Existing DC tram traction networks (DCTTN) provide intermittent regenerative energy and quasi-persistent surplus capacity as they are dimensioned for peak power demand. The deployment of DC microgrids is increasingly on the rise largely due to the compatibility with DC-native loads and avoiding AC grid congestion issues. In particular, the bipolar DC microgrid (BDCMG) is considered, as it enables different voltage levels, requires less copper for cabling and enhances reliability compared to unipolar architectures. Interlinking these two energy networks through isolated bidirectional DC-DC converters (IBDCs) offers a favorable opportunity to enhance the operation of the DC microgrid, particularly during periods of local demand fluctuations or utility disturbances.
This thesis considers three IBDC topologies for the interface: the CLLC resonant converter, single- and three-phase dual active bridges (DABs). Simulation-based comparisons of the 1-phase and 3-phase DAB demonstrate that the 3-phase DAB achieves higher efficiencies across operating points considered based on industry standards, requires smaller output capacitance, which enables a higher power density. This is advantageous for facilitating a modular architecture with high-frequency SiC MOSFETs, which not only allows passive components to be smaller in size and thus lower the overall system cost, but also ensures redundancy and scalability towards different energy demands of systems (DCTTN and BDCMG) at different locations or cities. To ensure robustness against the electrically demanding tram traction environment, it is necessary to consider a dedicated input circuit to protect the downstream components against damage from surge overvoltages as well as ensuring that the input circuit does not compromise the stability of the IBDCS. Simulation-based comparisons of two different passive damping methods (RC and RL) used on multi-stage LC filter topologies (part of the input circuit) were investigated. Results indicate that the RL damped filters require smaller footprint size, achieve lower output impedance and improved transient voltage attenuation. Based on Middlebrook’s impedance-based stability criterion, the inclusion of the input circuit and the DCTTN’s overhead line impedance did not compromise the stability of the system (input circuit + IBDC). Adopting a modular approach of stacking IBDCs, two distinct configurations of connecting the input circuit with the IBDCs were taken into consideration. A single input circuit for a 150 kW load showed a smaller size and weight and lower cost per kW compared to using three smaller input circuits for 50 kW loads each.

The rapid growth in electric vehicle (EV) adoption, particularly in densely populated urban areas, together with the global aim of reducing greenhouse gas emissions, is placing increasing stress on existing power distribution infrastructure. Bipolar DC microgrids present a robust and flexible solution for integrating daily DC loads such as lighting networks and EV chargers, offering higher efficiency due to fewer power conversion stages compared to the AC grid system. This project focuses on developing a cost-effective, reliable and easily deployable EV charger that enables seamless integration of DC microgrids into today’s AC-based urban systems, leveraging the existing Dutch public lighting network. By utilising the already-installed AC grid combi cables with 6mm2 auxiliary copper conductors, the proposed converter operating at ±350V (700V bipolar DC) and 15A with rated power of 11 kW can deliver power directly to dual street-side parking spots while preserving sufficient current capacity for lighting loads. The goal is to enable practical low-power charging in locations where vehicles remain parked for extended periods, such as overnight or during working hours. High reliability and minimal maintenance needed are key requirements to support the large-scale deployment of DC-based EV charging in urban environments.
A comprehensive review and comparison of wide-bandgap semiconductor technologies and isolated DC-DC converter topologies for dual-output, bidirectional operation led to the selection of a SiC-based Triple Active Bridge (TAB) as the most balanced solution in terms of performance, reliability, complexity and cost. The TAB converter is an evolution of the conventional Dual Active Bridge (DAB) topology, enabling direct power transfer between three ports. The inclusion of a third port, however, increases the complexity of power flow management as a single core transformer allocates the three windings. A detailed theoretical analysis, supported by open-loop simulations, guided the parameter design of the converter to ensure zero-voltage switching (ZVS) across all charging scenarios. The full mathematical expressions of the transformer’s AC currents, obtained through Fourier series decomposition of the full-bridge voltages, allowed comparing different current measurement approaches for closed-loop DC current control. Implementing the control strategies under real operating conditions required investigating scenarios such as the sudden disconnection of one EV and single-vehicle charging. These cases deviate from the typical three-port operation and are critical to analyze in order to ensure safe and efficient performance. Building on these results, a passively cooled hardware prototype was developed, integrating the power semiconductors, high-frequency transformer, filters, protections, and thermal management. Experimental validation confirmed the converter’s functionality, demonstrating a complete workflow from concept to prototype for an EV charger designed to be integrated in the emerging DC microgrid infrastructure.

The global shift towards sustainable energy sources, together with rapid technological advancements, has driven a significant increase in the demand for electric vehicles (EVs) over recent years. This surge in EV adoption highlights the critical need for robust supporting infrastructure, particularly in the deployment of efficient and accessible EV chargers. Gallium Nitride (GaN) is a wide bandgap semiconductor material that offers better performance characteristics compared to traditional silicon-based components in medium-power and high-frequency application. Its higher electron mobility, saturation electron velocity and breakdown electric field make GaN an ideal candidate for high-frequency power conversion applications in EV chargers. To explore the potential of GaN technology in EV charging applications, an 11kW GaN based isolated bidirectional multi-port EV charger with a two-stage architecture has been designed to interconnect bipolar DC grid and electric vehicles. The two stages of the charger consist of a three-level triple active bridge converter and a three-level interleaved buck converter. The TAB converter is an extension of conventional dual active bridge (DAB) converter, offering capability to direct power transmission among three ports. However, the addition of a third port introduces complexity in managing power flow. The TAB converter can operate under various modulation schemes, including three-level and five-level modulation. An analysis of the switching modes associated with each modulation scheme has been
erformed. In order or derive the soft switching range of the converter, the current expression
of series inductors of each port are derived based on generalized-harmonic approximation method and equivalent circuit models. A detailed analysis of the commutation process during critical switching transitions has been conducted, leading to the derivation of the soft
witching criteria for both three-level and five-level operations of the TAB converter. To validate the design and verify the soft switching range, simulations have been carried out. Moreover, the prototype of the 11 kW multi-port EV charger is designed based on calculation and simulation results. The prototype employs a top-side cooled GaN high-electron-mobility transistor (HEMT) to enhance thermal dissipation. Testing of the prototype indicates that the converter can transmit 1.2 kW from the primary to the secondary side at the rated voltage
of 700 V.