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Thesis (M.S. in Electrical Engineering)--Naval Postgraduate School, December 2009. / Thesis Advisor(s): Julian, Alexander L. Second Reader: Cristi, Roberto. "December 2009." Description based on title screen as viewed on January 28, 2010. Author(s) subject terms: Double Fed Induction Generator (DFIG), Space Vector Modulation (SVM), wind turbine, Field Programmable Gate Array (FPGA), bi-directional power flow. Includes bibliographical references (p. 75). Also available in print.

Thesis (M.S.)--Tennessee Technological University, 2009. / Title from title page screen (viewed on June 29, 2010). Bibliography: leaves 147-152.

Among the renewable energies, wind energy presents the highest growth in installed capacity and penetration in modern power systems. This is why reliability of wind turbines becomes an important topic in research and industry. To this end, condition monitoring (or health monitoring) systems are needed for wind turbines. The core of any condition monitoring system (CMS) are fault diagnosis algorithms whose task is to provide early warnings upon the occurrence of incipient (small magnitude) faults. Thanks to the use of CMS we can avoid premature breakdowns and reduce significatively maintenance costs. The present thesis deals with fault diagnosis in sensors of a doubly-fed induction generator (DFIG) for wind turbine (WT) applications. In particular we are interested in performing fault detection and isolation (FDI) of incipient faults affecting the measurements of the three-phase signals (currents and voltages) in a controlled DFIG. Although different authors have dealt with FDI for sensors in induction machines and in DFIGs, most of them rely on the machine model with constant parameters. However, the parameter uncertainties due to changes in the operating conditions will produce degradation in the performance of such FDI systems. In this work we propose a systematic methodology for the design of sensor FDI systems with the following characteristics: i) capable of detecting and isolating incipient additive (bias, drifts) and multiplicative (changes in the sensor gain) faults, ii) robust against changes in the references/disturbances affecting the controlled DFIG as well as modelling/parametric uncertainties, iii) residual generation system based on a multi-observer strategy to enhance the isolation process, iv) decision system based on statistical-change detection algorithms to treat the entire residual and perform fault detection and isolation at once. Three novel sensor FDI approaches are proposed. The first is a signal-based approach, that uses the model of the balanced three-phase signals (currents or voltages) for residual generation purposes. The second is a model-based approach that accounts for variation in the parameters. Finally, a third approach that combines the benefits of both the signal- and the model-based approaches is proposed. The designed sensor FDI systems have been validated using measured voltages, as well as simulated data from a controlled DFIG and a speed-controlled induction motor. In addition, in this work we propose a discrete-time multiple input multiple output (MIMO) regulator for each power converter, namely for the rotor side converter (RSC) and for the grid side converter (GSC). In particular, for RSC control, we propose a modified feedback linearization technique to obtain a linear time invariant (LTI) model dynamics for the compensated DFIG. The novelty of this approach is that the compensation does not depend on highly uncertain parameters such as the rotor resistance. For GSC control, a LTI model dynamics is derived using the ideas behind feedback linearization. The obtained LTI model dynamics are used to design Linear Quadratic Gaussian (LQG) regulators. A single design is needed for all the possible operating conditions.

Due to growing concerns over climate change, more and more countries are looking to renewable energy sources to generate electricity. Therefore, wind turbines are increasing in popularity, along with doubly-fed induction machines (DFIGs) used in generation mode. Current grids codes require DFIGs to provide voltage support during a grid fault. The fault ride-through (FRT) capability of DFIGs is the focus of this thesis, in which modifications to the DFIG controller have been proposed to improve the FRT capability. The static synchronous compensator (STATCOM) controller has been applied with proposed method to study its influence on the voltage at the point of common coupling (PCC). The proposed method was also compared with other FRT capability improvement methods, including the conventional crowbar method. The simulation of the dynamic behaviour of DFIG-based wind turbines during grid fault is simulated using MATLAB/Simulink. The results obtained clearly demonstrate the efficacy of the proposed method.

Στις μέρες μας το ενεργειακό πρόβλημα αποτελεί ένα από τα σπουδαιότερα προβλήματα του πλανήτη, το οποίο μαζί με την ραγδαία κλιματική αλλαγή οδηγούν στην ανάγκη για παραγωγή ηλεκτρικής ενέργειας με όσο πιο οικονομικό αλλά και φιλικό προς το περιβάλλον τρόπο. Συνεπώς οι Ανανεώσιμες Πηγές Ενέργειας και κυρίως η αιολική ενέργεια αποκτούν σπουδαίο ρόλο στο στόχο αυτό και αλλάζουν άρδην τόσο τον Ευρωπαϊκό όσο και τον Παγκόσμιο ενεργειακό χάρτη. Στην παρούσα διπλωματική εργασία μελετάμε μια διάταξη ανεμογεννήτριας, η οποία αποτελείται σε πρώτη φάση από μια επαγωγική μηχανή βραχυκυκλωμένου κλωβού (SCIG), από έναν μετατροπέα συχνότητας (back-to-back converter) και από ένα σε σειρά συνδεδεμένο τριφασικό R-L φίλτρο στην ac πλευρά μέχρι το ζυγό διασύνδεσης με το δίκτυο, ενώ στη συνέχεια προσθέτουμε, μετά το R-L φίλτρο, μια συστοιχία πυκνωτών συνδεδεμένων σε αστέρα, ένα τριφασικό R-L φορτίο στο ίδιο σημείο με τη συστοιχία πυκνωτών καθώς και τη γραμμή μεταφοράς για σύνδεση με το δίκτυο. Σκοπός λοιπόν αυτής της εργασίας είναι να μελετήσουμε το πλήρες σύστημα της ανεμογεννήτριας στο d-q στρεφόμενο σύστημα κάθετων αξόνων μέσω του μετασχηματισμού Park. Στη συνέχεια προχωρούμε στον υπολογισμό των εξισώσεων στο χώρο κατάστασης και τελικά προτείνουμε ένα σύστημα ελέγχου το οποίο βασίζεται στους PI ελεγκτές. Τέλος, προσομοιώνουμε το πλήρες σύστημα με το Simulink του MATLAB και στη συνέχεια παρουσιάζουμε τις αποκρίσεις των προσομοιώσεων και εξάγουμε συμπεράσματα. / Nowadays the energy problem is one of the most important global problems, which together with the rapid climate change lead to the need for power generation as more economical and environmentally friendly way. Therefore, the Renewable Energy Sources and especially wind power have a great role in this objective and radically change both the European and the global energy map. In this paper we study a wind turbine device, which consists at first of a squirrel cage induction generators (SCIG), a frequency converter (back-to-back converter) and an three-phase R-L filter on the grid side and then me add, after the filter, a capacitor bank connected in star, a three-phase R-L load at the same point with the capacitor banks and the transmission line for grid connection. Aim of this thesis is to study the complete wind turbine system in the d-q rotating vertical axes system through the Park transformation. Then we proceed to the calculation of the equations in state space and eventually propose a control system based on PI controllers. Finally, we simulate the complete system with the MATLAB/Simulink and then we present simulations of the responses and draw conclusions.

