Sharif Digital Repository

Hierarchical voltage control is one of the conventional kinds of coordinated voltage control schemes which has been studied in some countries in the last decades. In this method network is divided into several voltage control regions and in each region voltage profile is controlled near optimal level, satisfying the economic and secure goals of the network performance. In this thesis, after the explanation of the main concepts of this scheme, its effects on the network voltage control is assessed and its performance is improved. First the primary concepts of hierarchical voltage control are explained. Then, the methods of the control regions determination and pilot nodes selection are... 

In this thesis, dynamic behavior of the Iran’s power grid has been analyzed and inter-area oscillations of this grid have been studied. Important results of this thesis are inter-area oscillation modes characteristics, regions and coherent generators and tie-lines in expose of high amplitude oscillations. These results could be used in different applications. It is time consuming and complicated to apply complete model for dynamic analyzing, therefore in most of researches an equivalent dynamic model should be prepared. First step for constructing this grid is identifying coherent generators. There are different methods to identify coherent ones, but it is not possible to apply all of them... 

Permanent-magnet synchronous generators have attracted more attention in recent years due to their special advantages which make them a suitable option for variable-speed wind turbines.Low-voltage ride through capability is one of the main concerns in widespread use of wind turbines in electric grids. Nowadays wind turbines are expected to remain connected to the grid during voltage dips and support the voltage recovery process by supplying reactive power.This thesis aims at behavior analysis and performance improvement of permanent-magnet synchronous generator-based wind turbines during grid faults. A simple method is presented to overcome the over current problem of the grid-side converter... 

One of the main objectives in operating a power system is to maintain the system voltage properly to avoid equipment damage and transfer power efficiently. Automatic control of transmission network voltage provides significant improvements in security, quality and efficiency of power system operation. This thesis investigates coordinated voltage control of power systems in normal, alert and emergency states. The aim is to propose a comprehensive framework for real-time operation of practical power systems, ensuring secure operation of the system in the normal state and after occurrence of sever contingencies.
At the first, a new coordinated voltage control scheme is proposed to ensure... 

Poorly damped Low Frequency electromechanical Oscillations (LFOs) confine the capability of power system transmission lines. This phenomenon, which is the result of dynamic interactions among generator groups, may make the system prone to small- signal instability in some cases. Due to the use of some system controllers such as AVRs, these oscillations are inevitable.
Hence, electromechanical modes detection and assessing stability margin are important requirement in power system planning and operation, and can provide significant help to power system operators by preventing stability problems.
From the distant past, in order to monitor small-signal stability, power system operators... 

In recent years, the architecture of power systems has evolved in such a way to ease the use of Wide Area Measurement Systems (WAMS) to monitor real time situation of the grid. One of the most prevalent approaches for monitoring power systems is using data gained from Phasor Measurement Units (PMUs) and implementing advanced signal processing algorithms. By this way, tracking the electromechanical oscillatory modes of systems and stability evaluation become attainable. Unlike the model-based methods of estimating electromechanical modes, in which non-linear differential equations are linearized around the operation point, measurement-based methods lead to more accurate and continuously... 

In this study, control and protection of low voltage microgrids have been analyzed. Studied microgrid takes the advantages of inverter based distributed generation. One of the most important issues in inverter based microgrids is the control of its distributed generation. Since, the low voltage networks have single phase loads and many imbalances, these imbalances can affect the control of IBDGs (inverter based distributed generations). This issue in the islanded mode of operation is the most important, because in this mode of operation, IBDGs control the network voltage and frequency.
In this thesis, the different control strategies of IBDGs are reviewed, and then modified to address... 

Nowadays, development of renewable electricity generation technologies, the advent of small efficient generators and increasing number of grid-connected consumers, have caused increasing interest in energy production next to the consumers locations. Distributed generation (DG) is often defined as small-scale generators that produce several kilowatts to tens of megawatts of electricity. One primary reason for using DG is its potential to reduce power losses in transmission and distribution networks. Several loss reduction methods such as capacitor placement, network reconfiguration and load management are used in distribution systems. Employing DG results in the highest loss reduction and... 

This project aims to evaluate the performance of turbine generators of various kinds that are used in Manjil wind farm, based on the measured results over a period of operation and comparing them with simulation results of the same turbine generators. It also contains a comparative study with newer types of variable speed. The results of this study can be useful to examine the actual results of operation and design characteristics of the existing turbine, and also the possible advantages of using variable speed wind turbines at Manjil wind farm. Other goal of this project is to evaluate the performance of wind farm thorough turbines’ simulation results and measured result of plant’s fiders... 

The most important task of a power system is generating and transmitting electricity for consumers, according to electricity quality standards. One of these quality criteria is preservation of grid frequency close to nominal value with continuity of energy delivery to consumers. Electricity is a specific commodity that its generation and consumption should be equal continually; otherwise the network frequency will change and may become unstable, causing severe damage to the network equipment. There for different control and protective actions are deployed to maintain balance between generation and consumption, and thus to keep grid frequency close to nominal value. Characteristics of... 

