Significant advances in modern ac/ac power converter technologies and demands of industries have reached beyond standard ac/ac power converters with voltage-source inverters fed from diode rectifiers. Power electronics converters have been matured to stages toward compact realization, increased high-power handling capability, and improving utility interface. Modern ac/ac power converter topologies with various control strategies have been introduced for the further improvements, such as matrix converters, current-fed converters, PWM rectifiers, and active power filters. In this dissertation, several new converter topologies are proposed in conjunction with developed control schemes based on the modern ac/ac converters which enhance performance and solve the drawbacks of conventional converters. In this study, a new fault-tolerant PWM strategy is first proposed for matrix converters. The added fault-tolerant scheme would strengthen the matrix converter technology for aerospace and military applications. A modulation strategy is developed to reshape output currents for continuous operation, against fault occurrence in matrix converter drives. This study designs a hybrid, high-performance ac/ac power converter for high power applications, based on a high-power load commutated inverter and a mediumpower voltage source inverter. Natural commutation of the load commutated inverter is actively controlled by the voltage source inverter. In addition, the developed hybrid system ensures sinusoidal output current/voltage waveforms and fast dynamic response in high power areas. A new topology and control scheme for a six-step current source inverter is proposed. The proposed topology utilizes a small voltage source inverter, to turn off main thyristor switches, transfer reactive load energy, and limit peak voltages across loads. The proposed topology maximizes benefits of the constituent converters: highpower handling capability of large thyristor-based current source inverters as well as fast and easy control of small voltage source inverters. This study analyzes, compares, and evaluates two topologies for unity power factor and multiple ac/ac power conversions. Theoretical analyses and comparisons of the two topologies, grounded on mathematical approaches, are presented from the standpoint of converter kVA ratings, dc-link voltage requirements, switch ratings, semiconductor losses, and reactive component sizes. Analysis, simulation, and experimental results are detailed for each proposed topology.

Call number: LD2668 .T4 1984 S63 / Master of Science

published_or_final_version / Electrical Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Electrical Engineering / Master / Master of Philosophy

published_or_final_version / abstract / Electrical and Electronic Engineering / Master / Master of Philosophy

published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

No description available.

No description available.

This thesis presents and investigates Space Vector Modulation (SVM) switching strategies for (i) a multi-level Diode-Clamped Converter (DCC) and (ii) a multi-module Voltage-Sourced Converter (VSC) system in which each module is a conventional two-level VSC. Although the SVM strategies are general and applicable for n-level DCC and n-module VSC systems, this text only concentrates on five-level DCC and four-module VSC systems. For a five-level DCC, a computationally efficient SVM algorithm is proposed. The algorithm, that is based on a classifier Neural Network (NN), reduces the computational time for the SVM realization. Therefore, adequate saving of processor execution time, in each sampling period of SVM, is provided to carry out other functions, e.g. the calculations required for DC-capacitor voltage balancing task. The thesis also proposes a DC-capacitor voltage balancing strategy to counteract the voltage drift phenomenon of (i) a passive-front-end five-level DCC, and (ii) a back-to-back connected five-level DCC system. The proposed balancing strategy, that is based on augmenting the proposed SVM algorithm, takes advantage of the redundant switching states to minimize a quadratic cost function associated with voltage deviations of the DC-capacitors. The salient features of the proposed balancing strategy are (i) online calculation of SVM to select the best switching states, (ii) minimization of switching frequency, (iii) minimization of the THD content of the AC-side voltage, and (iv) no requirement for additional power circuitry. For a four-module VSC system a sequential sampling SVM strategy is proposed. The proposed strategy (i) provides harmonic cancellation/minimization at the net AC-side voltage of the multi-module VSC system, and (ii) offers a low switching frequency for each VSC module. Technical feasibility of the proposed SVM strategies for a five-level DCC and a four-module VSC system, as a STATCOM and a back-to-back HVDC system, are investigated and presented. The studies are conducted in the time-domain, in the PSCAD/EMTDC software environment.

This thesis investigates in detail a multilevel cascaded STATCOM employing indirect voltage control and square-wave control in combination as control strategy. An analytical steady-state model of a multilevel converter in harmonic domain is developed that can be used, for example, for resonance studies within a multilevel converter, or/and in studying interactions between a multilevel STATCOM and the host ac grid (like harmonic resonance). A systematic method is developed for STATCOM system design and optimization of STATCOM system parameters. The focus lies on minimizing losses, minimizing voltage. Total Harmonic Distortion (THD) and minimizing dc voltage ripple. Analytical formulae are presented that can be used to calculate the best value of each STATCOM system parameters. A discrete and an analytical dynamic converter model of a multilevel converter are developed to enable dynamic and/or stability studies. For the discrete model, the operating modes of a single-cell are analysed in detail and emulated using signal generators and integrators. The analytical multilevel converter model is segmented into a dynamic and static part in order to represent accurately all internal feedback connections. A general approach is developed for dynamic modelling of a STATCOM system. The dynamic system model has modular structure, and the controller gains are selected by analyzing the root locus of the analytical model to give optimum responses. The model is very accurate in the sub-synchronous range, and it is adequate for most control design applications and practical stability issues below 100 Hz. The controller robustness is also studied where the analytical STATCOM system model is used to perform eigenvalue analysis and to design controllers. Two different advanced control techniques are investigated and compared against the conventional method of proportional integral (PI) feedback voltage control.