ARBITRARY WAVEFORM MULTILEVEL GENERATOR FOR HIGH VOLTAGE HIGH FREQUENCY PLASMA ACTUATORS

Filopimin Andreas Dragonas - Ciclo XXVI - 2014

Abstract - This dissertation presents the theory and the conducted activity that lead to the construction of a high voltage high frequency arbitrary waveform voltage generator. The generator has been specifically designed to supply power to a wide range of plasma actuators. The system has been completely designed, manufactured and tested at the Department of Electrical, Electronic and Information Engineering of the University of Bologna. The generator structure is based on the single phase cascaded H-bridge multilevel topology and is comprised of 24 elementary units that are series connected in order to form the typical staircase output voltage waveform of a multilevel converter. The total number of voltage levels that can be produced by the generator is 49. Each level is 600 V making the output peak-to-peak voltage equal to 28.8 kV. The large number of levels provides high resolution with respect to the output voltage having thus the possibility to generate arbitrary waveforms. Maximum frequency of operation is 20 kHz. A study of the relevant literature shows that this is the first time that a cascaded multilevel converter of such dimensions has been constructed. Isolation and control challenges had to be solved for the realization of the system. The biggest problem of the current technology in power supplies for plasma actuators is load matching. Resonant converters are the most used power supplies and are seriously affected by this problem. The manufactured generator completely solves this issue providing consistent voltage output independently of the connected load. This fact is very important when executing tests and during the comparison of the results because all measures should be comparable and not dependent from matching issues. The use of the multilevel converter for power supplying a plasma actuator is a real technological breakthrough that has provided and will continue to provide very significant experimental results.

Peak-to-Peak Output Current Ripple Analysis in Multiphase and Multilevel Inverters

Jelena Loncarski- Ciclo XXVI - 2014

Abstract - Analysis of the peak-to-peak output current ripple amplitude for multiphase and multilevel inverters is presented in this PhD thesis. The current ripple is calculated on the basis of the alternating voltage component, and peak-to-peak value is defined by the current slopes and application times of the voltage levels in a switching period. Detailed analytical expressions of peak-to-peak current ripple distribution over a funda¬mental pe¬riod are given as function of the modulation in¬dex. For all the cases, refer¬ence is made to centered and symmetrical switching patterns, generated either by carrier-based or space vector PWM. Starting from the definition and the analysis of the output current ripple in three-phase two-level inverters, the theoretical developments have been extended to the case of multiphase inverters, with emphasis on the five- and seven-phase inverters. The instantaneous current ripple is introduced for a generic balanced multiphase loads consisting of series RL impedance and ac back emf (RLE). Simplified and effective expressions to account for the maximum of the output current ripple have been defined. The peak-to-peak current ripple diagrams are presented and discussed. The analysis of the output current ripple has been extended also to multilevel inverters, specifically three-phase three-level inverters. Also in this case, the current ripple analysis is carried out for a balanced three-phase system consisting of series RL impedance and ac back emf (RLE), representing both motor loads and grid-connected applications. The peak-to-peak current ripple diagrams are presented and discussed. In addition, simulation and experimental results are carried out to prove the validity of the analytical developments in all the cases. The cases with different phase numbers and with different number of levels are compared among them, and some useful conclusions have been pointed out. Furthermore, some application examples are given.

Sanjeevikumar Padmanaban- Ciclo XXIV - 2012

Abstract - In this thesis work a new version of multiphase-multilevel ac motor drive system is proposed. In detail, power section unit consists of four standard two-level voltage source inverters (VSIs) and asymmetrical six-phase induction motor (dual three-phase), with open-end stator windings. A dedicated control aspect based on synchronous reference frame was developed, which provides the sharing capabilities of the total motor power among the four VSIs in each switching cycle with three degree of freedom. Furthermore, fault-tolerant capabilities of the entire ac drive system was tested based on developed post-fault operating condition by exploiting three degree of freedom. Complete set of simulation results are provided with symmetrical/asymmetrical power sharing and faulty VSI conditions. Hardware prototype model of the quad-inverter conversion system was implemented with two three-phase passive loads in open-end configuration using two TMS320F2812 DSP controllers. The developed McBSP (multi-channel buffered serial port) communication algorithm was able to control the four VSIs for PWM communication and synchronization. Open-loop control scheme based on inverse three-phase decomposition approach was proposed to control entire quad-inverter configuration and tested in both balanced and unbalanced operating conditions with simplified PWM techniques.  

Darko Ostojic - Ciclo XXII - 2010

Abstract - A new conversion structure for three-phase grid-connected photovoltaic (PV) generation plants is presented and discussed in this Thesis. The conversion scheme is based on two insulated PV arrays, each one feeding the dc bus of a standard 2-level three-phase voltage source inverter (VSI). Inverters are connected to the grid by a traditional three-phase transformer having open-end windings at inverters side and either star or delta connection at the grid side. The resulting conversion structure is able to perform as a multilevel VSI, equivalent to a 3-level inverter, doubling the power capability of a single VSI with given voltage and current ratings. 

Gabriele Grandi - Ciclo VI - 1993

Abstract - In questa Tesi di Dottorato saranno analizzati alcuni tipi di azionamenti per macchine asincrone con elevate prerogative dinamiche. In particolare verranno descritte diverse Metodologie per il controllo della coppia sia con schemi del tipo ad "orientamento di campo" sia con schemi del tipo a "controllo diretto di coppia". Sarà quindi esaminata la possibilità di operare senza l'impiego di trasduttori di posizione e/o velocità nelle diverse condizioni di funzionamento della macchina. Per un corretto e stabile comportamento dell'azionamento, saranno studiate alcune tecniche di adattamento in linea dei parametri che ne caratterizzano maggiormente il funzionamento. Il lavoro teorico sarà dapprima supportato da simulazioni numeriche condotte grazie ad alcuni programmi di calcolo appositamente creati. I risultati di tali simulazioni saranno poi validati sperimentalmente grazie alla realizzazione di alcuni prototipi presso l'Istituto di Elettrotecnica dell'Università degli studi di Bologna.