DC Power Distribution Networks with Direct Current Transformers

The shift towards DC power distribution networks, enabled by power electronics technologies, is changing the nature of electrical power systems. Nowadays, DC power distribution networks can effectively support the high penetration of distributed energy resources and energy storage integration (both increasingly being DC by nature) in future electrical systems.

In this context, the DC transformer is an analogy to the AC transformer, playing a crucial role in developing advanced DC power systems by integrating various power sources and loads across different voltage levels. It facilitates the natural power flow between two DC buses, maintaining nearly constant gain under different loads, particularly at near-resonant frequencies. Consequently, the simple nature of AC transformers can be replicated in the DC transformer with some additional operational logic.

This thesis delves into the DC transformer's characteristics and its effect on DC power networks. It comprises two segments: the first focuses on the DC transformer, outlining strategies for effective open-loop operation, such as power reversal for bidirectional flow, soft start, and idle mode to reduce no-load losses. The feasibility of these methods is proven through low-voltage prototypes and a medium-voltage prototype. Besides that, the scalability aspect of the DC transformers by connecting several units in parallel is investigated with a sensitivity analysis of the resonant parameters.

The second part addresses system-level analysis in DC power distribution networks with DC transformers. It introduces mathematical tools and methodologies for DC systems, independent of the AC loop, using Modified Nodal Analysis and Harmonic Resonance Mode Analysis. Additionally, hardware-in-the-loop simulations assess various DC network architectures, voltage control speeds, and AC grid impedance impacts on resonant characteristics. This aids in understanding the planning, operability, and stability of future DC networks with DC transformers.

The main contributions of this thesis are the demonstration of the essential features of DC transformers and the development of a scalable methodology for system analysis of multi-converter DC power distribution networks with DC transformers. The modeling framework and methodology are described, discussed, and validated with experimental results. %Ultimately, all the developments present in this thesis contribute to the research and development of DC transformers for future DC grids.