Modelling and Control of Doubly Fed Induction Generator Based Stand-Alone Wind Energy Conversion System

The application of wound rotor induction machine is widely spread in wind energy generating stations because of its adaptability for variable speed wind turbines through which maximum possible extraction of wind energy is possible. Also among all the induction generator configurations for wind power systems the use of Doubly Fed Induction Generator (DFIG) configuration with back to back pulse width modulated voltage source converters (VSC) is one of the best topologies available and it is suitable for both grid connected systems as well as stand-alone systems. Here only stand-alone application of DFIG is considered. In this thesis mathematical modelling of doubly fed induction machine is presented. The control strategies for both stator side converter and rotor side converter are developed in stator flux oriented reference frame. The dynamics of dc link voltage build-up phase is also included. The stator side converter is used to control the output voltage in direct voltage control manner and the rotor side converter is current controlled where the power imbalance of the system is nullified using dc link voltage controller which modifies the quadrature axis rotor current reference value according to the changes in the wind speed as well as the load. Two control algorithms are presented out of which one gives the best performance for all kinds of loads (balance, unbalance, linear and non-linear) and the other gives poor load regulation and unwanted distortions in the output voltage for non-linear and unbalanced load

Modelling and simulation of a grid connected doubly fed induction generator for wind energy conversion system

The Doubly Fed Induction Generator (DFIG) based wind turbine with variable speed variable-pitch control scheme is the most popular wind power generator in the wind power industry. This machine can be operated either in grid connected or standalone mode. A thorough understanding of the modelling, control, and dynamic as well as the steady state analysis of this machine in both operation modes is necessary to optimally extract the power from the wind and accurately predict its performance. In this thesis, first a three phase PWM voltage source converter models expressed in the ABC and the DQO synchronous reference frame with its control schemes are developed and analysed. Then a DFIG-based wind turbine model connected to a constant voltage and frequency grid is developed in the Matlab/Simulink software in detail and its corresponding generator and turbine control structure is implemented. A thorough explanation of this control structure as well as the steady state behaviour of the overall wind energy conversion system which includes the aerodynamic models of the wind turbine, the DFIG models and the three-phase two-level PWM voltage source converter models are presented. A developed control schemes are also necessary to achieve useful output power from the WECS. These control schemes include the generator-side converter control, the grid-side converter control, the pitch angle control and the maximum power point tracking control. The grid-side converter controller is used to maintain the constant voltage across the capacitor and produce a unity power factor operation of the grid. The generator-side converter controller is used to regulating the torque, active power and reactive power

Modeling and Control of a Doubly-Fed Induction Generator for Wind Turbine-Generator Systems

Wind energy plays an increasingly important role in the world because it is friendly to the environment. During the last decades, the concept of a variable-speed wind turbine (WT) has been receiving increasing attention due to the fact that it is more controllable and efficient, and has good power quality. As the demand of controllability of variable speed WTs increases, it is therefore important and necessary to investigate the modeling for wind turbine-generator systems (WTGS) that are capable of accurately simulating the behavior of each component in the WTGS. Therefore, this thesis will provide detailed models of a grid-connected wind turbine system equipped with a doubly-fed induction generator (DFIG), which includes the aerodynamic models of the wind turbine, the models of the mechanical transmission system, the DFIG models and the three-phase two-level PWM voltage source converter models. In order to obtain satisfying output power from the WTGS, control strategies are also necessary to be developed based on the previously obtained WTGS models. These control schemes include the grid-side converter control, the generator-side converter control, the maximum power point tracking control and the pitch angle control. The grid-side converter controller is used to keep the DC-link voltage constant and yield a unity power factor looking into the WTGS from the grid-side. The generator-side converter controller has the ability of regulating the torque, active power and reactive power. The maximum power point tracking control is used to provide the reference values for the active power at the stator terminals. The pitch angle control scheme is used to regulate the pitch angle and thus keep the output power at rated value even when the wind speed experiences gusts. Various studies in the literature have reported that two-level converters have several disadvantages compared with three-level converters. Among the disadvantages are high switching losses, high dv/dt, and high total harmonic distortion (THD). Hence, the models and field oriented control schemes for three-level neutral-point-clamped (NPC) converters are also investigated and applied to a WTGS. Besides, an advanced modulation technology, namely, space vector PWM (SVPWM), is also investigated and compared to traditional sinusoidal PWM in a WTGS

