Browsing by Author "Tadesse, Teshale"

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    Extraction, Characterization and Evaluation of Reinforcing Nanocellulose in Polyvinyl Alcohol (PVA)
    (Addis Ababa University, 2017-06) Tadesse, Teshale; Woldeyes, Belay (PhD)
    Incorporating nanocellulose into different materials have become an attractive research topics for production of nanocomposite materials. The objective of this study was to extract nanocellulose from the bagasse and apply as reinforcement in the PVA polymer, and evaluate the effectiveness of the reinforcement. The extraction of nanocellulose was performed by acid hydrolysis with 50 % wt. H2SO4 at 45°C for 45 minutes and 40 % yield was obtained. The particle size analysis by dynamic light scattering (DLS) showed the mean particle size found was 348.6 nm, confirming the product was nanocellulose. The Fourier transform infrared spectroscopy (FTIR) indicated the effectiveness of pretreatment in removing lignin and hemicellulose. The nanocellulose was reinforced in PVA polymer by solvent casting process. The water absorption results revealed that reduction in percentage of water absorption from 82 % for neat PVA to 59 % PVA/ 7 %NC. This results were confirmed by increment in tensile strength from 33 MPa for neat PVA to 53 MPa for PVA/ 5 %NC and reduction in elongation at break about 2.4 folds with respect to neat PVA. The SEM micrograph showed uniform distribution and formation of network structure through hydrogen bonding. The comparative and cluster analysis showed the nanocellulose reinforced PVA films exhibited highest tensile strength, lower elongation at break and lower water absorption with respect to bagasse and cellulose reinforced PVA as well as with neat PVA film.
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    Study of Doubly Fed Induction Generator Control Under Grid Fault Conditions
    (Addis Ababa University, 2016-06) Tadesse, Teshale; Mamo, Mengesha (PhD)
    Wind power is growing rapidly around the world as a means of dealing with the world energy shortage and associated environmental problems. A wind electrical generation system is the most cost effective of all the environmentally clean and safe renewable energy sources in world. In this thesis, double fed induction generator and grid system are modeled under normal conditions of the grid system and under grid faulty conditions. Any abnormalities associates with grid are going to affect the system performance. Taking this into account, the performance of double fed induction generator (DFIG) variable speed wind turbine under network faults is studied using simulation developed in MATLAB/SIMULINK results show the fault behavior of the double fed induction generator when a sudden short circuit and voltage dip on the grid side. After the clearance the short circuit fault and voltage dip the proportional integral controller manages to restore the wind turbine‘s normal operation. The three-phase fault model is done by giving a fault equivalent resistance value of 0.01Ω and voltage dip at grid side with the value of 0.45p.u. At this time, the current, voltage, active power and reactive power value of DFIG is fluctuating between 0.2p.u and 1.36p.u, 0.7p.u and 1.12p.u, 0 and 6MW, 0.75 and 0.42MVAr, respectively. But in order to operate at normal operation the DFIG current and voltage value is at 1p.u. The maximum active power that generated from DFIG is 9MW. To stabilize the system the proportional integral controller compare the reference voltage and current with generated and minimize the error between voltage and current. The proportional integral controller minimizes the error by decreasing the rising time and makes the value of current and voltage to 1p.u. The detailed results of steady state and faulty or three-phase short circuit on grid system has been noted and analyzed with proper justification. In order to increase the fault ride through capabilities of the system, crowbar protection and series dynamic resistor could be added to the system. This thesis done by modeling and simulating of the controller using Matlab/Simulink in wind turbine system integrated with grid system. Finally, we observed and interpreted the result with Matlab/Simulink simulation software. Keywords:- , doubly fed induction generator; wind turbine; proportional integral controller; MATLAB/SIMULINK;