Sharif Digital Repository

The demand for independent power generation has been increased in recent years due to the growing demand in distributed networks, losses and power qaulity in power systems. These losses occur in transmission to distribution networks from the generation sites to the end-user loads which have harmonic losses besides fundamental losses.. The non-linear load makes the harmonics flow in the line and create the harmonic losses. However, in a distributed network with several DGs of different types and with all kinds of variable nature loads, an optimal power balance in the system has to be achieved. This optimal objective, which results in minimal energy losses over a specific period of time,... 

Hyderogels are hydrophilic polymers with three dimensional network structure, due to their high water absoption, their application to life and industry is increasing from day to day, such as tissue engineering, actuators and sensors, drug delivery and therapeutic treatment. In this project, it has been attempted to produce polyacrylamide/alginate double network hydrogels with the use of graphene oxide nanosheets to indicate that it is possible to combine toughness and self-healing in a hydrogel with the opposite nature of each other. To evaluate mechanical properties, uniaxial tensile test and loading-unloading test were used. The results indicate that increase the weight ratio of acrylamide... 

In recent years, there has been an increasing concern about harmonic distortion and the effects of harmonics on power systems. Nonlinear loads such as electronic power supplies, battery chargers, motor drives, UPS systems and … are the primary source of harmonic currents. Harmonic currents generated by nonlinear loads flow from the load into the power system due to the network low path of impedance Harmonic currents cause additional heat and losses in power system components and results in voltage distortion as well.To deal with the problems associated with harmonics and to reduce their effects on power networks, study of the effects of non-linear loads on distribution networks is essential.... 

Proliferation of Distributed Generations (DGs) introduces higher reliability, stability, and quality as well as lower power losses and operational costs to the power systems. Integrating smart-grid and microgrid technologies to the power systems will further improve the overall performance of the system. Moreover, DC microgrids create an infrastructure to supply DC loads by DC sources such as photovoltaic arrays, fuel cell modules, as well as storages, which in case, increases the energy conversion efficiency. Operating DC microgrids together with AC microgrids, therefore, will form a new distrib-ution system called hybrid AC/DC systems, in which the DC side is dynamically decoupled from the... 

Iran's electrical grid is connected to the Eastern Turkey grid via Khoy-Basqale 400KV transmission line. Harmonic mesurements indicate high level of the 5th harmonic voltage in Azarbayjan grid which is magnified in Van substation. According to Iran international obligations and economic losses caused by the high level of harmonics in Azerbaijan regional electricity grid, a comprehensive study is required. Early studies showed that this unprecedented increase in the 5th harmonic voltage is due to the small harmonic loads scattered throughout the network. The aim of this thesis is to perform a comprehensive harmonic analysis of Azerbaijan network due to scattered harmonic loads. In this... 

The use of wind energy and wind generators as an inexpensive type of renewable energy sources is increasing. Wind farms and distributed wind generators connected to the power transmission and distribution systems and microgrids, and wind generators in standalone mode of operation affect the stability and power quality of the grid. In this thesis, the performance of a matrix converter in grid connection of variable speed PMSG–driven wind turbine is improved. In this way, by transferring the matrix converter modulation matrix into the synchronous reference frame and decomposing it into the singular values, a new general method is proposed for analysis, modelling and modulation of a matrix... 

In this thesis first of all the different approaches of control and parallel of multi-modular inverter system were investigated. These methods have been categorized and examined separately, and they were compared with each other in terms of quality and quantity. Afterwards, Active current sharing technique due to its high reliability was selected as the fundamental control approach. Among different ways for active current sharing, Avarage load sharing was chosen for its acceptable stability compared with other method. Approaching this goal, for the first time optimal control strategy and LQR method was proposed to parallel 3-phase inverters.At first, the problem was edited to be compatible... 

converters, the different configurations for matrix converters are proposed. In this thesis, a one-phase to three-phase matrix converter is used to control in three-phase induction motor. Different control methods are proposed to control the induction motor. The novel control methods include close loop speed and converter output current controls of induction motor are proposed based on SPWM. The structure of indirect matrix converter for the mentioned application also investigated. Moreover, three usual method of control of induction motor: DTC (Direct Torque Control), FOC (Field Oriented Control) and Space Vector Modulation are applied to the single to three- phase matrix converter. Each of... 

Given the growing advantage of exploiting renewable energy resources over fossil fuels and using distributed generation systems, microgrids seem to be the best solution for reaching the energy-efficient world of the future. Hybrid microgrids are the best options among all other types of microgrids because they make it possible to use AC and DC sources and loads together and at the same time. Since connecting AC and DC sections of a hybrid microgrid leads to the interaction between them, proper control of interlinking converter(s) is crucial for the reliable and stable operation of the entire microgrid. In this study, small signal model of a hybrid AC/DC microgrid consisting of static and... 

Transformers are vital components of power networks, responsible for converting electricity from one voltage level to another. They are designed to operate under sinusoidal conditions, but the increasing use of non-linear loads and power electronic devices has caused a spike in harmonic pollution in power networks. These harmonics result in increased losses in transformers, leading to a reduction in capacity, accelerated aging, and a shortened life. The associated costs incurred by these issues include reduced energy sales for electricity companies and premature replacement costs. In this thesis, it is assumed that the transformer is affected by harmonics and ambient temperature, and other... 

