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In order to interconnect renewable energy sources, electric vehicles, battery storage and energy efficient loads, our electricity provision will increasingly rely upon power electronic converters. Power electronic converters are essential to connect photovoltaic modules to the grid, to charge electric vehicles and to supply heat pumps, to list some examples. These power electronic converters typically encompass an AC-DC conversion step, as the majority of the aforementioned applications finally operates on direct current (DC), while the electrical grid runs primarily on alternating current (AC). Our present electrical distribution system would however be more compatible with sources, loads and storage if DC, instead of AC, would have been adopted as the mainstream technology-of-choice, such as T. Edison envisioned back in the early days. A higher compatibility between the electrical infrastructure and the devices connected to it results in an appreciable simplification of the power conversion steps, yielding increased conversion efficiency, component-level reliability and compactness. Apart from that, DC enables transferring more power across the same cable, depending on the adopted voltage level.
For low-voltage DC (LVDC) distribution systems, two network configurations exist, namely the two-wire unipolar configuration and the bipolar three-wire configuration. The bipolar configuration encompasses a positive, neutral and negative conductor (operating at for example +/-380 V) and permits to transfer more power with less losses as compared to its unipolar counterpart. Furthermore, two voltage levels are available for connecting devices at an appropriate voltage level.
Although delivering decisive advantages over unipolar DC systems, additional complexity is introduced at the control level, because both the positive and negative pole-to-neutral voltage need to be controlled. Even in case of unbalanced loading conditions, the voltages require active control in order to avoid equipment from shutting down or being damaged because of under- or overvoltages.
The main objective of this thesis is to realize a stable and reliable bipolar LVDC system, subject to asymmetric loading conditions. Throughout this thesis, a strong emphasis is put on experimental validation. In the framework of this thesis a +/-500 V 100 kW bipolar LVDC test facility has been set-up, one of the five deployed worldwide at present. To stabilize the voltage, a dedicated DC/DC converter for balancing bipolar DC distribution systems is theoretically analyzed, prototyped and extensively tested. Furthermore, a voltage control strategy is proposed which enables to parallelize multiple of the aforementioned converters without any means of communication amongst them.