more complete background than is possible in the second chapter. Chapter II describes that study that I conducted on Rio Grande wild turkeys in the Texas Panhandle to assess their diet. This chapter is intended to be submitted to the Southwest Naturalist journal and is formatted in accordance with their policies.