THERMAL STRESS MITIGATION OF SINGLE-PHASE SINEWAVE INVERTER BY USING DOUBLE SWITCH H BRIDGE CONFIGURATION

The increasing demand for renewable energies and the ongoing advancement in the industry require continuously evolving power converters in terms of efficiency, power density, and reliability. Furthermore, power converters’ applications in harsh and remote environments such as offshore wind turbines demand robust and reliable designs to help reduce operational costs. Power switch failure is a critical reliability issue that leads to the converter going out of service, causing an unscheduled maintenance event. The main reason behind power switch failure is thermal cycling. Therefore, the first part of this thesis attempts to develop an effective double switch H bridge inverter topology aiming to lessen thermal cycling subjected to power switches, increasing the expected lifetime of power switches, improving the system\u27s overall reliability, and reducing operational costs. Meanwhile, the second focus of the thesis is to develop a visual interpretation of an empirical lifetime estimation model that enables the evaluation of the proposed inverter topology compared to the conventional topology. This is done by producing a novel lifetime improvement evaluation curve based on a common empirical lifetime estimation model using MATLAB®. Moreover, the interpretation of the empirical lifetime estimation models as a lifetime improvement evaluation curve helps to bridge the gap between any thermal condition change and its impact on the expected lifetime. The percentage reduction in the junction’s median temperature %_ and the percentage reduction in the temperature swing %Δ_ are taken as the main contributors to the change in the switch’s estimated cycles to failure . The effectiveness of the proposed topology was verified via simulation of the thermal parameters for the two topologies via PLECS® software. Several test scenarios were performed to illustrate the impact of shifting from the conventional topology to the proposed topology. Following that, numerous loading conditions were considered to perform an extensive comparative analysis between the proposed and the conventional topologies. Three power factor values were adopted at high, medium, and low values; to compare the two topologies while covering an adequate loading range for each power factor value. The assessment indices, namely, Life Prolonging Factor (LPF), and the average LPF (in a temperature range) obtained promising results, especially for high loading levels conditions. The LPF reached values more than ‘2’ under some conditions, indicating a more than double lifetime increase. Furthermore, the average LPF in a specific temperature range indicated promising results in general for common loading conditions with an advantage for higher loading conditions over lower loading conditions

Health Condition Monitoring and Fault-Tolerant Operation of Adjustable Speed Drives

Adjustable speed drives (ASDs) have been extensively used in industrial applications over the past few decades because of their benefits of energy saving and control flexibilities. However, the wider penetration of ASD systems into industrial applications is hindered by the lack of health monitoring and fault-tolerant operation techniques, especially in safety-critical applications. In this dissertation, a comprehensive portfolio of health condition monitoring and fault-tolerant operation strategies is developed and implemented for multilevel neutral-point-clamped (NPC) power converters in ASDs. Simulations and experiments show that these techniques can improve power cycling lifetime of power transistors, on-line diagnosis of switch faults, and fault-tolerant capabilities.The first contribution of this dissertation is the development of a lifetime improvement Pulse Width Modulation (PWM) method which can significantly extend the power cycling lifetime of Insulated Gate Bipolar Transistors (IGBTs) in NPC inverters operating at low frequencies. This PWM method is achieved by injecting a zero-sequence signal with a frequency higher than that of the IGBT junction-to-case thermal time constants. This, in turn, lowers IGBT junction temperatures at low output frequencies. Thermal models, simulation and experimental verifications are carried out to confirm the effectiveness of this PWM method. As a second contribution of this dissertation, a novel on-line diagnostic method is developed for electronic switch faults in power converters. Targeted at three-level NPC converters, this diagnostic method can diagnose any IGBT faults by utilizing the information on the dc-bus neutral-point current and switching states. This diagnostic method only requires one additional current sensor for sensing the neutral-point current. Simulation and experimental results verified the efficacy of this diagnostic method.The third contribution consists of the development and implementation of a fault-tolerant topology for T-Type NPC power converters. In this fault-tolerant topology, one additional phase leg is added to the original T-Type NPC converter. In addition to providing a fault-tolerant solution to certain switch faults in the converter, this fault-tolerant topology can share the overload current with the original phase legs, thus increasing the overload capabilities of the power converters. A lab-scale 30-kVA ASD based on this proposed topology is implemented and the experimental results verified its benefits