The proposed hybrid system generates AC power by combining solar and wind energy converted by a doubly-fed induction generator (DFIG). The DFIG, driven by a wind turbine, needs rotor excitation so the stator can supply a load or the grid. In a variable-speed wind energy system, the stator voltage and its frequency vary with wind speed, and in order to keep them constant, variable-voltage and variable-frequency rotor excitation is to be provided. A power conversion unit supplies the rotor, drawing power either from AC mains or from a PV panel depending on their availability. It consists of a multilevel inverter which gives lower harmonic distortion in the stator voltage. Maximum power point tracking techniques have been implemented for both wind and solar power. The complete hybrid renewable energy system is implemented in a PSIM-Simulink interface and the wind energy conversion portion is realized in hardware using dSPACE controller board.

In this thesis first the behavior of induction generators under unbalanced voltage conditions is studied. In this context, first the existing schemes for characterizing the unbalance in simple ways have been studied. The two schemes IEC, NEMA which are in use to quantify the unbalance, as an unbalance factor require the use of complex algebra. A new scheme to characterize voltage unbalance, without using any complex algebra has been proposed and its effectiveness is compared with the NEMA scheme. The performance of the induction generators under unbalanced voltage conditions is first studied using the symmetrical component approach and a detailed simulation. The machine currents for the same unbalanced conditions are estimated using the proposed scheme and its effectiveness is highlighted. Existing approximate schemes based on unbalance factor do not consider the phase differences between positive and negative sequence voltages while estimating currents. A method of including this feature is proposed here. A simple way to find the maximum line current among the three phases as a function of the positive and negative voltages has been proposed. Knowing the positive, negative sequence voltages and the angle between them, a scheme to compute the generator currents under unbalanced conditions is proposed. Finally a few affecting the machine heating due to unbalanced currents have been studied. The nature of the induction generator fault currents is studied to understand their implication on the system performance. First the fault currents that have been recorded from experiments on relatively small induction motors(26kW and 122kW) are compared with those obtained through detailed dynamic simulation, in order to validate the usage of the detailed dynamic simulations to study the fault current behavior of the induction machines in the absence of experimental or field records. In order to assess the impact of error in the measured machine parameters on the estimated fault current values, the sensitivity of the estimated values of fault currents to machine parameter variations is investigated. The fault currents in large grid connected induction generators (3MW to 500kW) have been estimated through detailed simulation. The effect of the instant of fault (w.r.t voltage cycle), input load levels and shunt capacitors on the fault current values is investigated. An eigen value analysis of the generator model has been carried out to understand the reasons for the observed behavior using the linearized machine model. Fitting functions have been adopted to quantify the fault currents, in order to facilitate comparison of fault currents. A method of including induction generators into the system fault studies has been proposed. Methods of choosing proper sequence impedances for representing the induction generator have been developed and validated using simulated values of balanced and unbalanced fault currents. Simulations of self excited generators have been used for determining the unbalanced fault currents, so as to ensure that the induction generator fault currents correspond to only those contributed by the machine. The impact of the induction generators penetration on the short circuit levels is investigated considering a 14 bus test system by incorporating the proposed induction generator models in a system level fault study.

Διάφοροι τύποι αιολικών συστημάτων . Έμφαση σε αιολικά συστήματα με ανεμογεννήτριες μεταβλητής ταχύτητας και κυρίως στην επαγωγική μαχανή διπλής τροφοδοσίας . Περιγραφή του ηλεκτρονικού μετατροπέα που χρησιμοποιείται σε αυτού του είδους το σύστημα . Έλεγχος της ισχύος και προσομοίωση για δύο τύπους ελεγκτών , PI και RST . Σύγκριση των αποκρίσεων . Απόκριση PI ελεγκτή για διάφορα κέρδη . / Different types of wind energy systems . Wind energy systems with variable speed wind turbines are studied . The doubly fed induction generator . Description of the power electronic converter which is used . Power control for two regulator types , PI and RST . We compare the dynamic performances . Dynamic performance for different values of the PI regulator parameters .