This thesis provides insight into state-of-the-art Modular Multilevel Converters (MMC) for medium and high voltage applications. Modular Multilevel Converters have increased in interest in many industrial applications, as they offer the following advantages: modularity, scalability, reliability, distributed location of capacitors, etc. In this study, the modeling, control and design considerations of modular based multilevel converters, with an emphasis on the reliability of the converter, is carried out. Both modular multilevel converters with half-bridge and full-bridge sub-modules are evaluated in order to provide a complete analysis of the converter. From among the family of modular based hybrid multilevel converters, the newly released Alternate Arm Converter (AAC) is considered for further assessment in this study. Thus, the modular multilevel converter with half-bridge and full-bridge power cells and the Alternate Arm Converter as a commercialized hybrid structure of this family are the main areas of study in this thesis. Finally, the DC fault analysis as one of the main issues related to conventional VSC converters is assessed for Modular Multilevel Converters (MMC) and the DC fault ride-through capability and DC fault current blocking ability is illustrated in both the Modular Multilevel Converter with Full-Bridge (FB) power cells and in the Alternate Arm Converter (AAC). Accordingly, the DC fault control scheme employed in the converter and the operation of the converter under the fault control scheme are explained. The main contributions of this study are as follows: The new D-Q model for the MMC is proposed for use in the design of the inner and outer loop control. The extended control scheme from the modular multilevel converter is employed to control the Alternate Arm Converters. A practical reliability-oriented sub-module capacitor bank design is described based on different reliability modeling tools. A Zero Current Switching (ZCS) scheme of the Alternate Arm Converter is presented in order to reduce the switching losses of the Director Switches (DS) and, accordingly, to implement the ZCS, a design procedure for the Arm inductor in the AAC is proposed. The capacitor voltage waveform is extracted analytically in different load power factors and the waveforms are verified by simulation results. A reliability-oriented switching frequency analysis for the modular multilevel converters is carried out to evaluate the effect of the switching frequency on the MMC's operation. For the latter, a DC fault analysis for the MMC with Full-Bridge (FB) power cells and the AAC is performed and a DC fault control scheme is employed to provide the capacitor voltage control and DC fault current limit, and is illustrated herein. / Master of Science

Στα πλαίσια της διδακτορικής διατριβής μελετήθηκε η αποτελεσματικότητα διαφόρων κυκλωμάτων ελέγχου που βασίζονται στην υπολογιστική νοημοσύνη με σκοπό την αντιμετώπιση βραχυκυκλωμάτων σε δίκτυα διασύνδεσης ανεμογεννητριών με το δίκτυο. Πιο συγκεκριμένα, τα προτεινόμενα συστήματα ελέγχου έχουν σκοπό τη διαμόρφωση κατάλληλων συνθηκών ώστε οι ανεμογεννήτριες να καταφέρουν να συνεχίσουν να είναι συνδεδεμένες στο δίκτυο κατά τη διάρκεια και αμέσως μετά τα βραχυκυκλώματα, συνεισφέροντας στη γρήγορη επαναφορά της τάσης στο Σημείο Κοινής Σύνδεσης με το Δίκτυο (ΣΚΣΔ). Στο πρώτο μέρος της διατριβής μελετήθηκε ένα προσαρμοζόμενο ασαφές σύστημα ελέγχου με σκοπό τη βελτιωμένη απόκριση Αιολικού Πάρκου (ΑΠ) με επαγωγικές γεννήτριες που τροφοδοτεί ένα ασθενές σύστημα ηλεκτρικής ενέργειας μέσω διασύνδεσης ΕΡ/ΣΡ/ΕΡ με Μετατροπείς Πηγής Τάσης (ΜΠΤ). Το