Sharif Digital Repository

Conventional induction motors are controlled by stator voltage, so the inverter should pass the full power of the motor. In the doubly-fed rotor wound induction motors, the nominal power of inverter can be reduced. Therefore, these types of machines have been concerned in recent decades and they are able to use as alternative for high power conventional induction machines in the future. Direct torque control is one of the well known methods for controlling the induction motors. Fast torque response and independence of machine parameters (except winding resistance). In this thesis, different sterategies of direct torque control of doubly fed induction motors are presented. In the first... 

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.... 

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... 

AC/DC/AC converters are widely being used in a variety of power applications. Continuity of service of these systems, as well as their reliability and performance are now of the major concerns. Indeed, the failure of the converter can lead to the total or partial loss of the control of the phase currents and can cause serious system malfunction or even shutdown. Thus, uncompensated faults can quickly endanger the system. Therefore, to prevent the spread of the fault to the other system components and to ensure continuity of service, fault tolerant converter topologies associated with quick and effective fault detection and compensation methods must be implemented. In this thesis, the... 

In this project, the main purpose is attaining the optimum controller for DFIG under permanent voltage unbalances. To achieve this purpose, firstly, the wind turbines’ control strategies, and main control subsystems including DFIG’s controller, Pitch control system, as well as Yaw’s control system are reviewed. The efficient control algorithms to achieve the control purposes for these three subsystems are introduced. To continue, it is focused on generator’s controller, and the different methods besides their performance are discussed. In order to improve DFIG’s controller performance under permanent grid voltage’s unbalance condition, at first, the problems of operating during such a... 

This study presents the application of the doubly fed induction generator (DFIG) to reduce the frequency deviation and improve frequency of a power system. This has been done by proposing a wind turbine model and a new fuzzy controller. The fuzzy controller has been suggested to change the speed of the wind turbine rotor and modify the impacts of wind speed fluctuations on the output power. The fuzzy controller parameters are optimized using the genetic algorithm (GA) to achieve the appropriate transient response. A smart controller is combined with fuzzy controller to optimize the usage of wind turbine’s kinetic energy. Furthermore, the DFIG with the proposed controller participates in the... 

Advanced control techniques are required to achieve a cost-effective and reliable use of the wind power generation. The wind turbines are generally controlled based on two control objectives: the turbine protection and the generation of acceptable power for the utility grid. These objectives are achieved if the control inputs are applied based on appropriate control logics. In this work, a nonlinear multivariable model of the wind turbine with a DFIG generator is considered. The rotor speed and the d-axis rotor current (as the control outputs) are controlled via manipulation of the two generator voltages (as the control inputs) in low wind velocity condition. For high wind velocity, the... 

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... 

Considerable attention has been given in recent years to renewable energy sources due to concerns about dwindling fuel reserves and the potential impact of conventional energy systems on the environment. Wind power is one form of renewable energy resources and also considered as Dispersed Generation (DG). Widespread utilization of wind power imposes many effects on planning and operation of power system. In other side, reliability evaluation and enhancement is an important factor in modern power system planning and operation. So, reliability assessment of wind turbines is of great importance and will receive more attention in the future according to increase of WT utilization. The... 

Concerns about fossil fuels and the consequent effects of burning such fuels on the environment have drawn attention of different countries around the world to the development of renewable energy systems, and it has encouraged researchers to focus on expanding the methods of using different sources of renewable energy. One of the most important renewable energies is wind energy, which is generated by wind turbines. A cluster of wind turbines forms a wind farm. Wind turbines can be equipped with a supervisory control system that sends the data measured by the sensors to the operator. Design of supervisory control algorithm for a wind farm requires a model of the wind farm system. The task of... 

The purpose of this project is designing a controller for a floating offshore wind turbine to exctract maximum power from wind energy in the first operating region of wind turbine. The NREL 5 MW OC3-Hywind floating wind turbine with BARGE platform will be used for this project. By using Turbsim software, proper wind profile for the first operating region of wind turbine will be produced. This profile will be excecuted for the FAST software that is simulates the mechanical part of wind turbine. By implementing the doubly fed induction generator in Simulink enviroment and designing the controller for doubly fed induction generator, the desired torque for maximizing the exctraction power of... 

Low-frequency oscillation damping in power system is main determinants of the stability of the system. According to increasing of wind farms with high penetration levels in the power system, participation and control of wind farms for increase damping inter area oscillations seems important. In this study a damping controller suggested for Doubly Fed Induction Generator (DFIG). Using this controller and only available local signal as input controller and combination of both active and reactive power modulation, can greatly increase damping inter area oscillations and stability of uncertainty and different working points. Using linear analysis system, identified modes oscillating system and...