Sharif Digital Repository

Current transformers are one of the key equipment of power networks that play a vital role in measuring, protecting and controlling the system. The proper operation of these equipments depends on the proper management of their temperature, because an increase in temperature can lead to a decrease in useful life, loss of efficiency and even irreparable failures. For this reason, analyzing and controlling the thermal behavior of these equipments, especially in critical conditions, is of particular importance. One of the biggest challenges in the operation of current transformers is the occurrence of a short-circuit fault, which leads to a sharp increase in temperature and can cause serious... 

Nowadays, credit risk is recognized as the most significant factor of bankruptcy in banking industry. Incorrect selection and unsuitable appropriation of financial resources to customers cause this risk. On the other hand, the detection of company operating and financial difficulties is a subject which has been fundamentally discussed in financial ratio analysis. As a result, this study tries to analyze existence and relationship between financial ratios of recipients and their reimbursement ability by evaluation of their conditions in awarding them credit. Furthermore, according to the credit risk context, some of financial ratios are selected and their values are obtained as the input... 

Following the recent developments in the field of decision making, a considerable number of problems involved with stochastic systems can be thought of whose analysis depends on a set of intricate mathematical relations. In such cases, simulation is one of the most popular tools that can be applied toward analysis of behavior of such stochastic systems. Not only does not the simulation model rely on such intricate mathematical relations, it also enjoys the added advantage of being free of any restricting assumptions which may normally be considered in a stochastic system.To analyze such problem, one may aim at determining the best combination of input variables to optimize the system... 

This work attempts to address the buffer allocation problem in an unreliable, linear production line. We try to determine the optimal sizes of buffers between adjacent work stations in such a way that a measure of costs is minimized and the production rate is simultaneously maximized. We resort to simulation optimization in order to determine the best combination of input parameters that leads to a near optimal performance for the system. To achieve this purpose, we employ a multi-objective genetic algorithm (NSGAII) in the optimization phase along with simulation as the tool for evaluating the objective function. To determine the merits of the proposed method, we compare the performance of... 

In this research, an integrated location-inventory-transportation problem with uncertainty in the parameters is analyzed using Robust Optimization techniques. Unknown demand distribution is the underlying assumption, which relates better to the reality. The model’s main purpose is to determine the number and locations of the Distribution Centers (DC) that connect the supplier to the retailers, and then to assign retailers to them. The problem’s mathematical model is developed on the basis of Robust Optimization techniques and the solution space is explored by Tabu Search. The results show that the Tabu Search method achieves optimality in small-scale cases, while providing desirable... 

Cache memory is one of the most important parts of the microprocessors. Caches improve performance by bringing data and instructions near the processors and decreasing the access time to data and instructions. But caches are also vulnerable to some kind of error called soft error. When an error happens in a cache, suddenly can propagate throughout the system and affect the integrity and reliability of the overall system. There are actually two types of errors in cache memories, permanent (hard) errors and transient (soft) errors. Previous studies have shown that about 92% of system reboots are initiated by soft-error occurring in cache memory. Soft-errors have two main sources, alpha... 

Due to increase in nonlinear loads in today’s applications, the percent of harmonic content has been increased. Transformers are one of the electrical system components that are subjected to these harmonics. The failure of such equipment is an important consideration. Such failure is because of the hotspot temperature rises over the normal temperature withstanding of internal insulation. The increments of hotspot temperature can decrease the transformer insulation life. Totally, because of the presence of many parameters, precise investigation of loss of life is very difficult. Hence, the only meaning of loss of life in this project is the loss of insulation life under heat stresses.... 

In the present study, the numerical simulation of incompressible laminar and turbulent flows using a high-order finite difference lattice Boltzmann method is presented. To handle curved geometries with non uniform grids, the incompressible form of lattice Boltzmann equation is transformed into the generalized curvilinear coordinates and the spatial derivatives of the resulting equation are discretized using the fifth-order WENO scheme. The advantage of using the WENO-LBM developed is that it needs less number of grid points and remains stable even at high Reynolds number flows. For the temporal term, the fourth-order explicit Rung-Kutta scheme is adopted for laminar flow calculations and... 

Reducing noise, vibration and damage to the vehicle's passenger compartment was always a problem that has been investigatedin many researches. Here engine mounts as connecting members between engine and chassis play a big role. Because both of engine and road vibration put on it. Many studies have been done to optimize the engine mounts. As a result, many of hydraulic engine mounts with different variety of system control. But a few researches did about optimization of location and orientation of engine mounts. Therefore this project is to provide a process to optimize the location and orientation of engine mounts. The optimization process is First the dynamic equations of dynamic system are... 

Unmanned Aerial Vehicles have been used for different applications despite their short life. In last recent years, the developements in technology which leads to producing smaller electronical and electromechanical deveices, attract more attention to the researches in these fields. Among these vehicles, unmanned helicopters are more important because of their special abilities and capabilities. However there are some drawbacks which makes using them more difficult.
In this project, the first principle modeling of a RC helicopter has been accomplished and a parametric model including different sections effects has been created. This model has been implemented for a Trex600 CF helicopter.... 

