Browsing by Author "Girma, Moges"

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    Modeling of Gird Connected Solar, Wind and Fuel Cell Distributed Generation Units and Power Flow Controlling System for Interfacing
    (AAU, 2018-03) Girma, Moges; Getachew, Bekele (PhD)
    The popularity of distributed generation system is growing faster in the last few years because of their higher operating efficiency and low emission levels. Distributed generators make use of several micro-sources for their operation like photovoltaic cells, wind generator and fuel cells. All the power generated by each is connected to a micro-grid for DG application. The microgrid must use an electric inverter for power conditioning and interfacing with the power system. Basically, the micro-grid inverter has two operation modes: Standalone mode and Grid-connected mode. In this thesis, the modeling of hybrid PV-Wind-FC distributed generation systems are done. Dynamic models for the major system components, namely, wind, PV and fuel cell system modes are developed by using HOMER tool. The simulation and optimization is done based on climatic data sources and economics of the power components in which the Net Present Cost (NPC) has to be minimized to have economically feasible system. Moreover, other parameters like capacity shortage, renewable fraction, excess electricity and Cost of Energy (COE) are also considered to check the technical capability so as to select the best system. HOMER simulation result displays the most economical feasible system sorted by NPC from top to down, the prime system ranked first has renewable fraction of 15 unit wind turbines with 100kW each rating power, 1000kW photovoltaic panel, 500kW fuel cell and 500kW converter are part of the system to fulfill the required 1.3832MW estimated load demand. Then, a simulation model for the proposed hybrid power system is developed by using Matlab/Simulink environment. This is done by creating a subsystem and masked block sets of the major dynamic component models and then cascading in to a single aggregate model. The final tasks are control system design and analysis of inverter interfaced micro-grid distributed generations with existing utility- grid system. The analysis of controller is done using the Matlab M-file. For both operation modes, a multi-loop controller is used. The voltage differential feedback inner loop is embedded in the outer voltage loop also an output voltage decoupling and current decoupling are implemented by using the output voltage feedback. The proposed control scheme possess very fast dynamic response at load step change and can also achieve good steady state performance at both linear and nonlinear loads.
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    A Study on Soil Nailing as a Deep Excavation Support in Railway Track
    (Addis Ababa University, 2019-10) Girma, Moges; Tensay, Gebremedhin (PhD)
    Soil nailing is one of the in-situ soil reinforcement and deep excavation support that has been used worldwide, with the main components as nail and facing. This study covers both conventional and state-of-the-art numerical analysis. It mainly focuses on the numerical analysis using PLAXIS 2D due to the drawback of deformation prediction in the conventional limit equilibrium method. The soil properties and initial wall parameters are taken from measurements on sites, were appropriately modeled and used in the finite element based software, PLAXIS 2D for the simulation. The global factor of safety as computed using limit equilibrium was greater than that of the finite element method. The effect of mesh density on the factor of safety and wall deflection was explained. Modeling of soil nail using a plate and geogrid elements were illustrated and from the analysis result, bending stiffness showed no effect on the global factor of safety at the end of the construction stage. However, it showed a significant effect on the other construction stages. A parametric study was conducted to investigate how the lateral displacement and the global factor of safety are influenced by varying the length, inclination, and spacing of the nails. When the nail length increases the lateral displacement decreases and the factor of safety increases. It was observed that when the nail inclination increases the global factor of safety increases however, for higher length to wall height ratio the global factor of safety decreases after 150 inclination. Based on the analysis result, the nail inclination angle between 100 and 150 is recommended as the optimal use of the nail.