σύστημα αυτό εντοπίζει τη σοβαρότητα του σφάλματος και διαμορφώνει ανάλογα την παλμοδότηση των βαλβίδων των ΜΠΤ κατά τη διάρκεια του σφάλματος. Επίσης, έχει την ιδιότητα να αυτορυθμίζεται κατά τη διάρκεια της μετασφαλματικής περιόδου, επιτυγχάνοντας εξασθένηση της ταλαντωτικής συμπεριφοράς του συστήματος που προκαλείται από τα βραχυκύκλωμα και την παραμονή των ανεμογεννητριών στο δίκτυο. Το ηλεκτρικό σύστημα που μελετήθηκε στο δεύτερο μέρος της διδακτορικής διατριβής περιλαμβάνει μια επαγωγική ανεμογεννήτρια διπλής τροφοδότησης (γνωστή με την ονομασία double-fed induction machine) η οποία τροφοδοτεί ένα δίκτυο ηλεκτρικής ενέργειας. Στη βιβλιογραφία που έχει δημοσιευθεί μέχρι σήμερα, για την αντιμετώπιση των βραχυκυκλωμάτων σε ανάλογα ηλεκτρικά συστήματα, προτείνονται διατάξεις οι οποίες βασίζονται είτε σε κατάλληλο μηχανικό εξοπλισμό όπως μπάρες βραχυκύκλωσης (crowbars) είτε σε κατάλληλο προγραμματισμό των ελεγκτών. Σε αυτό το μέρος της διατριβής προτείνεται ένα εναλλακτικό σύστημα ελέγχου που βασίζεται στον κατάλληλο προγραμματισμό των ελεγκτών, χωρίς να χρησιμοποιεί κάποιον εξοπλισμό προστασίας. Το σύστημα ελέγχου, το οποίο βασίζεται στους γενετικούς αλγορίθμους, συμβάλει στη βέλτιστη «συνεργασία» των δύο ΜΠΤ της γεννήτριας, επιτυγχάνοντας την εξασθένιση των διακυμάνσεων της τάσης στο ΣΚΣΔ και τη διατήρηση της σύνδεσης της γεννήτριας στο ηλεκτρικό δίκτυο. / This thesis studies the implementation of intelligent control techniques in the Fault Ride-Through (FRT) of grid connected Wind Turbines (WTs). The first part of the dissertation studies the issue of the fault ride-through capability of a wind farm of induction generators, which is connected to an ac grid through an HVDC link based on Voltage Sourced Converters (VSCs). This work proposes a control strategy which is implemented with adaptive fuzzy controllers and deals with every different type of fault with a corresponding appropriate action, blocking the converter valves for a time interval which depends on the severity of the fault. In addition, after the deblocking of the valves, the proposed control system activates a special controller, which alleviates the oscillations at the electrical system caused by the blocking of the valves. In this way, the overcurrents are limited, the wind turbines manage to remain connected and the ac voltage recovers quickly, as it is imposed by national grid codes. The second part of the dissertation proposes a Computational Intelligence–based control strategy, to enhance the low voltage ride-through capability of grid-connected WTs with doubly fed induction generators (DFIGs). The conventional crowbar-based systems that were initially applied in order to protect the rotor-side converter at the occurrence of grid faults, do not fulfill the recent requirement of the national GCs that the WTs should supply reactive power to the grid during and after the fault, in order to support the grid voltage. In order to conform to the above mentioned requirement, this work proposes a control scheme, which contributes to the optimal coordination of the two converters, aiming to attenuate the disturbances to the system caused by the fault and ensure system stability. Aiming to encounter the difficulties met due to the uncertainties of the system modeling and considering the non linearity of the system, the controllers were designed based on fuzzy logic and genetic algorithms, which are more efficient in such cases. By this concept the overcurrents at the rotor windings and the dc side overvoltages are effectively eliminated. In addition, the FRT requirement concerning the reactive power supply is fulfilled.