Electrical systems are prone to voltage or current unbalance due to unbalance loads or asymmetries in network elements such as untransposed lines. Voltage and current unbalance can cause malfunction or reduction in lifetime of different elements. Voltage unbalance has been studied in a variety of publications, due to its high importance and significant effects. However, current unbalance has not gained much attention despite its undeniable influence on transformers’ and line’s lifetime. Observing high values of current unbalance in LV systems is predictable and inevitable. On the other hand, HV systems must be much more balance due to their different structure and absence of large unbalance... 

Harmonic distortions are one of the most important power quality issues in power systems. Nowadays, vast peneteration of the power electronic devices and nonlinear loads, which are the harmonic-producing devices in a network, causes increase in harmonic distortion level in distribution systems, which has negative impacts. The high level of harmonic peneteration in power system can lead to increase in losses and reduction of network equipment’s life. So, system operators have to find and improve some solutions to control the harmonic level. In this way, investigation in harmonic contribution of major nonlinear loads and utilities is a criticl task, specially when a harmonic problem occures.... 

Power Quality can be simply defined as the quality of voltage at electrical loads. Detection and classification of voltage and current disturbances is of high importance in power system protection and monitoring. This procedure cannot be implemented by operators because of the high volume of the data which must be processed. So, it is needed to automate this procedure. Systems designed for this purpose usually contain three main parts: feature generation, feature selection and classifier design. The algorithms used for feature generation for power quality disturbances are mainly based on discrete Fourier transformation or discrete wavelet transformation. These approaches have shown some... 

Modeling and control of power electronic converters, due to their non-linear nature, are from important research subjects in power electronics. Power electronic systems contain both continuous-time dynamics and discrete-time inputs, and therefore, are categorized as hybrid systems. Conventionally, modeling and control of hybrid systems is done by linearizing and state-space averaging. However, these methods do not guaranty well operation of the system over a wide range of load or input variation. In this project, by employingMLD modeling method, a model for an inverter which is connected to a three-phase RL load through an LC filter, is derived. Then, this model is used in an optimization... 

In this thesis a new method to determine the accuracy of CVT in measuring harmonics has been proposed. Capacitive voltage transformers are known as an important tool in measuring the harmonics. However, the frequency response of CVT contains cut-off and pass characteristics, because of the resonances of the internal elements. One of the main problems is in determining the frequency response of HV voltage source capable of accurate change to apply to CVT. The proposed method reduces the voltage level of the HV source. Also, the source structure and the effect of the additional elements are described and the accuracy of the method has been evaluated by simulation and experimental results  

The need for independent power generation has been increased in recent years with the growing demand in micro grid systems. Losses in power systems are unavoidable. These losses occur in transmission to distributionnetworks from the generation sites to the end-user loads. Reduction of power losses has been one of the reasons for engineers to be attracted by micro grid systems which include small size independent power generation units. However, in a micro grid with several DGs of different types and with all kinds of loads of variable nature, an optimal power balance in the system has to be achieved. This optimal objective, which results in minimal energy losses over a specific period of... 

This research presents a new harmonic model for compact fluorescent lamps and other nonlinear loads. In recent years, nonlinear loads have gained widespread acceptance due to their characteristics. Although CFLs consume less energy, they draw extremely distorted current waveforms resulting in harmonic losses in distribution networks. The presented harmonic Norton models in this work are simple and accurate models which can consider the impact of supply voltage harmonics as well. The parameters of non linear appliances model are determined based on measurements. The characteristics of the proposed models make it suitable for analyzing the impacts of large number of CFLs or other non-linear... 

The extensive use of nonlinear loads such as motor drives and new generation of home appliances has caused a deterioration of power quality in power systems. The use of active power filters due to the fast variation of the loads has been recommended by power engineering. In this method, the harmonic profiles are controlled by applying active filters in a distribution feeder. Optimal placement of active filters in a distribution system due to the increase of harmonic generation loads has attracted researchers. However placement of each filter is the major concern that researchers face. In this paper, a new objective function is introduced to determine the optimal number and location of active... 

Nowdays, distributed generation (DG) systems based on renewable energy sources have significantly increased around the world as an alternative solution to enhance the power quality and reliability and reduce the need for extensive transmission system as well. An evolved recent concept called microgrid is developed by grouping a cluster of loads and DG units in a common local area which increases the integaration of sustainable energy generation systems. This thesis presents control of a medium voltage (MV) microgrid considering multi-hybrid distributed generation systems (MHGS) and renewable energy resources in island mode. The first part is focused on the development a control strategy for... 

In this thesis, a model is presented to detect and monitor the rotor bar's condition of large motors. This proposed model uses two diagnostic methods MCSA and ZCT, to extract the fault components. The input of the proposed model is only the motor current at two levels of 80% and 100% of the nominal motor load, which by using the two methods MCSA and ZCT and making changes in how to use them can be the disadvantages of other methods such as incorrect detection of rotor bars in Large motors with variable load, the harmonical stator voltage (or the presence of the drives) and asymmetric conditions. The extracted components are classified using two learnable algorithms, the k-NN algorithm and...