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- Fault Tolerant DC-DC ConvertersPublication . Bento, Fernando José Figueiredo; Cardoso, António João MarquesTechnology advancement verified in recent times is flagrant, specially in our home appliances. This advancement brought to us new electronic equipment and other DC-compatible appliances with improved capability for energy management, using electronic converters for such purpose. These loads have in common the fact that, at a certain point, they need to transform the AC energy of the grid to DC. Furthermore, an important increase in the distributed generation of energy has been witnessed. The majority of these systems produce energy in DC. These two statements, combined with the increased pressure related to the need of energy efficient systems, will certainly trigger, in a near future, the adoption of district-scale DC grids that connect DC generation plants and consumers, in an effort to reduce the number of conversion steps required to deliver power to a DC appliance and, at the same time, limit power losses arising from the energy transportation using conventional AC grids. In a future where DC grids will be used, several DC voltage levels will be required to allow the connection of the different load profiles that require DC. The inclusion of DC-DC converters will allow the creation of these voltage levels. The reliability of such converters plays a key role, as it ensures service continuity for the DC loads connected to them and, at the same time, preserve the quality of the energy delivered by these converters. With the reliability levels increase as a goal, this work uses an open-circuit fault-diagnostic method suitable for several DC-DC converter topologies. After detecting an open-circuit fault in any of the converter power switches, the control of the converter is re-adapted in order to minimize the adverse impacts of an open-circuit fault, namely the increase of the current ripple. To verify the effectiveness of these strategies, laboratory tests were conducted, using a three-phase interleaved boost converter prototype connected to a resistive load.
- Fault Tolerant DC–DC Converters at Homes and OfficesPublication . Bento, Fernando José Figueiredo; Cardoso, António João MarquesThe emergence of direct current (DC) microgrids within the context of residential buildings and offices brings in a whole new paradigm in energy distribution. As a result, a set of technical challenges arise, concerning the adoption of efficient, cost-effective, and reliable DC-compatible power conditioning solutions, suitable to interface DC microgrids and energy consuming elements. This thesis encompasses the development of DC–DC power conversion solutions, featuring improved availability and efficiency, suitable to meet the requirements of a comprehensive set of end-uses commonly found in homes and offices. Based on the energy consumption profiles and requirements of the typical elements found at homes and offices, three distinctive groups are established: light-emitting diode (LED) lighting, electric vehicle (EV) charging, and general appliances. For each group, a careful evaluation of the criteria to fulfil is performed, based on which at least one DC–DC power converter is selected and investigated. Totally, a set of five DC–DC converter topologies are addressed in this work, being specific aspects related to fault diagnosis and/or fault tolerance analysed with particular detail in two of them. Firstly, mathematical models are described for LED devices and EV batteries, for the development of a theoretical analysis of the systems’ operation through computational simulations. Based on a compilation of requirements to account for in each end-use (LED lighting, EV charging, and general appliances), brief design considerations are drawn for each converter topology, regarding their architecture and control strategy. Aiming a detailed understanding of the two DC–DC power conversion systems subjected to thorough evaluation in this work – interleaved boost converter and fault-tolerant single-inductor multiple-output (SIMO) converter – under both normal and abnormal conditions, the operation of the systems is evaluated in the presence of open-circuit (OC) faults. Parameters of interest are monitored and evaluated to understand how the failures impact the operation of the entire system. At this stage, valuable information is obtained for the development of fault diagnosis strategies. Taking profit of the data collected in the analysis, a novel fault diagnostic strategy is presented, targeting interleaved DC–DC boost converters for general appliances. Ease of implementation, fast diagnostic and robustness against false alarms distinguish the proposed approach over the state-of-the-art. Its effectiveness is confirmed through a set of operation scenarios, implemented in both simulation environment and experimental context. Finally, an extensive set of reconfiguration strategies is presented and evaluated, aiming to grant fault tolerance capability to the multiple DC–DC converter topologies under analysis. A hybrid reconfiguration approach is developed for the interleaved boost converter. It is demonstrated that the combination of reconfiguration strategies promotes remarkable improvements on the post-fault operation of the converter. In addition, an alternative SIMO converter architecture, featuring inherent tolerance against OC faults, is presented and described. To exploit the OC fault tolerance capability of the fault-tolerant SIMO converter, a converter topology targeted at residential LED lighting systems, two alternative reconfiguration strategies are presented and evaluated in detail. Results obtained from computational simulations and experimental tests confirm the effectiveness of the approaches. To further improve the fault-tolerant SIMO converter with regards to its robustness against sensor faults, while simplifying its hardware architecture, a sensorless current control strategy is presented. The proposed control strategy is evaluated resorting to computational simulations.
- Open-Circuit Fault Diagnosis and Fault Tolerant Operation of Interleaved DC–DC Boost Converters for Homes and OfficesPublication . Bento, Fernando; Cardoso, A. J. M.The use of electronic equipment and other dc-compatible appliances in homes and offices is increasing at a very fast rate, as well as the distributed generation of energy. These two statements will most likely trigger, in a near future, the adoption of district-scale dc grids, connecting dc microgeneration plants and consumers, in an effort to cut the number of conversion steps required to deliver power to dc appliances. DC-DC converters will be fundamental in these systems, allowing the establishment of several voltage levels. The reliability of these converters plays a critical role, ensuring service continuity of the loads connected to them. With the increment of reliability in mind, this paper proposes a simple yet effective open-circuit fault diagnostic method suitable for interleaved dc-dc boost converters, but equally functional for other dc-dc converter topologies. The improvements introduced in this paper greatly simplify the diagnostic action, while making the correct diagnostic of faults independent of the switching duty cycle. Moreover, an alternative reconfiguration scheme is proposed to mitigate the adverse impacts of such faults. To prove the effectiveness of these strategies, a three-phase interleaved boost converter prototype was used in the experiments.
- Thermal Analysis of a Directly Grid-Fed Induction Machine With Floating Neutral Point, Operating Under Unbalanced Voltage ConditionsPublication . Santiago, Fábio Emanuel Pereira; Bento, Fernando; Cardoso, A. J. M.; Gyftakis, Konstantinos N.Significant changes may occur in the thermal behavior of a directly grid-fed induction machine when subjected to unbalances in the voltage supply. This article studies and analyzes the thermal behavior of a low power, three-phase, squirrel-cage induction motor, connected in star configuration with floating neutral point, when subjected to different levels of unbalanced voltage. The dependence of the thermal motor behavior on the severity level of the unbalance is studied and analyzed. In addition to amplitude unbalances, this article focuses on the effects of phase unbalances, as well, which were not addressed in detail in previous published studies. Moreover, situations of mixed unbalance, where amplitude and phase unbalances occur simultaneously, are also studied. The finite element method was used to simulate the thermal behavior of the machine. The experimental setup consists of a three-phase programmable AC power supply, suitable to precisely emulate unbalanced conditions that may occur in real-scale power systems, supplying a 2.2 kW induction motor. Experimental data were acquired resorting to resistance temperature detectors PT100, placed in the machine phase whose supply current value changed the most. Finally, the simulation results are verified and critically discussed through experimentally obtained results.