Sharif Digital Repository

Inverter-based distributed generations (IBDGs) have gained special interest in the electricity supply chain due to the advancements in power electronics technology. Power electronic inverters provide high-efficiency energy conversion with full controllability over a wide operating range, and ancillary services such as reactive power support and active filtering. Considering the voltage and frequency control capability of power inverters, supplying inverter-based micro-grid (IBMG) loads is now pissible. Despite the significant opportunities associated with such sources, they introduce new challenges that should be addressed properly. Reliable and accurate protection against short circuit... 

Multilevel converters are the best solution in medium and high voltage applications because of their better output votage quality, redundant structure, less power semiconductor losses and common-mode voltages. Modular multilevel converters (MMCs) have additional advantages such as higher degree of modularity, transformerless operation capability and using a single common DC-bus. All these useful features make MMC the first alternative for HVDC systems. Fault detection, defected cells insulation and post fault operation control are necessary to ensure MMC continouse service. A simple, fast and accurate fault detection method is proposed for MMC semiconductor power switches in this thesis.... 

Condition monitoring and fault detection is an important way to provide economy in both time and costs. Many methods are developed for this purpose. Vibration analysis is one of the most important ways in this field. In this thesis, fault detection of mechanical rotary systems with nonlinear and non-stationary nature has been developed by use of empirical mode decomposition (EMD) and Hilbert transform. Firstly the measured signal is transformed into some intrinsic mode functions (IMF’s) by EMD, and then the Hilbert transform is implemented on each IMF. With Hilbert transform, the instantaneous frequency of system and then the mean frequency are obtained. Mean frequency is a key definition... 

Nowadays, industrial companies deal with a wide range of serious problems in the field of power transmission maintenance and also fault detection. A large amount of money and time is spent on these issues in order to solve them; consequently, there is an essential need for this subject. In this thesis, in order to tackle those major issues which were referred above, an artificial neural network is trained with only one hidden layer. Also, a suitable database for training an efficient neural network is needed. Thus, a one-stage gearbox system with appropriate degrees of freedom is used to set up referred database. In this system, a crack is imposed to a tooth of spur gear with different sizes... 

The accurate fault diagnosing system design helps the process safety and also helps increasing the products quality of the process. In this project, the fuzzy system and neural network have been used for fault detection and diagnosis of a yeast fermentation bioreactor. In one case, parameters of membership functions are selected in a conventional manner. In second case, the optimal values of these parameters have been obtained using the genetic algorithm. In another case, the neural network system is used for fault detection. These three cases are compared based on their performances in fault diagnosis of a yeast fermentation bioreactor for three different conditions. The results indicate... 

In this paper, an approach is proposed to estimate the location of the leaks in oil pipelines. The pipeline possesses a system of Partial Differential Equations (PDE). First, a closed-form solution is applied to the pipeline equations and the leakage location is found solving the steady-state equations. To tackle the case where the pressure and flowrate do not reach constant values, the initial estimations are provided by the closed-form solution. Then, the pressures, flow rates, and leakage coefficient are estimated by solving the system of differential equations. This system is then discretized with respect to the location variable to yield the system state equations which, in turn, are... 

Power transformer is one of the most important equipment in the power system which plays a key role in the reliability and quality of power delivery. Among numerous failure modes of a typical power transformer, the turn-to-turn fault (TTF) is one of the most prominent reasons for its unplanned outage. A turn-to-turn fault quite often begins with the degradation of the winding insulation between two adjacent turns, then in a short time, this low-level fault can progress to a large number of turns and results in the transformer costly damages. Therefore, it is vital to detect TTF at an early stage and prevent its progression to a catastrophic disaster. The conventional percentage differential... 

Inverters are the main utility in transmitting power from distributed generation (DG) systems toward grid and their safe and immune performance has become a major concern among the system designers. In addition to poor power transmittance, faults in inverter can harm other equipment such as DG and local load. Consequently different protections has been considered to increase the reliability of these systems. Some of these protections like fuses and varistors are implemented in the hardware while others are considered in the processor. In the later class of protections, fault detection and isolation (FDI) system has been the subject of intense investigation in inverters over the past two... 

Nowadays, condition monitoring and fault diagnosis are considered as one of the most important maintenance and diagnosis strategies in state-of-the-art industries. Serious damage such as machine and coupled equipment failure, machine failure in a neighborhood in case of severe damage, and production loss will be resulted in late fault detection. One of the critical information in condition monitoring is the machine's rotating speed. In many cases of condition monitoring, the rotating speed, and the machine's technical information (such as bearing characteristic frequency )is not available. In this study, a ruled-base model and an intelligent model are developed for machine running speed... 

The demand for safety operating systems is growing with the new developments in technology. One of the important and critical issues in system monitoring is the capability of the system to detect probable faults and hazards. In model-based fault detection one specific diagnostic signal called residual should be generated for each fault that is only sensitive to a particular fault and independent of other faults and disturbances. In this thesis we try to investigate the nonlinear geometric approach in fault detection of a class of mechanical systems. After a brief introduction to geometric tool for nonlinear inputaffine systems and conditions in which output is decoupled from input, we... 

Technology systems are vulnerable to various defects. Acuator failures reduce the performance of control systems and even they can damage the entire system. Incorrect reading from the sensors is the cause of many inefficiencies, because errors can reduce the quality and efficiency of a product line. In addition, failure of a single system's component can cause a system failure. Electromechanical energy conversion devices are composed of hundreds different components and subunits that are subject to varying adverse environmental conditions, and that is why the sensors, actuators, and components are likely to malfunction. Unexpected defects and unplanned repairs result in high operating costs... 

In the current era, the urgent need to reduce the use of fossil fuels and limit carbon emissions has highlighted the importance of shifting from traditional gasoline-powered vehicles to electric vehicles. The evolving landscape and challenges in the field of electric vehicles highlight the importance of ensuring the safety and reliability of energy storage systems. Lithium-ion batteries play a central role in electric vehicle technology, requiring thorough research efforts. Notably, the detection of internal short circuit, a common issue in lithium-ion batteries, has become a focus of researchers in recent years. This study aims to investigate the internal short circuit fault in lithium-ion...