Modular Power Converters for Smart Transformer Architectures
The large integration of renewable energy sources and intermittent loads (e.g. electric vehicles) into the utility grid has imposed new challenges to the electric distribution network with respect to the voltage regulation, power quality and reliability. The Smart Transformer (ST) has emerged as a technology able to solve most of these problems, providing a compact and efficient solution. Several Smart Transformer architectures have been discussed in literature. The three stages architecture (composed by a Medium Voltage AC-DC stage, DC-DC stage and a Low Voltage DC-AC stage) has been demonstrated to be the most compelling configuration, because it ensures better decoupling between the input and output voltage, when compared to other architectures. The conceptualization and design of the architecture are still very challenging, because of the broad options of power converters available for each stage, and how they can be connected. In this context, this thesis investigates the most promising architectures of the ST and power converters that should be used to fulfill all the requirements. To handle the MV level involved on the power conversion, the modular approach is used to share the voltage among the modules, allowing the use of low voltage rating devices (providing better performance and broad choice of devices). Additional advantages of the modular approach are: fault-tolerance, scalability in voltage and power. Each stage of the smart transformer architecture is evaluated and designed. The DC-DC stage is the most challenging one for implementing the system. Consequently, this work has a special focus on the DC-DC converters, and how to select and design them. Moreover, a new fault tolerant DC-DC converter is proposed to increase the reliability of the system. From the system level perspective, the optimum number of modules in function of the grid voltage is derived. Finally, the smart transformed is designed and experimentally evaluated.
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