Sharif Digital Repository

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... 

Considering the increasing development of renewable power plants including solar and wind power plants in the country, the need to investigate the effects of these power plants on the power grid is more needed and attention than before. Since these power plants are connected to the distribution network by means of power electronics, this problem has caused the injection of harmonics into the network, which can be transferred to higher voltage levels (upper distribution and transmission network). Harmonics in networks cause increased losses and reduced efficiency, false operation of protective relays, and damage to equipment due to the creation of harmonic intensifications. The purpose of... 

This dissertation is concened with the stability analysis and improvement in active power distribution grids. In active power grids, voltage stability challenges may occur due to the dynamic interactions among power electronic converters. In this research, power electronic converters are modeled in canonical form to investigate the effect of control loops on their impedance characteristics. The concept of dynamic interaction is explained from the impedance pint of view, which is the base of the stability criteria and imopedance coordinatioan. The concept of passivity is also explained and combined with the dynamic interaction to detect unstable modes from the DC bus impedance (Zbus)... 

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... 

With the increasing expansion of modern DC loads and utilizing energy storage systems along with distributed renewable energy resources, grid-following bidirectional voltage-sourced converters (GFBVSCs) with fast dynamic performance are required. Due to the presence of single-phase loads and asymmetrical short circuit faults, unbalanced grid voltage conditions are available in the distribution system. Under unbalanced conditions, an oscillating component with a frequency of twice the grid angular frequency appears on the DC side. Removing double-frequency ripple in the DC link voltage without third harmonic current injection and reactive power injection to the grid based on the grid codes to... 

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... 

Condition monitoring of a system refers to the process of measuring and analyzing the aging indicators of critical components of a system that can be harnessed to estimate the system’s health. One of the main interests in condition monitoring is to performing the scheduled maintenance activities for the system. Monitoring the status of the system reduces maintenance costs and prevents unwanted contingencies and outages in the system.
In power electronics, the capacitors are the most critical components of the system from the point of reliability. Capacitors are the leading cause of failure in the power electronic converters so, this essay attempts to attain a solution to monitor the... 

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... 

In urban rail (subway) substations, high voltage dc (750 V) is usually used to feed the traction motors of the wagons. This type of feeding raises issues, including the creation of stray currents in different parts of the structure. The cause of the stray current is the voltage difference between the two points of the rails, caused by the current flowing inside the rails. These currents accelerate the corrosion process in and around metallic equipment. One of the most dangerous issues is the corrosion of gas pipes near the subway that has caused disasters in the past. The stray current cannot be eliminated and its maximum reduction requires high costs. For this reason, different standards... 

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... 

Environmental problems and limitation in fossil fuels have increased the application of renewable energy sources (RES) and distributed generations (DG). Due to the DC nature of most loads and DGs, DC microgrids provide a suitable structure to integrate different kinds of DGs and loads. Undesirable voltage regulation and poor power sharing among the sources in conventional voltage-based control methods are solved by the superimposed droop frequency method. However, the superimposed droop frequency scheme suffers from three major problems which are (i) instability in terms of load variation, (ii) limitation in the system maximum loading which is due to the limitation in the transferred... 

In this research, a new step-by-step and comprehensive method for the design of back-to-back converters in the presence of nonlinear and constant power loads is proposed. Common methods for the design and control of back-to-back converters consider the design procedure independent of the linearity and dynamic of the load, also, the control system design and performance of the load-side and grid-side inverters of back-to-back converters are assumed to be independent of each other which is shown in this thesis that this assumption is not valid in the presence of nonlinear loads. These methods result in challenges and problems regarding stability and power quality indices of back-to-back... 

The growing usage of nonlinear loads has resulted in increasing voltage and current harmonic levels in distribution systems. Additionally the faults occurring in the distribution system cause voltage sag at the point of customer supply. These two power quality disturbances impose considerable costs on consumers through disrupting the correct performance of sensitive loads, causing unintended outages and decreasing the devices’ useful lifetime. Besides the aforementioned power quality issues, the electric power loss has a determining role in operational costs. In this thesis, based on feeder reconfiguration and shunt passive filters, a strategy is proposed in order to reduce the average... 

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... 

Electric motors used in electrical vehicles are varied, but in light electric vehicles such as electric motors and electric bikes, with limited space, permanent magnet motors due to their high power density and efficiency are very useful. These motors are divided into two types: trapezoidal and sinusoidal due to the waveform of the Back EMF voltage. The trapezoidal type, also known as the brushless DC motor , is more commonly used in light electric vehicles due to the simplicity of the control system and the lower cost. The control system of the BLDC motor is very important in its performance. Providing a controller for these engines that at low cost improves their performance is considered... 

Ilam refinery has three synchronous generator units each with a capacity of 26.5 MVA and two six-phase LCI-fed synchronous compressors, each with a capacity of 8 MW. Generators of Ilam gas refinery have instability issue in case of island operation. With launching one generator unit and then connecting two compressors the instability problem and tripping of the generators appear. Given the importance of this issue, various tests and corresponding measurements in both electrical and mechanical aspects were carried out at the plant in on-grid and island operations to analyze the cause of instability phenomena. The mechanical part of tests analyses the frequency of oscillations and vibration in... 

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... 

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.... 

Non-linear loads have been increasing in industries resulting in high levels of harmonics in power networks. Harmonics have several negative effects including higher energy losses and lifespan reduction in equipment due to thermal loss. To reduce the levels of harmonics and consequently reduce the losses in power networks different methods of harmonic suppression have been employed. These methods are normally used at the customer’s side. In all of these methods, the idea is to prevent harmonics from flowing into a power network at the Point of Common Coupling (PCC). This research tries to reduce the harmonic losses by controlling the harmonic flow in a power network. Therefore, the solution... 

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...