Nowadays, According to the improvement of technology, especially in power electronics, electric shipboards are preferred over traditional ones. Better maneuverability, lower emission, higher efficiency and reliability are the most conspicuous merits of this type of ships. Notwithstanding the fact that electric shipboards have many advantages, some serious problems may arise in these ships. Since the thruster motors of these ships are usually induction motors and they are fed by the microgrid of the ship, and taking into ACcount that thruster motors consume a huge amount of the generated power by the synchronous generators, the concept of infinite bus is inexistent in these microgrids. Hence,... 

Considering the continuous growth of renewable energy sources in electric grids, the importance of utilizing simple, generic, and upgradable models for different network stability studies is increasing. In recent years, generic and standard wind turbine models for transient and short-term frequency and voltage stability studies have been developed. In this regard, researchers are still working on promotion of the generic models, especially to expand them for unbalanced transient studies, improve the transient response, and include inertia response. In this thesis, different methods for generic modeling of wind turbines are surveyed and the development procedure of WECC and IEC generic model... 

In this thesis, a new wind turbine scheme based on Electrically-Excited Synchronous Generator (EESG) and diode-bridge rectifier is proposed and developed. This new scheme is mainly based on the idea of generator excitation current employment as a means of wind turbine control. That is why the new scheme is called Excitation Controlled Synchronous Generator (ECSG) wind turbine. The most important aspects of the proposed ECSG wind turbine scheme are cost-effectiveness, simplicity and higher reliability, as well as higher efficiency. Considerable cost reduction for the converter unit of ECSG wind turbine is so important since costly converter has been one of the main challenges of the... 

In this thesis, performance of doubly fed induction generators in wind turbine, under stator unbalanced voltage condition has been studied. Therefore, beside of presenting related equations and models for DFIG, drawbacks of DFIG performance under unbalanced voltage condition on variables such as electric torque, active and reactive stator power and stator current have been shown. Then, controller have been designed in the way that can eliminate electric torque oscillation, active power oscillation, negative sequences of stator current and rotor current. The controller is designed base on decoupling sequences in synchronous reference and stator voltage orientation method(SVO) that consist of... 

Variable speed wind turbines (VSWTs) provide the capability of wide speed operation and independent control of active and reactive power. In contrast to the fixed speed WTs, they have higher efficiency, power quality and controllability. VSWTs are mainly divided in two categories: WTs based on doubly fed induction generator (DFIG) and WTs based on full converter with permanent magnet synchronous generator.
At now, among the different alternatives to obtain VSWTs, DFIGs are the most commonly used. This is because the voltage source converter (VSC) in DFIG has to handle a fraction of the total power under steady state conditions, and thus the size and cost of the converter is reduced.... 

In this thesis, DC transmission of offshore wind farms is studied thoroughly. Various system configurations that are introduced in the literatures are discussed and their control scheme for normal conditions and grid faults are studied. Then, the HVDC transmission based on voltage source converter, located between a local offshore ac grid and the onshore ac system is investigated as the most practical choice. The HVDC linke circuit elements are calculated and the control strategies of the offshore and onshore converters are introduced. DFIGs are considered to be the best choice for the offshore wind turbine generators. Then, the system is simulated and the frequency of the offshore grid is... 

In Modular Multilevel converter, it is necessary to attain different objectives simultaneously. One of the most effective control methods which can cope with this kind of systems is the Model Predictive Control (MPC). This method predicts the values of control objectives based on different feasible states and determines the optimal one using cost functions. The number of feasible states can increase when the number of switches increases in the converter. This makes the MPC unable to be applied in the MMCs with hundereds of submodules (SMs) in each arm because of its heavy computational burden. In this thesis, a modified model predictive control is proposed to reduce the computational burden.... 

Nowadays, microgrids which comprise decentralized generation (DG) have attracted considerable attention. Establishment of DG-based microgrids that are normally connected to the main grid, is a practical method to reduce the costs of power outage and to increase the electricity supply reliability. The aim of this thesis is optimum planning of a microgrid containing wind turbines, photovoltaic systems and diesel generators, considering both the grid-connected and isolated modes of operation.
In the areas with considerable solar radiation and wind potential valuable opportunities exist in utilizing these resources. In addition to these renewable resources, the use of controllable resources... 

Implementation of various incentive-based demand response strategies has great potential to decrease peak load growth and customer electricity bill cost. Using advanced metering and automatic demand management makes it possible to optimize energy consumption, to reduce grid loss, and to release generation capacities for the sake of providing sustainable electricity supply, and reducing electricity bills for the customers. With these objectives, this thesis examines the previously suggested load scheduling programs in both residential and industrial sectors, and proposes new practical strategies for load management. This thesis has mainly focuses on three subdivisions of load management... 

Environmental problems as well as energy sustainability concerns, have forced many countries to provide part of their energy from Renewable Resources (RR). Solar energy has gained a lot of attention because of the huge energy stored in the sun and its availability. Since Energy conversion via PV panels causes less environmental problems, today, hundreds of large-scale PV parks are installed and planned worldwide. Since a single PV panel provides only up to 400 Watts of power, a PV park contains a large number of PV panels. Augmenting this large number of PV panels and converting their output power to AC using DC/DC and DC/AC converters should be in a form that not only the investment cost is...