This thesis deals with the modeling, analysis and control of a novel brushlessgenerator for wind power application. The generator is named as rotatingpower electronic brushless doubly-fed induction machine/generator (RPEBDFIM/G). A great advantage of the RPE-BDFIG is that the slip power recoveryis realized in a brushless manner. This is achieved by introducing an additionalmachine termed as exciter together with the rotating power electronicconverters, which are mounted on the shaft of a DFIG. It is shown that theexciter recovers the slip power in a mechanical manner, and delivers it backto the grid. As a result, slip rings and carbon brushes can be eliminated,increasing the robustness of the system, and reducing the maintenance costsand down-time of the turbine. To begin with, the dynamic model of the RPE-BDFIG is developed andanalyzed. Using the dynamic model, the working principle of the generatoris understood and its operation explained. The analysis is carried out atspeeds, ±20% around the synchronous speed of the generator. Moreover, thedynamics of the generator due to external load-torque disturbances are investigated.Additionally, the steady-state model is also derived and analyzed forthe machine, when operating in motor mode. As a next step, the closed-loop control of the generator is considered indetail. The power and speed control of the two machines of the generator andthe dc-link voltage control is designed using internal model control (IMC)principles. It is found that it is possible to maintain the stability of thegenerator against load-torque disturbances from the turbine and the exciter,at the same time maintain a constant dc-link voltage of the rotor converter.The closed-loop control is also implemented and the operation of the generatorwith the control theory is confirmed through experiments.In the third part of the thesis, the impact of grid faults on the behaviourof the generator is investigated. The operation of the generator and its responseis studied during symmetrical and unsymmetrical faults. An approachto successful ride through of the symmetrical faults is presented, using passiveresistive network (PRN). Moreover, in order to limit the electrical and mechanicaloscillations in the generator during unsymmetrical faults, the dualvector control (DVC) is implemented. It is found that DVC to a certain extentcan be used to safeguard the converter against large oscillations in rotorcurrents. Finally, for completeness of the thesis, a preliminary physical design ofthe rotating power electronic converter has been done in a finite elementsoftware called ANSYS. The thermal footprint and the cooling capability,with estimates of the heatsink and fan sizes, are presented. Besides, another variant of a rotating electronic induction machine whichis based on the Lindmark concept and operating in a single-fed mode is also investigated. It’s steady-state model is developed and verified through experiments. / <p>QC 20151006</p>

This thesis deals with the steady-state, dynamic and control aspects of new type of brushless configuration of a doubly-fed induction machine in which the slip rings and carbon brushes are replaced by rotating power electronics and a rotating exciter. The aim is to study the stability of this novel configuration of the generator under mechanical and grid disturbances for wind power applications. The derivation, development and analysis of the steady-state model of the brushless doubly-fed induction machine with a rotating excitor and the power electronic converters mounted on the shaft and rotating with it, is studied. The study is performed at rated power of the generator between ±20% slip range. Moreover unity power factor operation between ±20% speed range is also discussed. Furthermore dynamic modeling and control aspects of the generator are also analyzed. The controllers were designed using Internal Model Control principles and vector control methods were used in order to control the generator in a closed-loop system. It is shown that through the use of proper feedback control, the generator behaves in a stable state both at super-synchronous and sub-synchronous speeds. Moreover Low Voltage Ride Through of the generator during symmetrical and unsymmetrical voltage dips is also investigated. Passive Resistive Network strategy is employed for Low Voltage Ride Through of the generator during symmetrical voltage dips. On the other hand, Extended Vector Control is used in order to control the negative sequence currents during unsymmetrical voltage dips. Suppression of negative sequence currents is important as they cause extra heating in the windings and affects the lifetime of the mechanical and electrical components of the generator and system due to oscillations in power and torque. In addition to the above studies a steady-state model of a single-fed induction machine is also developed and investigated where the rotating exciter is removed and the rotor windings are short-circuited through the two rotating power electronic converters. In this way the slip power circulates in the rotor and with the help of the two rotating electronic converters, rotor current is used to magnetize the induction machine thereby improving the power factor. The steady state model is verified through experimental results. / <p>20120914</p> / Brushless Wind Generator with Rotating Power Electronic Converters