High-Power DC-DC Converter Technologies for Smart Grid and Traction Applications

Electric transformers, either utilized in electric power systems or within a power electronics-based converter, are fundamental components of modern highly-efficient energy supply chains, as they provide the voltage adaptation between networks with different voltage levels. In traction applications for example, the power for the drive system is provided through a single-phase Medium Voltage (MV) catenary line, whereas the traction machine is fed by a Low Voltage (LV) three-phase inverter. It is therefore necessary to step this MV down to the required LV level through a Low Frequency (LF) transformer which needs to be carried by the driving locomotive. Since the performance of these traction solutions is tightly linked to the weight of the energy supply system, which is to a large extent determined by the step-down and isolation transformer, lower weight energy supply concepts for modern traction solutions are highly desirable and cannot be fulfilled by classical concepts. In electric power systems, transformers also perform the fundamental task of adapting the voltage between the generation, transmission and distribution levels. In modern electric power systems, high flexibility in power delivery is becoming mandatory, as new types of sources, e.g. renewable energy generators, and dynamic loads are integrated into the power grid. Given the dynamic behavior of these sources/loads, a smart energy routing concept, i.e. a Smart Grid, which ensures a stable and reliable operation of the network is required. This task of fast and active response to variations in the grid conditions cannot be met by standard LF transformers, as they are intrinsically passive components. These limiting factors for future traction solutions and Smart Grid implementation can be overcome by integrating more controllable components within these supply chains. In the case of traction applications, this would mean the integration of the isolation stage within the converter system in order to enable the operation of this component at higher frequencies, and therefore a reduction in its weight. Considering Smart Grids energy routing solutions, the main requirement is the flexibility in power delivery, which results in the partial or total processing