Control Strategies for Improving Reliability and Efficiency in Modular Power Converters

The significance of modular power converters has escalated drastically in various applications such as electrical energy distribution, industrial motor drives and More Electric Aircraft (MEA) owing to the benefits such as scalability, design flexibility, higher degree of fault tolerance and better maintenance. One of the main advantages of modular systems is the ability to replace the faulty converter cells during maintenance instead of the entire system. However, such maintenance cycles can result in a system of converter cells with different aging. A system with cells having different aging arises the threats of multiple maintenance, lower reliability and availability, and high maintenance costs. For controlling the thermal-stress based aging of modular power converters, power routing strategy was proposed. The thesis focuses on the different implementation strategies of power routing for modular converters. Power semiconductors are one of the most reliability critical components in power converters, and thermal-stress has been identified as the main cause of their failure. This thesis work concentrates on the power semiconductor reliability improvement algorithms. For improving system lifetime, virtual resistor based power routing algorithms for single stage and multi-stage modular architectures have been investigated through simulations and validated with experiment. A unified framework for routing the power in complex modular converter architectures is defined based on graph theory. Popular converter architectures for Smart Transformer (ST) and MEA applications are modeled as graphs to serve as the basis for developing power flow optimization. The effectiveness of graph theory for optimizing the power flow in modular systems is demonstrated with the help of proposed algorithms

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

DC collection systems for offshore wind farms

Power generation through natural resources has found to be one of the best options to minimise climate change and global warming concerns. Among the naturally replenish sources, power generation from offshore wind accounts for a larger share. This has been showcased by the rapid development of offshore wind farms (OWF)s especial in the North sea. At the OWF collection system level, only alternating current (ac) technology is being used at present. Conversely, the use of direct current (dc) technology could provide additional benefits in terms of control flexibility, minimising system losses, and increasing power density of components. However, there are still a number of technical challenges that require addressing. One of the major aspects is the reliability of this concept as a whole. The research work presented in this thesis is aimed to address the existing challenges, in particular, from the component level to the system level from the perspective of reliability. The main contributions of this research work comprise of four parts, namely, (1) reliability analysis of semiconductors of dc-wind turbine machine side converter, (2) propose a new selection guideline based on reliability and costs to identify the most suitable multi-level converter topology for offshore wind power dc collection systems at different voltage levels and power levels, (3) identification of the most suitable dc collection system topology in terms of reliability and other economic factors, and (4) development of an analytical methodology to asses the availability of offshore wind farms considering the cable network dependency. One of the key building blocks of a dc collection system is the dc wind turbine (dcWT). The lifespan of a wind power system is highly influenced by the reliable operation of its power converter. A mission-profile based reliability assessment technique considering long-term and short-term thermal cycles are used to evaluate the lifetime of power electronic components of a dual active bridge based dcWT. Further, to ensure an effective lifetime evaluation of the entire converter system, a Monte Carlo method is used to generate the lifetime distributions and entire unreliability functions for power semiconductors. To utilise the full capacity of the dc technology in the context of the OWF collection system, the selection of a suitable power electronic converter topology is a key aspect. iii iv A selection criterion based on the optimal redundancy level with the consideration of the converter reliability, preventive maintenance interval, operational efficiency, the total cost of ownership and return on investment is proposed. The primary motivation of this work is to investigate the feasibility of utilising suitable multi-level voltage source converter topologies at different medium voltage dc levels and power levels. To select a suitable dc collection system topology, a comprehensive analytical reliability evaluation method based on Universal Generating Function (UGF) is proposed with associated economic factors. This strategy combines the stochasticity of wind with multiple power output states of a single wind turbine (WT). Subsequently, the relationship between the output states and corresponding state probabilities of WTs are combined using the UGF technique considering the network structure. To identify the best topology, the investment- and operating- costs (which includes network losses) are incorporated. The OWF collection system is made up of a considerable number of inter-array cables. The effectiveness of the OWF to export energy to the grid depends on the availability of that network. Therefore, it is imperative to include the reliability of the collection system in the overall availability assessment. However, this increases the number of components significantly, introducing the dimension curse. This combined with wind turbine output dependence makes the inclusion of the collection system in OWF availability assessment computationally intractable. An analytical reliability model based on the UGF technique is proposed accounting for the cable network dependency. Further, the impact of modelling wind farm components using a binary Markov model rather than a multi-state one is also investigate