Modular DC/DC Converter for DC Distribution and Collection Networks

A major change in the electrical transmission and distribution system is taking place in Europe at the moment. The shift from a centralised energy production to a distributed generation profoundly changes the behaviour of the grid. Environmental or social issues associated with the construction of new power lines to relieve bottlenecks, together with aged equipment dating from the 1960s, pose some serious challenges to government, the research community and the economy. Concepts of reactive compensation, harmonic cancellation, voltage stability, power quality and bulky low-frequency transformers need to be redefined for power exchange and transmission in the future. Photovoltaics, wind turbines, fuel cells, storage systems and uninterruptible power supplies use many power electronic interface circuits, where DC intermediate levels already exist. Large photovoltaic- or wind- powered installations, which are connected to a cable network, are characterised by non-negligible distances due to their low power-by-surface density. On the side of the consumer, current trends show an increasing use of DC in end-user equipment. In such a context, the numerous advantages of power electronics and DC cables may sometimes out-weigh their higher cost. In the future, high-power semiconductor devices that allow higher switching frequencies of the converters may make it possible to down-size even more the passive components. This would significantly reduce raw material consumption and therefore cost, something that is crucial for the market to accept the technology. In the first part of this PhD thesis, the advantages of DC distribution in terms of transmission losses are illustrated with the help of three case studies. The second part and the main contribution of this thesis is the analysis of a promising candidate for a power electronic transformer, the key component of any DC based grid. It is a bidirectional isolated DC/DC converter based on modular multilevel converters, which are well suited for medium or even high voltage range. The motivation was to investigate a converter operation with important voltage elevation ratios, capable of adapting the voltage level between low, medium and high voltage. A medium-frequency isolation stage provides the possibility of downsizing the passive components. Two modulation methods, a multilevel and a two-level operation, were analysed and compared in terms of losses. The modular DC/DC converter is an attractive solution for the sensitive aspect of the short-circuit behaviour of classical DC links and power lines. The converter can also handle short circuits without the need for additional protection devices, such as circuit breakers. Given the many advantages of DC systems (reduced environmental impact, reduced space requirements, reduced raw material use, high power quality, power flow control, low transmission losses), this new technology must, at least, be considered when assessing the extension or the renovation of conventional AC grids.