Optimized Modulation and Thermal Management for Modular Power Converters

The transition to a more and more decentralized power generation based on renewable energy generation is accompanied by high challenges. Modular power converters play a central role in facing these challenges, not only for grid integration but also to provide flexible services, highly efficient power transmission and safe storage integration. These goals are the key elements in becoming independent from fossil and nuclear power plants in near future. Even if the costs for renewable energy power plants like wind or photovoltaic systems are already competitive to conventional solutions, more flexible operation and further reduction in costs are required for faster global transformation towards sustainable energy systems. The further optimization of modular power converters can be seen as an ideal way to achieve these ambitious goals. It is therefore chosen as the focus of this work

Methods and Results of Power Cycling Tests for Semiconductor Power Devices

This work intends to enhance the state of the research in power cycling tests with statements on achievable measurement accuracy, proposed test bench topologies and recommendations on improved test strategies for various types of semiconductor power devices. Chapters 1 and 2 describe the current state of the power cycling tests in the context of design for reliability comprising applicable standards and lifetime models. Measurement methods in power cycling tests for the essential physical parameters are explained in chapter 3. The dynamic and static measurement accuracy of voltage, current and temperature are discussed. The feasibly achievable measurement delay tmd of the maximal junction temperature Tjmax, its consequences on accuracy and methods to extrapolate to the time point of the turn-off event are explained. A method to characterize the thermal path of devices to the heatsink via measurements of the thermal impedance Zth is explained. Test bench topologies starting from standard setups, single to multi leg DC benches are discussed in chapter 4. Three application-closer setups implemented by the author are explained. For tests on thyristors a test concept with truncated sinusoidal current waveforms and online temperature measurement is introduced. An inverter-like topology with actively switching IGBTs is presented. In contrast to standard setups, there the devices under test prove switching capability until reaching the end-of-life criteria. Finally, a high frequency switching topology with low DC-link voltage and switching losses contributing significantly to the overall power losses is presented providing new degrees of freedom for setting test conditions. The particularities of semiconductor power devices in power cycling tests are thematized in chapter 5. The first part describes standard packages and addressed failure mechanisms in power cycling. For all relevant power electronic devices in silicon and silicon carbide, the devices’ characteristics, methods for power cycling and their consequences for test results are explained. The work is concluded and suggestions for future work are given in chapter 6.:Abstract 1 Kurzfassung 3 Acknowledgements 5 Nomenclature 10 Abbreviations 10 Symbols 12 1 Introduction 19 2 Applicable Standards and Lifetime Models 25 3 Measurement parameters in power cycling tests 53 4 Test Bench Topologies 121 5 Semiconductor Power Devices in Power Cycling 158 6 Conclusion and Outlook 229 References 235 List of Publications 253 Theses 257Diese Arbeit bereichert den Stand der Wissenschaft auf dem Gebiet von Lastwechseltests mit Beiträgen zu verbesserter Messgenauigkeit, vorgeschlagenen Teststandstopologien und verbesserten Teststrategien für verschiedene Arten von leistungselektronischen Bauelementen. Kurzgefasst der Methodik von Lastwechseltests. Das erste Themengebiet in Kapitel 1 und Kapitel 2 beschreibt den aktuellen Stand zu Lastwechseltests im Kontext von Design für Zuverlässigkeit, welcher in anzuwendenden Standards und publizierten Lebensdauermodellen dokumentiert ist. Messmethoden für relevante physikalische Parameter in Lastwechseltests sind in Kapitel 3. erläutert. Zunächst werden dynamische und statische Messgenauigkeit für Spannung, Strom und Temperaturen diskutiert. Die tatsächlich erreichbare Messverzögerung tMD der maximalen Sperrschichttemperatur Tjmax und deren Auswirkung auf die Messgenauigkeit der Lastwechselfestigkeit wird dargelegt. Danach werden Methoden zur Rückextrapolation zum Zeitpunkt des Abschaltvorgangs des Laststroms diskutiert. Schließlich wird die Charakterisierung des Wärmepfads vom Bauelement zur Wärmesenke mittels Messung der thermischen Impedanz Zth behandelt. In Kapitel 4 werden Teststandstopologien beginnend mit standardmäßig genutzten ein- und mehrsträngigen DC-Testständen vorgestellt. Drei vom Autor umgesetzte anwendungsnahe Topologien werden erklärt. Für Tests mit Thyristoren wird ein Testkonzept mit angeschnittenem sinusförmigem Strom und in situ Messung der Sperrschichttemperatur eingeführt. Eine umrichterähnliche Topologie mit aktiv schaltenden IGBTs wird vorgestellt. Zuletzt wird eine Topologie mit hoch frequent schaltenden Prüflingen an niedriger Gleichspannung bei der Schaltverluste signifikant zur Erwärmung der Prüflinge beitragen vorgestellt. Dies ermöglicht neue Freiheitsgrade um Testbedingungen zu wählen. Die Besonderheiten von leistungselektronischen Bauelementen werden in Kapitel 5 thematisiert. Der erste Teil beschreibt Gehäusetypen und adressierte Fehlermechanismen in Lastwechseltests. Für alle untersuchten Bauelementtypen in Silizium und Siliziumkarbid werden Charakteristiken, empfohlene Methoden für Lastwechseltests und Einflüsse auf Testergebnisse erklärt. Die Arbeit wird in Kapitel 6 zusammengefasst und Vorschläge zu künftigen Arbeiten werden unterbreitet.:Abstract 1 Kurzfassung 3 Acknowledgements 5 Nomenclature 10 Abbreviations 10 Symbols 12 1 Introduction 19 2 Applicable Standards and Lifetime Models 25 3 Measurement parameters in power cycling tests 53 4 Test Bench Topologies 121 5 Semiconductor Power Devices in Power Cycling 158 6 Conclusion and Outlook 229 References 235 List of Publications 253 Theses 25