Simulation and Experimental Realization of Adaptive Controllers for Shunt Active Power Filter to improve Power Quality

As of late, the demand for electric power is increasing, which has developed a greater demand to maintain a higher level of power quality and continuity of power supply at the consumer end. But increased use of power electronic devices has imperatively degraded the overall power quality of the power system. Due to the non-linear nature of the power electronic devices various current and voltage harmonics are generated, causing harmonic distortion. These harmonics cause various undesirable effects such as equipment heating, nuisance tripping, overheating transformer, data losses, etc. Shunt Active Power Filters are a viable solution to mitigate these harmonics and thus improve the power quality. In this thesis work, various control strategies of shunt active power filter based on voltage and current controller has been presented to mitigate the current harmonics. To extract the three phase reference source current we have developed control algorithm based on Synchronous reference frame theory (id-iq) and Self Tuning Filter. For regulating the DC capacitor bus voltage various voltage controllers such as PI, PID, Fuzzy and Adaptive Fuzzy PID controllers has been developed. While to generate the gate signal of SAPF multiple current controllers such as Hysteresis band current controller, adaptive hysteresis band current controller, weighted adaptive hysteresis band current controller and Lyapunov function based stable current controller has been developed. To analyze their performance, simulation models of these controllers have been developed using Matlab/Simulink for different operating conditions. A complete hardware setup of the three phase shunt active power filter has been developed using dSPACE 1104 to verify the credibility of the proposed controllers

Modelling and control of united power flow controller for reinforcement of transmission systems

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.The work involved in the thesis is concentrated on modelling and control of UPFC. The overall objective is to provide effective methods and tools for assessing the impact of UPFC in the reinforcement of transmission systems. The thesis clarifies modelling and control of UPFC into several subproblems, in which the associated models, algorithms and control strategies of UPFC have been systematically reviewed. An electromagnetic transient prototype model of the UPFC has been set up by using its detailed power electronic device as well as its internal closed-loop controller. The problems encountered in the process of building such a model and the way of handling them by EMTP have been discussed. This EMTP-based simulator of SPWM UPFC implemented has provided a useful tool to assist the development and validation of more detailed and practical model of the UPFC for further studies. The steady-state modelling and control for the UPFC has been developed, including: (i) The power injection model of the UPFC suitable for its implementation in an optimal multiplier power flow computation method has been derived in rectangular form. The effectiveness of the proposed algorithm has been compared with the user defined model method. (ii) A systematic method for deriving the control capabilities of the UPFC has been proposed based on predicting the feasibility limit of the system. Using an index derived from optimal multiplier, three dimensional diagrams describing the ranges have been obtained. The results are also verified through the singular value decomposition algorithm. (iii) A power injection model based control method (PIM) has been proposed and implemented to directly derive the UPFC parameters as so to achieve the control objectives. The assumptions, algorithmic process and validation of the PIM have been investigated in detail. Its pros and cons are also discussed. (iv) Five internal limits of the UPFC device have been derived as the constraints to its performance. A complete set of control rules considering these limits as well as their implementation in the PlM have been constructed to form the basis of optimal UPFC control strategies for its steady-state local control. All the above proposed methods are tested and validated on the IEEE 30-bus system, a practical 306-bus system and a meshed network. The thesis concludes by suggesting the future research areas in further UPFC studies

High performance control of a three-phase PWM rectifier

Ph.DDOCTOR OF PHILOSOPH