In recent years health monitoring and prognostics of complex systems have been considered more than ever.. In the present researh, data - based approach has been selected among various prognostics and health monitoring approaches. One of the most challenging issues in data-based methods is how to map system sensor information to its health status. In this research, different methods of mapping are discussed. The results show that sensor fusion by principal component analysis (PCA) offers acceptable performance. This pattern produces a single-dimensional signal for health monitoring with high reliability.The second challenge is to predict the status and design of the prognostics module. For... 

Multiscale modeling is performed within the framework of homogenization methods for problems in which the scales are separated. The existence of representative volume element (RVE) is one of the main ingredients of homogenization methods. Due to non-existence of RVE and macroscale mesh sensitivity, the continuous homogenization method is not applicable for softening materials. Despite the non-existence of RVE for softening materials, it has been demonstrated that by performing the average over the active damage zone rather than the entire domain, objective responses with respect to RVE size could be obtained. That is why discontinuous homogenization is used instead of continuous... 

Nowadays, hybrid electric vehicles get more attention, not only for decreasing fossil fuel resources, but also for increasing environmental pollution. Using multi-propulsion system in hybrid vehicles causes, the low efficient performance of combustion engine is reduced compare to conventional vehicles. Fuel consumption and pollution that reduced in hybrid vehicles is related to how the multi-propulsion system works. Some researches that have been done about hybrid control algorithms, are completely software modeling and development. Despite importance of these researches in development and evaluation of power distribution algorithms, hardware-based evaluations are also necessary. In all... 

One of the most important unit operations in the production of dry powder biological products involves a freeze dryer apparatus. In this process, Liqueur, which is containing the α-amylase enzyme, undergoes a frigidity condition in which loses its moisture during sublimation. Under these circumstances, applying an adequate supply of sublimation enthalpy without raising the temperature of the frigid material up to melting, plays an important role in the quality of the product and preventing degradation of the α-amylase enzyme. Freeze drying depends on two main factors; time and energy. Furthermore, it is necessary to obtain an optimal temperature and pressure for different steps and... 

In this thesis, we consider design, modeling, and analysis of a single-phase Brushless Doubly-Fed Machine (BDFM). Generally, this type of machine is called the Brushless Doubly-Fed Induction Machine (BDFIM), that is a new class of induction machines. BDFM is a type of AC machines that requires two alternative power sources. One of the stators windings, called Power Winding, is connected to the network grid and the other one, which is called Control Winding, is connected via a converter, which is in the order of one-third of the nominal power of the machine, to the grid. Like the other types of induction machines, BDFM can also work as a generator or motor. The important criteria of this... 

The main objective of this project is to take the first step in designing torsional vibration damper (TVD) for engines in Iran. To this end a complete dynamic model of a 4 cylinder internal combustion engine comprising of all reciprocating parts plus the crankshaft was developed using Bond Graph method. Real firing data of the engine was used as input to the system and output torque to gearbox shaft as the output. The system equations in two cases, with TVD and without TVD were derived. MATLAB software was used to solve these equations in finding the quality of performance of the torsional vibration damper. Dynamic properties of crankshaft and designed TVD were obtained using finite element... 

Gearbox is one of the important components of power transmission chain in the car which the engagement of the gears inside it causes noise. The following thesis, is in line with the project that was defined as "NVH reduction of the Tiba X200 gearbox" and to increase the correlation between university and industry, the aim of which was to improve all the noise sources in the mentioned vehicle. The purpose of this project is to reduce the noise of the gearbox shell by using reinforcing blades called ribs. For this purpose, first the desired changes were made in Solidworks software on the 3D model of the gearbox shell, and then the finite element model of the gearbox shell was prepared in... 

With the environmental concerns and scarce supply of the crude oil, a revival in the development of bio-based and compostable materials is coming. They are in general being called the ''green'' materials, among which poly(lactic acid) (PLA) got the most extensive attention due to its good combined properties, including the renewable feedstock, the compostability, and its good mechanical properties, especially the young's modulus and stiffness. PLA has found numerous applications. It is considered having the highest potential to replace some regular petroleum based plastics. However PLA has its own shortcomings, its brittleness, low heat distortion temperature and relatively high cost witch... 

Due to the excessive failures of tires used on mining trucks, which lead to significant economic and environmental damages, mine operators are seeking replacements for them. The ASW mechanical wheel, introduced by the American company Global Air Cylinder Wheel, is recognized as the most well-known alternative to pneumatic tires for mining trucks. Although this company has been active since 2004, it has not yet released a commercial product. This study examines the structural, behavioral, and operational characteristics of the mechanical wheel and its impact on the performance of mining trucks. The structure of the mechanical wheel consists of a set of hydro-pneumatic actuators that connect a... 

Recently, the brushless doubly fed induction generator (BDFIG) has been proposed to be used in wind turbines due to variable speed operation with fractionally size power electronic converter and without brushes and slip rings. In this thesis, for the first time, LVRT capability of BDFIG has been assessed regarding the grid codes and several approaches have been suggested for improving it. Firstly, the steady-state performance of the BDFIG in different operating modes has been investigated and required tests have been performed. In addition, a new operating mode named as "double cascade mode" has been introduced. The fault-on performance of BDFIG has also been studied by means of torque-speed...