Advanced Silicon Carbide Based Fault-Tolerant Multilevel Converters

The number of safety-critical loads in electric power areas have been increasing drastically in the last two decades. These loads include the emerging more-electric aircraft (MEA), uninterruptible power supplies (UPS), high-power medical instruments, electric and hybrid electric vehicles (EV/HEV) and ships for military use, electric space rovers for space exploration and the like. This dissertation introduces two novel fault-tolerant three-level power converter topologies, named advanced three-level active neutral point clamped converter (A3L-ANPC) and advanced three-level active T-Type (A3L-ATT) converter. The goal of these converters is to increase the reliability of multilevel power converters used in safety-critical applications.These new fault-tolerant multilevel power converters are derived from the conventional ANPC and T-Type converter topologies. The topologies has significantly improved the fault-tolerant capability under any open circuit or certain short-circuit faults in the power semiconductor devices. In addition, under healthy conditions, the redundant phase leg can be utilized to share overload current with other main legs, which enhances the overload capability of the converter. The conduction losses in the power devices can be reduced by sharing the load current with the redundant leg. Moreover, unlike other existing fault-tolerant power converters in the literature, full output voltages can be always obtained during fault-tolerant operation. Experimental prototypes of both the A3L-ANPC and A3L-ATT converters were built based on Silicon Carbide (SiC) MOSFETs. Experimental results confirmed the anticipated performance of the novel three-level converter topologies.SiC MOSFET technology is at the forefront of significant advances in electric power conversion. SiC MOSFETs switch significantly faster than the conventional Silicon counterparts resulting in power converters with higher efficiency and increased switching frequencies. Low switching losses are one of the key characteristics of SiC technology. In this dissertation, hard and soft switching losses of a high power SiC MOSFET module are measured and characterized at different voltage and current operating points to determine the maximum operating frequency of the module. The purpose of characterizing the SiC MOSFET module is to determine the feasibility of very high frequency (200kHz-1MHz) power conversion which may not be possible to be implemented in the conventional Silicon based high power conversion. The results show that higher switching frequencies are achievable with soft switching techniques in high power converters