Submitted by Renata Lopes (renatasil82@gmail.com) on 2017-04-24T19:07:44Z No. of bitstreams: 1 pedromachadodealmeida.pdf: 10367147 bytes, checksum: 04b7cf913c75cb9f82395bf7b9769825 (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2017-04-25T15:25:19Z (GMT) No. of bitstreams: 1 pedromachadodealmeida.pdf: 10367147 bytes, checksum: 04b7cf913c75cb9f82395bf7b9769825 (MD5) / Made available in DSpace on 2017-04-25T15:25:19Z (GMT). No. of bitstreams: 1 pedromachadodealmeida.pdf: 10367147 bytes, checksum: 04b7cf913c75cb9f82395bf7b9769825 (MD5) Previous issue date: 2013-11-29 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / A presente tese contribui para a análise, modelagem e projeto de controladores discretos de um sistema de geração fotovoltaico de 30 kWp conectado à rede elétrica sem filtros passivos. O conversor fonte de tensão (VSC) de interface é interligado a rede elétrica usando somente as indutâncias de dispersão de um banco de transformadores monofásicos como filtros harmônicos. Modelos discretos são desenvolvidos tanto para o lado CC quanto para o lado CA do conversor. A modelagem do lado CA foi feita nos sistemas de coordenadas αβ0 e dq0. Já a modelagem da dinâmica do lado CC foi feita no sistema de coordenadas dq de acordo com balanço de potência entre os terminais do VSC. Baseado nos modelos obtidos, duas estratégias básicas foram investigadas e discutidas para projetar os compensadores discretos usados para controlar as correntes sintetizadas por um sistema de geração fotovoltaico no modo de corrente. Resultados experimentais mostram que o uso apenas de controladores lineares, proporcional–integral (PI) e proporcional–ressonante (PR), sintonizados na componente fundamental não é suficiente para manter a qualidade das correntes geradas dentro dos padrões internacionais, devido a operação não linear do transformador de conexão. Para contornar o problema anterior duas soluções foram investigadas: (i) inclusão de múltiplos controladores ressonantes nas coordenadas αβ; e (ii) inclusão de um controlador repetitivo em paralelo com o controlador PI nas coordenadas dq. Resultados experimentais mostraram que ambas estratégias são adequadas para compensar as componentes harmônicas. Finalmente, foi proposta uma estratégia para controlar o conversor durante faltas assimétricas (Fault–ride through) e eliminar as oscilações no barramento CC durante condições de desbalanço. O controlador proposto é composto por uma parcela PI e duas parcelas ressonantes, as quais controlam as componentes média e oscilante, através da injeção correntes de sequencia positiva e negativa na rede, respectivamente. Resultados de simulação mostram que o controlador proposto é adequado para eliminar as oscilações no barramento CC sem prejudicar as estabilidade do sistema. / The current thesis contributes to the analysis, modelling and design of discrete time controllers which aim is to control a 30 kWp photovoltaic dispersed generation system connected to the electric grid without passive filters. In fact, the interface voltage– sourced converter (VSC) is connected to the grid using only the leakage inductance of a single–phase transformer bank as harmonic filters. Initially, discrete time models are developed to the converter’s DC–side as well as to the AC–side. The AC–side modelling is performed on αβ0 and dq0 coordinate systems. On the other hand, the DC–side dynamics are modeled on the dq frame according to the power balance between the converter’s terminals. Based on the models obtained, strategies to control the converter in the current mode control on the αβ and dq are developed and a methodology to design the controllers are addressed in details. Experimental results shown that only the use of linear controllers, proportional–integral (PI) and proportional–resonant (PR), tuned on the fundamental component are not sufficient to guarantee the quality of the generated currents according to international standards. This is due to the operation of the connection transformer in a nonlinear region. In order to overcome this drawback, two solutions are taken into account: (i) inclusion of several parallel resonant controller in αβ frame; and (ii) inclusion of a repetitive controller in parallel with the PI controller in the dq frame. Experimental results shown that both strategies are suitable to compensate the harmonic components on the output current. Finally, a strategy is proposed to control the system under asymmetrical faults (fault–ride through) and to mitigate the voltage oscillation on the DC–side during unbalance conditions. The proposed controller is composed of a PI part and two resonant parts, which controls the average and the oscillating voltage components, through the injection of positive and negative sequence currents into the grid, respectively. Simulation results shown that the proposed controller is suitable to mitigate the DC–side voltage oscillations without jeopardizing the system stability.

Adding Power Electronic Interfaced Devices (PEID) generation to grids is an increasing trend because of the concurrent development of better power electronic converters and a greater interest in a better utilisation of energy resources. Small and dispersed energy sources that would previously not be worth introducing into the grid is becoming more and more viable and other potential benefits such as better control of voltage levels and smoothing out load changes also spur this development. But while there are great potential benefits of the controllability of these devices there are also risks when existing protection systems are made for the linear behaviour of traditional synchronous generators. This thesis describes the peculiarities of the short circuit behaviour of PEID generators and how this affects the short circuit energy levels in terms of short circuit current, I2t and incident arc energy. Using simulation, it is shown that in the case of the specific mine grid studied, the incident arc energy increases substantially and that this need to be considered when evaluating installation of PEID generation. / Användningen av nätansluten omformardriven generering ökar alltmer i takt med att bättre omformare utvecklas och intresset för ett effektivare nyttjande av energiresurser ökar. Små och utspridda energiresurser som tidigare inte var värda att ta vara på tillgängliggörs alltmer, och fördelar som bättre spänningsreglering och lastutjämning driver på utvecklingen. Men med de fördelar som kommer av omformarnas reglerbarhet så kommer också risker beroende av deras olinjäritet, eftersom existerande skyddssystem är anpassade till det linjära beteendet hos traditionella synkrongeneratorer. Den här avhandlingen behandlar säregenheterna i kortslutningsbeteendet hos effektelektroniska omformare och hur det påverkar kortslutningsenergin i bemärkelsen I2t, händelseenergin vid ljusbågar samt kortslutningsströmmen. Via dynamisk simulering så visas att händelseenergin i vissa fall kan öka avsevärt och att detta behöver övervägas vid installation av omformardriven generering.

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.

In order to reduce emissions and achieve sustainable energy systems, renewable energy is increasingly being integrated into the power grid. However, the integration of renewable energy into the grid poses several challenges, including maintaining a stable power supply under changing and unpredictable conditions. Low Voltage Ride Through (LVRT) assesses a generator's ability to maintain stable voltage during grid voltage drops, which is crucial for renewables due to their low inertia and vulnerability to voltage disruptions caused by changes in wind or sunlight. LVRT requirements are defined by regional grid codes and regulations, which vary in their diversity. A study was conducted using Matlab Simulink to model and simulate the LVRT phenomenon on Siemens Energy's medium gas turbine generator. The entire power system generation system was simulated to observe the system's response and the generator's behavior during LVRT events. A previous gas turbine power plant project in Romania, delivered by Siemens Energy in Finspång, was simulated for analysis and compared against the grid code requirements. The findings indicated that the Siemens Energy gas turbine model SGT-750 satisfies the Romanian LVRT grid code requirements.