الفهرس 
COVEROnly 14 pages are availabe for public view
 |  | from | 116 |  |  |
| | | from | 116 | | |
Abstract
Several studies have been conducted in this thesis; these studies can be summa-rized as follows:
In chapter 1: presents an introduction about wind energy, its advantages, his-torical review on the progress of wind energy, wind energy in Egypt, Furthermore explains the different component of the wind turbine and its different generator technologies
The aerodynamic model of wind turbine is developed in chapter 2. Further-more, a review of maximum power point tracking algorithms for wind turbine DFIG is illustrated.
Chapter 3 explain the theory of operation of DFIM. There are four modes of operation depending on the machine’s mechanical speed and the produced torque. The feature of the four modes are illustrated, furthermore three different models for the induction machine (1st order, 3rd order and 5th order models) are presented. These models are capable of predicting the behaviour of induction machines.
In chapter 4 the different control and protection schemes of the wind turbine based DFIG are explained and implemented on PSCAD/EMTDC software. The control schemes include, pitch angle control and power converter control. The pitch angle control is used to maintain the machine‘s power to its rated value in high wind speed. The power converter is made up of a back-to-back PWM con-verter connecting the rotor circuit and the grid. The rotor side converter is used to control active and reactive powers. Grid side converter is used to regulate the DC-link voltage at constant value. Furthermore different protection schemes are mod-elled, namely, the crowbar model for over current protection and DC chopper cir-cuit model for over current protection. The dynamic behaviour of the wind turbine system and its controls was investigated for step changes in wind speed variations.
In chapter 5 the steps of the proposed aggregation technique are given. The technique is used to simplify the full system representation (a system consisting of three identical wind turbines) into single turbine representation (aggregated mod-el). The validity of the proposed aggregation method was proved by the compari-
88
son of the full system representation and single turbine representation results. The dynamic behaviour of the systems was investigated when the wind turbines are
c) Subjected to the same wind speeds and speed variations.
d) Subjected to different wind speeds with different speed variations.
The dynamic behaviour of the wind turbine under different levels of voltage dips is studied in chapter 6. This study is carried out for different feeder length.
Based on the results of the thesis, the following conclusions have been drawn:
• The aerodynamic model, the DFIM models, wind turbine system with variable-speed variable-pitch control model (turbine model, generator model, converter model and the relay protection model) and the aggregated model of wind tur-bines are developed on PSCAD/EMTDC software.
• The Dynamic behaviour and steady state model of the asynchronous machine are developed The 1st order model (the steady-state performance) are repre-sented when the rotor of the machine is fed with different direct- and quadra-ture-axis components of control voltages, The 3rd and 5th order models are simulated to determine the dynamic behaviour of the asynchronous machine (a) when it is connected to an infinite bus and operating with normal conditions and (b) when the induction machine is subjected to three-phase symmetrical faults.
• 1st order model is simpler ,less computational effort and gives the same steady – state results of the 3rd and 5th order models.
• The 5th order model provides more accurate results for the stability analysis.
• The 5th order model requires computation time and simulation efforts more than that requires in 3rd order model.
• The results demonstrate the importance of selecting the machine model espe-cially in wind turbine systems.
• The dynamic behaviour of the wind turbine system was investigated during step change in wind variations. The injected rotor voltages are derived from PI con-trollers that regulate the active and reactive powers delivered by the generator. The speed is adjusted to maximize the power generated at a given wind speed, the pitch angle controller is used to regulate the turbine power not to exceed its maximum rated value
• Tuning the controllers associated with different wind turbine system greatly en-hances their dynamic performance.
89
• When the same wind speed conditions applied to the single turbine representa-tion (STR) is also applied to the full system representation (FSR), the simula-tion results of the STR and the FSR are very close. This proves the validity of the method of aggregation used to develop the STR of a wind farm.
• When using the FSR and the STR, the outputs of real and reactive powers at the point of common coupling match for both representations. Thus, modelling the wind farm with an equivalent representation preserves the basic response of the wind farm.
• A simplified single‐machine equivalent model of a large wind farm (that in-cludes an equivalent collector system model) preserves the net steady state and dynamic behaviour of the actual installation. The method of aggregation can be extended to derive equivalent models for a wind farm with different types and sizes of wind turbine. The post transient region is the more important period of simulation because it gives an indication of survivability of the system. In the post transient response, generally the STR and FSR show the same response (damping, settling time, etc.).
• The maximum time before losing the stability of WT generators due to a fault is reduced as higher voltage dips occur. Also, the effect of diversity of cable length is negligible. This can be attributed to the fact that the feeder impedance is not the dominant impedance between the generator and the fault if compared to the impedance of the LV/MV and MV/HV transformers.
• Thus, assuming the relay protection at each individual turbine is set to the same settings among the turbines in a wind farm, the turbines disconnected from the grid during a fault will depend on the severity of the fault and the location of the turbine within the wind farm.